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High strength steel bolted connections with filler plates
Journal of Constructional Steel Research 66 (2010) 75–84
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High strength steel bolted connections with filler plates Peter Dusicka ∗ , Gregory Lewis Portland State University, Department of Civil and Environmental Engineering, P.O. Box 751, Portland, OR 97207, United States
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Article history: Received 1 February 2009 Accepted 31 July 2009 Keywords: Splice Filler plate High performance steel Oversize
abstract The effects of undeveloped filler plates were experimentally evaluated for high strength steel bolted connections using standard size holes, oversize holes and multiple fillers. Filler plates up to 51 mm (2 in.) thick were utilized in connecting high performance A709 HPS 480W (HPS 70W) grade steel plates using 22 mm (0.88 in.) A490 bolts. With increasing filler thickness, the connection strength was found to decrease up to a limit and then recover for thicker fillers in standard and oversized hole connections. Multiple filler connections experienced the greatest strength decrease as the bolt failure shifted to the threads located outside the shear planes. Deformations also exhibited a limit, however the use of oversize holes resulted in 1.7 times larger deformations than standard or multiple fillers. Recommendations were developed for strength reduction and deformation amplification as function of filler thickness and bolt diameter. Published by Elsevier Ltd
1. Introduction Filler plates are used in steel bolted connections to compensate for dimensional differences, including flange plate thickness in bridge girders and chord to diagonal member depths in long span building trusses. Filler plates may be developed to act together with the connection plates or undeveloped where no special accommodations are made to make the filler part of the connection [1]. Developed fillers can be achieved by welding or by bolting an extended portion of the filler beyond the connection, causing the plates to act as one unit in sharing the imposed stresses. Undeveloped fillers merely provide a common faying surface and can move independently with respect to the connecting members as stresses build. Designs of connections with undeveloped fillers rely on data from tests on connections with single and multiple fillers up to 19 mm (0.75 in.) thick [2]. Those specimens utilized A325M (A325) bolts with A514M grade 345 MPa (50 ksi) splice plates and A36 filler plates. The results indicated that the greater the undeveloped filler thickness, the greater the deformation of the connection. The consequent bending of the bolt was found to reduce the strength of the connection and increase the deformation at failure. Multiple fillers reduced the capacity to a greater degree than single fillers, although the differences between the ultimate load and maximum deformation for three 6 mm (0.25 in.) filler plates and one 19 mm (0.75 in.) filler plate were modest. Based on these results an empirical strength reduction factor was proposed that related the
reduction in shear strength of the bolt to the filler plate thickness t. Design guides and specifications in the United States (US) further modified this linear relationship such that no strength reduction resulted from fillers of 6 mm (0.25 in.) thickness or lower, leading to a unit dependent equation 1 − 0.015(t − 6) for thickness in mm (1 − 0.4(t − 0.25) for thickness in inches) for up to 19 mm (0.75 in.), a limitation imposed by the limited test data [3]. Alternatively, the design guidelines required fillers over 6 mm (0.25 in.) thickness to be fully developed via welding or through extending the filler to add additional bolts. European design guides specify a reduction for loose fillers of βp = 9d/(8d + 3tp ) ≤ 1, where d is the nominal bolt diameter and tp is the thickness of the single thickest filler the bolt passes through [4]. As steel structures continue to evolve, increasingly higher grade steels are being implemented in design [8]. For newer designs the use of filler thicknesses greater than 19 mm (0.75 in.) may be needed. In an effort to expand the understanding of the behavior of bolted connections with fillers, this paper summarizes the results from an experimental study conducted to investigate the influence of filler plates on connections utilizing higher strength steels and a wider range of filler thicknesses than previously considered. The objectives of the study were to quantify the effects of filler plates on the performance of bolted connections. Considerations were also made to evaluate the use of oversized holes and multiple fillers, which could occur intentionally in design or through a field modification. 2. Connection details
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Corresponding author. Tel.: +1 503 725 9558; fax: +1 503 725 5950. E-mail address: [email protected] (P. Dusicka).
0143-974X/$ – see front matter. Published by Elsevier Ltd doi:10.1016/j.jcsr.2009.07.017
The test specimens were designed to induce bolt failure with bolt threads excluded from the shear planes, by ensuring the
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Table 1 Material properties. Plates Plate thickness (mm (in.))
Designation, metric (US)
Yield strength (MPa (ksi))
Tensile strength (MPa (ksi))
6 (0.25) 13 (0.5) 25 (1) 51 (2) 29 (1.125) 44 (1.75)
572M GR 345 (A572 GR50) 572M GR 345 (A572 GR50) 572M GR 345 (A572 GR50) 572M GR 345 (A572 GR50) 709M GR 480W (A709 GR 70) 709M GR 480W (A709 GR 70)
380 (55) 365 (53) 441 (64) 510 (74) 586 (85) 555 (81)
483 (70) 488 (71) 696 (101) 779 (113) 686 (100) 671 (97)
Bolts M22 (0.88 in.) Bolt length (mm (in.)) 140 (5.5) 152 (6) 178 (7) 229 (9)
Designation, metric (US) A490M (A490) A490M (A490) A490M (A490) A490M (A490)
Yield strength (MPa (ksi)) 1207 (175) 1207 (175) 1262 (183) 1276 (185)
Tensile strength (MPa (ksi)) 1386 (201) 1386 (201) 1427 (207) 1441(209)
Table 2 Connection test arrangements.
Fig. 1. Bolted assembly layout.
connection design bearing strength and design net and gross section of the plate far exceeded the bolt shear strength. For one bolt the design shear strength was 200 kN (45 kip) while the design bearing strength was 1270 kN (290 kip) and for three bolts the design shear strength was 600 kN (135 kip) while the design bearing strength was 5770 kN (1300 kip). Each connection comprised of 152 mm (6 in.) wide pull plates, splice plates and filler plates bolted to create a developed and an undeveloped side of the connection as illustrated in Fig. 1. The 152 mm (6 in.) wide pull plates and splice plates were fabricated from high performance steel A709M HPS grade 480W MPa (70 ksi) of 44 mm (1.75 in.) and 29 mm (1.125 in.) thickness respectively. The filler plates were made using A588M 345W MPa (50 ksi) when available and A572M grade 345 MPa (50 ksi) otherwise. Measured values of plate yield and tensile strength are detailed in Table 1. The choice of filler material was deemed to be consistent with design practice where filler plates of lower grade than the primary connection can be utilized, especially when taking into consideration the material availability of thinner plates. A 22 mm (0.88 in.) bolt diameter was used as representative of a common size used in field splice connections for steel plate girder bridges. In bridge design, bolts of 22 to 25 mm (0.88 to 1 in.) diameter can be found connecting flange plates of 51 mm (2 in.) thickness and also include filler plates up to 19 mm (0.75 in.). Bolts of the each considered length were obtained
Filler thickness (mm (in.))
Bolt length (mm (in.))
Hole size
0 13 (0.5) 2 × 6 (2 × 0.25) 25 (1) 4 × 6 (4 × 0.25) 51 (2)
140 (5.5) 152 (6) 152 (6) 178 (7) 178 (7) 229 (9)
Standard Standard Standard Standard Standard Standard
Oversize Oversize – Oversize – Oversize
from the same production lot to help reduce deviations in material properties and behavior. The focus of the experiments were on connections utilizing A490M (A490) bolts although a selected number of tests were initially conducted using A325M (A325) bolts in order to verify test results with previously published data. Bolt holes were spaced at 76 mm (3 in.) on center with a 38 mm (1.5 in.) edge distance, which is equal to or exceeds current design provisions [3,4]. Two types of bolt holes were considered, standard and oversized. Standard size bolt holes are typically 1.5 mm (0.06 in.) larger than the nominal bolt diameter resulting in a 24 mm (0.94 in.) diameter hole. Oversize holes have a 27 mm (1.06 in.) diameter and were chosen as the maximum size that would still have sufficient bearing area under the washer to maintain the bolt pre-tension force [1]. Measured values of bolt yield and tensile strength are detailed in Table 1. Connections consisting of a single bolt assembly and of a multibolt assembly of three bolts were considered. Each connection specimen included an undeveloped end and a developed end. The undeveloped end was the focus of the study while the developed end was used to simulate the thicker side of the connection. The developed end was achieved via two additional bolts outside of the connection that secured the filler to act together with the pull plate. The primary variable in the investigation was the filler thickness, which ranged from no filler to 51 mm (2 in.) thick. When considered, multiple fillers consisted of stacks of 6 mm (0.25 in.) thick plates to make up the total required filler thickness. The resulting test matrix for the assembly tests is summarized in Table 2. Each connection configuration was tested twice, each time with new plates and bolts. Mill scale was removed from the faying surfaces of the connection plates as well as the fillers. The surfaces were shot blasted to SP-10, also known as NACE 2 near-white metal blast cleaning [5]. The plates were scrubbed with a degreasing solvent and air dried prior to assembly. An effort was made at each bolt installation to maximize the movement in the holes during the test. To do so the connection was assembled in reverse bearing relative to the applied force in order to create a conservative measurement of the connection deformations. A complete reverse bearing was not always possible with multibolt assemblies given the tolerances in fabrication.
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Fig. 2. Test setup.
3. Test setup A modular self-reaction frame was specially designed for the bolted assembly tests to produce tensile forces on the pull plates of the specimens. Tests using applied tension were chosen for conservatism in the outcomes over compression loads, which are easier to produce in the laboratory environment. Tension was selected since bolted splice connections subjected to tensile loads can exhibit approximately 10% lower shear strength capacities due to splice plate prying [6]. To generate the required forces, a mechanical leverage with a 2:1 ratio was used to translate the compressive force from the hydraulic actuator to a tensile force on the pull plates as illustrated in Fig. 2. The connection assembly was bolted to pivoting arms, which were inserted between two beams at the top and the bottom of the frame. The fillers were assembled symmetrically around the connection. Since a more common configuration of a girder flange splice has asymmetric fillers (i.e. on only on one side of the flange), further testing would be required to measure the effect of the eccentricity on the results reported in this study. The hydraulic ram included a rotational bearing while the pivoting arms rested on rocker bearings to produce a tensile force in the specimen that was directly proportional to the force in the actuator. Displacement transducers were used to measure the relative deformation of the plates as illustrated in Fig. 1. The overall connection deformation was primarily evaluated using transducer LVDT 5, which captured the relative movement of the opposing pull plates. 4. Calibration of bolt tension For each connection test, the clamping force from the tensioned bolts was needed in order to effectively study the connection slip resistance. Turn-of-nut procedures rather than torque-totension relationships were used to develop bolt tension. Turnof-nut was selected because it had been found to result in more consistent bolt tension values [1]. While the pre-tension force does not significantly affect the bolt’s shear strength [6], the actual magnitude of the normal force does affect the force at which the connection will slip. Therefore a consistent force developed in the bolt was important in the evaluation of normalized slip force in the connection. The turn-of-nut procedure elongates the bolt by specifying the number of turns required to achieve the minimum pre-tension value. Using a similar approach, the bolt tension force was obtained by developing the required turn-of-nut relationships for the bolts used in the experiments.
The approach taken for the assembly tests was to tighten each bolt until the plateau of the bolt force to deformation relationship was achieved. This effectively yielded the bolt and thereby provided for a consistent estimation of clamping force in each connection test. Potential underestimation of the bolt pre-tension may result using this procedure from uneven plates, whereby part of the turn of the nut past snug would be used up to bend uneven plates together instead of elongating the bolt. To decrease the variability in the estimation of the normal force, the number of plates used was as close a number as to the number encountered in the connection. Bolt threads were excluded from the shear planes while investigating various filler thicknesses among the different connection options and consequently required different lengths depending on the filler thickness used. The A490M bolts have been shown to have the same ultimate shear stress regardless of shear plane location [6], but keeping the threads out of the shear plane was chosen for the higher shear force capacity. Bolts of lengths ranging from 152 to 229 mm (6 to 9 in.) were used as summarized in Table 3. The bolts were acquired such that each bolt length originated from the same production lot, thereby minimizing strength variability within a bolt length. A relationship of bolt tension versus a fraction of a turn was developed for each bolt length using the Skidmore–Wilhelm bolt tension calibrator. For each bolt length, plates were used to make up the required total thickness expected in the connection tests. Bolts were tightened using a breaker bar to a snug tight position as defined in current design provisions [1]. Additional bolt rotation was then applied with the aid of a torque multiplier, which replicated the method in assembly of the connections. Five samples of each bolt length were tested, except for the 140 mm and 178 mm (5.5 in. and 7 in.) long A490M bolts for which only three bolts were available without compromising available bolts for the subsequent connection tests. Each bolt was tightened by quarter turn increments resulting in the average bolt tension values of Fig. 3 for each of the bolt lengths. The Skidmore–Wilhelm tests were taken to failure, leading to obtained values and calculated variability representing the bolt strengths. Also included are the results from using oversized holes on 152 mm (6 in.) long A490 bolts as a representative case for oversize holes. For each length, the bolt tension consistently reached a force plateau after 3/4 turn past snug tight, except for the 178 mm (7 in.) long A490M (A490) bolts, which were found to plateau after 1/2 turn past snug tight. Based on these tests, the bolts in the connections were always tightened to just beyond 3/4 turn past snug tight. The average value and standard deviation of bolt
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Table 3 Bolt properties. Length (mm (in.))
Thread length (mm (in.))
140 (5.5) 152 (6) 178 (7) 229 (9)
38 (1.5) 38 (1.5) 38 (1.5) 38 (1.5)
Tension at 3/4 turn Average (kN (kip))
Std. Dev. (kN (kip))
295 (66.3) 287 (64.5) 293 (65.8) 298 (67.0)
11 (2.5) 12 (2.7) 5.6 (1.3) 5.1 (1.1)
Fig. 3. Measured bolt tension.
tension at 3/4 turn was used as the bolt pre-tension value for each respective length and are summarized in Table 3. Past the 3/4 turn the oversized hole bolt tension results were similar to the standard size hole values, confirming that the oversize hole tests would not adversely affect the bolt tension force. 5. Force deformation behavior The connection behavior at the undeveloped end of the assembly is shown by a single representative sample of force versus deformation in Fig. 4 for the one bolt assembly tests and Fig. 5 for the three bolts tests using standard and oversize holes. The general trend of the bolted connection assembly was characterized by an initial slip, connection movement resulting in bolt-toplate bearing, inelastic deformation, and finally failure of the bolts. The deformation behavior of the connections, which utilized high strengths structural steels and bolts, were similar to those reported for lower grades [2]. Nonstructural bolts of poor ductility characteristics would be expected to result in poor connection deformation performance. After the initial slip, an increase in deformation was signified by only a nominal increase and at times decrease in force until the bolts engaged the plate holes in bearing. Once in bearing, the connection stiffness increased resulting in nonlinear behavior as both the bolts and the bearing plates deformed plastically. Regions of plastic deformations were observed by the permanent deformation of the hole edges by the plates and in the final shape of the bolts. As per the specimen design objectives, the ultimate failure occurred by fracture in the bolts. One or both ends of the fractured bolts often flew out of the connection at failure. The presence of fillers affected the force deformation response and thereby the performance of the bolted connections. The following sections further discuss the results of the tests in terms of the individual metrics of connection deformation, ultimate strength and slip resistance. 6. Connection deformation The recorded deformation at failure consisted of a combined effect of slip deformation needed to engage the bolts in bearing,
Fig. 4. Force deformation for representative one bolt connections.
shear and flexural deformation of the bolts, and the hole deformations in the plates. The failure mode for the three bolt connection was typically a near-simultaneous failure of all bolts. The total maximum recorded deformations are summarized in Fig. 6 for the single bolt and the three bolt connections. Discrete points represent the test results and continuous lines connect through the average values. In general, comparing Figs. 4 and 5 the one and three bolt assemblies deformed with similarities between the individual connection arrangements (i.e. standard size hole, oversize hole and multiple filler).
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Fig. 6. Connection deformation at failure. Fig. 5. Force deformation for representative three bolt connections.
An increase in connection deformation was observed for connections with fillers as compared to those without. The values peaked with the 25 mm (1 in.) filler thickness, resulting in a 3 times larger deformations on average for standard size holes. The corresponding deformation peak for the oversized holes was 2.5 times larger than the non-filler connections with oversize holes and 5 times larger than the non-filler connections with standard size holes. The utilization of 51 mm (2 in.) thick fillers did not exhibit a further increase in deformation. Bolts from connections without fillers remained straight, with fractures along the shear planes between plates. With filler thickness increasing up to 25 mm (1 in.), the middle of the bolt increasingly deformed relative to the ends via a stepped deformation primarily within the length of the filler as shown in Fig. 7. The deformation indicated wedging of the bolt within the hole along the filler thickness. The bolts from connections with 51 mm (2 in.) fillers had a similar total deformation as the 25 mm (1 in.), except that the wedging was distributed over a longer length. Nonetheless, the high strength
bolts and plates used for these experiments resulted in plastic deformations with total connection displacements greater than the hole clearance, which was 1.6 mm (1/16 in.) for standard and 4.8 mm (3/16 in.) for oversized holes. Displacements greater than the hole clearance are the result of plate bearing plus bolt shear and flexural deformations. Introduction of the filler plates only served to increase the ultimate deformation of the connection. The connections with oversize holes experienced up to 1.7 times larger deformations than connections with standard size holes when comparing the same filler thicknesses. Two components contributed to the increased deformation as deduced from the force deformation plots in Figs. 4 and 5. Plastic deformations of the holes and the bolt contributed, but the more significant portion was caused by the deformation needed to engage the bolts in bearing following the initial slip. After the bearing was initiated in the oversized holes, the load increased at a lower rate than in the standard size holes. The deformations of connections with oversized holes exceeded even those recorded in multiple filler connections, which were at maximum 1.2 times larger on average than the standard size holes as measured for the three bolt connections with 25 mm (1 in.) fillers.
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Fig. 7. Sample of deformed bolts.
7. Connection strength The effects of grip length and bolt preload have been found to have little effect on the ultimate bolt shear strength [6]. To reduce variability, bolts of the same length were ordered from the same production lot. The maximum force recorded in each experiment is summarized in Fig. 8 for the single bolt and three bolt connections. The discrete data represents the recorded strengths, while the continuous line connects the average values. The strengths from the one bolt connections were higher than for the three bolt connections when normalized to strength per bolt, but exhibited similar characteristics. One possible explanation for this observation can be the result of splice plate prying. Uneven bearing is created at each plate as the bolt deforms, creating a resisting force that is not at the centerline of the plate [6]. This eccentric force then pushes the connection’s splice plates away from the connection, creating a tensile force along with shear in the bolt. The tensile force is partially resisted by the flexural stiffness of the plate and since the same plate dimensions were used in all of the tests, the relatively higher stiffness of the plate compared to a single bolt would reduce the effect on the bolt. The strengths of standard size hole connections with fillers were lower than those without fillers. However, the ultimate strength did not exhibit continued decrease with increasing filler thickness. The lowest strengths were recorded for fillers 25 mm (1 in.) thick, where the strength decreased on average by 6% and 9% for the one bolt and three bolt assemblies respectively. These reductions were less than the current guidelines which suggested a 15% lower strength for 19 mm (0.75 in.) filler thicknesses [1]. The assemblies with 51 mm (2 in.) thick fillers were stronger than any of the connections using thinner fillers, approaching the original strengths of connections without fillers. The basis for this behavior is illustrated in the bolts shown in Fig. 7, where the bolt deformation and fracture pattern correlated to the ultimate strength of the connections. The bolts from connections without fillers sheared perpendicular to the bolt at least one of the connection shear planes. Bolts from connections with fillers of 13 mm (0.5 in.) and 25 mm (1 in.) exhibited increasing bolt deformation within the filler hole and the fracture planes were no longer as clearly sheared. The reduction of strength for connections with fillers was attributed to the combined effect of flexural deformations imposed on the bolt along with the shear. For single filler connections, the influence of flexural deformation increased as the bolt wedged within the constraints of the holes
Fig. 8. Maximum strength.
for filler thickness up to 25 mm (1 in.). The 51 mm (2 in.) thick fillers reduced the flexural influence as the bolt deformations were distributed over the hole within the thicker filler and the failure pattern started to approximate that of the shear dominated behavior in connections without any fillers. The connections with oversize holes and without fillers exhibited slightly lower strengths than the same connections with standard size holes. Although the results from the single bolt connections exhibit little trend, the three bolt connections indicated 3% lower strength than the standard size holes over the range of filler thicknesses considered. Similar to the trends observed for the standard size holes, the oversized holes with 25 mm (1 in.) thick fillers exhibited the lowest strength and the 51 mm (2 in.) thick fillers tended to improve the strength to levels comparable to that of the connection without filler plates. The detrimental influence of the bolt flexural deformation was especially evident for the multiple fillers, which exhibited just 3% lower strengths at 13 mm (0.5 in.) filler, but further increased to 20% for the 25 mm (1 in.) filler in the three bolt connections. Since the individual plies shifted and rearranged as the deformation increased, the bolt was minimally restrained in the hole within the thickness of the filler. Dotted lines superimposed over the
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Fig. 9. Sample of bolt failure in multiple filler connection.
fractured bolts in Fig. 9 indicate the location of the shear planes between the plates. The shear planes are outside of the threaded area, yet the bolts failed in the threads. Once slip occurred, the bolts were deformed in such a way that a combination of bending and tensile forces resulting from catenary action, rather than shear, caused the bolt failure. For tests using 4 × 6 mm (4 × 0.25 in.) filler plates, all failures were controlled by a combination of bending and tensile forces. The failure surface through the threads of the bolts in the 4 × 6 mm (4 × 0.25 in.) tests indicated similar patterns to bolts tested in tension during the bolt tensile force calibration. Consequently, the connection ultimate strength was significantly lower when compared to single filler connections and would be expected to decrease further for thicker fillers where multiple thin fillers are used. The arrangement of four equally thin fillers used to make up 25 mm (1 in.) thickness as considered in these experiments represented an extreme case. Multiple fillers can also be used by providing a combination of thicker and thinner fillers, for example using a 19 mm and 6 mm (0.75 in and 0.25 in.) filler. For such a combination, the reduction in strength is not expected to be as severe as 4 × 6 mm (4 × 0.25 in.) fillers as the bolt would be more constrained within the hole although further tests are needed. Current US design guidelines for connections with fillers are based on the resisting load at a deformation limit of 6 mm (0.25 in.), which had been used to develop the shear strength reduction design equations [3,2]. This limit was chosen to represent a performance level beyond which a connection would experience excessive deformations and would not be considered useful [7]. In an effort to compare the current design recommendations to the results from this research, the recorded force was extracted at 6 mm (0.25 in.) deformation and is summarized in Fig. 10 for both the single bolt and three bolt connections. The values for connections with oversize holes are significantly lower and exhibit larger scatter because of increased deformations needed to engage the bolts in bearing as compared to standard size holes. Consequently, the force at the 6 mm (0.25 in.) deformation had just started to increase after bearing had been established against the oversize holes. The average experimental data for the various connections were normalized to the averaged ultimate force value found in the 3 bolt, standard size hole, no filler plate connection and compared to the current design guidelines in Fig. 11. The data were in close correlation to the design equation for filler thicknesses up to 25 mm (1 in.), indicating that the equations are applicable for connections with higher strength steels. The force in the connection further decreased for 51 mm (2 in.) fillers, but not in the linear manner suggested by the US design equations [3]. In comparison with the European design equation [4], the strength recovery was accounted for with a close correlation even for thicker filler sizes. From these results, the US design equations are conservative for fillers thicker than those previously considered. Although the
Fig. 10. Shear force at 6 mm (0.25 in.) deformation.
recorded forces were significantly lower in connections with oversized holes, similar characteristic of a reduced effect for 51 mm (2 in.) filler thickness was also observed at 6 mm (0.25 in.) deformation. The force in connections with multiple fillers decreased at a larger gradient than for the single fillers, resulting in reductions that could be underestimated using the current design equations for 25 mm (1 in.) fillers. 8. Normalized slip force Slip critical joints rely on the slip resistance between the surfaces of the plates rather than the shear strength of the bolts in the connection. The slip was identified by the displacement transducer measuring deformation of the splice plate relative to the pull plate. A sudden movement or reduction in load did not always accompany the slip, but the point at which the slip occurred was readily identified from the instrumentation. The force measured at slip was normalized to the test using the values from Table 3 as the normal force. The normalized slip force values are summarized in Fig. 12 for the single bolt and three bolt connections. An expected normalized slip force was estimated using a design equation for slip critical connections calculated as
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Fig. 11. Normalized shear force at 6 mm (0.25 in.) deformation.
Fn /(N0 )av g = µDu hsc Tb Ns /(N0 )av g , where Fn is the unfactored maximum slip load, µ = 0.5 for Class B surface (unpainted, blast cleaned), Du = 1.13 to account for higher than required minimum bolt pre-tension found in the field, Ns = 2 for the number of slip planes, Tb is the minimum bolt tension, and (N0 )av g is the average normal force of the bolt per Table 3 [3]. Per AISC hsc = 1.0 is for standard holes and hsc = 0.85 is for oversized holes. Since the values reflect the negative consequences of slip as opposed to any relationship to the performance of oversized holes, hsc was kept equal to 1.0. The expected slip values are also included in Fig. 12 for comparison to the test data, as calculated using bolt tension values established in Table 2 for Tb . The design value Du accounts for installation uncertainty in pretension force. Since all bolts were tightened to a calibrated turn-of-nut described in section Calibration of Bolt Tension, the value of Du was reduced to unity for the purposes of comparing them to the test result. The desired surface was intended to be Class B, however majority of the recorded normalized slip force values were well below the expected. Sample tests were conducted on 3 different samples using test procedures recommended by Kulak et al. [1], which include the use of a universal testing machine, constant controlled normal force representing the bolt and a measured slip force applied to one plate sandwiched by two other plates in a symmetric setup. These slip test samples were obtained from previously tested 32 mm (1.25 in.) thick splice plates and cut to 76 mm × 127 mm (5 in. × 3 in.) sections. Using the measured normal force and the measured slip force, an average slip coefficient of 0.18 was obtained. Despite the low sample numbers and the reuse of surface on the plate for these slip tests, the results indicate that a slip coefficient was present that is much lower than would be expected for Class B surface. More extensive research beyond the scope of this study would be needed to determine if the reason is related to the improper application of the surface treatment or the reaction of the HPS 480W (HPS 70W) steel grade to the treatment. In general, the variability in normalized slip force made deterministic evaluations difficult with just two data points per filler thickness, but general trends were observed. In both the single bolt and the three bolt assemblies, the normalized slip force for standard size holes were consistently lower for connections with fillers as compared to connections without fillers. The decrease in the normalized slip force was further exaggerated for multiple fillers, which exhibited the lowest normalized slip values. The exact cause
Fig. 12. Comparison of normalized slip force.
for the decrease in the normalized slip force was difficult to ascertain, but one of the possible reasons could be related to the surface finish observed on the filler plates. Imperfections in the surface, as illustrated by the example filler plate in Fig. 13, limited the faying surface’s contact near the bolt holes. The reduced contacts in the high stress areas may have contributed to the lower than expected slip values for connections with fillers. This effect may have been multiplied for the multiple fillers at the various faying surfaces. The normalized slip force for oversize holes along with fillers did not exhibit appreciable differences when compared to the standard size holes. The normalized slip values remained approximately constant across the different filler thicknesses. When no fillers were used, the oversize holes were found to have lower normalized slip force as compared to the standard size holes. 9. Design recommendations Current design equations for bolted connections with fillers are based on connection strengths at a predetermined deformation
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Fig. 13. Faying surface of 51 mm (2 in.) filler plate.
Fig. 14. Proposed strength reduction comparison.
and had been found conservative for thick fillers in these experiments. Since the intended application of a bolted connection can vary, the following strength design recommendations are based on ultimate values and not on forces at a specific deformation so as to provide an increased level of design freedom. The test results had indicated that the lowest strengths occurred for connections with 25 mm (1 in.) fillers and nearly fully recovered when 51 mm (2 in.) fillers were used. The previously described observations of bolt deformation and failure indicated that wedging of the bolt within the filler thickness affected the failure mode. When thin fillers were introduced, the bolt failure became a combination of shear and flexure as highlighted by the plastic deformations in Fig. 7. As the filler thickness increased, the relative deformation of the bolt within the filler thickness was distributed over a longer length, resulting in the failure mode reverting back to shearing at the ends of the filler. These bolt deformation observations combined with the strength reduction and recovery from the tests suggests that the strength reduction be based on filler thickness relative to the bolt diameter instead of the filler thickness alone. Using a general form of a quadratic equation, the expression for the strength reduction fr of bolted connections with fillers was derived as a unit independent relationship shown in Eq. (1).
fr = 1 +
kf d2b
(0.75t − db )2 − kf .
83
(1)
The value of fr may not become less than zero for any combination of filler thickness t and bolt diameter db . The factor kf varies depending on the type of bolted connection and represents the value of maximum reduction, with kf = 0.10 for standard size holes and kf = 0.13 for oversize hole connections to account for the slightly higher strength reduction for oversized holes. The factor of 0.75 was chosen such that the peak of the strength reduction is assumed to occur at a filler thickness of 4/3 the bolt diameter and no reduction occurs for fillers larger than 8/3 the bolt diameter. The equations are compared in Fig. 14 to the measured strengths F , which had been normalized to the values of the average standard size connection strength without fillers (F0 )av g . An emphasis needs to be placed on the penalty of oversized holes on connection performance in terms of deformations.
Although the amplification in deformation was found to be approximately the same for both standard and oversize holes when compared to their respective connections without fillers, the connections with oversize holes exhibited much larger overall deformations. The connection deformations did not continue to increase past 25 mm (1 in.) fillers and if normalized to the bolt diameter, the deformation amplification Ub for bolted connections with fillers could be expressed using a linear relationship shown in Eq. (2).
Ub = kd
1+
2t db
.
(2)
The value of Ub may not exceed 3kd . The factor kd represents the effect of the oversize holes on the connection. For standard size holes kd = 1 and for oversize holes kd = 1.7 since the oversize hole connection was found to be approximately 1.7 times the deformation across the different filler or non-filler configuration. The slope of the line of 2 was chosen to encompass the intermediate results. The equation is compared in Fig. 15 to the measured deformations D, which have been normalized to the average recorded deformation of the three bolt standard size hole connections without fillers (D0 )av g . The above relationships are not applicable for multiple fillers, which can exhibit significantly greater reduction in strength and no evidence exists that the strength recovers for thicker fillers when numerous thin plies are used. The recommendations are based on test results involving 22 mm (0.88 in.) diameter bolts and need to be verified for other diameters, but the equation presents a framework that is based on bolt deformation and failure observations along with the recorded values and may become more refined as more data becomes available. Additional research is required to quantify the effect of long connections and further study the observed connection slip trends. 10. Conclusions Bolted connections made using high performance A709M HPS 480W MPa (70W) steel and 22 mm (0.88 in.) diameter A490M (A490) bolts were used to conduct over 50 individual experiments
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observed at the 25 mm (1 in.) filler thickness and nearly recovered at the 51 mm (2 in.) filler thickness. • Use of fillers with either oversize hole or standard size hole connections produce a negligible effect on the ultimate strength of the connection. Connections with oversize holes and fillers exhibit lower ultimate strength by approximately 3% as compared to standard size holes. • The use of fillers consisting of multiple thin plies induces bending and tensile stresses in the bolt that can result in a shift of the failure mode from the shank to the threads and result in up to a 20% reduction in connection capacity for designs where the bolt threads were intended to be excluded from the shear plane. • The force at which the connection slipped does not appreciably vary with filler thickness, although lower slip values can result from multiple ply fillers. Acknowledgements
Fig. 15. Proposed deformation amplification comparison.
to failure for filler thicknesses up to 51 mm (2 in.). The following findings are based on the observations and analyses of the results:
• The connection deformations increase as the filler thickness increase up to a limit, which was found to occur at 25 mm (1 in.) filler thickness for the arrangements considered. • Oversize holes significantly increase connection deformations, resulting in up to 1.7 times the standard size hole deformations at failure across the filler thicknesses as compared to multiple ply fillers that were at maximum 1.2 times larger for the same total thickness. • The use of fillers does not always result in a decrease in connection strength for standard and oversized hole connections. With increasing filler thickness, the detrimental influence peaks and can return to the strength of the connection without fillers. In the arrangements considered, the lowest strength values were
The research was conducted by Portland State University. The authors would like to acknowledge the financial support of the Research Council on Structural Connections and Fought & Company Inc, in Portland, Oregon. However, any opinions, findings, conclusions and recommendations presented in this paper are those of the authors and do not necessarily reflect the views of any of the sponsors. References [1] Kulak GL, Fisher JW, Struik JHA. Guide to design criteria for bolted and riveted joints. 2nd ed. Chicago (IL): Research Council on Structural Connections; 2001. [2] Yura JA, Hansen MA, Frank KH. Bolted splice connections with undeveloped fillers. Journal of the Structural Division 1982;108:ST12. [3] American Institute of Steel Construction Inc. Steel construction manual. 13th ed. 2005. [4] EN 1993-1-8 (2002). Eurocode 3: Design of steel structures — Design of joints. European Committee for Standardisation. [5] National Association of Corrosion Engineers. Joint surface preparation standard: Near-white metal blast cleaning. 1999. [6] Wallaert JJ, Fisher JW. Shear strength of high-strength bolts. Journal of the Structural Division 1965;91:No. ST3. [7] Perry WC. Bearing Strength of Bolted Connections. Thesis presented to the University of Texas, at Austin in partial fulfillment of the requirements for the degree of Master of Science. 1981. [8] Wright WJ. High performance steel: research to practice. Federal Highway Administration: Public Roads, Spring 1997, 1997. pp. 34–38.
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High strength steel bolted connections with filler plates Peter Dusicka ∗ , Gregory Lewis Portland State University, Department of Civil and Environmental Engineering, P.O. Box 751, Portland, OR 97207, United States
article
info
Article history: Received 1 February 2009 Accepted 31 July 2009 Keywords: Splice Filler plate High performance steel Oversize
abstract The effects of undeveloped filler plates were experimentally evaluated for high strength steel bolted connections using standard size holes, oversize holes and multiple fillers. Filler plates up to 51 mm (2 in.) thick were utilized in connecting high performance A709 HPS 480W (HPS 70W) grade steel plates using 22 mm (0.88 in.) A490 bolts. With increasing filler thickness, the connection strength was found to decrease up to a limit and then recover for thicker fillers in standard and oversized hole connections. Multiple filler connections experienced the greatest strength decrease as the bolt failure shifted to the threads located outside the shear planes. Deformations also exhibited a limit, however the use of oversize holes resulted in 1.7 times larger deformations than standard or multiple fillers. Recommendations were developed for strength reduction and deformation amplification as function of filler thickness and bolt diameter. Published by Elsevier Ltd
1. Introduction Filler plates are used in steel bolted connections to compensate for dimensional differences, including flange plate thickness in bridge girders and chord to diagonal member depths in long span building trusses. Filler plates may be developed to act together with the connection plates or undeveloped where no special accommodations are made to make the filler part of the connection [1]. Developed fillers can be achieved by welding or by bolting an extended portion of the filler beyond the connection, causing the plates to act as one unit in sharing the imposed stresses. Undeveloped fillers merely provide a common faying surface and can move independently with respect to the connecting members as stresses build. Designs of connections with undeveloped fillers rely on data from tests on connections with single and multiple fillers up to 19 mm (0.75 in.) thick [2]. Those specimens utilized A325M (A325) bolts with A514M grade 345 MPa (50 ksi) splice plates and A36 filler plates. The results indicated that the greater the undeveloped filler thickness, the greater the deformation of the connection. The consequent bending of the bolt was found to reduce the strength of the connection and increase the deformation at failure. Multiple fillers reduced the capacity to a greater degree than single fillers, although the differences between the ultimate load and maximum deformation for three 6 mm (0.25 in.) filler plates and one 19 mm (0.75 in.) filler plate were modest. Based on these results an empirical strength reduction factor was proposed that related the
reduction in shear strength of the bolt to the filler plate thickness t. Design guides and specifications in the United States (US) further modified this linear relationship such that no strength reduction resulted from fillers of 6 mm (0.25 in.) thickness or lower, leading to a unit dependent equation 1 − 0.015(t − 6) for thickness in mm (1 − 0.4(t − 0.25) for thickness in inches) for up to 19 mm (0.75 in.), a limitation imposed by the limited test data [3]. Alternatively, the design guidelines required fillers over 6 mm (0.25 in.) thickness to be fully developed via welding or through extending the filler to add additional bolts. European design guides specify a reduction for loose fillers of βp = 9d/(8d + 3tp ) ≤ 1, where d is the nominal bolt diameter and tp is the thickness of the single thickest filler the bolt passes through [4]. As steel structures continue to evolve, increasingly higher grade steels are being implemented in design [8]. For newer designs the use of filler thicknesses greater than 19 mm (0.75 in.) may be needed. In an effort to expand the understanding of the behavior of bolted connections with fillers, this paper summarizes the results from an experimental study conducted to investigate the influence of filler plates on connections utilizing higher strength steels and a wider range of filler thicknesses than previously considered. The objectives of the study were to quantify the effects of filler plates on the performance of bolted connections. Considerations were also made to evaluate the use of oversized holes and multiple fillers, which could occur intentionally in design or through a field modification. 2. Connection details
∗
Corresponding author. Tel.: +1 503 725 9558; fax: +1 503 725 5950. E-mail address: [email protected] (P. Dusicka).
0143-974X/$ – see front matter. Published by Elsevier Ltd doi:10.1016/j.jcsr.2009.07.017
The test specimens were designed to induce bolt failure with bolt threads excluded from the shear planes, by ensuring the
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Table 1 Material properties. Plates Plate thickness (mm (in.))
Designation, metric (US)
Yield strength (MPa (ksi))
Tensile strength (MPa (ksi))
6 (0.25) 13 (0.5) 25 (1) 51 (2) 29 (1.125) 44 (1.75)
572M GR 345 (A572 GR50) 572M GR 345 (A572 GR50) 572M GR 345 (A572 GR50) 572M GR 345 (A572 GR50) 709M GR 480W (A709 GR 70) 709M GR 480W (A709 GR 70)
380 (55) 365 (53) 441 (64) 510 (74) 586 (85) 555 (81)
483 (70) 488 (71) 696 (101) 779 (113) 686 (100) 671 (97)
Bolts M22 (0.88 in.) Bolt length (mm (in.)) 140 (5.5) 152 (6) 178 (7) 229 (9)
Designation, metric (US) A490M (A490) A490M (A490) A490M (A490) A490M (A490)
Yield strength (MPa (ksi)) 1207 (175) 1207 (175) 1262 (183) 1276 (185)
Tensile strength (MPa (ksi)) 1386 (201) 1386 (201) 1427 (207) 1441(209)
Table 2 Connection test arrangements.
Fig. 1. Bolted assembly layout.
connection design bearing strength and design net and gross section of the plate far exceeded the bolt shear strength. For one bolt the design shear strength was 200 kN (45 kip) while the design bearing strength was 1270 kN (290 kip) and for three bolts the design shear strength was 600 kN (135 kip) while the design bearing strength was 5770 kN (1300 kip). Each connection comprised of 152 mm (6 in.) wide pull plates, splice plates and filler plates bolted to create a developed and an undeveloped side of the connection as illustrated in Fig. 1. The 152 mm (6 in.) wide pull plates and splice plates were fabricated from high performance steel A709M HPS grade 480W MPa (70 ksi) of 44 mm (1.75 in.) and 29 mm (1.125 in.) thickness respectively. The filler plates were made using A588M 345W MPa (50 ksi) when available and A572M grade 345 MPa (50 ksi) otherwise. Measured values of plate yield and tensile strength are detailed in Table 1. The choice of filler material was deemed to be consistent with design practice where filler plates of lower grade than the primary connection can be utilized, especially when taking into consideration the material availability of thinner plates. A 22 mm (0.88 in.) bolt diameter was used as representative of a common size used in field splice connections for steel plate girder bridges. In bridge design, bolts of 22 to 25 mm (0.88 to 1 in.) diameter can be found connecting flange plates of 51 mm (2 in.) thickness and also include filler plates up to 19 mm (0.75 in.). Bolts of the each considered length were obtained
Filler thickness (mm (in.))
Bolt length (mm (in.))
Hole size
0 13 (0.5) 2 × 6 (2 × 0.25) 25 (1) 4 × 6 (4 × 0.25) 51 (2)
140 (5.5) 152 (6) 152 (6) 178 (7) 178 (7) 229 (9)
Standard Standard Standard Standard Standard Standard
Oversize Oversize – Oversize – Oversize
from the same production lot to help reduce deviations in material properties and behavior. The focus of the experiments were on connections utilizing A490M (A490) bolts although a selected number of tests were initially conducted using A325M (A325) bolts in order to verify test results with previously published data. Bolt holes were spaced at 76 mm (3 in.) on center with a 38 mm (1.5 in.) edge distance, which is equal to or exceeds current design provisions [3,4]. Two types of bolt holes were considered, standard and oversized. Standard size bolt holes are typically 1.5 mm (0.06 in.) larger than the nominal bolt diameter resulting in a 24 mm (0.94 in.) diameter hole. Oversize holes have a 27 mm (1.06 in.) diameter and were chosen as the maximum size that would still have sufficient bearing area under the washer to maintain the bolt pre-tension force [1]. Measured values of bolt yield and tensile strength are detailed in Table 1. Connections consisting of a single bolt assembly and of a multibolt assembly of three bolts were considered. Each connection specimen included an undeveloped end and a developed end. The undeveloped end was the focus of the study while the developed end was used to simulate the thicker side of the connection. The developed end was achieved via two additional bolts outside of the connection that secured the filler to act together with the pull plate. The primary variable in the investigation was the filler thickness, which ranged from no filler to 51 mm (2 in.) thick. When considered, multiple fillers consisted of stacks of 6 mm (0.25 in.) thick plates to make up the total required filler thickness. The resulting test matrix for the assembly tests is summarized in Table 2. Each connection configuration was tested twice, each time with new plates and bolts. Mill scale was removed from the faying surfaces of the connection plates as well as the fillers. The surfaces were shot blasted to SP-10, also known as NACE 2 near-white metal blast cleaning [5]. The plates were scrubbed with a degreasing solvent and air dried prior to assembly. An effort was made at each bolt installation to maximize the movement in the holes during the test. To do so the connection was assembled in reverse bearing relative to the applied force in order to create a conservative measurement of the connection deformations. A complete reverse bearing was not always possible with multibolt assemblies given the tolerances in fabrication.
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Fig. 2. Test setup.
3. Test setup A modular self-reaction frame was specially designed for the bolted assembly tests to produce tensile forces on the pull plates of the specimens. Tests using applied tension were chosen for conservatism in the outcomes over compression loads, which are easier to produce in the laboratory environment. Tension was selected since bolted splice connections subjected to tensile loads can exhibit approximately 10% lower shear strength capacities due to splice plate prying [6]. To generate the required forces, a mechanical leverage with a 2:1 ratio was used to translate the compressive force from the hydraulic actuator to a tensile force on the pull plates as illustrated in Fig. 2. The connection assembly was bolted to pivoting arms, which were inserted between two beams at the top and the bottom of the frame. The fillers were assembled symmetrically around the connection. Since a more common configuration of a girder flange splice has asymmetric fillers (i.e. on only on one side of the flange), further testing would be required to measure the effect of the eccentricity on the results reported in this study. The hydraulic ram included a rotational bearing while the pivoting arms rested on rocker bearings to produce a tensile force in the specimen that was directly proportional to the force in the actuator. Displacement transducers were used to measure the relative deformation of the plates as illustrated in Fig. 1. The overall connection deformation was primarily evaluated using transducer LVDT 5, which captured the relative movement of the opposing pull plates. 4. Calibration of bolt tension For each connection test, the clamping force from the tensioned bolts was needed in order to effectively study the connection slip resistance. Turn-of-nut procedures rather than torque-totension relationships were used to develop bolt tension. Turnof-nut was selected because it had been found to result in more consistent bolt tension values [1]. While the pre-tension force does not significantly affect the bolt’s shear strength [6], the actual magnitude of the normal force does affect the force at which the connection will slip. Therefore a consistent force developed in the bolt was important in the evaluation of normalized slip force in the connection. The turn-of-nut procedure elongates the bolt by specifying the number of turns required to achieve the minimum pre-tension value. Using a similar approach, the bolt tension force was obtained by developing the required turn-of-nut relationships for the bolts used in the experiments.
The approach taken for the assembly tests was to tighten each bolt until the plateau of the bolt force to deformation relationship was achieved. This effectively yielded the bolt and thereby provided for a consistent estimation of clamping force in each connection test. Potential underestimation of the bolt pre-tension may result using this procedure from uneven plates, whereby part of the turn of the nut past snug would be used up to bend uneven plates together instead of elongating the bolt. To decrease the variability in the estimation of the normal force, the number of plates used was as close a number as to the number encountered in the connection. Bolt threads were excluded from the shear planes while investigating various filler thicknesses among the different connection options and consequently required different lengths depending on the filler thickness used. The A490M bolts have been shown to have the same ultimate shear stress regardless of shear plane location [6], but keeping the threads out of the shear plane was chosen for the higher shear force capacity. Bolts of lengths ranging from 152 to 229 mm (6 to 9 in.) were used as summarized in Table 3. The bolts were acquired such that each bolt length originated from the same production lot, thereby minimizing strength variability within a bolt length. A relationship of bolt tension versus a fraction of a turn was developed for each bolt length using the Skidmore–Wilhelm bolt tension calibrator. For each bolt length, plates were used to make up the required total thickness expected in the connection tests. Bolts were tightened using a breaker bar to a snug tight position as defined in current design provisions [1]. Additional bolt rotation was then applied with the aid of a torque multiplier, which replicated the method in assembly of the connections. Five samples of each bolt length were tested, except for the 140 mm and 178 mm (5.5 in. and 7 in.) long A490M bolts for which only three bolts were available without compromising available bolts for the subsequent connection tests. Each bolt was tightened by quarter turn increments resulting in the average bolt tension values of Fig. 3 for each of the bolt lengths. The Skidmore–Wilhelm tests were taken to failure, leading to obtained values and calculated variability representing the bolt strengths. Also included are the results from using oversized holes on 152 mm (6 in.) long A490 bolts as a representative case for oversize holes. For each length, the bolt tension consistently reached a force plateau after 3/4 turn past snug tight, except for the 178 mm (7 in.) long A490M (A490) bolts, which were found to plateau after 1/2 turn past snug tight. Based on these tests, the bolts in the connections were always tightened to just beyond 3/4 turn past snug tight. The average value and standard deviation of bolt
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Table 3 Bolt properties. Length (mm (in.))
Thread length (mm (in.))
140 (5.5) 152 (6) 178 (7) 229 (9)
38 (1.5) 38 (1.5) 38 (1.5) 38 (1.5)
Tension at 3/4 turn Average (kN (kip))
Std. Dev. (kN (kip))
295 (66.3) 287 (64.5) 293 (65.8) 298 (67.0)
11 (2.5) 12 (2.7) 5.6 (1.3) 5.1 (1.1)
Fig. 3. Measured bolt tension.
tension at 3/4 turn was used as the bolt pre-tension value for each respective length and are summarized in Table 3. Past the 3/4 turn the oversized hole bolt tension results were similar to the standard size hole values, confirming that the oversize hole tests would not adversely affect the bolt tension force. 5. Force deformation behavior The connection behavior at the undeveloped end of the assembly is shown by a single representative sample of force versus deformation in Fig. 4 for the one bolt assembly tests and Fig. 5 for the three bolts tests using standard and oversize holes. The general trend of the bolted connection assembly was characterized by an initial slip, connection movement resulting in bolt-toplate bearing, inelastic deformation, and finally failure of the bolts. The deformation behavior of the connections, which utilized high strengths structural steels and bolts, were similar to those reported for lower grades [2]. Nonstructural bolts of poor ductility characteristics would be expected to result in poor connection deformation performance. After the initial slip, an increase in deformation was signified by only a nominal increase and at times decrease in force until the bolts engaged the plate holes in bearing. Once in bearing, the connection stiffness increased resulting in nonlinear behavior as both the bolts and the bearing plates deformed plastically. Regions of plastic deformations were observed by the permanent deformation of the hole edges by the plates and in the final shape of the bolts. As per the specimen design objectives, the ultimate failure occurred by fracture in the bolts. One or both ends of the fractured bolts often flew out of the connection at failure. The presence of fillers affected the force deformation response and thereby the performance of the bolted connections. The following sections further discuss the results of the tests in terms of the individual metrics of connection deformation, ultimate strength and slip resistance. 6. Connection deformation The recorded deformation at failure consisted of a combined effect of slip deformation needed to engage the bolts in bearing,
Fig. 4. Force deformation for representative one bolt connections.
shear and flexural deformation of the bolts, and the hole deformations in the plates. The failure mode for the three bolt connection was typically a near-simultaneous failure of all bolts. The total maximum recorded deformations are summarized in Fig. 6 for the single bolt and the three bolt connections. Discrete points represent the test results and continuous lines connect through the average values. In general, comparing Figs. 4 and 5 the one and three bolt assemblies deformed with similarities between the individual connection arrangements (i.e. standard size hole, oversize hole and multiple filler).
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Fig. 6. Connection deformation at failure. Fig. 5. Force deformation for representative three bolt connections.
An increase in connection deformation was observed for connections with fillers as compared to those without. The values peaked with the 25 mm (1 in.) filler thickness, resulting in a 3 times larger deformations on average for standard size holes. The corresponding deformation peak for the oversized holes was 2.5 times larger than the non-filler connections with oversize holes and 5 times larger than the non-filler connections with standard size holes. The utilization of 51 mm (2 in.) thick fillers did not exhibit a further increase in deformation. Bolts from connections without fillers remained straight, with fractures along the shear planes between plates. With filler thickness increasing up to 25 mm (1 in.), the middle of the bolt increasingly deformed relative to the ends via a stepped deformation primarily within the length of the filler as shown in Fig. 7. The deformation indicated wedging of the bolt within the hole along the filler thickness. The bolts from connections with 51 mm (2 in.) fillers had a similar total deformation as the 25 mm (1 in.), except that the wedging was distributed over a longer length. Nonetheless, the high strength
bolts and plates used for these experiments resulted in plastic deformations with total connection displacements greater than the hole clearance, which was 1.6 mm (1/16 in.) for standard and 4.8 mm (3/16 in.) for oversized holes. Displacements greater than the hole clearance are the result of plate bearing plus bolt shear and flexural deformations. Introduction of the filler plates only served to increase the ultimate deformation of the connection. The connections with oversize holes experienced up to 1.7 times larger deformations than connections with standard size holes when comparing the same filler thicknesses. Two components contributed to the increased deformation as deduced from the force deformation plots in Figs. 4 and 5. Plastic deformations of the holes and the bolt contributed, but the more significant portion was caused by the deformation needed to engage the bolts in bearing following the initial slip. After the bearing was initiated in the oversized holes, the load increased at a lower rate than in the standard size holes. The deformations of connections with oversized holes exceeded even those recorded in multiple filler connections, which were at maximum 1.2 times larger on average than the standard size holes as measured for the three bolt connections with 25 mm (1 in.) fillers.
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Fig. 7. Sample of deformed bolts.
7. Connection strength The effects of grip length and bolt preload have been found to have little effect on the ultimate bolt shear strength [6]. To reduce variability, bolts of the same length were ordered from the same production lot. The maximum force recorded in each experiment is summarized in Fig. 8 for the single bolt and three bolt connections. The discrete data represents the recorded strengths, while the continuous line connects the average values. The strengths from the one bolt connections were higher than for the three bolt connections when normalized to strength per bolt, but exhibited similar characteristics. One possible explanation for this observation can be the result of splice plate prying. Uneven bearing is created at each plate as the bolt deforms, creating a resisting force that is not at the centerline of the plate [6]. This eccentric force then pushes the connection’s splice plates away from the connection, creating a tensile force along with shear in the bolt. The tensile force is partially resisted by the flexural stiffness of the plate and since the same plate dimensions were used in all of the tests, the relatively higher stiffness of the plate compared to a single bolt would reduce the effect on the bolt. The strengths of standard size hole connections with fillers were lower than those without fillers. However, the ultimate strength did not exhibit continued decrease with increasing filler thickness. The lowest strengths were recorded for fillers 25 mm (1 in.) thick, where the strength decreased on average by 6% and 9% for the one bolt and three bolt assemblies respectively. These reductions were less than the current guidelines which suggested a 15% lower strength for 19 mm (0.75 in.) filler thicknesses [1]. The assemblies with 51 mm (2 in.) thick fillers were stronger than any of the connections using thinner fillers, approaching the original strengths of connections without fillers. The basis for this behavior is illustrated in the bolts shown in Fig. 7, where the bolt deformation and fracture pattern correlated to the ultimate strength of the connections. The bolts from connections without fillers sheared perpendicular to the bolt at least one of the connection shear planes. Bolts from connections with fillers of 13 mm (0.5 in.) and 25 mm (1 in.) exhibited increasing bolt deformation within the filler hole and the fracture planes were no longer as clearly sheared. The reduction of strength for connections with fillers was attributed to the combined effect of flexural deformations imposed on the bolt along with the shear. For single filler connections, the influence of flexural deformation increased as the bolt wedged within the constraints of the holes
Fig. 8. Maximum strength.
for filler thickness up to 25 mm (1 in.). The 51 mm (2 in.) thick fillers reduced the flexural influence as the bolt deformations were distributed over the hole within the thicker filler and the failure pattern started to approximate that of the shear dominated behavior in connections without any fillers. The connections with oversize holes and without fillers exhibited slightly lower strengths than the same connections with standard size holes. Although the results from the single bolt connections exhibit little trend, the three bolt connections indicated 3% lower strength than the standard size holes over the range of filler thicknesses considered. Similar to the trends observed for the standard size holes, the oversized holes with 25 mm (1 in.) thick fillers exhibited the lowest strength and the 51 mm (2 in.) thick fillers tended to improve the strength to levels comparable to that of the connection without filler plates. The detrimental influence of the bolt flexural deformation was especially evident for the multiple fillers, which exhibited just 3% lower strengths at 13 mm (0.5 in.) filler, but further increased to 20% for the 25 mm (1 in.) filler in the three bolt connections. Since the individual plies shifted and rearranged as the deformation increased, the bolt was minimally restrained in the hole within the thickness of the filler. Dotted lines superimposed over the
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Fig. 9. Sample of bolt failure in multiple filler connection.
fractured bolts in Fig. 9 indicate the location of the shear planes between the plates. The shear planes are outside of the threaded area, yet the bolts failed in the threads. Once slip occurred, the bolts were deformed in such a way that a combination of bending and tensile forces resulting from catenary action, rather than shear, caused the bolt failure. For tests using 4 × 6 mm (4 × 0.25 in.) filler plates, all failures were controlled by a combination of bending and tensile forces. The failure surface through the threads of the bolts in the 4 × 6 mm (4 × 0.25 in.) tests indicated similar patterns to bolts tested in tension during the bolt tensile force calibration. Consequently, the connection ultimate strength was significantly lower when compared to single filler connections and would be expected to decrease further for thicker fillers where multiple thin fillers are used. The arrangement of four equally thin fillers used to make up 25 mm (1 in.) thickness as considered in these experiments represented an extreme case. Multiple fillers can also be used by providing a combination of thicker and thinner fillers, for example using a 19 mm and 6 mm (0.75 in and 0.25 in.) filler. For such a combination, the reduction in strength is not expected to be as severe as 4 × 6 mm (4 × 0.25 in.) fillers as the bolt would be more constrained within the hole although further tests are needed. Current US design guidelines for connections with fillers are based on the resisting load at a deformation limit of 6 mm (0.25 in.), which had been used to develop the shear strength reduction design equations [3,2]. This limit was chosen to represent a performance level beyond which a connection would experience excessive deformations and would not be considered useful [7]. In an effort to compare the current design recommendations to the results from this research, the recorded force was extracted at 6 mm (0.25 in.) deformation and is summarized in Fig. 10 for both the single bolt and three bolt connections. The values for connections with oversize holes are significantly lower and exhibit larger scatter because of increased deformations needed to engage the bolts in bearing as compared to standard size holes. Consequently, the force at the 6 mm (0.25 in.) deformation had just started to increase after bearing had been established against the oversize holes. The average experimental data for the various connections were normalized to the averaged ultimate force value found in the 3 bolt, standard size hole, no filler plate connection and compared to the current design guidelines in Fig. 11. The data were in close correlation to the design equation for filler thicknesses up to 25 mm (1 in.), indicating that the equations are applicable for connections with higher strength steels. The force in the connection further decreased for 51 mm (2 in.) fillers, but not in the linear manner suggested by the US design equations [3]. In comparison with the European design equation [4], the strength recovery was accounted for with a close correlation even for thicker filler sizes. From these results, the US design equations are conservative for fillers thicker than those previously considered. Although the
Fig. 10. Shear force at 6 mm (0.25 in.) deformation.
recorded forces were significantly lower in connections with oversized holes, similar characteristic of a reduced effect for 51 mm (2 in.) filler thickness was also observed at 6 mm (0.25 in.) deformation. The force in connections with multiple fillers decreased at a larger gradient than for the single fillers, resulting in reductions that could be underestimated using the current design equations for 25 mm (1 in.) fillers. 8. Normalized slip force Slip critical joints rely on the slip resistance between the surfaces of the plates rather than the shear strength of the bolts in the connection. The slip was identified by the displacement transducer measuring deformation of the splice plate relative to the pull plate. A sudden movement or reduction in load did not always accompany the slip, but the point at which the slip occurred was readily identified from the instrumentation. The force measured at slip was normalized to the test using the values from Table 3 as the normal force. The normalized slip force values are summarized in Fig. 12 for the single bolt and three bolt connections. An expected normalized slip force was estimated using a design equation for slip critical connections calculated as
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Fig. 11. Normalized shear force at 6 mm (0.25 in.) deformation.
Fn /(N0 )av g = µDu hsc Tb Ns /(N0 )av g , where Fn is the unfactored maximum slip load, µ = 0.5 for Class B surface (unpainted, blast cleaned), Du = 1.13 to account for higher than required minimum bolt pre-tension found in the field, Ns = 2 for the number of slip planes, Tb is the minimum bolt tension, and (N0 )av g is the average normal force of the bolt per Table 3 [3]. Per AISC hsc = 1.0 is for standard holes and hsc = 0.85 is for oversized holes. Since the values reflect the negative consequences of slip as opposed to any relationship to the performance of oversized holes, hsc was kept equal to 1.0. The expected slip values are also included in Fig. 12 for comparison to the test data, as calculated using bolt tension values established in Table 2 for Tb . The design value Du accounts for installation uncertainty in pretension force. Since all bolts were tightened to a calibrated turn-of-nut described in section Calibration of Bolt Tension, the value of Du was reduced to unity for the purposes of comparing them to the test result. The desired surface was intended to be Class B, however majority of the recorded normalized slip force values were well below the expected. Sample tests were conducted on 3 different samples using test procedures recommended by Kulak et al. [1], which include the use of a universal testing machine, constant controlled normal force representing the bolt and a measured slip force applied to one plate sandwiched by two other plates in a symmetric setup. These slip test samples were obtained from previously tested 32 mm (1.25 in.) thick splice plates and cut to 76 mm × 127 mm (5 in. × 3 in.) sections. Using the measured normal force and the measured slip force, an average slip coefficient of 0.18 was obtained. Despite the low sample numbers and the reuse of surface on the plate for these slip tests, the results indicate that a slip coefficient was present that is much lower than would be expected for Class B surface. More extensive research beyond the scope of this study would be needed to determine if the reason is related to the improper application of the surface treatment or the reaction of the HPS 480W (HPS 70W) steel grade to the treatment. In general, the variability in normalized slip force made deterministic evaluations difficult with just two data points per filler thickness, but general trends were observed. In both the single bolt and the three bolt assemblies, the normalized slip force for standard size holes were consistently lower for connections with fillers as compared to connections without fillers. The decrease in the normalized slip force was further exaggerated for multiple fillers, which exhibited the lowest normalized slip values. The exact cause
Fig. 12. Comparison of normalized slip force.
for the decrease in the normalized slip force was difficult to ascertain, but one of the possible reasons could be related to the surface finish observed on the filler plates. Imperfections in the surface, as illustrated by the example filler plate in Fig. 13, limited the faying surface’s contact near the bolt holes. The reduced contacts in the high stress areas may have contributed to the lower than expected slip values for connections with fillers. This effect may have been multiplied for the multiple fillers at the various faying surfaces. The normalized slip force for oversize holes along with fillers did not exhibit appreciable differences when compared to the standard size holes. The normalized slip values remained approximately constant across the different filler thicknesses. When no fillers were used, the oversize holes were found to have lower normalized slip force as compared to the standard size holes. 9. Design recommendations Current design equations for bolted connections with fillers are based on connection strengths at a predetermined deformation
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Fig. 13. Faying surface of 51 mm (2 in.) filler plate.
Fig. 14. Proposed strength reduction comparison.
and had been found conservative for thick fillers in these experiments. Since the intended application of a bolted connection can vary, the following strength design recommendations are based on ultimate values and not on forces at a specific deformation so as to provide an increased level of design freedom. The test results had indicated that the lowest strengths occurred for connections with 25 mm (1 in.) fillers and nearly fully recovered when 51 mm (2 in.) fillers were used. The previously described observations of bolt deformation and failure indicated that wedging of the bolt within the filler thickness affected the failure mode. When thin fillers were introduced, the bolt failure became a combination of shear and flexure as highlighted by the plastic deformations in Fig. 7. As the filler thickness increased, the relative deformation of the bolt within the filler thickness was distributed over a longer length, resulting in the failure mode reverting back to shearing at the ends of the filler. These bolt deformation observations combined with the strength reduction and recovery from the tests suggests that the strength reduction be based on filler thickness relative to the bolt diameter instead of the filler thickness alone. Using a general form of a quadratic equation, the expression for the strength reduction fr of bolted connections with fillers was derived as a unit independent relationship shown in Eq. (1).
fr = 1 +
kf d2b
(0.75t − db )2 − kf .
83
(1)
The value of fr may not become less than zero for any combination of filler thickness t and bolt diameter db . The factor kf varies depending on the type of bolted connection and represents the value of maximum reduction, with kf = 0.10 for standard size holes and kf = 0.13 for oversize hole connections to account for the slightly higher strength reduction for oversized holes. The factor of 0.75 was chosen such that the peak of the strength reduction is assumed to occur at a filler thickness of 4/3 the bolt diameter and no reduction occurs for fillers larger than 8/3 the bolt diameter. The equations are compared in Fig. 14 to the measured strengths F , which had been normalized to the values of the average standard size connection strength without fillers (F0 )av g . An emphasis needs to be placed on the penalty of oversized holes on connection performance in terms of deformations.
Although the amplification in deformation was found to be approximately the same for both standard and oversize holes when compared to their respective connections without fillers, the connections with oversize holes exhibited much larger overall deformations. The connection deformations did not continue to increase past 25 mm (1 in.) fillers and if normalized to the bolt diameter, the deformation amplification Ub for bolted connections with fillers could be expressed using a linear relationship shown in Eq. (2).
Ub = kd
1+
2t db
.
(2)
The value of Ub may not exceed 3kd . The factor kd represents the effect of the oversize holes on the connection. For standard size holes kd = 1 and for oversize holes kd = 1.7 since the oversize hole connection was found to be approximately 1.7 times the deformation across the different filler or non-filler configuration. The slope of the line of 2 was chosen to encompass the intermediate results. The equation is compared in Fig. 15 to the measured deformations D, which have been normalized to the average recorded deformation of the three bolt standard size hole connections without fillers (D0 )av g . The above relationships are not applicable for multiple fillers, which can exhibit significantly greater reduction in strength and no evidence exists that the strength recovers for thicker fillers when numerous thin plies are used. The recommendations are based on test results involving 22 mm (0.88 in.) diameter bolts and need to be verified for other diameters, but the equation presents a framework that is based on bolt deformation and failure observations along with the recorded values and may become more refined as more data becomes available. Additional research is required to quantify the effect of long connections and further study the observed connection slip trends. 10. Conclusions Bolted connections made using high performance A709M HPS 480W MPa (70W) steel and 22 mm (0.88 in.) diameter A490M (A490) bolts were used to conduct over 50 individual experiments
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observed at the 25 mm (1 in.) filler thickness and nearly recovered at the 51 mm (2 in.) filler thickness. • Use of fillers with either oversize hole or standard size hole connections produce a negligible effect on the ultimate strength of the connection. Connections with oversize holes and fillers exhibit lower ultimate strength by approximately 3% as compared to standard size holes. • The use of fillers consisting of multiple thin plies induces bending and tensile stresses in the bolt that can result in a shift of the failure mode from the shank to the threads and result in up to a 20% reduction in connection capacity for designs where the bolt threads were intended to be excluded from the shear plane. • The force at which the connection slipped does not appreciably vary with filler thickness, although lower slip values can result from multiple ply fillers. Acknowledgements
Fig. 15. Proposed deformation amplification comparison.
to failure for filler thicknesses up to 51 mm (2 in.). The following findings are based on the observations and analyses of the results:
• The connection deformations increase as the filler thickness increase up to a limit, which was found to occur at 25 mm (1 in.) filler thickness for the arrangements considered. • Oversize holes significantly increase connection deformations, resulting in up to 1.7 times the standard size hole deformations at failure across the filler thicknesses as compared to multiple ply fillers that were at maximum 1.2 times larger for the same total thickness. • The use of fillers does not always result in a decrease in connection strength for standard and oversized hole connections. With increasing filler thickness, the detrimental influence peaks and can return to the strength of the connection without fillers. In the arrangements considered, the lowest strength values were
The research was conducted by Portland State University. The authors would like to acknowledge the financial support of the Research Council on Structural Connections and Fought & Company Inc, in Portland, Oregon. However, any opinions, findings, conclusions and recommendations presented in this paper are those of the authors and do not necessarily reflect the views of any of the sponsors. References [1] Kulak GL, Fisher JW, Struik JHA. Guide to design criteria for bolted and riveted joints. 2nd ed. Chicago (IL): Research Council on Structural Connections; 2001. [2] Yura JA, Hansen MA, Frank KH. Bolted splice connections with undeveloped fillers. Journal of the Structural Division 1982;108:ST12. [3] American Institute of Steel Construction Inc. Steel construction manual. 13th ed. 2005. [4] EN 1993-1-8 (2002). Eurocode 3: Design of steel structures — Design of joints. European Committee for Standardisation. [5] National Association of Corrosion Engineers. Joint surface preparation standard: Near-white metal blast cleaning. 1999. [6] Wallaert JJ, Fisher JW. Shear strength of high-strength bolts. Journal of the Structural Division 1965;91:No. ST3. [7] Perry WC. Bearing Strength of Bolted Connections. Thesis presented to the University of Texas, at Austin in partial fulfillment of the requirements for the degree of Master of Science. 1981. [8] Wright WJ. High performance steel: research to practice. Federal Highway Administration: Public Roads, Spring 1997, 1997. pp. 34–38.