Progressive collapse behaviour of extended endplate connection to square hollow column via blind Hollo-Bolts

Thin-Walled Structures 131 (2018) 681–694

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Full length article

Progressive collapse behaviour of extended endplate connection to square hollow column via blind Hollo-Bolts

T

⁎

Wei Wanga,c, Ling Lib, , Dabiao Chena,c, Ting Xua,c a

State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China School of Engineering, RMIT University, Melbourne 3000, Australia c Department of Structural Engineering, Tongji University, Shanghai 200092, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Hollo-Bolt Square hollow section column Blind bolted extended endplate connection Progressive collapse Catenary mechanism Stiffness

This paper investigated behaviour of two blind bolted extended endplate connections using Hollo-Bolts to square hollow section (SHS) column in full-scale beam-joint-beam assemblies under a scenario of inner column removal. The characteristics of the Hollo-Bolt were firstly examined by nine direct tension tests and six double shear tests, which demonstrated low pre-tension and anti-sliding capacity under recommended torques in comparison with the requirement for standard high strength slip-critical bolts. In two specimens with special configuration of reduced beam section (RBS) or thin column (TC), Hollo-Bolts were pulled out from column wall with different failure mechanism, one exhibiting shear failure of sleeve and the other combining closure of sleeve legs in HolloBolts and enlarged bolt holes of column wall. Strain intensities at the connection region and load transfer mechanisms of flexural resistance and catenary mechanism were analysed in details. Results indicated that the premature failure of thin column wall disabled the full utilization of Hollo-Bolts strength and hence resulted in low level of stiffness and catenary resistance of the assembly, which implied the necessity of connected plates with sufficient thickness for satisfactory performance of Hollo-Bolt connections. In the comparison against other blind bolted connection and welded connection in normalised vertical resistance - normalised beam rotation curves, extended endplate connections using Hollo-Bolts presented lower initial normalised stiffness due to their limited pre-tensions, and lower level of catenary mechanism development due to the reduced strength of HolloBolt under large deformation.

1. Introduction Tubular section is a popular option for columns owing to its inherent mechanical advantages of closed sections over the open sections, with the relatively high stiffness and capacity of flexural and torsional performance. The widely adopted concrete filled steel tubular (CFST) columns also promote the application of tubular sections. There are other advantages preferred in architectural design of buildings, such as simple and elegant appearance, and low cost of surface painting and maintenance. In practice, tubular columns are connected to beams by welding or bolting. Welded connections to column are usually configured with inner-diaphragms, outer-diaphragms or through-diaphragms and they provide large stiffness for the panel zone. It is highly demanded of significant construction effort and welding quality on site in order to avail a satisfactory performance of the welded connection and avoid damages like what happened in 1994 Northridge and 1995 Hyogoken-

⁎

Nanbu earthquake [1]. Comparatively, the bolted connections are much more convenient in assembling structural components and meanwhile it is easier to control the construction quality of bolted connections. However, the inaccessibility into the internal space of a normal-sized tubular column limits the application of bolted connections to hollow column in practice. With the aim of installing and fastening the bolts from outside of a column, blind-bolting technology [2] has been developed over past decades with a number of blind bolts (or one-side bolts) invented. The commercially available products include Blind Oversized Mechanically Locked Bolt (BOM), High Strength Blind Bolt (HSBB), Ultra-Twist [3], Flowdrill [4], Hollo-Bolt (HB) [5], Reverse Mechanism HB (RMH) [6], Extended HB (EHB), Molabolt [7], Blind bolt [8] and Ajax-Oneside [9]. The products of blind bolts have different mechanisms to achieve oneside installation, and therefore they exhibit different characteristics as well as different applicability such as certain clamping thickness of connected plate, diameter of bolt hole or pre-tension.

Corresponding author. E-mail address: [email protected] (L. Li).

https://doi.org/10.1016/j.tws.2018.07.043 Received 12 April 2018; Received in revised form 24 June 2018; Accepted 23 July 2018 Available online 06 August 2018 0263-8231/ © 2018 Elsevier Ltd. All rights reserved.

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Hollo-Bolts were tested in full scale on a planar beam-joint-beam (B-JB) [35] assembly under an inner column removal scenario. Prior to that, characteristics of Hollo-Bolts were firstly examined in direct tension tests and double shear tests. Based on the test results of the individual Hollo-Bolts, two different configurations of extended endplate connections were designed for the B-J-B assemblies. Special configurations of reduced beam section (RBS) and thin column section were included in the connection design in order to compare different failure mechanisms. The test results of load-displacement relationship, damage evolution, strain development and load transfer mechanism were presented in detail. Initial stiffness and ultimate capacity of the tested connections were discussed through comparison with other beam-tocolumn connections.

Mourad et al. carried out monotonic tests [10,11] and cyclic tests [12] on endplate connections with HSBB or BOM for the flexural behaviour, based on which a quantitative procedure for detailing and designing blind bolted extended endplate were proposed [13]. Tabsh et al. [14] developed resistance factors for Ultra-Twist via reliability analyses on the blind bolted connections. Nakajima et al. [15] experimentally verified the fatigue strength and other basic characteristics of Ultra-Twist bolted joints. France et al. [16–18] carried out static tests on twenty-six endplate connections using Flowdrill process to investigate the strength and rotational stiffness characteristics of joints. Barnett et al. [19] compared the behaviour of endplate connections adopting Hollo-Bolts or RMH and found that RMH exhibited higher stiffness but lower ductility due to the fact that legs of the sleeve turned plastic and then failed almost at the same time. Yeomans [20] investigated the endplate connections using Hollo-Bolts under tension and bending moment. Elghazouli et al. [21] conducted monotonic and cyclic tests on Hollo-Bolts bolted angle connections. Liu et al. [22] examined the performance of blind bolted connection using Hollo-Bolts under shear load, involving various connection types. Tao et al. [23–25] experimentally studied blind bolted connections to CFST columns using Hollo-Bolts and EHBs, respectively concerning different stiffening elements inside the columns [23], stainless steel and floor slab [24] and fire performance [25]. Tizani et al. [26–29] studied the behaviour of blind bolted endplate connections to concrete-filled square hollow sections using EHB. In eight full size connection specimens under monotonic moment load, fracture occurred in the shank between the bolt head and threaded cone nut [26]. While under cyclic moment loading, two failure modes of bolt shank facture and partial bolt pullout were observed [27]. Following those, ten T-stub to CFST joints with different fastening systems were tested under monotonic tension load [28] and component-based models were subsequently proposed [29]. Lee et al. [30] tested blind bolted T-stub connections to unfilled hollow section columns using Ajax-Oneside in the tension and compression regions under static loading. Gardner et al. [31] discussed the features of five Ajax-Oneside bolted connections to the concrete-filled circular hollow column under cyclic tension. It can be seen that researches on blind bolted connections so far were focused on the behaviour under bending moment, pure tension or shear load. In general, most blind bolts are not able act as high strength slip-critical bolts, except AjaxOneside whose specification [9] states that the product exhibits the similar pre-tightening force and ultimate capacity as standard high strength bolts. As the progressive collapse prevention becomes a new concern for structural design, it is of significant value examining the progressive collapse behaviour of blind bolted connections. Under a typical progressive collapse scenario of the inner column removal, the responses of connections in the extracted beam-to-column assembly are featured by a transition of flexural mode (mechanism) at the early stage towards catenary mode (mechanism) at the later stage [32–34]. The catenary mechanism is significant for assembly to obtain vertical resistance as flexural mechanism declines, and its development relies on two factors, i.e. beam axial force and beam deflection. To be more precise, satisfactory connections for effective development of catenary mechanism need to maintain sufficient connection strength and meanwhile provide adequate deformation (rotation) capacity [35]. Therefore, the blind bolted connection to hollow column should be able to keep certain integrity under large rotation for the sake of catenary mechanism development after it successfully undergoes flexural resisting stage. As the response of connection under this type of loading path cannot be represented by the previous loading condition of bending moment, pure tension or shear force, such behaviour needs to be investigated in a beam-to-column assembly extracted from the frame which is directly affected by the removed column. In this work, Hollo-Bolt was selected for blind bolted connections due to its wide availability and application in practice. Two extended endplate connections to square hollow section (SHS) columns using

2. Characteristics of Hollo-Bolts (HB) used for connections Prior to the tests on the beam-to-column assembly, characteristics of Grade 8.8 Hollo-bolts of three types, M12, M16 and M20, were firstly examined through direct tension test and double shear test, although only M16 and M20 bolts were adopted in the extended endplate connections. 2.1. General Hollo-Bolt (HB) was developed by Lindapter International of UK. As shown in Fig. 1(a), a HB consists of a collar, a sleeve (with four legs), a cone and a bolt. For the update products, a High Clamping Force (HCF) mechanism is placed between the collar and sleeve in order to apply a greater pre-tension. During the installation illustrated in Fig. 1(b), legs of sleeve are expanded during the moving of the cone towards the connected plates and the bolt hole is then locked by the expanded sleeve when the pre-tension is applied. The easy installation without requiring special device facilitates the wide application of HB in structural steel. In the tests conducted by Yeomans [20] on Hollo-Bolts bolted endplate connections to rectangular hollow section column of different thicknesses under tension or bending moment, it was found that connections had lower capacity but larger ductility with smaller column wall thickness. Endplate connections failed with greater plastic deformation and subsequent pull-out of Hollo-Bolt, but distinct features were found at different column wall, being that Hollo-bolts were pulled out due to bolt hole cracking at column wall of small thickness (5 mm, 8 mm) or due to shear failure of sleeves in the bolts connecting column wall of large thickness (12.5 mm). Elghazouli et al. [21] conducted monotonic and cyclic tests on Hollo-Bolts bolted angle connections to rectangular hollow section columns, in order to assess the influence of various salient connection properties and geometric parameters on the connection behaviour of initial stiffness and bearing capacity under bending moment. Liu et al. [22] experimentally examined the stiffness, capacity and failure mechanism of Hollo-Bolts bolted connections under shear load, involving angle connections, combined channel/angle connections as well as single and double lap joints. Wang et al. [23] proposed four different strengthening methods for the concrete-filled steel tubular (CFST) column with Hollo-Bolts as shown in Fig. 2, (a) using inner binding bars, (b) welding two internal HCF mechanism Collar

Cone

Sleeve

Expanded Legs of sleeve

Bolt

(a) Components . Fig. 1. Hollo-bolts. 682

Interlocked Cone

(b) Installation [5].

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Fig. 2. Strengthening methods for blind bolted connection to CFST column with Hollo-Bolts [23].

mechanism. Except HB12-T-3 and HB20-T-3, Hollo-bolts were tested with pressure sensor. The specimens had different durations of pretension relaxation (2 h, 0.5 h or 0 h) before testing. HB16-T-2, HB20-T-2 and HB20-T-3 were applied with torques greater than the recommended values (asterisked in Table 1) by the manufacturer [5] for obtaining the behaviours under over-torques.

rings, (c) externally welding two C-shaped channels, and (d) internally embedding a short segment of I-section in the steel tube. Six tests on the blind-bolted joints were accordingly conducted to evaluate the efficiency of the strengthening methods. Test results indicated the effectiveness of C-shaped channels and embedded I-section in enhancing the performance of the joints and the significant improvement of strength and deformation capacity with large bending deformation developed in the endplates.

2.2.2. Test results According to records of the pressure sensors, the process of pretension relaxation of Hollo-Bolt was completed within 15 min from the peak pre-tension was reached under applied torque and thereafter maintained a stable level. In the direct tension tests, shear failure of sleeve legs (Mode A) and fracture of bolt shank (Mode B) were observed in the Hollo-Bolts, as demonstrated in Fig. 4, except that HB16-T-1 was unfinished due to malfunction of loading equipment. The results of direct tension tests are summarised in Table 2 and the load-deformation curves are plotted in Fig. 5. The force at separation represents the applied tension force when connected plates were observed to separate, and this force together with the ultimate capacity is marked on the corresponding curve in Fig. 5. It can be seen from Table 2 that the stable pre-tensions of HolloBolts under recommended torques were much less than the required value P for standard high strength slip-critical bolts [36]. Applied with an over-torque, Hollo-Bolts in HB20-T-2 reached a stable pre-tension (114.4 kN) which was comparable with the required pre-tension P (125 kN). However, an over-torque may result in failure of the HCF mechanism as what happened to HB16-T-2 (under two times of the recommended torque) ending up with a lower stable pre-tension (2.60 kN) than that (7.55 kN in HB16-T-3) under a recommended torque by Hollo-bolt manufacturer [5]. In Fig. 5, a lower bound values Tb for tensile capacity of bolt shank [36] is marked in each figure, and results indicate that the ultimate capacities of Hollo-Bolts are close to or slightly exceeded Tb. For M16 Hollo-Bolts, the failure of HCF mechanism in HB16-T-2 under an over-

2.2. Direct tension tests 2.2.1. Test programs In the direct tension test, each individual Hollo-Bolt was pulled apart by the connected plate made of steel Q345B after the torque had been applied to the Hollo-Bolt. A pressure sensor was adopted between the connected plates for monitoring clamping force of the Hollo-Bolt from the installation until the end of test loading. The connected plates were required to be rigid to exclude their effect on the behaviour of the Hollo-Bolt. To this end, connected plates were designed as thick as possible within the range of recommended clamping thickness for each Hollo-Bolt [5]. Accordingly, test setups equipped with and without pressure sensor were proposed as illustrated in Fig. 3(a) and (b), respectively. Tension force was applied to Hollo-Bolt through the top and bottom frames by the actuator in a displacement control, and axial deformations were measured with two displacement transducers D1 and D2. Due to the occupancy of sensor, the thickness of connected plates ① in test setup of Fig. 3(a) was reduced, in comparison with that of Fig. 3(b), so that the recommended clamping thickness of Hollo-Bolts was matched. Nine direct tension tests were performed on three sizes of HolloBolts, as summarised in Table 1, where the specimen reference is expressed with HB following by the size of bolt shank, T (representing tension) and a serial number. Among the Hollo-Bolts, M12 is the type without HCF mechanism, while M16 and M20 are with HCF

(a) With pressure sensor.

(b) Without pressure sensor.

Fig. 3. Test setup and instrumentation for direct tension tests of Hollo-Bolts. 683

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Table 1 Summary of direct tension test programs for Hollo-Bolts. Reference HB12-T-1 HB12-T-2 HB12-T-3 HB16-T-1 HB16-T-2 HB16-T-3 HB20-T-1 HB20-T2 HB20-T3 a

Bolt shank (Grade 8.8)

Diameter of bolt hole (mm)

Torque(Nm) a

M12

21.5

80

M16 (HCF)

27.5

M20 (HCF)

34.5

190a 380 (over-torque) 190a 300a 400 (over-torque) 400 (over-torque)

Plate ① thickness (mm)

With pressure Sensor

Duration ofpre-tension relaxation (h)

10 10 20 12 12 12 16 16 28

Yes Yes No Yes Yes Yes Yes Yes No

2 0.5 0 0 2 2 2 0.5 0

The values are the recommended torques by the manufacturer [5].

Fig. 4. Failure modes of Hollo-Bolts under tension force. Table 2 Summary of results of direction tension tests on Hollo-Bolts. Reference

Failure mode

Force at separation (kN)

Ultimate capacity (kN)

Stable pretension (kN)

Required pretension P [36] (kN)

HB12-T-1 HB12-T-2 HB12-T-3 HB16-T-1 HB16-T-2 HB16-T-3 HB20-T-1 HB20-T-2 HB20-T-3

A A A / B B A A B

17 23 17 4 6 18 89 117 132

77 74 76 / 133 147 200 220 228

3.67 10.05 / / 2.60 7.55 71.58 114.40 /

45

Fig. 6. Test setup and instrumentation for double shear tests on Hollo-Bolts.

2.3. Double shear tests

75

2.3.1. Test programs Double shear tests were conducted on Hollo-Bolts in the test setup illustrated in Fig. 6 to investigate the anti-sliding behaviour of HolloBolts. Three plates made of steel Q345B were connected by a Hollo-Bolt equipped with a sensor on one end and by two standard Grade 10.9 high strength slip-critical bolts ④ on the other end. Contact surfaces were treated with sand blasting. Two transducers D1 and D2 were arranged for measurement of relative displacements. All bolts were applied with torque before starting shear load. A total of six Hollo-Bolts were tested and the program were summarised in Table 3, where the specimen reference is expressed with HB

125

torque resulted in a lower force at separation and a lower ultimate capacity than those in HB16-T-3 under the recommended torque. For M20 Hollo-Bolts, over-torque but no failure of HCF mechanism brought a higher force at separation and a higher ultimate capacity. It is implied that greater torque applied to the Hollo-Bolt could be beneficial for improved behaviour which meanwhile relies on the normal work of HCF mechanism.

Fig. 5. Load-deformation curves of direct tension tests on Hollo-Bolts. 684

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HB16-S-1 under the recommended torque. Under an over-torque of 200 Nm (2.5 times of the recommended value of 80 Nm), Hollo-Bolt in HB12-S-1 obtained an anti-sliding capacity of 6.5 kN which was even greater than that in HB16-S-1 under the recommended torque. As is clearly implied in Fig. 9, the magnitude of applied torque influences significantly the behaviour of Hollo-Bolts, even though the anti-sliding capacity still cannot reach the design values Sb specified by code [37] for standard high strength slip-critical bolts as marked in the figures. It is revealed by the direct tension tests and double shear tests that, under recommended torques, Hollo-Bolts cannot provide pre-tensions and anti-sliding capacities as required for standard high strength slipcritical bolts in the codes [36,37]. Consequently, cautions should be taken when using of Hollo-Bolts for the slip-critical connections.

Table 3 Summary of programs and results of double shear tests on Hollo-Bolts. Reference

HB12-S-1

Bolt shank (Grade 8.8)

Thickness of plate (mm) ①③

②

M12

10

5

Bolts ④ (Grade 10.9)

Torque (Nm)

Stable pretension (kN）

Antisliding capacity (kN)

M20

200 (overtorque) To ultimate 190a To ultimate 300a To ultimate

4.08

6.5

12.96

/

5.25 64.81

3.7 42.8

22.97 84.59

40.00 /

HB12-S-2 HB16-S-1 HB16-S-2

M16

10

5

M24

HB20-S-1 HB20-S-2

M20

14

7

M24

a

3. Testing programs for Hollo-Bolts bolted endplate connections after a column loss

The values are the recommended torques by the manufacturer [5].

3.1. Design of specimens Full-scale test specimens of beam-joint-beam (B-J-B) assemblies were designed to represent the relevant beam-to-column connection region under a column removal scenario. As illustrated in Fig. 10, the BJ-B assembly assumes that the inflection points locates at the mid-span of the original beam members when a central column is removed [35]. In each specimen, a square hollow section (SHS) column and two Isection beams are connected by steel extended endplate connections (thickness of endplates is 24 mm). The performance of Grade 8.8 HolloBolts in the beam-to-column connections was investigated in two specimens the details of which are summarised in Table 4. In Specimen HB20-RBS, four rows of M20 Hollo-Bolts were used to connect the endplate and the column section SHS250 × 12 on each side with a special configuration of reduced beam section (RBS) near the connected beam ends so that plastic deformation concentrates at the relatively weak beams. Whereas in Specimen HB16-TC (TC representing “Thin Column”), five rows of M16 Hollo-Bolts and a relatively weak column section SHS250 × 6 were used in order to direct the failure to occur at the column wall. The details of the connection configuration of these two specimens are demonstrated in Fig. 11. The Hollo-Bolts were applied with the recommended torques [5]. The span of the beams (l0) was 4.5 m, providing span-to-depth ratios of 18 and 15 to the Specimens HB20-RBS and HB16-TC, respectively. The measured material properties of SHS columns, I-section beams and endplates are summarised in Table 5. Based on the measured ultimate capacities of M16 and M20 HolloBolts under tension (refer Table 2) and the yield strength of components (Table 5), the ultimate flexural capacities provided by Hollo-Bolts for the extended endplate connections are greater than the full plastic flexural capacities Mp of critical beam sections. The design philosophy of “strong panel - weak beam” was satisfied by Specimen HB20-RBS, but not by Specimen HB16-TC in which plastic flexural capacity of column wall is lower than Mp. The thin column wall in Specimen HB16-

Fig. 7. Fracture of bolt shanks under over-torque (HB20-S-2).

followed by the size of bolt shank, S (representing shear) and a serial number. There were two specimens for each size of Hollo-Bolt, one of which was installed with a specified torque while the other was with an increasing torque until the torque cannot be applied any more. In tests HB12-S-2 and HB20-S-2, bolt shanks fractured when applying overtorques of 250 Nm and 1040 Nm, respectively, as demonstrated in Fig. 7. Hollo-Bolt in HB16-S-2 survived from the over-torque so that it continued to be tested under shear load. The pre-tensions reached under applied torques are listed in Table 3, which were all less than the required value P (refer to Table 2) for standard high strength slip-critical bolts. 2.3.2. Test results When installations of bolts were completed, marking lines were painted horizontally across three plates around the connecting bolts so that sliding between the plates could be directly observed. In the double shear tests, sliding was evident at the interfaces between the plates connected by Hollo-bolts, as shown in Fig. 8(a), while no sliding was observed at the other end adopting standard high strength slip-critical bolts shown in Fig. 8(b). Load-displacement curves of the four completed double shear tests are shown in Fig. 9, where the anti-sliding capacity was marked at the load when the relative displacement obviously increased. Results indicate that the anti-sliding capacity of Hollo-Bolt in HB16-S-2 was greatly improved with the over-torque in comparison with that in

(a) Evident slidingusing Hollo-Bolts.

(b) No sliding using standard high strength bolts.

Fig. 8. Sliding between connected plates in the double shear tests on Hollo-Bolts. 685

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Fig. 9. Load-displacement curves of double shear tests on Hollo-Bolts.

F

Sections W2 and E2 were expected to represent the most critical sections near the connection. As illustrated in Fig. 13(c) and (d), the strains at the connection region were also monitored by the strain gauges at the column wall (T1 and T2), the stiffeners on the west side (T3 and T4) and the endplate near the edge of connected beam bottom flange on the east side (T5).

l0 Fig. 10. A beam-joint-beam (B-J-B) assembly for an inner column removal scenario [35].

4. Progressive collapse behaviour of Hollo-Bolts bolted extended endplate connections

TC was designed to turn plastic prior to the beam section for the purpose of presenting different response of extended endplate connection in the weak panel.

4.1. General behaviour 4.1.1. Load-displacement relationships and damage evolution Fig. 14 shows the load (F) - displacement (δ) relationships of the central columns in B-J-B assemblies, where a nominal beam plastic load Fp represents the vertical load at the central column causing the formation of plastic hinges at the critical Sections W2 and E2, and the beam chord rotation θ is evaluated by dividing the vertical displacement δ against the distance of 2.25 m between the column and the pin supports. Key stages of the damage evolution are marked on the curves in Fig. 14 and subsequently depicted with corresponding photos in Fig. 15 and Fig. 16. For Specimen HB20-RBS, the top flanges of RBSs (Sections W2 and E2) on two sides yielded (Points “HR-1″ on the load-displacement curve in Fig. 14 with corresponding photo in Fig. 15) when the applied vertical displacements reached 50 mm (θ = 0.022 rad) and the vertical loads were close to the plastic load Fp. Afterwards, the load increased with a slower rate. At a displacement of 274 mm (0.122 rad), tensile failure of Hollo-Bolts by shear failure of sleeves happened at the lowest row on the west side (point “HR-2″), which induced a drop of the load from 182.1 kN (2.56Fp) to 147.2 kN (2.07Fp). Afterwards, the HolloBolts were gradually pulled out from the column wall while the load recovered up to the maximum peak value of 231.7 kN (3.26Fp) before a slight drop due to the shear failure of the sleeves and therefore pull-out of Hollo-Bolts at the lowest row on the east side (Point “HR-3″). The subsequent failure of Hollo-Bolts at the second lowest row on the west side (Point “HR-4″) at a displacement of 384 mm (0.171 rad) and on the east side (point “HR-5″) at 425 mm (0.189 rad) also caused the corresponding load drops. In spite of a certain recovery of load following each of the successive failure of Hollo-Bolts, Specimen HB-RBS had lost a great deal of the vertical resistance when the maximum movement

3.2. Test setup An overall view of the test setup is illustrated in Fig. 12. Each B-J-B assembly was pin-supported in a horizontally self-balanced support frame with latch-type rollers at the two ends of the beams. The distance between two pin supports is 4500 mm to fit the span of the beams. The test specimens were vertically loaded in a quasi-static manner (with a displacement rate of less than 7 mm/min) at the top of the central column and were guided along the bottom part within a sliding support which allows a maximum vertical free movement of approximately 450 mm. The test was terminated when the specimen lost most of its capacity or the maximum vertical displacement at the central column was reached. 3.3. Instrumentations The arrangements of displacement transducers and strain gauges for the two specimens are shown in Fig. 13. Displacement transducers (see Fig. 13(a)) were used to measure the deflection of the beam-column assembly along the beam length and any possible movements of the two pin supports. Strain gauges were arranged at four beam sections shown in Fig. 13. (b), including two sections near the pin supports (Sections W1 and E1) and two sections near the beam-to-column connections (Sections W2 and E2). Sections W1 and E1 were anticipated to remain elastic during the loading and their strains would be used to calculate internal forces of the beam sections and thereby the reaction forces at the pin supports. Table 4 Geometrical characteristics of specimens (dimensions in mm). Specimen

Column section

Beam section

Hollo-Bolts (Grade 8.8)

Special configuration

HB20-RBS HB16-TC

SHS250 × 12 SHS250 × 6

H250 × 125 × 7 × 8 H300 × 150 × 6 × 8

8 ×M20 10 ×M16

Reduced beam section (RBS) Thin column (TC)

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(a) Specimen HB20-RBS.

(b) Specimen HB16-TC.

Fig. 11. Connection configurations of B-J-B assemblies.

range for the central column within the sliding support was reached. In Specimen HB16-TC, when the applied displacement of central column was 85 mm (0.038 rad), gaps between the endplate and column wall at the lowest row were already noticeable. This was accompanied by the convex deformation of the column wall and closure between the originally expanded sleeve legs of Hollo-Bolts (Points “HT-1″ on the load-displacement curve in Fig. 14 with corresponding photo in Fig. 16). The load then kept increasing up to 76 kN (0.48Fp) at a displacement of 210 mm (0.093 rad), in spite of the accumulative plastic convex deformation of the column wall near the lowest row (Point “HT2″) which allowed the gradual pull-out of the Hollo-Bolts. The load increased to 115 kN (0.72Fp) when at least half length of the Hollo-Bolts was pulled out (Point “HT-3″) at a displacement of 344 mm (0.153 rad) following which the load slightly decreased and then recovered again. When the test was terminated at the maximum vertical displacement for the specimen, the Hollo-Bolts at the lowest row were totally pulled out from the column wall (Point “HT-4″) while the load still kept an increasing trend.

Table 5 Material properties of specimens. Specimen

HB20-RBS

HB16-TC

Component

Yield strength fy (MPa)

Ultimate strength fu (MPa)

Elongation δ (％)

Endplate (te = 24 mm) SHS250 × 12 Beam flange (tf = 8 mm) Beam web (tw = 7 mm) SHS250 × 6 Beam flange (tf = 8 mm) Beam web (tw = 6 mm)

351

510

34

465 300

615 450

22 29

315

465

33

460 310

605 460

32 30

320

460

30

Vertical reaction frame

Vertical actuator Horizontal support frames B-J-B assembly Pin supports

Base beams Sliding support

Fig. 12. Test setup for a B-J-B assembly. 687

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(a) Displacement transducers.

(b) Four beam sections for strain gauges.

(c) Arrangement of strain gauges for Specimen HB20-RBS.

(d) Arrangement of strain gauges for Specimen HB16-TC. Fig. 13. Layouts of instrumentations for B-J-B assemblies.

34.5 mm (refer to Table 1) than the bolt shank M20. As illustrated in Fig. 17(a), there was no crack at the column wall in this connection, implying a sufficiently strong column wall (thickness of 12 mm) in comparison with the Hollo-Bolts. On the contrary, in Specimen HB16-TC, the interaction between the thin column wall (6 mm) and Hollo-Bolts was not as strong as in Specimen HB20-RBS so that Hollo-Bolt only achieved a state of closure between legs of sleeve but without further shear failure. The Hollo-Bolt with closed sleeve was still blocked at the bolt hole and therefore forced the thin column wall to undergo severe plastic deformation near the bolt hole as illustrated in Fig. 17(b). In the end, enlarged diameter of

4.1.2. Different failure mechanisms of Hollo-Bolts’ pull-out from column wall The extended endplate connections in two specimens presented different failure mechanisms during pull-out of the Hollo-Bolts from the column wall, in spite of the fact that both failure modes started from the damage of sleeves. The difference is supposed to result from the strength relationship between the Hollo-Bolts and the column wall. In Specimens HB20-RBS, shear failure happened to the Hollo-Bolt’ sleeve which experienced compression by the column wall at the bolt hole, and the subsequent absence of sleeve allowed the bolt to easily pass through hole which was machined with a greater diameter of 688

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Fig. 14. Load-displacement curves of B-J-B assemblies.

approximately 0.36 when the specimen started to present obvious nonlinear performance (before HT-1 occurred in Fig. 14(b) and Fig. 16). At Stage II, strain intensity grew with a much greater rate than at Stage I, indicating that the plastic deformation was much contributed to by the column wall. When F/Fp exceeded 0.68 (before HT-3 in Fig. 14(b) and Fig. 16), strain intensities stopped growing and got to their plateaus, as little force was transferred from the connecting Hollo-Bolts at the lowest row next to the measuring points T1 and T2 (refer to Fig. 13(d)) which started to be pulled out and thereby quit connecting. In Specimen HB20-RBS, the strain intensities kept a proportionally growing rate with respect to F/Fp, which is assumed to be just in their first stage (Stage I). Comparing the development of strain intensities in the first stage, growing rates with respect of F/Fp were much greater in Specimen HB16-TC than in HB20-RBS. This clearly has the implication that the endplate connection to thin column was less capable of transferring the load at the top of the column to the connected beams when experiencing severe plastic deformation of column wall.

bolt hole allowed the closed sleeves to pass through it.

4.2. Strain development at connection regions Referring to Fig. 13(c) and (d), the strains at the connection regions were monitored by strain gauges arranged at vulnerable locations of the column wall (T1 and T2), stiffeners (T3 and T4) and endplates (T5). At each spot, three strain gauges were used for calculation of strain intensity εi according to the method described in literature [38]. The developments of strain intensities at the stiffeners, column wall and endplates are demonstrated in Fig. 18 with the corresponding plastic strains marked. In Specimen HB20-RBS, column walls (see Fig. 18(a)) and stiffeners (Fig. 18(b)) turned plastic while the endplate remained elastic (Fig. 18(c)). In Specimen HB16-TC, strain intensities up to 0.025 at column walls (Fig. 18(a)) were much greater than those at the elastic stiffeners (Fig. 18(b)) and endplates (Fig. 18(c)), signifying that the column wall experienced severe plasticity and were the most vulnerable components of the connection. Strain intensities of column walls were plotted with respect to the normalised load F/Fp (refer to Fig. 14 for the values of Fp) in Fig. 18(d), where three stages can be clearly identified for the curves of Specimen HB16-TC, named by Stages I, II and III. Strain intensities of column wall increased with a relative slow rate at Stage I which ended at F/Fp of

4.3. Load transfer mechanisms Load resistance mechanisms, being flexural mechanism and catenary mechanism, could be evaluated by the internal forces analysis in the B-J-B assembly. According to the analysis method proposed by [35],

West

East

HR-1: Flanges of

HR-2: Shear failure of sleeves at the lowest

HR-3: Pull-out of bolt at the lowest row

RBSs yielded

row on the west side (2.56Fp, 0.122 rad)

on the east side (3.26Fp, 0.159 rad)

West

West

East

HR-4: Pull-out of bolt at second lowest

HR-5: Pull-out of bolts at second lowest row on the east side and

rows on the west side (3.09Fp, 0.171 rad)

at third lowest row on the west side (1.87Fp, 0.189 rad)

Fig. 15. Damage evolution at the connection region of Specimen HB20-RBS. 689

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HT-1: Gap between

HT-2: Plastic convex deformation

HT-3: Pull-out of

HT-4: Total pull-out

endplate and column

of column wall and gradual

half-length of bolts at

of the bolts at the

at the lowest row

pull-out of bolts at the lowest row

the lowest rows

lowest rows

(0.038 rad, 0.30Fp)

(0.093 rad, 0.48Fp)

(0.153 rad, 0.72Fp)

(0.189 rad, 0.80Fp)

Fig. 16. Damage evolution at the connection region of Specimen HB16-TC.

(a) Specimen HB- RBS.

(b) Specimen HB- TC.

Fig. 17. Different failure mechanisms of Hollo-Bolts’ pull-out from column wall.

(a) Column walls.

(b) Stiffeners.

(c) Endplates.

(d) εi - F/Fp relationship at column wall.

Fig. 18. Strain intensities at the tested extended endplate connections. 690

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(a) Specimen HB20-RBS

(b) Specimen HB16-TC

Fig. 19. Normalised axial forces and bending moments at the critical beam Sections W2/E2.

its full capacity Mp.

strain readings at the expected elastic beam Sections W1/E1 (see Fig. 13. (c) and (d)) are interpreted into the axial force N, bending moment M and shear force V. Based on equilibrium equations, reaction forces at the pin supports and the internal forces of critical beam sections could be obtained from the internal forces of the Sections W1/E1. By decomposing internal forces of Sections W1/E1, flexural resistance Ff and catenary resistance Fc are calculated to be vertical resistances arising from shear force and axial force, respectively. The sum of flexural resistance and catenary resistance is assumed theoretically equal to the applied load at the top of the central column, but the difference between them is inevitable due to the measuring error of the strains at Section W1/E1.

4.3.2. Flexural resistances and catenary resistances The normalised catenary resistance Fc/Fp and flexural resistance Fc/ Fp are demonstrated in Fig. 20. It can be seen that the catenary resistance did not start to develop until θ reached 0.07 rad, just like the axial force N in Fig. 19. As shown in Fig. 20(a), Specimen HB20-RBS effectively utilized the full flexural capacities of the critical beam sections to obtain a flexural resistance of 1.21Fp, and at the later stage developed a catenary resistance up to 1.91Fp during the pull-out of Hollo-Bolts. In Fig. 20(b), the flexural resistance Ff of Specimen HB16TC merely reached 0.53Fp and the catenary resistance had a maximum value of 0.63Fp at the end of the test. The low resistance from these two load transfer mechanisms developed in Specimen HB16-TC resulted from the relatively low stiffness and capacity of extended endplate connection to the thin column wall.

4.3.1. Bending moments and axial forces at critical beam sections Normalised axial force N/Np and normalised bending moment M/ Mp of critical beam Sections W2/E2 are demonstrated in Fig. 19, where the Np and Mp are respectively the nominal full plastic tensile capacity and flexural capacity of the sections. In Specimen HB20-RBS, the bending moment M was able to exceed the full plastic capacity Mp before they gradually decreased along with the growth of the axial force N. It is noted in Fig. 19(a) that successive pull-out of Hollo-Bolts over 0.16 rad impeded the development of axial force and made it drop to a level of 0.4 Np from the maximum values of 0.55 Np. For Specimen HB16-TC with thin column section, bending moment M in Fig. 19(b) was merely able to reach 0.54Mp and the axial force N just increased up to 0.20 Np when the tests were terminated at a chord rotation of 0.20 rad. It is clear that the thin column wall resulted in too low a strength of the connection to allow the critical beam section to develop

5. Initial stiffness and ultimate capacities of beam-to-column connections against progressive collapse Li et al. [39] proposed an approach to compare ability of various types of steel beam-to-column assembly under push-down loading through the relationship of normalised vertical resistance (F/Fp) against the normalised beam rotation (θ/θp), where θp is the plastic rotation corresponding to the state when the nominal beam plastic load Fp is reached by the B-J-B assembly. By means of normalisation, effects of beam sections, beam spans as well as material yield strengths are filtered out. This approach is based on the seismic design philosophy of

(b) Specimen HB16-TC

(a) Specimen HB20-RBS

Fig. 20. Normalised load resistances of B-J-B assemblies. 691

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used SCBBs in the extended endplate connections between I-section beams and square hollow section (SHS) columns [38]. In Specimen TE [38], relatively thin endplate was adopted. In the connection region of Specimen SBF [38], bottom beam flanges were strengthened by bolted bottom cover plates through slotted bolt holes. Specimen CO-WB and CO-W [35] connected I-section beams to circular hollow section (CHS) columns with outer-diaphragms through welded flange - bolted web method and welded flange–welded web method, respectively. In Specimen SI-WB and SI-WB-2 [40], I-section beams were connected to SHS columns with inner-diaphragm through welded flange - bolted web method, where four bolts were arranged at the beam web in one line and two lines, respectively. Specimen SI-W [41] and SI-W-RBS [42] used welded flange - welded web method to connect I-section beams to SHS columns with inner-diaphragms and latter specimen had a special configuration of reduced beam section. Table 6 summarises the initial normalised stiffness Ki, ultimate normalised capacity (F/Fp)u and ultimate normalised beam chord rotation (θ/θp)u of the comparing specimens. Ki is obtained as the stiffness of linear portion of the F/Fp - θ/θp curve and (F/Fp)u is the maximum value of F/Fp at which point (θ/θp)u is meanwhile identified. The listed values (F/Fp)u and (θ/θp)u have the greater-than sign (>) for the specimens exhibiting still-rising resistance when the test was terminated due to the limited range of vertical displacement for the central column. Theoretically, a B-J-B assembly adopting ideally rigid and moment connections possesses Ki = 1 and (F/Fp)u > 1. In Table 6, the greatest value of Ki is 0.794 from Specimen SI-WB-2 and greatest (F/Fp)u is 4.03 from Specimen RBS. Some features are revealed by Table 6 as follow.

Fig. 21. F/Fp-θ/θp curves of B-J-B assemblies with blind bolted extended endplate connections.

“strong panel - weak beam” by which the plastic capacity of beam-tocolumn connection is theoretically determined by that of the beam section. 5.1. Comparisons between tested connections using Hollo-Bolts Plotting F/Fp - θ/θp curves of two tested specimens in Fig. 21, it can clearly be seen that Specimen HB16-TC presented a much lower normalised stiffness and normalised ultimate capacity than Specimen HB20-RBS. The failure of combining closure of sleeve legs and enlarged bolt holes at the thin column wall in Specimen HB16-TC did not exploit the full capacity of Hollo-Bolt and hence resulted in the poor performance of the extended endplate connection. The comparison quantitatively verifies the necessity of connected plates with sufficient thickness to obtain satisfactory performance of Hollo-Bolts bolted connections. It is effective by using strong columns to reduce plastic deformation and improve both stiffness and capacity.

(1) Connections with inner-diaphragms inside the SHS column (specimens with reference “SI”) had greater stiffness than connections without strengthening inside the hollow column. One common cause of the reduced stiffness in the latter connections was the evident deformation of the hollow column. (2) Stiffness of extended endplate connections were generally lower than those of welded connections, being attributed to the severe local plastic deformation of the hollow column around the bolt holes. In spite of that, some of them (HB20-RBS, RBS, TE and SBF) were able to achieve much higher capacities (F/Fp)u than the welded connections by means of the effectively utilized catenary mechanism under the larger deformation capacities (θ/θp)u which has benefited from the ductile failure of connections. (3) Among the extended endplate connections, Specimens HB20-RBS, RBS and SBF with the concentrated plastic deformation at critical beam sections relieved the plastic deformation at the connection region and thereby obtained improved initial normalised stiffness and ultimate normalised capacity. On the contrary, the thin column wall resulted in both low stiffness and capacity for Specimens HB16-TC and TC through the severe plastic deformation of column wall. Specimen TE configured with thin endplates, in spite of the relatively low initial stiffness, obtained a high ultimate normalised capacity of 3.77 by maintaining the strength of bolted connection during the ductile plastic deformation of endplate which facilitated the development of catenary mechanism.

5.2. Comparisons among various connections Two published blind bolted extended endplate connections adopting novel Slip-Critical Blind Bolts (SCBBs) [38] are also demonstrated in Fig. 21 for comparison against the tested connections in the present work. Specimens RBS and TC [38], respectively having the same beam, column and connection configuration with that of Specimens HB20-RBS and HB16-TC, performed better than their counterparts, because the blind bolts SCBB provided sufficient pre-tensions and capabilities. However, the comparison between the special configurations of RBS and thin column (TC) were the same as observed in the tested connections using Hollo-Bolts. A further comparison are made in Table 6 concerning stiffness and capacities of beam-to-column assemblies under push-down loading, including the two tested connections in this work and other four blind bolted extended endplate connections together with six welded flange welded or bolted web connections. Specimens RBS, TC, TE and SBF Table 6 Initial normalised stiffness and ultimate normalised capacities of B-J-B assemblies.

Welded flanged – welded or bolted web connections

Blind bolted extended endplate connections Reference

Ki

(F/Fp)u

(θ/θp)u

Reference

Ki

(F/Fp)u

(θ/θp)u

HB20-RBS HB16-TC RBS [38] TC [38] TE [38] SBF [38]

0.315 0.094 0.466 0.149 0.274 0.392

3.24 > 0.89 4.03 > 1.25 > 3.77 > 3.73

20 > 18.3 18.2 > 19.6 > 19.9 > 19.9

CO-WB [35] CO-W [35] SI-WB [40] SI-W [41] SI-WB-2 [40] SI-W-RBS [42]

0.593 0.583 0.757 0.662 0.794 0.726

> 1.27 1.36 > 1.79 1.16 > 1.30 > 2.96

> 13.6 8.5 > 13.1 5.4 > 14.7 > 21.7

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(4) Extended endplate connections in Specimens HB20-RBS and HB16TC using Hollo-Bolts exhibited stiffness merely about 2/3 of those in their counterparts of Specimens RBS and TC.

the authors and do not necessarily reflect the views of the sponsors.

In general, the blind bolted extended endplate connections performed with lower initial normalised stiffness than the welded connections, but they were still able to provide considerable ultimate normalised capacity for a B-J-B assembly by means of catenary mechanism with ductile failure mode. As for Hollo-Bolts, the limited pretension as well as its gradual loss during damage evolution disabled it to offer adequate stiffness and maintain sufficient strength of connection essential for developing catenary mechanism under large deformation. Therefore, cautions should be taken when using Hollo-Bolts in blind bolted connection with a requirement of preventing progressive collapse.

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6. Conclusions This paper experimentally investigated the behaviour of blind bolted extended endplate connections using Hollo-Bolts to square hollow section (SHS) columns in beam-joint-beam (B-J-B) assemblies under the scenario of inner column removal. Special configurations of reduced beam section (RBS) and thin column (TC) were designed at the connection region to present different failure mechanisms. Load-displacement relationship, damage evolution, and strain intensity development at the connection regions were presented in detail. Load transfer mechanism, initial stiffness and ultimate capacity were also discussed. Prior to the tests on B-J-B assemblies, the characteristics of HolloBolt (M12, M16 and M20) were firstly examined by nine direct tension tests and six double shear tests. Hollo-Bolts failed in two patterns under tension, which are shear failure of sleeve legs and fracture of bolt shank. The pre-tension and anti-sliding capacity of Hollo-Bolts under recommended torque did not satisfy the requirement for standard high strength slip-critical bolts. Although it is not advisable in practice to apply torques greater than the recommended values by the manufacturer, there is still a chance of benefit from the over-torque to improve the tensile capacity and anti-sliding capacity of Hollo-Bolt on the premise the High Clamping Force (HCF) mechanism works normally under the over-torque. Under column removal scenario, the extended endplate connections exhibited pull-out of Hollo-Bolts from the column wall but with different failure mechanism. Hollo-Bolts in Specimen HB20-RBS experienced shear failure of sleeves before they were pulled out. Connecting thin column wall in Specimen HB16-TC, Hollo-Bolts were pulled out due to combination of the closure of sleeve legs and enlarged bolt hole at column wall resulting from the large plastic deformation. The premature failure of thin column wall did not allow the full utilization of the Hollo-Bolts’ strength and hence resulted in a low level of stiffness and catenary mechanism development of the assembly. This implies the necessity of connected columns with sufficient thickness for satisfactory performance of Hollo-Bolt connections. By comparison using normalised vertical resistance - normalised beam rotation curve, extended endplate connections using Hollo-Bolts exhibited lower stiffness and ultimate capacities in the B-J-B assemblies against progressive collapse, resulting from the limited pre-tension as well as its gradual loss during damage evolution. Acknowledgments The research presented in this paper was supported by the Natural Science Foundation of China (NSFC) through Grant No. 51378380, "Shuguang Program" through Grant No. 15SG19 and the Sustainable Structural Engineering Research Funds through Grant No. 2018 from Tongji Architectural Design (Group) Co. Ltd. Any opinions, findings, conclusions, and recommendations expressed in this paper are those of 693

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