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Experimental study on semi-rigid composite joints with steel beams and precast hollowcore slabs
Journal of Constructional Steel Research 62 (2006) 771–782 www.elsevier.com/locate/jcsr
Experimental study on semi-rigid composite joints with steel beams and precast hollowcore slabs F. Fu, D. Lam ∗ School of Civil Engineering, University of Leeds, Leeds, LS2 9JT, UK Received 8 July 2005; accepted 30 November 2005
Abstract The concept of semi-rigid composite connection has been widely researched in the past; however, most of the researches are limited to composite joints with metal deck flooring and solid concrete slabs. Composite construction incorporating precast concrete hollowcore slabs (HCU) is a recently developed composite floor system for buildings. The research on the structural behaviour of the semi-rigid composite joints with HCU is new and without any previous experimental database. In this paper, eight full-scale tests of beam-to-column semi-rigid composite joints with steel beams and precast hollowcore slabs are reported. The variables are stud spacing, degree of the shear connections, area of the longitudinal reinforcement and slab thickness. The test set-up and instrumentation is described in detail. The experimental behaviour is analysed and based on the test data the structural behaviour of these semi-rigid composite joints is discussed. Based on the experimental data, a simplified method to predict rotation and moment capacity for this type of composite connection is proposed. c 2005 Elsevier Ltd. All rights reserved. Keywords: Semi-rigid; Composite; Joints; Precast; Hollowcore; Steel; Connections; Beam–column
1. Introduction In the area of composite construction, extensive research works have been focused on semi-rigid connection design since it was first proposed by Barnard [1] in the 70s. They showed these forms of connections when used in design will lead to reduction in beam sizes, which in turn will reduce the beam depth, the overall building height and cladding cost, etc. The moment rotation characteristic of the semi-rigid composite connections was first investigated by Johnson and Hope-Gill [2] in 1972, they found that neither simple nor rigid beam–column connections are ideal. Simple joints are too unpredictable while rigid joints are often too stiff in relation to their strength and are expensive; therefore, the semi-rigid joint with a large rotation capacity and a predictable flexural strength that does not require site welding or accurate fitting is needed. Numerous researches have been carried out on semi-rigid composite connections [3, 4], the most common types of floor slab used being solid R.C. slabs or profiled metal deck floors. ∗ Corresponding author.
E-mail address: [email protected] (D. Lam). c 2005 Elsevier Ltd. All rights reserved. 0143-974X/$ - see front matter doi:10.1016/j.jcsr.2005.11.013
Composite beams incorporating precast hollowcore floor slabs are a newly developed composite floor system for buildings. Compared with the other two types of floor system, it has the following advantages: HCU can be manufactured up to 500 mm in depth, meaning that simply supported floor spans of up to 20 m are possible; however, the most common depths are 150–400 mm. HCU has an excellent structural capacity to selfweight ratio, with span/depth ratios in the order of 35 being possible for normal office loadings. The volume of the hollow cores accounts for up to 50% of the cross section, therefore a 10 m span floor only weighs 3.5 kN/m2 . The standard width of the units is 1.2 m, enabling fixing rates of around 2000 m2 per week. The floor system does not require a structural screed to carry horizontal diaphragm forces thus further reducing the dead weight of the floor. Welding of the shear studs may be carried out in a factory or at ground level on site with mobile welding equipment using the semi-automatic drawn arc process. Although the use of precast hollowcore slabs dates back to the 1940s, research on composite construction incorporating steel beams with precast hollowcore slabs is relatively new. The first commercial testing in this area was carried out at Salford University and reported by Hamilton [5]. Tests were carried out
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Fig. 1. General arrangement of test set-up.
Fig. 3. Test specimen before concrete infill.
Fig. 2. End plate connection.
using 150 mm deep slabs and 406 × 178 × 60 UB with 19 mm diameter × 125 mm long headed shear studs. The results showed more than 70% increase in ultimate moment capacity when compared with its bare steel counterpart. Mode of failure was due to shearing off of the headed studs. Research on shear connector strength in precast solid concrete planks was carried out by Moy and Tayler [6] in 1996. Twenty-seven push-off tests were carried out and the results showed a reduction in strength as the gap of in situ concrete decreased. It is recommended that the width of in situ concrete should be a minimum of 100 mm to avoid reductions in shear strength of the shear connectors. It is also recommended that two layers of reinforcement must be used in the structural topping to avoid any tensile splitting. A test on composite beams with precast solid planks was also conducted by Jolly [7] at Southampton University. A 16 m span composite beam with 110 mm deep precast concrete planks was tested. The results showed that the dynamic response of long span, shallow composite construction complied with the requirements of BS5950 [8] without the need to increase the minimum number of shear connectors as specified in the code. Shim et al. [9] studied the behaviour of headed shear studs in a precast post-tensioned bridge deck at the Seoul National University. Push-out tests were carried out to determine the structural behaviour of the shear connection in the precast deck. It is found that as the thickness of the bedding layer increases, the ultimate strength of the shear connection decreases. Horizontal push-off tests with precast hollowcore slabs were first performed by Lam et al. [10] in 1998. They showed that the shear capacity of the stud for this type of construction was not only affected by the tensile capacity of the stud itself,
Fig. 4. Load versus slip curve from push-off test.
Fig. 5. LVDT’s position for measuring beam rotation and interface slip.
but also affected by the gap width, the amount of transverse reinforcement, the strength of the in situ concrete infill and the presence of the longitudinal and transverse joints. Three full scale simply supported composite beams with variable parameters were also carried out by Lam et al. [11] to study the flexure behaviour and was compared with the non-composite bare steel beams. The results showed two modes of failure; failure due to loss of shear studs and failure of the concrete slab due to yielding of the transverse reinforcement. Nevertheless, the residual moment capacity of all the beams remained at least 40% above the moment capacity of the bare steel section only. A beam with a pre-cracked in situ/precast concrete joint was also tested. Modelling of the headed studs in steel-precast composite beams using the finite element analysis software
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Fig. 6. Strain gauge on steel reinforcement.
Fig. 8. Bolt strain gauges.
Fig. 7. Strain gauge mounted on steel beam flanges and web.
ABAQUS [12] was carried out by El-Lobody and Lam [13], good agreement was obtained when compared with the test results. The use of the precast hollowcore slabs in current buildings design is so far limited to simple beam–column connections. As semi-rigid connection has many advantages over the simple connection, the main objective of this research is to investigate the structural behaviour of the composite semi-rigid connections with precast hollowcore slabs and to determine whether these kinds of joints can provide sufficient moment capacity and rotation capacity to develop the mid-span plastic hinge for the plastic analysis uses in the composite beam design. 2. Test arrangement 2.1. Test specimens All specimens were of cruciform arrangement as shown in Fig. 1 to simulate the internal beam–column joints in a semirigid composite frame. The specimen was assembled from two 3300 mm long; 457 × 191 × 89 kg/m; grade S275 universal
Fig. 9. Test set-up and loading arrangement.
beams and one 254 × 254 × 167 kg/m; grade S275 universal column to form the cruciform arrangement. The beams are connected to the column flanges using 10 mm thick flush endplates with two rows of M20 Grade 8.8 bolts as shown in Fig. 2. The steel connection is a typical connection currently used in UK practice for simple joints, this is to ensure that the enhanced performance of the composite joint is not provided by the bare steel connection. A single row of 19 mm diameter headed shear studs is pre-welded to the top flange of the steel beams. Finally,
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Table 1 Strength of concrete from test results Ref. CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
Compressive strength (N/mm2 ) 7 days Test day 30.9 43.1 40.6 35.7 30.6 33.2 32.8 35.2
40.1 49.6 47.9 43.8 41.0 37.3 40.2 42.96
28 days
Tensile strength (N/mm2 ) 7 days Test day
28 days
Density (kg/m3 )
45.0 53.5 57.1 41.8 46.8 44.4 44.3 44.7
2.25 2.15 2.63 2.15 2.75 2.16 2.43 2.55
2.50 2.90 3.19 3.14 3.45 2.45 2.99 3.17
2330 2384 2367 2340 2336 2318 2352 2346
two 305 × 102 × 28 kg/m universal beams were connected to the column web to make up the full joint arrangement. Fig. 3 shows the semi-rigid composite joint before casting, it can be seen that the ends of the HCUs are chamfered and the top of the alternative cores of the units opened for a length of 500 mm for the placement of transverse reinforcement. A gap of 65 mm is formed between the ends of the units. In situ concrete is used to fill the gap between the HCUs and the opened slots. A nominal concrete strength of 40 N/mm2 is used for the in situ infill. The HCUs are tied together transversely by the 16 mm diameter high strength reinforcing bars 1 m long. Longitudinal reinforcement is provided by the high strength steel reinforcing bars across the joint for continuity.
2.49 2.49 3.55 2.50 3.05 2.48 2.40 2.63
Table 2 The mechanical properties for the reinforcing bar Ref.
Yield strength (N/mm2 )
Ultimate tensile strength (N/mm2 )
Cross section area (mm2 )
T16 T20
536 534
621 634
195.9 305.4
Table 3 Tensile test results for steel beam Ref.
Yield strength (N/mm2 )
Ultimate tensile strength (N/mm2 )
Yield strain (µε)
Web Flange
382.1 341.7
549.4 503.0
2379 2285
2.2. Material properties The compressive and tensile strength of the in situ concrete is shown in Table 1. The in situ infill concrete strength is tested at the 7th day, the test day and the 28th day. The characteristic concrete strength for the precast hollowcore units is taken to be 55 N/mm2 as specified by the manufacturer. The tensile strengths of the T20 and T16 reinforcing bars were determined in accordance with BS 4449 [14] and the results are shown in Table 2. Test coupons of the web and flange with thickness of 10.9 mm and 17.7 mm respectively were cut from the ends of the steel beams after the tests where stresses had been low. Tensile tests of coupons were conducted according to BS EN 10002-1 [15] and the test results are shown in Table 3. Tensile tests for M20 Grade 8.8 bolts were performed and an average ultimate strength of 678 N/mm2 is obtained. 19 mm diameter × 125 mm height TRW-Nelson headed shear connectors are used with an average ultimate strength of 610.5 N/mm2 . Horizontal push-off tests were conducted to determine the load–slip characteristics of the head shear stud and the average load–slip curve is shown in Fig. 4, the result is used to determine the shear capacity and degree of shear connection for the semi-rigid joint tests.
measure the in situ concrete strain at the slab’s surface. The strain gauges used to monitor the strain and yielding of the rebar and steel beam were of the type FLA-5-11 with a gauge length of 6 mm. The gauges have 120 ±0.3 resistances with a gauge factor of 2.13. Strain gauges are placed on the longitudinal bar and on the transverse reinforcement as shown in Fig. 6. The strain gauges on the surface of the rebar were coated with epoxy to protect them from damage during concreting. Strain gauges were also placed on the top flange of the steel beam and on the web to measure the strain on the steel beam to determine the position of the neutral axis throughout the test (Fig. 7). Strain in the bolts was measured using BTM-1C bolt strain gauges as shown in Fig. 8 and their gauge parameters are shown in Table 4. The gauges were calibrated before the test and used to measure the bolt forces and elongation of the bolts. Load is applied by hydraulic jacks simultaneously to each ends of the steel beams as shown in Fig. 9. An elastic test is carried out before test to failure to check the instrumentation and the system. The load was applied at 10 kN intervals and continued until failure occurred. 2.4. Test parameters
2.3. Instrumentation and loading procedure Instrumentation comprised of linear voltage displacement transducers (LVDTs) for measuring slip, beam deflection and beam rotation is shown in Fig. 5. Electrical resistance strain gauges (ERSGs) are used to measure strain in reinforcing bars, steel beams and bolts at the joints. Demac gauges are used to
In order to investigate different variables affecting the behaviour of the composite joint, different emphases are adopted for each test. For the first five tests, the main variables are stud spacing and the degree of the shear connections. In CJ6 and CJ7, the main variables are the cross sectional area of the longitudinal bar and for the CJ8, the variable is the thickness of
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Fig. 10. General arrangement and position of the strain gauges for Test CJ1.
Fig. 11. General arrangement and position of the strain gauges for Test CJ2.
Fig. 12. General arrangement and position of the strain gauges for Test CJ3.
Fig. 13. General arrangement and position of the strain gauges for Test CJ4.
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Table 4 Parameters of the BLM bolt strain gauge Type BTM-1C
Gauge (mm) Length
Width
Base (mm) Length
Width
Gauge centre a
b
Hole Dia. (mm)
1
0.7
5.6
1.4
1.8
3.8
1.6
Table 5 Test arrangement Reference
In situ concrete cube strength (N/mm2 )
Longitudinal bars and cross section area (mm2 )
Hollowcore slabs thickness (mm)
Studs spacing (mm)
Position of first stud (mm)
No. of shear stud per beam
CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
39 50 48 44 41 37.3 40.2 42.9
2 T20(628) 2 T20(628) 2 T20(628) 2 T20(628) 2 T20(628) 4T16(800) 2T16(400) 4T16(800)
200 200 200 200 200 200 200 250
300 600 900, 1200a 400 500 310 1200 450
235 235 990, 540a 510, 710a 645 465 900 705
7 4 2 3 3 6 2 4
19φ × 125 mm long headed shear connectors were used for all the tests. a Stud on the east side.
Fig. 14. General arrangement and position of the strain gauges for Test CJ5.
Fig. 15. General arrangement and position of the strain gauges for Test CJ6.
the precast hollowcore slabs. The test arrangement is shown in Table 5 and Figs. 10–17. Strain gauges were put on the surface of the shank of the stud near the weld collar to measure the stresses in the studs. In CJ6, CJ7 and CJ8, bolt strain gauges were used at the connection to monitor the bolt forces and elongation of the bolt.
3. Test results Results of all eight composite joint tests are shown in Table 6 and Fig. 18. All tests except Test CJ3 failed in a ductile manner with beam rotation well in excess of 30 mrad and obtained a moment capacity above 0.3 Mp of the beams,
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Fig. 16. General arrangement and position of the strain gauges Test CJ7.
Fig. 17. General arrangement and position of the strain gauges Test CJ8. Table 6 Test result Reference
CJ1
CJ2
CJ3
CJ4
CJ5
CJ6
CJ7
CJ8
Moment capacity (kN m) Rotation capacity (mrad) Long. reinf. capacity—yield (kN) Long. reinf. capacity—Ult. (kN) Shear connector capacity (kN) Degree of shear connection (%)a Degree of shear connection (%)b Max. strain in long. reinf. (µε) Maximum end slip (mm) Failure mode
370 35.4 326 387 896 >100 >100 26,000 0.34 RF
363 33.5 326 387 512 >100 >100 23,000 0.8 RF
250 6.1 326 387 256 78.5 66 2031 5.8 CF & SF
368 37.4 326 387 384 >100 98 16,000 3.5 CF
363 31.7 326 387 384 >100 98 13,706 3.5 CF
425 46.8 424 486 512 >100 >100 26,000 0.84 RF
274 30 212 243 256 >100 >100 23,000 0.4 RF
439 42.3 424 486 512 >100 >100 23,000 1.6 RF
RF – reinforcement fracture; CF – connector fracture; SF – slab shear failure. a Calculated using the yield strength of longitudinal steel bar. b Calculated using the ultimate strength of longitudinal steel bar.
it can be concluded that these types of joints can provide sufficient moment capacity and rotation capacity. Tests CJ1, CJ2, CJ6, CJ7, and CJ8 failed due to the fracture of longitudinal reinforcement while Tests CJ3, CJ4 and CJ5 failed by fracture of the shear connectors. No yielding or buckling to the column was observed. For all the tests conducted, no bond failure between the in situ and the precast concrete was observed. It can therefore be concluded that the in situ and the precast concrete are acting compositely throughout. The mode of failure can be divided into two main categories: (a) fracture of the longitudinal bar as shown in Fig. 19, and (b) fracture of shear studs as shown in Fig. 20. No other mode of failure was observed in any of the tests. Fig. 21 shows the strain measurement of the longitudinal rebars during the test, it can be seen that with the only exception of Test CJ3, the longitudinal bars in all tests developed strain hardening with the longitudinal rebars in Tests CJ1, CJ2, CJ6,
Fig. 18. Moment versus rotation curves.
CJ7 and CJ8 fractured at the end of the tests. For Tests CJ4 and CJ5, stud fracture occurred before fracture of the longitudinal
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Fig. 21. Strain measurement of the longitudinal reinforcement. Fig. 19. Mode of failure due to fracture of the longitudinal reinforcing bars.
Fig. 20. Mode of failure due to stud fracture. Fig. 22. Typical strain profile of longitudinal reinforcing bar along the length.
bar due to partial shear connection. From these results, it can concluded that full shear connection should be provided to enable full mobilisation of the longitudinal reinforcement and partial shear connection will lead to low moment and rotation capacity as the longitudinal bars cannot be fully mobilised. A typical strain profile of the longitudinal rebar along the beam is shown in Fig. 22. It shows that yielding of the longitudinal reinforcement occurred at the distance between the centre line of the column and the second stud position, the strain in the other part of the steel bar is very small and remained elastic. This further demonstrates that the position of the headed studs played an important role in rotation capacity of the composite connections. Fig. 23 shows the end slip for all the tests. As predicted, larger slip is obtained for the partial shear connection tests CJ3, CJ4 and CJ5 where the amounts of slip are smaller for the full shear connection tests. For CJ6 and CJ8, as the ratios of the force in the longitudinal rebar to the longitudinal shear force provided by the studs are larger than that of CJ1, CJ2 and CJ7, larger connector slip resulted. Strain is measured at the steel top and bottom flange of the steel section for all the tests and the steel beam of all eight tests remained elastic throughout. The development of the crack pattern around the concrete slab was similar to the one shown in Fig. 24 for all the composite joint tests. The first cracks were visible at the column flange tips and propagated
Fig. 23. Moment versus end slip for all the tests.
towards the slab edges before eventually forming the dominant transverse cracks across the complete breadth of the slab. As the widths of these cracks developed further, a significant loss of connection stiffness resulted. This in turn led to a considerable widening of these cracks, causing a high localised strain in the longitudinal reinforcement. Consequently, fracture of the longitudinal reinforcement occurred. Fig. 25 shows the strain measurement of the transverse bar for all the joint tests. It showed that the strain in the transverse bar is very small. The maximum strains measured in all the tests
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4.2. Effect of stud spacing From the comparison of Tests CJ1 and CJ2, it can be seen that although different stud spacings and numbers of studs are used, no distinct difference of moment and rotation capacity was found from the test results. Therefore, it can be concluded that provided the same degree of shear connection and position of first stud is used, the numbers of studs and their spacing did not have much influence on the moment capacity and rotation capacity of the connection. 4.3. Effect of the degree of shear connection Fig. 24. Typical crack pattern of the joint test.
Fig. 25. Strain measurement of the transverse reinforcement for all the tests.
were less than 900 µε and hence the transverse rebars remained elastic. It can be concluded that the transverse bar has little effect on the connection moment capacity. 4. Discussions 4.1. Effect of distance between first stud and column flange The formation of cracks would appear to be related to the position of the first stud. Fewer cracks are formed in Tests CJ1 and CJ2 with a main crack opening near the column faces and eventually leading to the fracture of the longitudinal reinforcement. At failure, little interface slip is recorded and hence little steel connection deformation is observed. The amounts of slip observed in Tests CJ1 and CJ2 were very small. When the specimen was dismantled after the test, the studs were found to be intact. In Tests CJ4 and CJ5, the first stud position is placed further away from the face of the column. After the formation of the first crack, additional cracks were formed between the column face and the position of the first stud. Cracks between the column face and the first stud distributed evenly rather than concentrated at a single crack around the column face, this led to lesser demand on the percentage of elongation required by the longitudinal reinforcement. Large interface slip is also observed.
In this series of experiments, two methods for calculating the degree of shear connection are employed. The first method is based on the yield strength of the longitudinal rebars as assumed by many researchers and ignoring the ultimate strength of the reinforcing bars. The second method is based on the ultimate strength of the longitudinal rebars to determine the degree of shear connection, which takes into consideration the ultimate tensile strength of the longitudinal bars. In accordance with method 1, if the shear stud capacity is greater than the yield strength of the longitudinal rebars, then it should be classified as full shear connection. However, as shown in Table 6, the mode of failure in Tests CJ3, CJ4 and CJ5 is due to stud fracture. It is because the shear stud capacities were less than the force of longitudinal rebars as calculated using method 2. Therefore, it suggested that the ultimate strength of the longitudinal rebars should be taken into consideration when determining the degree of shear connection. In test CJ3, the values of interface slip increased with lower degree of shear connection, which led to low moment and rotation capacity. Comparison between CJ3 and CJ7 shows that although only two studs were used in both the tests, Test CJ3 has a very low degree of shear connection in compared with Test CJ7 and hence a small amount of rotation was recorded. It can be concluded that for a joint with partial shear connection, as long as the shear stud can allow the longitudinal bar to be mobilised until the yielding stage, there will be no obvious deduction of the moment and rotation capacity. Otherwise low degrees of the shear connection will lead to low moment and rotation capacity as yielding and elongation cannot occur in the longitudinal rebars. 4.4. Effect of amount of longitudinal reinforcement Comparison among Tests CJ1, CJ2, CJ6 and CJ7 showed that with full shear connection and same slab thickness, increases in the amount of longitudinal reinforcement led to higher moment and rotation capacity. Therefore, it can be concluded that increases in the amount of longitudinal reinforcement will not only lead to increases in moment capacity but also larger rotation capacity. The analysis of the test results showed that the inclusion of a large amount of localised reinforcement is one of the most effective ways of achieving good rotation capacity. The reinforcement enables cracks to be distributed evenly in the
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concrete, thereby reducing localised strains and allowing high average strains to develop in the reinforcement. Appropriate connection details can be used to ensure that the average strain occurs over a substantial length, for example by placing the first shear stud on the beam further away from the column face. High strains over a substantial length result in large deformation of the reinforcement. The reinforcement also helps to develop large compressive forces in the lower parts of the steel section, causing plastification which leads to large strains in the steel. Large tensile deformations in the reinforcement and compressive deformations in the steel beam will in turn equate to significant connection rotation. 4.5. Effect of concrete cracking on the shear connectors’ capacity Shear studs placed within the concrete in the hogging moment region did not show any deterioration in its strength. The shear stud capacity in the tests agreed well with the push tests carried out in the horizontal push test [16]. Although the concrete was severely cracked, the reinforcing bars were effective and able to transfer the tensile force. Good interaction between the shear studs with the concrete and the concrete with the longitudinal rebars were observed, which enables the transmission of the longitudinal shear forces.
Fig. 26. Force diagram and the components of the composite joint. Table 7 Comparison of test results and the proposed method for rotation capacity References
Test results (mrad)
Calculated results (mrad)
CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
35.4 33.5 6.1 37.4 31.7 46.8 30 42.3
28.8 37.5 8.2 39.7 36.3 40.1 50.4 47.8
4.6. Effect of precast slab thickness With the only difference between Test CJ6 and CJ8 being the depth of the precast hollowcore slabs, the result showed that by using deeper slabs, an increase in moment capacity was obtained. The increases in moment capacity are purely due to the increases in the lever arm, but a slight reduction of the rotation capacity with the Test CJ8 was observed with deeper slabs. 5. Proposed method to calculate rotation capacity A simple calculation method [17] based on the component method is proposed to predict the rotation capacity, φu for this form of composite joints as shown in Eq. (1) and Fig. 26. The strain profile of the longitudinal reinforcing bar taken from the test data shows that the steel bar yielded only in the region between the column centre line and the second shear stud, the strain in the other part of the steel bar is small and remained elastic. Therefore, the elongation zone of the longitudinal reinforcement, L can be taken as between the centre line of the column and the position of the second stud. Hence, it can assumed that the length for calculating the elongation is p1 + p2 + D/2, where p1 is the distance between the column face and the centre line of the first stud; p2 is the distance between the centre line of the first stud and the second stud and D is the depth of the column. L Slip + Db + Dr Db D L = εsh p1 + p2 + . 2
φu =
(1) (2)
For full shear connection, reinforcement strain, εsh is taken as the ultimate strain developed in the longitudinal reinforcement. It is because for full shear connection, the longitudinal reinforcing bars can developed into the strain hardening whereas for the partial shear connection, εsh is taken as the maximum strain developed in the longitudinal bar. For simplicity, it can be taken as the yield strain of the steel bars if enough shear studs are provided to enable yielding of the longitudinal bars. Composite joints with partial shear connection unable to develop yielding of the longitudinal reinforcing bars will lead to low rotation capacity as demonstrated in Test CJ3. Table 7 shows the comparison between the test results and the calculated results using the proposed method. The results show that the method gave good prediction of the test results on composite connection with steel beams and precast hollowcore slabs. 6. Proposed method to calculate moment capacity No calculation method for the prediction of moment capacity of composite connection with precast hollowcore slabs is currently available. A method to predict the moment capacity for this type of semi-rigid connection is proposed [17]. The proposed method assumes that R f ≥ Rb + Rr , where, R f = compressive resistance of the bottom flange of the steel beam, Rr = the lesser of the ultimate tensile strength of the longitudinal reinforcement or the studs capacity,
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Table 8 Comparison of test results and the proposed method for moment capacity
precast hollowcore slabs, the following conclusions can be made:
References
Test results (kN m)
Calculated results (kN m)
CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
370 362 250 368 363 425 274 439
365.8 365.8 284.5 365.0 366.6 422.3 274.0 446.7
1. Semi-rigid composite joints with precast hollowcore slabs can provide sufficient moment and rotation capacity as required by the design code as a suitable type of connection for plastic design. 2. Two failure modes are observed from the joint tests. Mode of failure was either fracture of the longitudinal rebars or fracture of the headed studs. 3. Adequate shear connection should be provided to enable full mobilisation of the longitudinal reinforcement. Partial shear connection in the hogging moment region will lead to low moment and rotation capacity as the longitudinal bars cannot be fully mobilised. It is recommended that the minimum percentage of shear connection provided should enable the longitudinal reinforcement to develop yield. 4. The ultimate tensile strength of the longitudinal reinforcement should be taken into consideration when calculating the degree of shear connection. 5. Close spacing of the shear stud near to the column face will affect the crack pattern and demanded high degree of elongation from the longitudinal reinforcement to achieve the same rotation capacity with the same joint with larger stud spacing between the column flange and the first stud. It is recommended that the first stud should be placed at a spacing equivalent to two times the column width. 6. The spacing between other shear studs has little effect on the moment capacity and rotation capacity of the connection provided the degree of shear connection is the same. 7. The in situ concrete infill and the precast hollowcore slabs acted compositely throughout, no separation was observed from any of the tests carried out. 8. A simple method to predict the moment and rotation capacity is proposed and compared with the test results, it is found to be acceptable for use to predict the behaviour of this form of composite joints.
Rb = effective tensile resistance of the bolt group. The moment resistance of the composite connection, Mcc Mcc = Rr (Db + Dr − 0.5t f ) + Rb (Db − r1 − 0.5t f )
(3)
where Db is the depth of the beam; r1 is the distance of the first row of bolts below the top of the beam; Dr is the distance of the reinforcement above the top of the beam and t f is the flange thickness of the steel beam. For R f < Rb + Rr , (R +R −R ) The neutral axis, yc = r tw bPy f where, tw is the web thickness and p y is the design strength of the steel section. The moment resistance of the composite connection, Mcc Mcc = Rr (Db + Dr − 0.5t f ) + Rb (Db − r1 − 0.5t f ) − Rw
yc 2
(4)
where Rw = yc tw p y . The comparison of the test results and the results from the proposed method above is shown in Table 8. The results showed that the moment capacity of the semi-rigid composite connections is dependent on the strength and the ability to mobilize the longitudinal reinforcing bars. The influential factor to their mobilization is dependent on the degree of the shear connection between the slabs and the steel beams, which is determined by the number and the capacity of the shear studs in the hogging moment region. In all five tests carried out, all factors but the shear studs were kept constant. All tests with the exception of Test CJ3, the shear connection capacity is larger than the yield strength of the longitudinal reinforcement, therefore the tensile strength of the longitudinal bars can be mobilized, and hence adequate moment and rotation capacity can be achieved. 7. Conclusions The behaviour of eight full-scale semi-rigid composite connections with precast hollowcore slabs was examined. Different levels of shear connection, spacing and position of first studs from the column face have been examined. Tests showed these joints combine simple and efficient construction and yet provide worthwhile levels of moment capacity, rotational stiffness and ductility with the introduction of longitudinal reinforcement across the column. From the experimental study of the semi-rigid composite joints with
Acknowledgements The authors would like to acknowledge the financial support from International Precast Hollowcore Association (IPHA) and Overseas Research Scholarship (ORS), the support provided by Severfield—Revee Structures Ltd. for supplying the steel specimens and Bison Concrete Products Ltd. for supplying the precast hollowcore slabs. The skilled assistance provided by the technical staff in the School of Civil Engineering at Leeds University is also appreciated. References [1] Barnard PR. Innovations of composite floor systems. In: Canadian structural engineering conference. Canadian Steel Industries Construction Council; 1970. p. 13–21. [2] Johnson RP, Hope-Gill M. Semi-rigid joints in composite frames. In: International association for bridge and structural engineering, ninth congress. 1972. p. 133–44.
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[3] Bernuzzi C, Salvatore N, Zandonini R. Semi-rigid composite joints: Experimental studies. In: Connections in Steel Structures II: Behaviour, strength and design conference. 1991. [4] Li TQ, Moore DB, Nethercot DA, Choo BS. The experiment behaviour of a full-scale, semi-rigid connected composite frame: Overall consideration. Journal of Constructional Steel Research 1996;39(3):167–91. [5] Hamilton TR. Composite steel and precast concrete slab construction. Chartered Membership thesis. UK: The Institution of Structural Engineers; 1989. [6] Moy SSJ, Tayler C. Composite steel and precast concrete slab construction. Journal of Constructional Steel Research 1996;36(3): 201–13. [7] Jolly CK. Long span composite beams for car parks. In: Proceeding of the joint IStructE/City university international seminar. London: City University; 1996. [8] BS5950, Part 3-1, Structural use of steelwork in building: Code of practice for design of simple and continuous composite beams. London: British Standards Institution; 1990. [9] Shim CS, Lee PG, Chung CH. Design of shear connections in composite steel and concrete bridges with precast decks. Journal of Constructional
Steel Research 2001;57:203–19. [10] Lam D, Elliott KS, Nethercot DA. Push-off tests on shear studs with hollow-cored floor slabs. The Structural Engineer 1998;76(9):167–74. [11] Lam D, Elliott KS, Nethercot DA. Experiments on composite steel beams with precast concrete hollow core floor slabs. Proceedings of the Institution of Civil Engineers: Structures and Buildings 2000;140(May): 127–38. [12] ABAQUS. Version 6.2. Hibbitt, Karlson and Sorensen, Inc.; 2001. [13] El-Lobody E, Lam D. Modelling of headed stud in steel—precast composite beams. Steel & Composite Structures 2002;2(5):355–78. [14] BS4449. Specification for carbon steel bars for the reinforcement of concrete. London: British Standards Institution; 1997. [15] BS EN 10002-1. Metallic materials: Tensile testing—Part 1: Method of test at ambient temperature. London: British Standards Institution; 2001. [16] Lam D. New test for shear connectors in composite construction. In: Composite construction in steel and concrete, IV. American Society of Civil Engineers; 2000. p. 404–14. [17] Fu F, Lam D. Modelling semi-rigid composite joints with precast hollowcore slabs in hogging moment region (under review).
Experimental study on semi-rigid composite joints with steel beams and precast hollowcore slabs F. Fu, D. Lam ∗ School of Civil Engineering, University of Leeds, Leeds, LS2 9JT, UK Received 8 July 2005; accepted 30 November 2005
Abstract The concept of semi-rigid composite connection has been widely researched in the past; however, most of the researches are limited to composite joints with metal deck flooring and solid concrete slabs. Composite construction incorporating precast concrete hollowcore slabs (HCU) is a recently developed composite floor system for buildings. The research on the structural behaviour of the semi-rigid composite joints with HCU is new and without any previous experimental database. In this paper, eight full-scale tests of beam-to-column semi-rigid composite joints with steel beams and precast hollowcore slabs are reported. The variables are stud spacing, degree of the shear connections, area of the longitudinal reinforcement and slab thickness. The test set-up and instrumentation is described in detail. The experimental behaviour is analysed and based on the test data the structural behaviour of these semi-rigid composite joints is discussed. Based on the experimental data, a simplified method to predict rotation and moment capacity for this type of composite connection is proposed. c 2005 Elsevier Ltd. All rights reserved. Keywords: Semi-rigid; Composite; Joints; Precast; Hollowcore; Steel; Connections; Beam–column
1. Introduction In the area of composite construction, extensive research works have been focused on semi-rigid connection design since it was first proposed by Barnard [1] in the 70s. They showed these forms of connections when used in design will lead to reduction in beam sizes, which in turn will reduce the beam depth, the overall building height and cladding cost, etc. The moment rotation characteristic of the semi-rigid composite connections was first investigated by Johnson and Hope-Gill [2] in 1972, they found that neither simple nor rigid beam–column connections are ideal. Simple joints are too unpredictable while rigid joints are often too stiff in relation to their strength and are expensive; therefore, the semi-rigid joint with a large rotation capacity and a predictable flexural strength that does not require site welding or accurate fitting is needed. Numerous researches have been carried out on semi-rigid composite connections [3, 4], the most common types of floor slab used being solid R.C. slabs or profiled metal deck floors. ∗ Corresponding author.
E-mail address: [email protected] (D. Lam). c 2005 Elsevier Ltd. All rights reserved. 0143-974X/$ - see front matter doi:10.1016/j.jcsr.2005.11.013
Composite beams incorporating precast hollowcore floor slabs are a newly developed composite floor system for buildings. Compared with the other two types of floor system, it has the following advantages: HCU can be manufactured up to 500 mm in depth, meaning that simply supported floor spans of up to 20 m are possible; however, the most common depths are 150–400 mm. HCU has an excellent structural capacity to selfweight ratio, with span/depth ratios in the order of 35 being possible for normal office loadings. The volume of the hollow cores accounts for up to 50% of the cross section, therefore a 10 m span floor only weighs 3.5 kN/m2 . The standard width of the units is 1.2 m, enabling fixing rates of around 2000 m2 per week. The floor system does not require a structural screed to carry horizontal diaphragm forces thus further reducing the dead weight of the floor. Welding of the shear studs may be carried out in a factory or at ground level on site with mobile welding equipment using the semi-automatic drawn arc process. Although the use of precast hollowcore slabs dates back to the 1940s, research on composite construction incorporating steel beams with precast hollowcore slabs is relatively new. The first commercial testing in this area was carried out at Salford University and reported by Hamilton [5]. Tests were carried out
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Fig. 1. General arrangement of test set-up.
Fig. 3. Test specimen before concrete infill.
Fig. 2. End plate connection.
using 150 mm deep slabs and 406 × 178 × 60 UB with 19 mm diameter × 125 mm long headed shear studs. The results showed more than 70% increase in ultimate moment capacity when compared with its bare steel counterpart. Mode of failure was due to shearing off of the headed studs. Research on shear connector strength in precast solid concrete planks was carried out by Moy and Tayler [6] in 1996. Twenty-seven push-off tests were carried out and the results showed a reduction in strength as the gap of in situ concrete decreased. It is recommended that the width of in situ concrete should be a minimum of 100 mm to avoid reductions in shear strength of the shear connectors. It is also recommended that two layers of reinforcement must be used in the structural topping to avoid any tensile splitting. A test on composite beams with precast solid planks was also conducted by Jolly [7] at Southampton University. A 16 m span composite beam with 110 mm deep precast concrete planks was tested. The results showed that the dynamic response of long span, shallow composite construction complied with the requirements of BS5950 [8] without the need to increase the minimum number of shear connectors as specified in the code. Shim et al. [9] studied the behaviour of headed shear studs in a precast post-tensioned bridge deck at the Seoul National University. Push-out tests were carried out to determine the structural behaviour of the shear connection in the precast deck. It is found that as the thickness of the bedding layer increases, the ultimate strength of the shear connection decreases. Horizontal push-off tests with precast hollowcore slabs were first performed by Lam et al. [10] in 1998. They showed that the shear capacity of the stud for this type of construction was not only affected by the tensile capacity of the stud itself,
Fig. 4. Load versus slip curve from push-off test.
Fig. 5. LVDT’s position for measuring beam rotation and interface slip.
but also affected by the gap width, the amount of transverse reinforcement, the strength of the in situ concrete infill and the presence of the longitudinal and transverse joints. Three full scale simply supported composite beams with variable parameters were also carried out by Lam et al. [11] to study the flexure behaviour and was compared with the non-composite bare steel beams. The results showed two modes of failure; failure due to loss of shear studs and failure of the concrete slab due to yielding of the transverse reinforcement. Nevertheless, the residual moment capacity of all the beams remained at least 40% above the moment capacity of the bare steel section only. A beam with a pre-cracked in situ/precast concrete joint was also tested. Modelling of the headed studs in steel-precast composite beams using the finite element analysis software
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Fig. 6. Strain gauge on steel reinforcement.
Fig. 8. Bolt strain gauges.
Fig. 7. Strain gauge mounted on steel beam flanges and web.
ABAQUS [12] was carried out by El-Lobody and Lam [13], good agreement was obtained when compared with the test results. The use of the precast hollowcore slabs in current buildings design is so far limited to simple beam–column connections. As semi-rigid connection has many advantages over the simple connection, the main objective of this research is to investigate the structural behaviour of the composite semi-rigid connections with precast hollowcore slabs and to determine whether these kinds of joints can provide sufficient moment capacity and rotation capacity to develop the mid-span plastic hinge for the plastic analysis uses in the composite beam design. 2. Test arrangement 2.1. Test specimens All specimens were of cruciform arrangement as shown in Fig. 1 to simulate the internal beam–column joints in a semirigid composite frame. The specimen was assembled from two 3300 mm long; 457 × 191 × 89 kg/m; grade S275 universal
Fig. 9. Test set-up and loading arrangement.
beams and one 254 × 254 × 167 kg/m; grade S275 universal column to form the cruciform arrangement. The beams are connected to the column flanges using 10 mm thick flush endplates with two rows of M20 Grade 8.8 bolts as shown in Fig. 2. The steel connection is a typical connection currently used in UK practice for simple joints, this is to ensure that the enhanced performance of the composite joint is not provided by the bare steel connection. A single row of 19 mm diameter headed shear studs is pre-welded to the top flange of the steel beams. Finally,
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Table 1 Strength of concrete from test results Ref. CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
Compressive strength (N/mm2 ) 7 days Test day 30.9 43.1 40.6 35.7 30.6 33.2 32.8 35.2
40.1 49.6 47.9 43.8 41.0 37.3 40.2 42.96
28 days
Tensile strength (N/mm2 ) 7 days Test day
28 days
Density (kg/m3 )
45.0 53.5 57.1 41.8 46.8 44.4 44.3 44.7
2.25 2.15 2.63 2.15 2.75 2.16 2.43 2.55
2.50 2.90 3.19 3.14 3.45 2.45 2.99 3.17
2330 2384 2367 2340 2336 2318 2352 2346
two 305 × 102 × 28 kg/m universal beams were connected to the column web to make up the full joint arrangement. Fig. 3 shows the semi-rigid composite joint before casting, it can be seen that the ends of the HCUs are chamfered and the top of the alternative cores of the units opened for a length of 500 mm for the placement of transverse reinforcement. A gap of 65 mm is formed between the ends of the units. In situ concrete is used to fill the gap between the HCUs and the opened slots. A nominal concrete strength of 40 N/mm2 is used for the in situ infill. The HCUs are tied together transversely by the 16 mm diameter high strength reinforcing bars 1 m long. Longitudinal reinforcement is provided by the high strength steel reinforcing bars across the joint for continuity.
2.49 2.49 3.55 2.50 3.05 2.48 2.40 2.63
Table 2 The mechanical properties for the reinforcing bar Ref.
Yield strength (N/mm2 )
Ultimate tensile strength (N/mm2 )
Cross section area (mm2 )
T16 T20
536 534
621 634
195.9 305.4
Table 3 Tensile test results for steel beam Ref.
Yield strength (N/mm2 )
Ultimate tensile strength (N/mm2 )
Yield strain (µε)
Web Flange
382.1 341.7
549.4 503.0
2379 2285
2.2. Material properties The compressive and tensile strength of the in situ concrete is shown in Table 1. The in situ infill concrete strength is tested at the 7th day, the test day and the 28th day. The characteristic concrete strength for the precast hollowcore units is taken to be 55 N/mm2 as specified by the manufacturer. The tensile strengths of the T20 and T16 reinforcing bars were determined in accordance with BS 4449 [14] and the results are shown in Table 2. Test coupons of the web and flange with thickness of 10.9 mm and 17.7 mm respectively were cut from the ends of the steel beams after the tests where stresses had been low. Tensile tests of coupons were conducted according to BS EN 10002-1 [15] and the test results are shown in Table 3. Tensile tests for M20 Grade 8.8 bolts were performed and an average ultimate strength of 678 N/mm2 is obtained. 19 mm diameter × 125 mm height TRW-Nelson headed shear connectors are used with an average ultimate strength of 610.5 N/mm2 . Horizontal push-off tests were conducted to determine the load–slip characteristics of the head shear stud and the average load–slip curve is shown in Fig. 4, the result is used to determine the shear capacity and degree of shear connection for the semi-rigid joint tests.
measure the in situ concrete strain at the slab’s surface. The strain gauges used to monitor the strain and yielding of the rebar and steel beam were of the type FLA-5-11 with a gauge length of 6 mm. The gauges have 120 ±0.3 resistances with a gauge factor of 2.13. Strain gauges are placed on the longitudinal bar and on the transverse reinforcement as shown in Fig. 6. The strain gauges on the surface of the rebar were coated with epoxy to protect them from damage during concreting. Strain gauges were also placed on the top flange of the steel beam and on the web to measure the strain on the steel beam to determine the position of the neutral axis throughout the test (Fig. 7). Strain in the bolts was measured using BTM-1C bolt strain gauges as shown in Fig. 8 and their gauge parameters are shown in Table 4. The gauges were calibrated before the test and used to measure the bolt forces and elongation of the bolts. Load is applied by hydraulic jacks simultaneously to each ends of the steel beams as shown in Fig. 9. An elastic test is carried out before test to failure to check the instrumentation and the system. The load was applied at 10 kN intervals and continued until failure occurred. 2.4. Test parameters
2.3. Instrumentation and loading procedure Instrumentation comprised of linear voltage displacement transducers (LVDTs) for measuring slip, beam deflection and beam rotation is shown in Fig. 5. Electrical resistance strain gauges (ERSGs) are used to measure strain in reinforcing bars, steel beams and bolts at the joints. Demac gauges are used to
In order to investigate different variables affecting the behaviour of the composite joint, different emphases are adopted for each test. For the first five tests, the main variables are stud spacing and the degree of the shear connections. In CJ6 and CJ7, the main variables are the cross sectional area of the longitudinal bar and for the CJ8, the variable is the thickness of
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Fig. 10. General arrangement and position of the strain gauges for Test CJ1.
Fig. 11. General arrangement and position of the strain gauges for Test CJ2.
Fig. 12. General arrangement and position of the strain gauges for Test CJ3.
Fig. 13. General arrangement and position of the strain gauges for Test CJ4.
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Table 4 Parameters of the BLM bolt strain gauge Type BTM-1C
Gauge (mm) Length
Width
Base (mm) Length
Width
Gauge centre a
b
Hole Dia. (mm)
1
0.7
5.6
1.4
1.8
3.8
1.6
Table 5 Test arrangement Reference
In situ concrete cube strength (N/mm2 )
Longitudinal bars and cross section area (mm2 )
Hollowcore slabs thickness (mm)
Studs spacing (mm)
Position of first stud (mm)
No. of shear stud per beam
CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
39 50 48 44 41 37.3 40.2 42.9
2 T20(628) 2 T20(628) 2 T20(628) 2 T20(628) 2 T20(628) 4T16(800) 2T16(400) 4T16(800)
200 200 200 200 200 200 200 250
300 600 900, 1200a 400 500 310 1200 450
235 235 990, 540a 510, 710a 645 465 900 705
7 4 2 3 3 6 2 4
19φ × 125 mm long headed shear connectors were used for all the tests. a Stud on the east side.
Fig. 14. General arrangement and position of the strain gauges for Test CJ5.
Fig. 15. General arrangement and position of the strain gauges for Test CJ6.
the precast hollowcore slabs. The test arrangement is shown in Table 5 and Figs. 10–17. Strain gauges were put on the surface of the shank of the stud near the weld collar to measure the stresses in the studs. In CJ6, CJ7 and CJ8, bolt strain gauges were used at the connection to monitor the bolt forces and elongation of the bolt.
3. Test results Results of all eight composite joint tests are shown in Table 6 and Fig. 18. All tests except Test CJ3 failed in a ductile manner with beam rotation well in excess of 30 mrad and obtained a moment capacity above 0.3 Mp of the beams,
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Fig. 16. General arrangement and position of the strain gauges Test CJ7.
Fig. 17. General arrangement and position of the strain gauges Test CJ8. Table 6 Test result Reference
CJ1
CJ2
CJ3
CJ4
CJ5
CJ6
CJ7
CJ8
Moment capacity (kN m) Rotation capacity (mrad) Long. reinf. capacity—yield (kN) Long. reinf. capacity—Ult. (kN) Shear connector capacity (kN) Degree of shear connection (%)a Degree of shear connection (%)b Max. strain in long. reinf. (µε) Maximum end slip (mm) Failure mode
370 35.4 326 387 896 >100 >100 26,000 0.34 RF
363 33.5 326 387 512 >100 >100 23,000 0.8 RF
250 6.1 326 387 256 78.5 66 2031 5.8 CF & SF
368 37.4 326 387 384 >100 98 16,000 3.5 CF
363 31.7 326 387 384 >100 98 13,706 3.5 CF
425 46.8 424 486 512 >100 >100 26,000 0.84 RF
274 30 212 243 256 >100 >100 23,000 0.4 RF
439 42.3 424 486 512 >100 >100 23,000 1.6 RF
RF – reinforcement fracture; CF – connector fracture; SF – slab shear failure. a Calculated using the yield strength of longitudinal steel bar. b Calculated using the ultimate strength of longitudinal steel bar.
it can be concluded that these types of joints can provide sufficient moment capacity and rotation capacity. Tests CJ1, CJ2, CJ6, CJ7, and CJ8 failed due to the fracture of longitudinal reinforcement while Tests CJ3, CJ4 and CJ5 failed by fracture of the shear connectors. No yielding or buckling to the column was observed. For all the tests conducted, no bond failure between the in situ and the precast concrete was observed. It can therefore be concluded that the in situ and the precast concrete are acting compositely throughout. The mode of failure can be divided into two main categories: (a) fracture of the longitudinal bar as shown in Fig. 19, and (b) fracture of shear studs as shown in Fig. 20. No other mode of failure was observed in any of the tests. Fig. 21 shows the strain measurement of the longitudinal rebars during the test, it can be seen that with the only exception of Test CJ3, the longitudinal bars in all tests developed strain hardening with the longitudinal rebars in Tests CJ1, CJ2, CJ6,
Fig. 18. Moment versus rotation curves.
CJ7 and CJ8 fractured at the end of the tests. For Tests CJ4 and CJ5, stud fracture occurred before fracture of the longitudinal
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Fig. 21. Strain measurement of the longitudinal reinforcement. Fig. 19. Mode of failure due to fracture of the longitudinal reinforcing bars.
Fig. 20. Mode of failure due to stud fracture. Fig. 22. Typical strain profile of longitudinal reinforcing bar along the length.
bar due to partial shear connection. From these results, it can concluded that full shear connection should be provided to enable full mobilisation of the longitudinal reinforcement and partial shear connection will lead to low moment and rotation capacity as the longitudinal bars cannot be fully mobilised. A typical strain profile of the longitudinal rebar along the beam is shown in Fig. 22. It shows that yielding of the longitudinal reinforcement occurred at the distance between the centre line of the column and the second stud position, the strain in the other part of the steel bar is very small and remained elastic. This further demonstrates that the position of the headed studs played an important role in rotation capacity of the composite connections. Fig. 23 shows the end slip for all the tests. As predicted, larger slip is obtained for the partial shear connection tests CJ3, CJ4 and CJ5 where the amounts of slip are smaller for the full shear connection tests. For CJ6 and CJ8, as the ratios of the force in the longitudinal rebar to the longitudinal shear force provided by the studs are larger than that of CJ1, CJ2 and CJ7, larger connector slip resulted. Strain is measured at the steel top and bottom flange of the steel section for all the tests and the steel beam of all eight tests remained elastic throughout. The development of the crack pattern around the concrete slab was similar to the one shown in Fig. 24 for all the composite joint tests. The first cracks were visible at the column flange tips and propagated
Fig. 23. Moment versus end slip for all the tests.
towards the slab edges before eventually forming the dominant transverse cracks across the complete breadth of the slab. As the widths of these cracks developed further, a significant loss of connection stiffness resulted. This in turn led to a considerable widening of these cracks, causing a high localised strain in the longitudinal reinforcement. Consequently, fracture of the longitudinal reinforcement occurred. Fig. 25 shows the strain measurement of the transverse bar for all the joint tests. It showed that the strain in the transverse bar is very small. The maximum strains measured in all the tests
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4.2. Effect of stud spacing From the comparison of Tests CJ1 and CJ2, it can be seen that although different stud spacings and numbers of studs are used, no distinct difference of moment and rotation capacity was found from the test results. Therefore, it can be concluded that provided the same degree of shear connection and position of first stud is used, the numbers of studs and their spacing did not have much influence on the moment capacity and rotation capacity of the connection. 4.3. Effect of the degree of shear connection Fig. 24. Typical crack pattern of the joint test.
Fig. 25. Strain measurement of the transverse reinforcement for all the tests.
were less than 900 µε and hence the transverse rebars remained elastic. It can be concluded that the transverse bar has little effect on the connection moment capacity. 4. Discussions 4.1. Effect of distance between first stud and column flange The formation of cracks would appear to be related to the position of the first stud. Fewer cracks are formed in Tests CJ1 and CJ2 with a main crack opening near the column faces and eventually leading to the fracture of the longitudinal reinforcement. At failure, little interface slip is recorded and hence little steel connection deformation is observed. The amounts of slip observed in Tests CJ1 and CJ2 were very small. When the specimen was dismantled after the test, the studs were found to be intact. In Tests CJ4 and CJ5, the first stud position is placed further away from the face of the column. After the formation of the first crack, additional cracks were formed between the column face and the position of the first stud. Cracks between the column face and the first stud distributed evenly rather than concentrated at a single crack around the column face, this led to lesser demand on the percentage of elongation required by the longitudinal reinforcement. Large interface slip is also observed.
In this series of experiments, two methods for calculating the degree of shear connection are employed. The first method is based on the yield strength of the longitudinal rebars as assumed by many researchers and ignoring the ultimate strength of the reinforcing bars. The second method is based on the ultimate strength of the longitudinal rebars to determine the degree of shear connection, which takes into consideration the ultimate tensile strength of the longitudinal bars. In accordance with method 1, if the shear stud capacity is greater than the yield strength of the longitudinal rebars, then it should be classified as full shear connection. However, as shown in Table 6, the mode of failure in Tests CJ3, CJ4 and CJ5 is due to stud fracture. It is because the shear stud capacities were less than the force of longitudinal rebars as calculated using method 2. Therefore, it suggested that the ultimate strength of the longitudinal rebars should be taken into consideration when determining the degree of shear connection. In test CJ3, the values of interface slip increased with lower degree of shear connection, which led to low moment and rotation capacity. Comparison between CJ3 and CJ7 shows that although only two studs were used in both the tests, Test CJ3 has a very low degree of shear connection in compared with Test CJ7 and hence a small amount of rotation was recorded. It can be concluded that for a joint with partial shear connection, as long as the shear stud can allow the longitudinal bar to be mobilised until the yielding stage, there will be no obvious deduction of the moment and rotation capacity. Otherwise low degrees of the shear connection will lead to low moment and rotation capacity as yielding and elongation cannot occur in the longitudinal rebars. 4.4. Effect of amount of longitudinal reinforcement Comparison among Tests CJ1, CJ2, CJ6 and CJ7 showed that with full shear connection and same slab thickness, increases in the amount of longitudinal reinforcement led to higher moment and rotation capacity. Therefore, it can be concluded that increases in the amount of longitudinal reinforcement will not only lead to increases in moment capacity but also larger rotation capacity. The analysis of the test results showed that the inclusion of a large amount of localised reinforcement is one of the most effective ways of achieving good rotation capacity. The reinforcement enables cracks to be distributed evenly in the
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concrete, thereby reducing localised strains and allowing high average strains to develop in the reinforcement. Appropriate connection details can be used to ensure that the average strain occurs over a substantial length, for example by placing the first shear stud on the beam further away from the column face. High strains over a substantial length result in large deformation of the reinforcement. The reinforcement also helps to develop large compressive forces in the lower parts of the steel section, causing plastification which leads to large strains in the steel. Large tensile deformations in the reinforcement and compressive deformations in the steel beam will in turn equate to significant connection rotation. 4.5. Effect of concrete cracking on the shear connectors’ capacity Shear studs placed within the concrete in the hogging moment region did not show any deterioration in its strength. The shear stud capacity in the tests agreed well with the push tests carried out in the horizontal push test [16]. Although the concrete was severely cracked, the reinforcing bars were effective and able to transfer the tensile force. Good interaction between the shear studs with the concrete and the concrete with the longitudinal rebars were observed, which enables the transmission of the longitudinal shear forces.
Fig. 26. Force diagram and the components of the composite joint. Table 7 Comparison of test results and the proposed method for rotation capacity References
Test results (mrad)
Calculated results (mrad)
CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
35.4 33.5 6.1 37.4 31.7 46.8 30 42.3
28.8 37.5 8.2 39.7 36.3 40.1 50.4 47.8
4.6. Effect of precast slab thickness With the only difference between Test CJ6 and CJ8 being the depth of the precast hollowcore slabs, the result showed that by using deeper slabs, an increase in moment capacity was obtained. The increases in moment capacity are purely due to the increases in the lever arm, but a slight reduction of the rotation capacity with the Test CJ8 was observed with deeper slabs. 5. Proposed method to calculate rotation capacity A simple calculation method [17] based on the component method is proposed to predict the rotation capacity, φu for this form of composite joints as shown in Eq. (1) and Fig. 26. The strain profile of the longitudinal reinforcing bar taken from the test data shows that the steel bar yielded only in the region between the column centre line and the second shear stud, the strain in the other part of the steel bar is small and remained elastic. Therefore, the elongation zone of the longitudinal reinforcement, L can be taken as between the centre line of the column and the position of the second stud. Hence, it can assumed that the length for calculating the elongation is p1 + p2 + D/2, where p1 is the distance between the column face and the centre line of the first stud; p2 is the distance between the centre line of the first stud and the second stud and D is the depth of the column. L Slip + Db + Dr Db D L = εsh p1 + p2 + . 2
φu =
(1) (2)
For full shear connection, reinforcement strain, εsh is taken as the ultimate strain developed in the longitudinal reinforcement. It is because for full shear connection, the longitudinal reinforcing bars can developed into the strain hardening whereas for the partial shear connection, εsh is taken as the maximum strain developed in the longitudinal bar. For simplicity, it can be taken as the yield strain of the steel bars if enough shear studs are provided to enable yielding of the longitudinal bars. Composite joints with partial shear connection unable to develop yielding of the longitudinal reinforcing bars will lead to low rotation capacity as demonstrated in Test CJ3. Table 7 shows the comparison between the test results and the calculated results using the proposed method. The results show that the method gave good prediction of the test results on composite connection with steel beams and precast hollowcore slabs. 6. Proposed method to calculate moment capacity No calculation method for the prediction of moment capacity of composite connection with precast hollowcore slabs is currently available. A method to predict the moment capacity for this type of semi-rigid connection is proposed [17]. The proposed method assumes that R f ≥ Rb + Rr , where, R f = compressive resistance of the bottom flange of the steel beam, Rr = the lesser of the ultimate tensile strength of the longitudinal reinforcement or the studs capacity,
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Table 8 Comparison of test results and the proposed method for moment capacity
precast hollowcore slabs, the following conclusions can be made:
References
Test results (kN m)
Calculated results (kN m)
CJ1 CJ2 CJ3 CJ4 CJ5 CJ6 CJ7 CJ8
370 362 250 368 363 425 274 439
365.8 365.8 284.5 365.0 366.6 422.3 274.0 446.7
1. Semi-rigid composite joints with precast hollowcore slabs can provide sufficient moment and rotation capacity as required by the design code as a suitable type of connection for plastic design. 2. Two failure modes are observed from the joint tests. Mode of failure was either fracture of the longitudinal rebars or fracture of the headed studs. 3. Adequate shear connection should be provided to enable full mobilisation of the longitudinal reinforcement. Partial shear connection in the hogging moment region will lead to low moment and rotation capacity as the longitudinal bars cannot be fully mobilised. It is recommended that the minimum percentage of shear connection provided should enable the longitudinal reinforcement to develop yield. 4. The ultimate tensile strength of the longitudinal reinforcement should be taken into consideration when calculating the degree of shear connection. 5. Close spacing of the shear stud near to the column face will affect the crack pattern and demanded high degree of elongation from the longitudinal reinforcement to achieve the same rotation capacity with the same joint with larger stud spacing between the column flange and the first stud. It is recommended that the first stud should be placed at a spacing equivalent to two times the column width. 6. The spacing between other shear studs has little effect on the moment capacity and rotation capacity of the connection provided the degree of shear connection is the same. 7. The in situ concrete infill and the precast hollowcore slabs acted compositely throughout, no separation was observed from any of the tests carried out. 8. A simple method to predict the moment and rotation capacity is proposed and compared with the test results, it is found to be acceptable for use to predict the behaviour of this form of composite joints.
Rb = effective tensile resistance of the bolt group. The moment resistance of the composite connection, Mcc Mcc = Rr (Db + Dr − 0.5t f ) + Rb (Db − r1 − 0.5t f )
(3)
where Db is the depth of the beam; r1 is the distance of the first row of bolts below the top of the beam; Dr is the distance of the reinforcement above the top of the beam and t f is the flange thickness of the steel beam. For R f < Rb + Rr , (R +R −R ) The neutral axis, yc = r tw bPy f where, tw is the web thickness and p y is the design strength of the steel section. The moment resistance of the composite connection, Mcc Mcc = Rr (Db + Dr − 0.5t f ) + Rb (Db − r1 − 0.5t f ) − Rw
yc 2
(4)
where Rw = yc tw p y . The comparison of the test results and the results from the proposed method above is shown in Table 8. The results showed that the moment capacity of the semi-rigid composite connections is dependent on the strength and the ability to mobilize the longitudinal reinforcing bars. The influential factor to their mobilization is dependent on the degree of the shear connection between the slabs and the steel beams, which is determined by the number and the capacity of the shear studs in the hogging moment region. In all five tests carried out, all factors but the shear studs were kept constant. All tests with the exception of Test CJ3, the shear connection capacity is larger than the yield strength of the longitudinal reinforcement, therefore the tensile strength of the longitudinal bars can be mobilized, and hence adequate moment and rotation capacity can be achieved. 7. Conclusions The behaviour of eight full-scale semi-rigid composite connections with precast hollowcore slabs was examined. Different levels of shear connection, spacing and position of first studs from the column face have been examined. Tests showed these joints combine simple and efficient construction and yet provide worthwhile levels of moment capacity, rotational stiffness and ductility with the introduction of longitudinal reinforcement across the column. From the experimental study of the semi-rigid composite joints with
Acknowledgements The authors would like to acknowledge the financial support from International Precast Hollowcore Association (IPHA) and Overseas Research Scholarship (ORS), the support provided by Severfield—Revee Structures Ltd. for supplying the steel specimens and Bison Concrete Products Ltd. for supplying the precast hollowcore slabs. The skilled assistance provided by the technical staff in the School of Civil Engineering at Leeds University is also appreciated. References [1] Barnard PR. Innovations of composite floor systems. In: Canadian structural engineering conference. Canadian Steel Industries Construction Council; 1970. p. 13–21. [2] Johnson RP, Hope-Gill M. Semi-rigid joints in composite frames. In: International association for bridge and structural engineering, ninth congress. 1972. p. 133–44.
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