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Shear-bond capacity of composite slabs made with profiled sheeting
The International Journal of Cement Composites and Lightweight Concrete, Volume 8, Number4
November 1986
Shear-bond capacity of composite slabs made with profiled sheeting K. C. G. Ong* and M. A. Mansurt
S y n o p s i s An experimental study conducted to determine the shear-bond capacity of concrete slabs reinforced with locally available profiled steel decks is reported in this paper. Ten one-way slab specimens were tested under third point loading. In the light of these test data, the currently available design methods have been reviewed, and an empirical relationship is proposed to estimate the shear-bond capacity of this type of slab with and without end anchorages under third point loading. K e y w o r d s Concrete slabs, shear tests, composite structures, reinforced concrete, shear bond capacity, design criteria, regression analysis, bond stress, ductility, brittle failure, design standards, cracking (fracturing)
INTRODUCTION A floor system for tall buildings created by integrating the structural properties of concrete and profiled steel decks has been gaining increasing popularity throughout the world. This is especially so in developed countries where, among other factors, high development charges and cost of labour have made this type of construction more attractive to developers, architects and engineers in recent years. In this type of construction, the steel deck serves not only as a self-sustaining form supporting the construction and fresh concrete loads but also as the principal reinforcement for the bottom fibres of the slab. However, the successful functioning of this type of slab depends entirely on reliable bond between the concrete and the steel deck at their interface. Without shear connectors, bond is generally insufficient to ensure composite action up to failure [1]. It is thus necessary to employ mechanical means or so-called shear transferring devices to achieve positive interlocking between the two materials. These can take the form of closely spaced indentations or embossments, transverse wires welded to the top of the corrugations, holes punched on the sides of the deck for the formation of concrete shear keys and surface bonding with suitable deck profile. Previous investigations [1,2] have shown that even if shear transferring devices are provided, sudden failure by the destruction of interracial bond still occurs. Known as shear-bond failure, this type of sudden collapse is characterised by the formation of major diagonal cracks in concrete, usually at the loading points, due tO slippage * Lecturer and t Senior Lecturer, Department of Civil Engineering, National Universityof Singapore @ Longman Group UK Ltd 1986 0262-5075/86/08403231/$02.00
of the steel deck relative to the concrete. No rational method has yet been developed to predict the shearbond strength of steel-deck reinforced concrete slabs. In 1976, the American Iron and Steel Institute (AISI) sponsored a research programme at Iowa State University [2] with the principal objective of developing a unified design criteria for steel deck-concrete floor systems, Based on extensive study, Porter et al. [3] have recommended the following equation for use in the design of such slabs under concentrated line loads: VuS/bd = m pd/L' + k' ~
(1)
taking f~ = 0.78 fcu and rearranging Equation 1, we have VuS/bd where
and
= m (pd/L' ~/-f-c~ + k
(2)
Vu is the maximum shear at shear-bond failure in Newtons S is the centre-to-centre spacing of shear transfer devices other than embossments in mm b is the width of the slab in mm d is the effective depth in mm fc is the cylinder compressive strength of concrete in N/mm 2 fcu is the cube strength of concrete in N/mm 2 p is the percentage of steel L' is the shear span in mm k and m are regression constants.
The equation can be corrected [3] to include the effects of dead weight and of the shoring conditions. The values of the constants k and m should be empirically obtained for each type of steel deck profile because of its own shear transfer characteristics. The European Convention for Constructional Steelwork, ECCS [4] carried out an extensive review on the shear-bond strength of such slabs in an attempt to
231
Shear bond capacity of composite slabs made w~thprofiled sheeting
---21 mm ~ I
Ong and Mansur
$22 Gauge fWetded steel deck mesh
'.I -~ ~ - ' ---= I i2 mm 4 1"-32l--~ 5ram
Figure 1 Typical cross-section of test specimens
mmif:r==
610 mm rationalise the methods of design. The results have been published in the form of design recommendations [4]. As the review was based on Swiss and North American works, these recommendations are suitable for the type of decks used in those regions. The most commonly used steel decks in the AsiaPacific region are marketed under the trade names 'Bondek', 'Klip-lok' and 'Spandek'. Among these decks, mechanical interlocking is incorporated only in Bondek by its profile as shown in Figure 1. This was developed by John Lysaght (SEA) Pte Ltd based on the work of Resvesky [51. The other two types are normally used as roofing or as structural form in slab construction, and they serve no structural function after the concrete has hardened. The study reported in this paper was conducted on composite steel-deck reinforced concrete slabs using 'Bondek' profiles with the main objective of determining the shear-bond capacity. Ten one-way slab specimens were tested under third point loading. The major parameters of the study were the span and thickness (reinforcement ratio) of the slab. The results of these tests are presented and discussed in this paper. Based on the present test data the current design methods are reviewed and an 6mpirical relationship is proposed to estimate the shear-bond capacity of this type of slab under third point loading.
of the supporting steel girder in actual construction. The second group P, contained no end anchorage. The second letter, A, B or C indicates the span at which the slabs were tested, namely 2.3m, 2.85m and 3.45m respectively. The number following the letter symbols indicates the specimen number. The details of test specimens can be found in Table 1. The concrete used was designed for a 28 day compressive strength of 30 MPa. The mix ratio by weight of cement, sand and aggregates (20mm maximum size) was 1:1.84:2.76 and the water-cement ratio was 0.53. A layer of welded wire mesh was used as secondary reinforcement (Figure 1). The wire mesh consisted of 5mm diameter wires with a square grid of 200mm. The average yield strength (0.2% proof stress) of the wires was 571 MPa and the modulus of elasticity was 1.97 x 105 MPa. The slabs were cast using wooden side forms with a central prop. The surface of the decks was thoroughly cleaned before placing the fresh concrete. Compaction
s J/--
16mml~ shear studs Concrete slab
j=Lo
.--,- S E
TEST PROGRAMME Test specimens In all, ten 0.61 m wide specimens were tested. They were of three different lengths, namely 2.4m, 3.0m and 3.6m and of two different thicknesses; 100mm and 120mm. Figure 1 shows a typical cross-section of the slab. The steel deck employed is commercialy known as 'Bondek'. It is made up of 0.75mm thick galvanised steeJ plate. The average yield strength and modulus of elasticity for the steel deck were found to be 626 MPa and 2.02 x 10 ~ MPa, respectively. The test specimens were designated by two letters followed by a number. The first letter A or P, indicates the series name according to the type of end anchorage used. For the specimens in A-series, anchorage was provided by three 16mm diameter studs through a 10ram thick flat steel bearing plate at each end of the specimen (Figure 2). The steel plate simulated the flange
232
A ~
Bearing ' plate
or mm
(a) Longitudinal section 25 mm S
I,
_T/
Shear stud
T/ Bearing plate
(b) Section A-A Figure2 Slabs with end anchorage
!
Shear-bond capacity of compos/te slabs made with profiled sheeting
Ong and Mansur
Table1 Slab details and summary of test results
Slab designation PA 1 PB 1 PB2 PC1 PC2 AA1 AA2 AB1 AB2 AC 1
Total depth (ram)
Total length (m)
Span L (m)
Cube compressive strength fo~ N/ram 2
100 120 120 120 120 100 100 120 120 120
2.40 3,00 3.00 3.60 3.60 2.40 2.40 3.00 3.00 3.60
2.30 2.85 2.85 3.45 3,45 2,30 2,30 2.85 2.85 3.45
36.2 45.0 47.9 38.1 44.1 37.7 39.5 48.8 48.0 42.3
Properties of concrete Cylinder splitting Modulus of strength elasticity fsp E~ N/ram 2 kN/mm 2 2.76 3.38 4.41 3,75 3.78 2.33 2.26 4,80 4.37 3.65
29.1 25.0 24.5 23.9 244 29.1 29.1 27.1 26.9 23.9
Total load at shear-bond failure Po (kN) 20.0 25.8 29.8 21.6 21.6 26.0 26.0 35.8 33.8 27.9
A: End anchorage P: Without shear transferring devices
was achieved by using a poker type vibrator. Three 150mm cubes and three 150mm by 300mm cylinders were cast for each slab to determine the compressive strength and modulus of elasticity of the concrete. The slabs were cured under damp hessian at ambient temperature for about 2 weeks and then air-dried in the laboratory before testing. The compressive strengths of the concrete at the time of slab tests are shown in Table 1.
Test setup and instrumentation The specimens were simply supported and tested in a self-balanced loading frame under third point line loading (Figure 3). Strains were measured at the bottom of the steel deck at midspan by means of electrical resistance strain gauges. A series of dial gauges (accuracy of +0.1 ram) was used to measure deflection along the length of the slab. The load was applied in increments varying between 2 and 5 per cent of the estimated ultimate load. At each increment, the dial and strain gauge readings were recorded. When shear-bond failure occurred the specimens were reloaded until the applied load became stable again. This was repeated until final collapse:
DISCUSSION OF TEST RESULTS Behaviour of specimens The load-deflection Curves of typical specimens are shown in Figure 4. The curves were initially linear at the early stages of loading. With the formation of cracks, they deviated slightly from tinearity. Although not initially visible, the formation of cracks was accompanied by crackling which became louder and more frequent with
Figure 3 Test setup
the formation of a major diagonal crack in the concrete at or near one of the loading points. This crack widened with increasing load until failure occurred by a loss of bond between concrete and steel deck, All the specimens failed in this manner. The loads at which shear-bond failure occurred are indicated in Figure 4. It can be seen that this type of failure was accompanied by a sudden drop in the applied load for most of the specimens. For the specimens in Group P, attempts to increase the load resulted in increasing deformations without a significant load recovery. The specimens eventually collapsed due to vertical as well as horizontal separation of the steel deck from concrete (Figures 5 and 6). Porter and Ekberg [2J postulated that the failure of a steel-deck-reinforced concrete slab initiates by irregular development of the flexural and bond stresses between the concrete and steel deck. The bond stress in the cracked zone becomes very high and depends on the
233
Shear-bond capacity of composite slabs made with pro filed sheeting
Ong and Mansur
rate of change of the concrete stresses adjacent to the steel deck. An increase in concrete stress gradient results in an increase in local bond stress. This continues until the bond strength is exceeded and local bond failure occurs. A redistribution of stresses to the adjacent sound section nearer to the supports then takes place. When the loss of bond progressively reaches the free ends of the slabs, slippage between the concrete and steel occurs with eventual collapse. In contrast, Group A specimens continued to sustain a significant amount of additional load after shear-bond 55
!
,
,
,
l
I
50
S~abs without any sheartransferringdevice • Slabs with end anchorage
/.5
~
I
•
Figure 6 Typical end slip
Load at shear- bond foiture
///'~
40
35
/.~'
Figure 7 Tearing-off failure of steel deck
[~" / 5~t'
0
2.30 m span -- -- 2.85 m span
5
10 15 20 25 30 Midsl:~n deflection (mm)
Figure 4 Load-deflection curves
35
failure. Composite action was not completely lost as the shear studs acted as anchors to transfer forces from the concrete to the steel deck. The specimens however deflected considerably before the new load-carrying mechanism had been fully mobilised. Collapse finally occurred due to tearing-off of the steel deck at shear stud connections (Figure 7). It is evident from the preceding discussion that slabs in Group P would have failed catastrophically if the load level was maintained at the shear-bond failure load. On the other hand, slabs with end anchorages would give adequate warning through excessive deflection prior to failure.
Comparison
of
test
results
with
available
design
methods
Figure5 Typical specimen after failure
234
The design methods currently used in the United States of America, Australia and Switzerland [4] are based on three criteria that must be satisfied in the design of such slabs. These methods specify safe limits for the concrete compressive stress, tensile stress in steel deck and the interfacial bond stress. The design method adopted in Switzerland considers in addition, the tensile stress in concre[e.
Shear-bond capacity of composite slabs made with profiled sheeting
Design methods practised in USA, UK and parts of Europe follow closely the conventional reinforced concrete design. A fully cracked section is assumed when determining the stress in the concrete and steel deck. Bond stress is assumed to be resisted around the periphery of the ribs only. This assumption was verified by Resevsky [5} who showed that the contribution of the valley of the deck to bond strength was negligible. Australian design practice follows closely the method used in the States. However, in the calculation of cross-sectional properties, the stiffness and bending strength of the steel deck are taken into consideration. Hence the values of the calculated stresses in the concrete and the deck become smaller. Bond stress is assumed to develop at the periphery of the ribs only. A refinement is however made by assuming that this bond stress acts at the centre of gravity of the deck section. Swiss practice is to treat the composite slab as uncracked and all bending stresses are assumed to be carried by the concrete alone. Thus the strength contributed by the steel deck is ignored. In other words, the steel deck is treated merely as formwork to support fresh concrete and construction loads. The contribution of the deck profile to the properties of the whole section is very small and hence, bond stress calculated as unit shear stress between the concrete and steel deck is very small and rarely affects the design. Table 2 shows a comparison of present test data with various design methods as discussed above. The permissible load for each slab has been computed in accordance with the limits for bending and bond stresses specified by the various design methods. It can be seen that bond stress governs the design in all cases except for the method employed in Switzerland. The Australian method is more conservative than that employed in the US. Being based purely on concrete tensile stress without giving due consideration to shear-bond failure, the Swiss method has been found to be the most conservative among the methods considered in the
Ong and Mansur
present discussion. However, all the methods have been found to be safe. For comparison the permissible load for each slab has also been computed in accordance with the manufacturer's manual [6, 7] and are recorded in Table 2. It can be seen that the manufacturer's design manual yields shear-bond failure loads closer to those given by the method employed in the US.
Shear-bond parameters and equation Based on Equation (2) two regression plots were obtained within the scope of the present experimental data. A linear regression line together with a l~ne ,representing minus 15% deviation from each regression line are shown in Figures 8 and 9 for slabs without shear transferring devices (Group P) and slabs with end anchorage (Group A) respectively. The reduced regression line of minus 15% was used to account for the fluctuation of test results due to variations in steel deck profile, the size and spacing of the shear transferring devices, the specimen dimensions and the natural scatter resulting from strength variations as recommended by Porter et al. [3]. The shear-bond parameters obtained from Figures 8 and 9 for the slab specimens with 0.75mm thick 'Bondek' profile under third point line loading are: VuS/bd ~/fcu = 0.5 pd/L' vf~:u + 0.016 for slabs without shear transferring devices (Group P) VuS/bd ~
= 0.4 pd/L' ~
+ 0.024
The permissible loads calculated for each slab by using Equations 3 and 4 are shown in Table 2. Compared to the design methods discussed in this paper, the proposed
Method practised in/proposed by
PAl PB1 PB2 PC1 PC2 AA1 AA2 AB1 AB2 AC1
USA, UK and parts of Europe
Australia
Switzerland
Manufacturer
Authors
1.20 1.23 1.42 i 03 1.03 1.56 1.56 1.70 1.61 1.33
2.00 1.49 1.72 1.25 1.25 2.60 2.60 2.07 1.95 1.61
2.86 4.16 4.73 4.60 4.32 2.89 2.67 4.71 4.45 3.88
1.25 1.12 1.30 1.34 1.34 1.63 1.63 1.56 1.47 1.73
1.14 1.16 1.32 1.10 1.06 1.17 1.16 1.27 1.20 109
A: End Anchorage P: Without shear transferring devices
'
(4)
for slabs with end anchorages (Group A)
Table 2 Ratio of experimental shear-bond capacity to that predicted by various methods
Slab designation
(3)
235
Shear-bond capacity of composite slabs made with profiled sheeting
I
I
I
Ong and Mansur
I
Figure 8 Regression plot for slabs without shear transferring device
I
0.05
~0.04 L~..~ o.03 ~" 002
>= 0.01
0
I
I
I
I
i
0.005
0.01
0.015
0.02
0.025
0.03
pd/(L"Tfcu ) (mm/'J'N) 0.05
I
[•
i
I
I
Figure 9 Regression plot for slabs with end anchorage
I
0434
L,~o 0-03 -,~
0.02 0.01
-
0
t
IT" ;;~L I 0.005
i 0.01
I
I 0.015
.pd/¢ 1'
i
L
I
4 L .~
I
I
0.02
0.025
-
" 0.03
) ¢mm/ fN")
equations give good predictions of the shear-bond capacity of the slabs.
rages. An excellent prediction of the shear-bond capacity has been obtained.
SUMMARY AND CONCLUSIONS This study was conducted to determine the shear-bond capacity of concrete slabs reinforced with locally available steel decks (Bondek). Two types of slabs were investigated, one without any shear transferring devices and the other with 16mm studs as end anchorage. All the slabs failed in the shear-bond mode. Slabs without any shear transferring devices are more prone to this type of failure under third point loading. A comparison with the current design methods showed that the strength of all the slabs was underestimated. Two linear regression plots were also obtained using the current experimental data and two reduced aggression lines of minus 15% deviation were used to provide a conservative estimate of the shearbond strength of the slabs with and without end ancho-
ACKNOWLEDGEMENTS
236
The work described in this paper was supported in part by the National University of Singapore under Grant, RP 97/82. The authors gratefully acknowledge the assistance of their students in conducting the tests. John Lysaght (SEA) Pte Ltd has donated the decks required for the study. REFERENCES
1. Schuster, R. M. 'Composite steel-deck concrete floor systems,' Journal of the Structural Division, Proceedings, American Society of Civil Engineers, Vol. t02, No. ST5, May 1976, pp. 899-917. 2. Porter, M. L. and Ekberg, C. E. 'Design recommendations for steel-deck floor slabs,' Journal of
Shear-bond capacity of composite slabs made with profiled sheeting
the Structural Division, Proceedings, American Society of Civil Engineers, Vol. 102, No. ST11, November 1976, pp. 2133-6. 3. Porter, M. L., Ekberg, C. E., Greimann, L. F. and Elleby, H. A. 'Shear-bond analysis of steel-deckreinforced slabs,' Journal of the Structural Division, Proceedings, American Society of Civil Engineers, Vol. 102, No. ST12, December 1976, pp. 2255-68. 4. European Convention for Construction Steelwork Committee 17 'European recommendations for the design of composite floors with profiled steel
Ong and Mansur
sheet', Constrado, Croydon, England, 1977. Resevsky, C. G. 'Composite slab construction theories and practice,' MSc Thesis, University of Sydney, 1970, 58 pp. 6. Lysaght Brownbuilt Industries 'Bondek structural decking technical design manual, BD 14-6,' SfB (23) Gd 2, John Lysaght (SEA) Pte Ltd, November 1979, 22 pp. 7. Lysaght 'Bondek structural decking technical design manual, JLSEA-BD 3,' John Lysaght (SEA) Pte Ltd, October 1983, 28 pp. 5.
237
November 1986
Shear-bond capacity of composite slabs made with profiled sheeting K. C. G. Ong* and M. A. Mansurt
S y n o p s i s An experimental study conducted to determine the shear-bond capacity of concrete slabs reinforced with locally available profiled steel decks is reported in this paper. Ten one-way slab specimens were tested under third point loading. In the light of these test data, the currently available design methods have been reviewed, and an empirical relationship is proposed to estimate the shear-bond capacity of this type of slab with and without end anchorages under third point loading. K e y w o r d s Concrete slabs, shear tests, composite structures, reinforced concrete, shear bond capacity, design criteria, regression analysis, bond stress, ductility, brittle failure, design standards, cracking (fracturing)
INTRODUCTION A floor system for tall buildings created by integrating the structural properties of concrete and profiled steel decks has been gaining increasing popularity throughout the world. This is especially so in developed countries where, among other factors, high development charges and cost of labour have made this type of construction more attractive to developers, architects and engineers in recent years. In this type of construction, the steel deck serves not only as a self-sustaining form supporting the construction and fresh concrete loads but also as the principal reinforcement for the bottom fibres of the slab. However, the successful functioning of this type of slab depends entirely on reliable bond between the concrete and the steel deck at their interface. Without shear connectors, bond is generally insufficient to ensure composite action up to failure [1]. It is thus necessary to employ mechanical means or so-called shear transferring devices to achieve positive interlocking between the two materials. These can take the form of closely spaced indentations or embossments, transverse wires welded to the top of the corrugations, holes punched on the sides of the deck for the formation of concrete shear keys and surface bonding with suitable deck profile. Previous investigations [1,2] have shown that even if shear transferring devices are provided, sudden failure by the destruction of interracial bond still occurs. Known as shear-bond failure, this type of sudden collapse is characterised by the formation of major diagonal cracks in concrete, usually at the loading points, due tO slippage * Lecturer and t Senior Lecturer, Department of Civil Engineering, National Universityof Singapore @ Longman Group UK Ltd 1986 0262-5075/86/08403231/$02.00
of the steel deck relative to the concrete. No rational method has yet been developed to predict the shearbond strength of steel-deck reinforced concrete slabs. In 1976, the American Iron and Steel Institute (AISI) sponsored a research programme at Iowa State University [2] with the principal objective of developing a unified design criteria for steel deck-concrete floor systems, Based on extensive study, Porter et al. [3] have recommended the following equation for use in the design of such slabs under concentrated line loads: VuS/bd = m pd/L' + k' ~
(1)
taking f~ = 0.78 fcu and rearranging Equation 1, we have VuS/bd where
and
= m (pd/L' ~/-f-c~ + k
(2)
Vu is the maximum shear at shear-bond failure in Newtons S is the centre-to-centre spacing of shear transfer devices other than embossments in mm b is the width of the slab in mm d is the effective depth in mm fc is the cylinder compressive strength of concrete in N/mm 2 fcu is the cube strength of concrete in N/mm 2 p is the percentage of steel L' is the shear span in mm k and m are regression constants.
The equation can be corrected [3] to include the effects of dead weight and of the shoring conditions. The values of the constants k and m should be empirically obtained for each type of steel deck profile because of its own shear transfer characteristics. The European Convention for Constructional Steelwork, ECCS [4] carried out an extensive review on the shear-bond strength of such slabs in an attempt to
231
Shear bond capacity of composite slabs made w~thprofiled sheeting
---21 mm ~ I
Ong and Mansur
$22 Gauge fWetded steel deck mesh
'.I -~ ~ - ' ---= I i2 mm 4 1"-32l--~ 5ram
Figure 1 Typical cross-section of test specimens
mmif:r==
610 mm rationalise the methods of design. The results have been published in the form of design recommendations [4]. As the review was based on Swiss and North American works, these recommendations are suitable for the type of decks used in those regions. The most commonly used steel decks in the AsiaPacific region are marketed under the trade names 'Bondek', 'Klip-lok' and 'Spandek'. Among these decks, mechanical interlocking is incorporated only in Bondek by its profile as shown in Figure 1. This was developed by John Lysaght (SEA) Pte Ltd based on the work of Resvesky [51. The other two types are normally used as roofing or as structural form in slab construction, and they serve no structural function after the concrete has hardened. The study reported in this paper was conducted on composite steel-deck reinforced concrete slabs using 'Bondek' profiles with the main objective of determining the shear-bond capacity. Ten one-way slab specimens were tested under third point loading. The major parameters of the study were the span and thickness (reinforcement ratio) of the slab. The results of these tests are presented and discussed in this paper. Based on the present test data the current design methods are reviewed and an 6mpirical relationship is proposed to estimate the shear-bond capacity of this type of slab under third point loading.
of the supporting steel girder in actual construction. The second group P, contained no end anchorage. The second letter, A, B or C indicates the span at which the slabs were tested, namely 2.3m, 2.85m and 3.45m respectively. The number following the letter symbols indicates the specimen number. The details of test specimens can be found in Table 1. The concrete used was designed for a 28 day compressive strength of 30 MPa. The mix ratio by weight of cement, sand and aggregates (20mm maximum size) was 1:1.84:2.76 and the water-cement ratio was 0.53. A layer of welded wire mesh was used as secondary reinforcement (Figure 1). The wire mesh consisted of 5mm diameter wires with a square grid of 200mm. The average yield strength (0.2% proof stress) of the wires was 571 MPa and the modulus of elasticity was 1.97 x 105 MPa. The slabs were cast using wooden side forms with a central prop. The surface of the decks was thoroughly cleaned before placing the fresh concrete. Compaction
s J/--
16mml~ shear studs Concrete slab
j=Lo
.--,- S E
TEST PROGRAMME Test specimens In all, ten 0.61 m wide specimens were tested. They were of three different lengths, namely 2.4m, 3.0m and 3.6m and of two different thicknesses; 100mm and 120mm. Figure 1 shows a typical cross-section of the slab. The steel deck employed is commercialy known as 'Bondek'. It is made up of 0.75mm thick galvanised steeJ plate. The average yield strength and modulus of elasticity for the steel deck were found to be 626 MPa and 2.02 x 10 ~ MPa, respectively. The test specimens were designated by two letters followed by a number. The first letter A or P, indicates the series name according to the type of end anchorage used. For the specimens in A-series, anchorage was provided by three 16mm diameter studs through a 10ram thick flat steel bearing plate at each end of the specimen (Figure 2). The steel plate simulated the flange
232
A ~
Bearing ' plate
or mm
(a) Longitudinal section 25 mm S
I,
_T/
Shear stud
T/ Bearing plate
(b) Section A-A Figure2 Slabs with end anchorage
!
Shear-bond capacity of compos/te slabs made with profiled sheeting
Ong and Mansur
Table1 Slab details and summary of test results
Slab designation PA 1 PB 1 PB2 PC1 PC2 AA1 AA2 AB1 AB2 AC 1
Total depth (ram)
Total length (m)
Span L (m)
Cube compressive strength fo~ N/ram 2
100 120 120 120 120 100 100 120 120 120
2.40 3,00 3.00 3.60 3.60 2.40 2.40 3.00 3.00 3.60
2.30 2.85 2.85 3.45 3,45 2,30 2,30 2.85 2.85 3.45
36.2 45.0 47.9 38.1 44.1 37.7 39.5 48.8 48.0 42.3
Properties of concrete Cylinder splitting Modulus of strength elasticity fsp E~ N/ram 2 kN/mm 2 2.76 3.38 4.41 3,75 3.78 2.33 2.26 4,80 4.37 3.65
29.1 25.0 24.5 23.9 244 29.1 29.1 27.1 26.9 23.9
Total load at shear-bond failure Po (kN) 20.0 25.8 29.8 21.6 21.6 26.0 26.0 35.8 33.8 27.9
A: End anchorage P: Without shear transferring devices
was achieved by using a poker type vibrator. Three 150mm cubes and three 150mm by 300mm cylinders were cast for each slab to determine the compressive strength and modulus of elasticity of the concrete. The slabs were cured under damp hessian at ambient temperature for about 2 weeks and then air-dried in the laboratory before testing. The compressive strengths of the concrete at the time of slab tests are shown in Table 1.
Test setup and instrumentation The specimens were simply supported and tested in a self-balanced loading frame under third point line loading (Figure 3). Strains were measured at the bottom of the steel deck at midspan by means of electrical resistance strain gauges. A series of dial gauges (accuracy of +0.1 ram) was used to measure deflection along the length of the slab. The load was applied in increments varying between 2 and 5 per cent of the estimated ultimate load. At each increment, the dial and strain gauge readings were recorded. When shear-bond failure occurred the specimens were reloaded until the applied load became stable again. This was repeated until final collapse:
DISCUSSION OF TEST RESULTS Behaviour of specimens The load-deflection Curves of typical specimens are shown in Figure 4. The curves were initially linear at the early stages of loading. With the formation of cracks, they deviated slightly from tinearity. Although not initially visible, the formation of cracks was accompanied by crackling which became louder and more frequent with
Figure 3 Test setup
the formation of a major diagonal crack in the concrete at or near one of the loading points. This crack widened with increasing load until failure occurred by a loss of bond between concrete and steel deck, All the specimens failed in this manner. The loads at which shear-bond failure occurred are indicated in Figure 4. It can be seen that this type of failure was accompanied by a sudden drop in the applied load for most of the specimens. For the specimens in Group P, attempts to increase the load resulted in increasing deformations without a significant load recovery. The specimens eventually collapsed due to vertical as well as horizontal separation of the steel deck from concrete (Figures 5 and 6). Porter and Ekberg [2J postulated that the failure of a steel-deck-reinforced concrete slab initiates by irregular development of the flexural and bond stresses between the concrete and steel deck. The bond stress in the cracked zone becomes very high and depends on the
233
Shear-bond capacity of composite slabs made with pro filed sheeting
Ong and Mansur
rate of change of the concrete stresses adjacent to the steel deck. An increase in concrete stress gradient results in an increase in local bond stress. This continues until the bond strength is exceeded and local bond failure occurs. A redistribution of stresses to the adjacent sound section nearer to the supports then takes place. When the loss of bond progressively reaches the free ends of the slabs, slippage between the concrete and steel occurs with eventual collapse. In contrast, Group A specimens continued to sustain a significant amount of additional load after shear-bond 55
!
,
,
,
l
I
50
S~abs without any sheartransferringdevice • Slabs with end anchorage
/.5
~
I
•
Figure 6 Typical end slip
Load at shear- bond foiture
///'~
40
35
/.~'
Figure 7 Tearing-off failure of steel deck
[~" / 5~t'
0
2.30 m span -- -- 2.85 m span
5
10 15 20 25 30 Midsl:~n deflection (mm)
Figure 4 Load-deflection curves
35
failure. Composite action was not completely lost as the shear studs acted as anchors to transfer forces from the concrete to the steel deck. The specimens however deflected considerably before the new load-carrying mechanism had been fully mobilised. Collapse finally occurred due to tearing-off of the steel deck at shear stud connections (Figure 7). It is evident from the preceding discussion that slabs in Group P would have failed catastrophically if the load level was maintained at the shear-bond failure load. On the other hand, slabs with end anchorages would give adequate warning through excessive deflection prior to failure.
Comparison
of
test
results
with
available
design
methods
Figure5 Typical specimen after failure
234
The design methods currently used in the United States of America, Australia and Switzerland [4] are based on three criteria that must be satisfied in the design of such slabs. These methods specify safe limits for the concrete compressive stress, tensile stress in steel deck and the interfacial bond stress. The design method adopted in Switzerland considers in addition, the tensile stress in concre[e.
Shear-bond capacity of composite slabs made with profiled sheeting
Design methods practised in USA, UK and parts of Europe follow closely the conventional reinforced concrete design. A fully cracked section is assumed when determining the stress in the concrete and steel deck. Bond stress is assumed to be resisted around the periphery of the ribs only. This assumption was verified by Resevsky [5} who showed that the contribution of the valley of the deck to bond strength was negligible. Australian design practice follows closely the method used in the States. However, in the calculation of cross-sectional properties, the stiffness and bending strength of the steel deck are taken into consideration. Hence the values of the calculated stresses in the concrete and the deck become smaller. Bond stress is assumed to develop at the periphery of the ribs only. A refinement is however made by assuming that this bond stress acts at the centre of gravity of the deck section. Swiss practice is to treat the composite slab as uncracked and all bending stresses are assumed to be carried by the concrete alone. Thus the strength contributed by the steel deck is ignored. In other words, the steel deck is treated merely as formwork to support fresh concrete and construction loads. The contribution of the deck profile to the properties of the whole section is very small and hence, bond stress calculated as unit shear stress between the concrete and steel deck is very small and rarely affects the design. Table 2 shows a comparison of present test data with various design methods as discussed above. The permissible load for each slab has been computed in accordance with the limits for bending and bond stresses specified by the various design methods. It can be seen that bond stress governs the design in all cases except for the method employed in Switzerland. The Australian method is more conservative than that employed in the US. Being based purely on concrete tensile stress without giving due consideration to shear-bond failure, the Swiss method has been found to be the most conservative among the methods considered in the
Ong and Mansur
present discussion. However, all the methods have been found to be safe. For comparison the permissible load for each slab has also been computed in accordance with the manufacturer's manual [6, 7] and are recorded in Table 2. It can be seen that the manufacturer's design manual yields shear-bond failure loads closer to those given by the method employed in the US.
Shear-bond parameters and equation Based on Equation (2) two regression plots were obtained within the scope of the present experimental data. A linear regression line together with a l~ne ,representing minus 15% deviation from each regression line are shown in Figures 8 and 9 for slabs without shear transferring devices (Group P) and slabs with end anchorage (Group A) respectively. The reduced regression line of minus 15% was used to account for the fluctuation of test results due to variations in steel deck profile, the size and spacing of the shear transferring devices, the specimen dimensions and the natural scatter resulting from strength variations as recommended by Porter et al. [3]. The shear-bond parameters obtained from Figures 8 and 9 for the slab specimens with 0.75mm thick 'Bondek' profile under third point line loading are: VuS/bd ~/fcu = 0.5 pd/L' vf~:u + 0.016 for slabs without shear transferring devices (Group P) VuS/bd ~
= 0.4 pd/L' ~
+ 0.024
The permissible loads calculated for each slab by using Equations 3 and 4 are shown in Table 2. Compared to the design methods discussed in this paper, the proposed
Method practised in/proposed by
PAl PB1 PB2 PC1 PC2 AA1 AA2 AB1 AB2 AC1
USA, UK and parts of Europe
Australia
Switzerland
Manufacturer
Authors
1.20 1.23 1.42 i 03 1.03 1.56 1.56 1.70 1.61 1.33
2.00 1.49 1.72 1.25 1.25 2.60 2.60 2.07 1.95 1.61
2.86 4.16 4.73 4.60 4.32 2.89 2.67 4.71 4.45 3.88
1.25 1.12 1.30 1.34 1.34 1.63 1.63 1.56 1.47 1.73
1.14 1.16 1.32 1.10 1.06 1.17 1.16 1.27 1.20 109
A: End Anchorage P: Without shear transferring devices
'
(4)
for slabs with end anchorages (Group A)
Table 2 Ratio of experimental shear-bond capacity to that predicted by various methods
Slab designation
(3)
235
Shear-bond capacity of composite slabs made with profiled sheeting
I
I
I
Ong and Mansur
I
Figure 8 Regression plot for slabs without shear transferring device
I
0.05
~0.04 L~..~ o.03 ~" 002
>= 0.01
0
I
I
I
I
i
0.005
0.01
0.015
0.02
0.025
0.03
pd/(L"Tfcu ) (mm/'J'N) 0.05
I
[•
i
I
I
Figure 9 Regression plot for slabs with end anchorage
I
0434
L,~o 0-03 -,~
0.02 0.01
-
0
t
IT" ;;~L I 0.005
i 0.01
I
I 0.015
.pd/¢ 1'
i
L
I
4 L .~
I
I
0.02
0.025
-
" 0.03
) ¢mm/ fN")
equations give good predictions of the shear-bond capacity of the slabs.
rages. An excellent prediction of the shear-bond capacity has been obtained.
SUMMARY AND CONCLUSIONS This study was conducted to determine the shear-bond capacity of concrete slabs reinforced with locally available steel decks (Bondek). Two types of slabs were investigated, one without any shear transferring devices and the other with 16mm studs as end anchorage. All the slabs failed in the shear-bond mode. Slabs without any shear transferring devices are more prone to this type of failure under third point loading. A comparison with the current design methods showed that the strength of all the slabs was underestimated. Two linear regression plots were also obtained using the current experimental data and two reduced aggression lines of minus 15% deviation were used to provide a conservative estimate of the shearbond strength of the slabs with and without end ancho-
ACKNOWLEDGEMENTS
236
The work described in this paper was supported in part by the National University of Singapore under Grant, RP 97/82. The authors gratefully acknowledge the assistance of their students in conducting the tests. John Lysaght (SEA) Pte Ltd has donated the decks required for the study. REFERENCES
1. Schuster, R. M. 'Composite steel-deck concrete floor systems,' Journal of the Structural Division, Proceedings, American Society of Civil Engineers, Vol. t02, No. ST5, May 1976, pp. 899-917. 2. Porter, M. L. and Ekberg, C. E. 'Design recommendations for steel-deck floor slabs,' Journal of
Shear-bond capacity of composite slabs made with profiled sheeting
the Structural Division, Proceedings, American Society of Civil Engineers, Vol. 102, No. ST11, November 1976, pp. 2133-6. 3. Porter, M. L., Ekberg, C. E., Greimann, L. F. and Elleby, H. A. 'Shear-bond analysis of steel-deckreinforced slabs,' Journal of the Structural Division, Proceedings, American Society of Civil Engineers, Vol. 102, No. ST12, December 1976, pp. 2255-68. 4. European Convention for Construction Steelwork Committee 17 'European recommendations for the design of composite floors with profiled steel
Ong and Mansur
sheet', Constrado, Croydon, England, 1977. Resevsky, C. G. 'Composite slab construction theories and practice,' MSc Thesis, University of Sydney, 1970, 58 pp. 6. Lysaght Brownbuilt Industries 'Bondek structural decking technical design manual, BD 14-6,' SfB (23) Gd 2, John Lysaght (SEA) Pte Ltd, November 1979, 22 pp. 7. Lysaght 'Bondek structural decking technical design manual, JLSEA-BD 3,' John Lysaght (SEA) Pte Ltd, October 1983, 28 pp. 5.
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