- Email: [email protected]
Experimental studies on composite deck slabs to determine the shear-bond characteristic (m–k) values of the embossed profiled sheet
Journal of Constructional Steel Research 63 (2007) 791–803 www.elsevier.com/locate/jcsr
Experimental studies on composite deck slabs to determine the shear-bond characteristic (m–k) values of the embossed profiled sheet V. Marimuthu a,∗ , S. Seetharaman a , S. Arul Jayachandran a , A. Chellappan a , T.K. Bandyopadhyay b , D. Dutta b a Structural Engineering Research Centre, CSIR Campus, Chennai - 600 113, India b Institute for Steel Development And Growth (INSDAG), Ispat Niketan, 52/1A, Ballygunge Circular Road, Kolkata - 700 019, India
Received 15 March 2006; accepted 19 July 2006
Abstract Composite deck slab floors are gaining wide acceptance in many countries as they lend themselves to faster, lighter and economical construction in buildings. The cold formed profile sheeting which is an integral part of the deck slab is provided with embossments to improve their shear bond characteristics. However, the shear behaviour of composite deck slab is a complex phenomenon and therefore experimental methods are often resorted to establish their shear strength under flexural loads. An experimental study has been carried out to investigate primarily the shear bond behaviour of the embossed composite deck slab under simulated imposed loads and to evaluate the m–k values. Totally 18 composite slab specimens were cast using M20 grade concrete. The 18 numbers of specimens were split into six sets of three specimens each in which three sets were tested for shorter shear span loading and the other three sets for longer shear span loading. The specimens were tested as per the general provisions in Eurocode 4 [Eurocode 4. Design of composite steel and concrete structures. Part 1.1. General rules and rules for buildings]. This paper presents details of the experimental investigations conducted on the composite deck slabs and the evaluation of m–k values for the embossed profiled sheet. c 2006 Elsevier Ltd. All rights reserved.
Keywords: Profiled sheet; Embossments; Composite deck slab; Shear bond failure; m–k value
1. Introduction Cold formed profiled steel sheets with embossments are widely used for composite floor decking wherein they remain permanently in place as an integral part of the floor system. They perform two functions: they act as the formwork while concreting and in the composite slab, they act as the tension reinforcement. The only additional steel that needs to be provided is for taking care of shrinkage and temperature. In the case of continuous slabs, reinforcing steel is required to resist the negative bending moment at the supports. This type of flooring results in faster construction, lighter floors and rational use of construction materials. They also provide certain other advantages such as easy handling, a good ceiling surface and convenient ducting for routing utility services. Finally, the ∗ Corresponding author. Tel.: +91 044 22549142; fax: +91 044 2254 1508.
E-mail addresses: [email protected], mari [email protected] (V. Marimuthu). c 2006 Elsevier Ltd. All rights reserved. 0143-974X/$ - see front matter doi:10.1016/j.jcsr.2006.07.009
thin sheeting is extremely light and hence can be transported conveniently, and handled and placed easily by the construction personnel. Some of the disadvantages pointed out for the system are inadequate fire rating, the need for proper bonding between the steel deck and the concrete and also the protection needed against damage from high local loads. Even though the steel deck is galvanized, it is advisable to apply anticorrosive paints on the exposed side of the sheet. Effects of ponding and edge deformation also need to be taken care of during design. The term ‘composite steel deck floor slab’ means that there is a provision in the system for bonding between the steel deck and the concrete by some mechanical means. In other words, for the steel deck and concrete to act compositely, a mechanical interlocking is needed. This is provided essentially by various ‘shear transferring devices’ such as rolled embossments, transverse wires, holes etc. Examples of composite steel deck floor slab systems are illustrated in Fig. 1. One of the efficient ways of achieving the interlocking between the steel deck and concrete is by means of embossments on the profiled
792
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
List of symbols As b d Vs Ls Lo
Cross sectional area of the profiled sheet (mm2 ) Width of the profiled sheet (mm) Average depth of the composite deck slab (mm) Shear force (N) Shear span (mm) Length of overhang of the composite slab from centre line of support (mm)
ρ
As bd
f c0 , f cm Cube compressive strength of the concrete (N/mm2 ) m slope of the ultimate shear bond regression line k Intercept of the ultimate shear bond regression line Φ Capacity reduction factor (0.8) s Parameter denoting supporting condition during casting. Nc , Nc f Compressive force in concrete (N) Np Tensile force in sheeting (N)
steel sheeting. The deck profile must provide the resistance to vertical separation and horizontal slippage between the contact surface of the steel and the concrete. Additional composite action may be achieved by attaching studs or similar shear devices. The shear bond characteristic of the embossed sheeting is rated by two empirical parameters ‘m’ and ‘k’, where ‘m’ represents the mechanical interlocking between steel and concrete and ‘k’ stands for friction between them. This paper deals with the experimental evaluation of m–k values for composite deck slabs using cold formed profiled
sheets with rectangular dishing type embossments. Totally 18 composite slab specimens were cast using M 20 grade concrete. Using the values of ‘m’ and ‘k’ as determined by parametric tests, ultimate load carrying capacity of the composite deck could be calculated. The longitudinal shear strength of the composite slab calculated using m–k method is verified with the results obtained by partial shear connection method in Eurocode 4 [1, Annex E]. 2. Load carrying mechanism of composite profiled sheet deck floors In a composite steel floor deck, the hardened concrete slab, acting compositely with the profiled steel decks, spans the supporting beams and carries the imposed live loads. The composite action depends upon adequate transfer of horizontal shear forces between the concrete slab and the steel deck to enable the deck to act as the tensile reinforcement. In addition to horizontal shearing forces, the bending action also leads to vertical separation between the steel and the concrete. The profiled sheet, therefore, has to be designed to resist vertical separation, in addition to transferring the horizontal shears. Resistance to vertical separation is achieved by suitable shape in trapezoidal profile and also by the embossments. There are three distinct phases in the structural action of a composite deck system [3]. In the first phase i.e. during the construction phase, the steel sheeting must rigidly support the wet concrete during casting. In the composite slab action phase, the composite steel concrete slab should support the imposed loads on the slab and in the composite beam action phase, the steel beams, which act compositely with concrete through the stud shear connectors, must support the imposed loads in the transverse direction. This paper deals with the study
Fig. 1. Composite deck slabs using different types of profiled sheets.
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
of composite slab action phase, wherein the behaviour of the composite action of the steel sheet and the overlying concrete is focused. The three primary failure modes important for design of a composite deck slab are: (1) flexure, (2) shear at support and (3) shear bond mode. Failure of the slab is said to be ductile if the failure load exceeds the load causing first recorded end slip by more than 10%. The failure load is taken as the load at midspan deflection of L/50 unless failure has already taken place. One of the principal modes of failure of steel deck slabs is by the shear bond. The shear bond mode of failure is characterized by the formation of diagonal tension crack in the concrete at or near the load points, followed by a loss of bond between the steel deck and the concrete. There is a slippage between the steel and concrete causing a loss of composite action in the shear span region, which lies between the support reactions and the concentrated load. Slippage usually occurs when the load reaches its ultimate value and this is followed by a significant drop in loading. 3. Review of literature Crisinel and Marimon [2] have presented a new design approach for the prediction of composite slab behaviour. This new approach combines results from standard materials tests and small-scale tests with a simple calculation model (referred to here as the “New Simplified Method”) to obtain the moment–curvature relationship at the critical cross-section of a composite slab. The New Simplified Method facilitates the calculation of the load-carrying capacity of composite slabs by considering three phases of the M–θ behaviour observed in composite slab critical cross-sections. It requires knowledge of the geometric dimensions of the slab, the material properties (steel and concrete) and the characteristic behaviour of the steel–concrete connection as determined based on tests on small-scale specimen. In order to study the shear-bond action in composite slabs, Chen [3] tested seven simply supported one-span composite slabs and two continuous composite slabs using different end restraints in the simply supported slabs. The slabs with end anchorage of steel shear connectors were found to bear higher shear-bond strength than that of slabs without end anchorage. To enable an effective end anchorage, however, it is the shearbond slip rather than the strength of anchored studs that governs the contribution of the end restraints to the shear-bond resistance in composite slabs. Burnet and Oehlers [4] presented a new form of push-test that simulates the bond characteristics more accurately and which is used in 33 tests to determine the main parameters that affect both the chemical bond and mechanical bond strengths of dovetailed and trapezoidal rib shear connectors. The effects of the geometry of the cross-section, embossments, sheet thickness and surface treatment on the bond strengths are presented in a form that can be used as guidelines in the development of new forms of profiled sheets for slabs, beams and walls.
793
Makelainen and Sun [5] studied the shear-connection behaviour of composite slabs with a particular profiled steel sheeting having a depth of 153 mm. Twenty-seven push-out test specimens of different shapes, sizes, locations of embossments and different steel sheeting thicknesses are carried out in two test series. The embossments are first made on the slant faces based on standard design norms. Thereafter, the embossed sheets are profiled as per the requirement suiting to the standards of the manufacturer. It is found that the shear-connection behaviour of composite slabs is significantly affected by the depth of embossments. For the profiled steel sheeting with indented embossments, the reduction of Young’s modulus caused by the penetrated embossments is an important factor that affects the determination of the depth and width of the embossments. Finally, a new type of profiled steel sheeting, which can offer longitudinal shear strength in composite slabs up to 0.6 N/mm2 , is proposed for further research. Tenhovuori and Leskela [6] studied the behaviour of composite slabs with profiled steel sheeting as affected by the bond failure in the longitudinal shear connection. The effect of various important parameters is considered and the critical factors are reviewed on the basis of numerical data obtained from non-linear calculation by the method of finite elements. A thorough study is carried out to compare the present methods of analysis for the bond failures in the ENV Eurocode 4 [1] Part l-l, and it is shown that they can be improved and simplified and finally unified so as to get a clearly comprehensible system describing when the bond failure in a composite slab is possible in the design and what is to be done for it. Calixto et al. [7] carried out an experimental investigation on the behaviour and strength of full-scale one-way single span composite slabs with ribbed decking. Several aspects were studied, including different steel deck thickness, total slab height, as well as shear span length. The effect of connectors (stud bolt type) on the end anchorage was also investigated. Normal procedures for batching and mixing of the concrete were used. Throughout the monotonic loading tests, midspan deflections, end slips and strains in steel decking were measured. The test results indicate clearly the better performance of the composite slabs built with stud bolt connectors. In this study the slabs fabricated with plain sheeting and stud bolts attained in all cases a higher ultimate load when compared to the respective specimens built with ribbed decking only. The floors constructed with ribbed decking and stud bolts showed a different behaviour characterized by no drop in the load during the entire monotonic loading procedure. In all cases the failure mode was by shear bond even in the slabs fabricated with end anchorage and ribbed sheeting. The experimental results are also compared with the partial interaction design method specified in Eurocode 4 [1]. The current design equations do not separate explicitly the resistance of the mechanical interlocking from the friction at the concrete-decking interface over the supports. Depending on the position and shape of the embossments on the ribbed decking (AXE type II decks for instance), the contribution of each resistance mechanism plays a different role. Therefore a procedure which explicitly takes into consideration the
794
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Fig. 2. Shape, size and frequency of embossments.
effects of the mechanical interlocking and friction separately is presented. The proposed method is compared with the current test results and those obtained in other investigations. The comparisons show good correlation. Evans and Wright [8] and Wright et al. [9] have carried out more than 200 tests on composite deck slab elements and compared the results with the available design methods. They have studied the aspects of construction phase, composite slab action phase and the composite beam action phase in detail. The studies have shown that the variation in concrete strength has little effect on the ultimate load capacity. The crucial parameter that has significant effect on the ultimate strength is the height of the embossment. They have concluded that though the present design procedures are safe, they are very conservative in certain cases. They have recommended that the system as a whole, namely, slab span, beam span and stud connectors, should be considered for obtaining maximum economy in design. Porter and Ekberg [10] have carried out a large number of experimental studies on cold formed steel deck floor slabs. The work primarily involved one way full scale slab elements and tested up to failure, emphasizing the ultimate strength design concept. Porter et al. [11] have further conducted experimental studies on the shear bond failure characteristics of one-way slab elements and reported several observations on the significant parameters influencing the behaviour. They have also reported √ a linear regression relationship between Vu s/bd f c0 and √ 0 ρd/L s f c to determine the slope and intercept constants needed for design. Design procedures for the design of composite steel deck floor slabs based on the ultimate strength concepts have been recommended by Porter and Ekberg [12]. The capacity is based on the shear bond strength of the deck slab. The design equations for the shear bond capacity are derived from the data collected from a series of performance tests on the slabs and establishing the linear regression relationship as mentioned above by Porter et al. [11]. A separate regression is recommended for each deck profile, each gauge thickness of the sheeting, steel surface coating and concrete strength. In the construction phase the sheeting is designed for the loads due to the wet concrete and its self weight. The review of literature shows that analysis of the composite deck slab behaviour is complex. The extent of shear bond achieved depends upon many parameters, like the height, shape,
orientation and frequency of the embossment pattern and the geometry and flexibility of the profiled sheet itself. Currently an accurate determination of strength is possible only by performance testing. Performance tests need to be carried out as each steel deck profile has its own unique shear transferring mechanism. The purpose of the tests is to provide data for the ultimate strength design equations. Particularly, a series of tests is needed for getting ultimate experimental shears for a linear regression analysis of the parameters that affect the shear bond capacity. 4. Experimental studies on composite steel deck floor slabs The specimens were split into six sets of three specimens each in which three sets were tested for shorter shear span loading and the other three sets for longer shear span loading. The shape, size and frequency of the embossment are given in Fig. 2. In the shorter shear span loading, shear spans of 320 mm, 350 mm and 380 mm were chosen, while in the longer shear span loading, 850 mm, 950 mm and 1150 mm were adopted. For each set of three specimens, one specimen was tested to failure under monotonic loading, test duration being not less than one hour; the other two specimens were tested for cyclic loading for 5000 cycles each for duration of three hours, followed by a static test. The details of the profiled sheet and the moment carrying capacity of the composite slab worked out as per Eurocode 4 [1] are given in Appendix A. 4.1. Preparation of the composite slab specimen 4.1.1. Casting of slab The composite slab was cast with the profiled sheet as the base. The sheet was thoroughly cleaned before concreting. The casting was carried out in a fully supported condition. Mild steel reinforcing bar meshes (using 6 mm diameter bars) of the required size with the spacing of bars at 250 mm c/c in both directions were prepared. These meshes were placed 25 mm from the top surface of the profiled sheet (Fig. 3). The concrete mix of M 20 (28 day cube compressive strength of 20 MPa) grade designed as per the relevant Indian Standard [13] code was chosen for concreting. The coarse aggregate size used in the concrete was 20 mm down. The slabs were cast and cured for 14 days. Figs. 3 and 4 show the view of embossed sheet with the steel bar mesh and detailing of the composite deck
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Fig. 3. Embossed sheet with the 6 mm diameter reinforcement mesh.
795
Roller and hinge supports were specially fabricated for the study. The schematic view of the roller and hinge supports is shown in Fig. 6. The photographs of the hinged and roller supports are shown in Fig. 7. Fig. 8 shows the complete experimental set up. The midspan deflection and the end slip in the concrete at the hinged support were measured using LVDTs (Fig. 9). The LVDTs were connected to a computer which automatically stored the data for the given interval of time. The slip of the steel sheet and the concrete at the roller support end and steel sheet at the hinged support end were measured using electronic dial gauges (Fig. 10). The readings were noted down at specific load intervals up to the maximum load. The load was applied using a computer-controlled servohydraulic 25 ton MTS actuator under displacement control. The loading history was recorded automatically in the computer for the given interval of time. The load was applied as two line loads distributed across the width of the slab by transferring the actuator load through a distribution beam section (ISMB 300, I beam of depth 300 mm) to two smaller sections (ISMB 150, I beam of depth 150 mm) placed across the width of the slab. 4.3. Shear span
Fig. 4. Detailing of the composite deck slab.
slab. After 28 days, the slabs were transferred from the casting yard to the testing laboratory using proper supports so as not to transfer any flexural load to the slab. 4.2. Experimental set up The schematic view of the experimental setup is shown in Fig. 5.
The shear span is defined as the distance between the centres of support at either end to the point of application of the line load in the slab. Two schemes of loading were chosen, namely, one with shorter shear spans and the other with longer shear spans forming the two ends of the shear span zone. The tests were conducted by varying the shear span with three sets of shorter shear span loadings and three sets of longer shear span loadings.
Fig. 5. Schematic view of the experimental set-up.
Fig. 6. Schematic view of the supporting arrangements.
796
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Fig. 7. Actual view of roller and hinged support. Fig. 10. Dial gauges to measure the end slip.
Fig. 11. View of a loading point in a shorter shear span.
Fig. 8. View of test in progress.
thickness of the composite slab. Hence shorter shear spans of 320 mm, 350 mm and 380 mm were chosen (>315 mm, three times the slab thickness). Fig. 11 shows shear span of 380 mm. For each shear span, three numbers of composite slab specimens were tested, one monotonically till failure and the other two by initial cyclic loading for 5000 cycles and then loaded to failure by monotonic loading. 4.5. Longer shear span The criterion for the longer shear span is that the shear span should be as long as possible while still providing failure in longitudinal shear. And hence for longer shear spans, values of 850 mm, 950 mm and 1150 mm were chosen. Here too, for each shear span, one monotonic test, and two cyclic tests followed by monotonic test to failure were carried out. Fig. 12 shows the details of the set up for longer shear span loading.
Fig. 9. LVDT at the mid span of the slab.
4.4. Shorter shear span Three sets of shorter shear spans were chosen in such a way that the shear span was greater than three times the total
4.6. Static test The specimen was placed over the supporting roller-hinge arrangements and the shear span and loading points were marked. The load was applied incrementally by displacement control. The rate of loading was adjusted in such a way that
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
797
(a) Shear span L s = 320 mm. Fig. 12. View of longer span loading points.
(b) Shear span L s = 350 mm.
Fig. 13. Sequence of cyclic loading.
failure did not occur in less than one hour. The rate of loading adopted for static test was 0.1 mm/s. For data acquisition in the computer, the time interval was set as 5 s. The server computer automatically records the mid span deflection and the slip in the concrete at the hinged end at every 5 s. A maximum mid span deflection limit was set to shut down the actuator to avoid sudden collapse of the specimen.
(c) Shear span L s = 380 mm. Fig. 14. Load vs central deflection for shorter shear span specimens.
5. Results and discussion 4.7. Cyclic test 5.1. Static test Two specimens under each shear span were subjected to preliminary cyclic loading. This preliminary cycling ensures that any kind of chemical adhesion formed between steel and concrete is removed and the static load that is later applied would provide the true indication of the mechanical bond formed by the embossment. For cyclic loading, the slab was subjected to 5000 cycles of loading applied in a time span of three hours. Fig. 13 shows the loading pattern for cyclic loading. The slabs were subjected to cyclic load ranging from 0.5Wq to 1.5Wq, where Wq is the anticipated value of the characteristic load which will be acting on the slab. For the present study, Wq was assumed as 3 kN/m2 (as per IS-875 (Part 2)) [14], the uniformly distributed load value recommended for commercial buildings. This uniformly distributed load was converted in to a concentrated total central load and this load was applied as per specification. For the present case, the load range applied was 3.69–11.07 kN. This load range was applied with the frequency of 0.5 Hz.
5.1.1. Shorter shear span specimens 5.1.1.1. Load deflection behaviour. Two stages of load deflection behaviour were observed in the case of shorter shear span specimens. Fig. 14(a)–(c) show the load–deflection behaviour for shorter shear span specimens. At first, the shear cracks formed near the loading points and a load drop was observed (Region A–B in Fig. 14). Secondly, there was a load pick-up and subsequent flexural failure of the specimen (Region B–C). The rate of deflection was high in the second stage of behaviour. Fig. 15 shows typical crack formation in the shorter shear span specimens. Table 1 shows the capacity and behaviour characteristics of the slab for shorter shear span loading. 5.1.1.2. Slip behaviour of slabs. The slip was observed from the early stage of loading. In 320 mm shear span slab, the slip in the initial loading was very minimal. The zigzag portion in the
798
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
(a) 320 mm shear span.
Fig. 15. Crack formation in shorter shear span loading. Table 1 Details of shorter shear span loading and its behaviour No
Shear span L s (mm)
Failure load (kN)
Behaviour
1
320
55.625
First stage: Shear cracks were formed near the loading point and sudden drop in the capacity. Region A–B in Fig. 14
2
350
52.191
Second stage: Carried additional load by reinforcement mesh provided at the mid-depth of the concrete. Also flexural cracks were formed in between the loading points. Region B–C in Fig. 14
3
380
47.340
Slip: Slip was observed from the early stage of loading and the rate of slip was higher after first stage.
curves depicts gradual debonding of the slab. In all shorter span loadings, the slip up to the first crack appearance is gradual and then the rate of slip increases. After the first crack appears, the slip was in reverse direction (see Fig. 16(a) and (b)). This shows that the bond between the profiled sheeting and the concrete slab is removed and both the sheeting and concrete slab acts independently. Fig. 17 shows the differential movement of the concrete slab and profiled sheet.
(b) 350 mm shear span.
(c) 380 mm shear span. Fig. 16. Load slip curve for shorter shear span specimens.
5.1.2. Longer shear span specimens 5.1.2.1. Load deflection behaviour. As in the case of shorter shear span loading, visible cracks were formed in between the loading points and drop in load carrying capacity. A load pickup with the aid of nominal reinforcement mesh provided at the mid-depth of the concrete was observed and the flexure cracks widened with a higher rate of central deflection. In Fig. 18(a)–(c), the point A denotes when visible flexural cracks start forming. The portion A–B shows the drop of the load and the region B–C shows regaining of load to some extent. Table 2 shows the capacity and behaviour of longer shear span specimens. Fig. 19 shows the crack patterns formed in a longer shear span loaded specimen.
Fig. 17. End slip in shorter shear span loading.
5.1.2.2. Slip behaviour of slabs. In all the longer shear span specimens, slip was more compared to the shorter shear span specimens. Here also the trends of the load-mid span
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
(a) Shear span L s = 850 mm.
799
Fig. 19. Crack formation in longer shear span specimens.
(b) Shear span L s = 950 mm.
(a) 850 mm shear span.
(c) Shear span L s = 1150 mm. (b) 950 mm shear span. Fig. 18. Load deflection curve for longer shear span specimens.
Table 2 Details of longer shear span loading and its behaviour No
Shear span L s (mm)
Failure load (kN)
Behaviour
1
850
22.612
Flexure cracks were formed in between the loading points accompanied by a sudden drop in the capacity.
2
950
26.920
Additional load was resisted by nominal reinforcement mesh provided at the mid-depth of the concrete.
3
1150
16.391
Slip: Rate of slip is comparatively higher.
deflection and load-end slip curves are same as those for the short shear span loading. After the first stage, the rate of slip is more (Fig. 20). As in the case of shorter shear span specimens, the zigzag pattern of the curve shows the differential movements of the slab and the profiled sheet. In this case, the reverse slip is not observed due to the
(c) 1150 mm shear span. Fig. 20. Load end slip behaviour for longer shear span specimens.
longer shear span. As the shear span is larger, the load position moves towards the midspan and the load will try to merge the slab with the profiled sheet. The load carrying capacity of the composite slab increased with the help of the profiled sheet. Fig. 20 shows the load–slip behaviour of the slab. Tables 1 and 2 present the capacities and behaviour of the specimens in static test for shorter shear span and longer shear span loading respectively.
800
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Table 3 Parameters to plot m–k curve No
Shear span, L s (mm)
Load, P (kN)
Shear, Vu (kN)
Vu /bd (N/mm2 )
As /bL s
1 2 3 4 5 6
320 350 380 850 950 1150
55.625 52.191 47.340 22.612 26.920 16.391
22.250 20.876 18.936 9.045 10.768 6.556
0.3501 0.3285 0.2979 0.1423 0.1694 0.1031
0.0036 0.0033 0.0030 0.0013 0.0012 0.0010
5.2. Cyclic test Two specimens from each set were subjected to preliminary cyclic loading. All the shorter shear span specimens withstood 5000 cycles of loading. After three hours of cyclic loading, the specimens were subjected to static load in such a way that the specimens did not fail in less than one hour time. The behaviour and the capacity were more or less the same as obtained in the case of the static loading. In the case of longer shear span specimens, especially the specimen with 1150 mm shear span could not withstand cyclic loading, because its ultimate static load carrying capacity itself was only 8.20 kN. Hence, it failed before reaching the upper limit of cyclic loading. The other two shear span specimens, namely, 850 mm and 950 mm withstood 5000 cycles of loading for three hours and behaved in the same way in the subsequent static tests as in the case of pure static loading.
(2)
loaded) is calculated for each set of specimen. For calculating Vu , a capacity reduction factor, Φ = 0.8 is applied to the average failure load. Table 3 shows the calculation of necessary parameters for plotting the curve. A further reduction of 10% is applied to obtain the reduced regression line based on which the values of the regression constants m and k are computed. The values are compared with those for other profiled sheets with chevron type embossments @ 0◦ and 90◦ configuration reported in literature (Table 4). It was inferred that each profiled sheet has its own unique properties. As specified in Eurocode 4 [1], the m and k values may be determined as shown in Fig. 21. Using the m and k values obtained, the longitudinal shear strength of the composite slab is found out and compared with the τv method. This method is an alternate method to m–k method to calculate the longitudinal shear resistance of the composite slab. This method is given in Annex E of Eurocode 4 [1]. The comparison is given in Table 5 and the detailed calculations are given in Appendix B. It was found that the values obtained by τv method were slightly higher compared to the values obtained by the m–k method.
(3)
6. Conclusions
5.3. Evaluation of m–k value The main objective of the present testing programme is to determine the m–k values which define the shear transferring capacity of the profiled sheet. The recommended design equation for shear bond capacity of composite deck slabs is given by Porter et al. [12]. p bd mρd 0 Vu = + k fc (1) s Ls where s is the parameter depending on the type of shoring during the casting of the composite deck slabs. Eq. (1) may be written as follows p Vu s ρd =m + k f c0 bd Ls p Vu As =m + k f c0 bd bL s
where ρ =
As bd
Fig. 21. m–k curve.
which is in the form of an equation for a straight line y = mx + c. As the casting of the slab has been carried out in a continuously supported condition, the weight of concrete and other dead loads were added to the applied load [2]. After completing all the static and cyclic tests, the total load at failure is calculated by adding the values of self-weight of the slab and weight of the distribution beams to the applied load at failure for each specimen. The average value of the total load at failure (average of one statically loaded and two cyclically
From the experimental investigations conducted on 18 specimens with varying shear spans, the following conclusions are derived: • The behaviour of the embossed profiled composite steel deck slab depends mainly on the shear span; • For the shorter shear spans, strength of the slab is governed by shear bond failure; • If the shear span is large enough but not greater than 1.25 m, the behaviour of the slab is governed by flexural failure;
801
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803 Table 4 Comparison of m–k values with other profile sheets Author
Type of profile
Embossment type
m
k
H.D. Wright et al. (1987)
Trapezoidal
Chevron embossements @90◦
107.527
0.0401
S. Chen (2003)
Trapezoidal
Chevron embossements @0◦
84.665
0.0221
Present work
Trapezoidal
Rectangular dishing type of embossments
87.956
0.0322
Table 5 Comparison of longitudinal shear strength (τu ) Shear span, L s (mm)
Longitudinal shear strength, τu (N/mm2 ) m–k method τv method
320 350 380 850 950 1150
0.281 0.285 0.241 0.122 0.112 0.097
0.318 0.303 0.284 0.156 0.167 0.118
during experimental investigations. They express their grateful thanks to Institute for Steel Development And Growth (INSDAG), Kolkata, for sponsoring the project. The paper is published with the kind permission of Director, Structural Engineering Research Centre, Chennai. Appendix A A.1. Properties of the embossed profiled sheet (approximate) The following are the properties of the embossed sheet:
• In all the specimens, the slip is observed from an early stage of loading. This inadequacy may be improved by varying the embossment details such as depth, width and position of embossment. This may also be improved by increasing the depth of concrete portion and providing stud connectors at the ends. Their efficacy can be verified by carrying out experimental investigations further; • The cyclic loading carried out as per the provisions in Eurocode [1] does not affect the load carrying capacity of the slabs; • The linear regression constants ‘m’ and ‘k’ for rectangular dishing type embossments are found to be: m = 87.956 k = 0.03 • As the m and k value differs for each profiled sheet, experimental verifications are necessary for each type of profiled sheet. • The longitudinal shear carrying capacities calculated by m–k method and partial shear connection method differ by about 26% in the average.
Length of the sheet = 3000 mm Width of the sheet, b = 820 mm Spacing of embossments = 65 mm Width of embossments = 21 mm Shape of embossments = Rectangular dishing type Length of embossments = 25 mm Thickness of the sheet = 0.8 mm (20 gauge) Area of cross section of the sheet, A p = 9.5382 cm2 Weight of the sheet = 6.91 kg/m Modulus of section, Z e = 16.79 cm3 Plastic section modulus, Z p = 17.350 cm3 Yield stress of the sheet, f y = 250 N/mm2 Effective area of sheet, Aeff = 4.20 cm2 Moment of inertia, I x x = 28.8 cm4 . Fig. A.1 shows the cross sectional view of the embossed sheet and Fig. A.2 shows the view of embossments provided in the sheet: Flexural resistance of the composite slab = 14.52 kN m.
Acknowledgements The first four authors thank Dr N. Lakshmanan, Director, and Dr C.V. Vaidyanathan, Advisor Management, Structural Engineering Research Centre, Chennai, for their kind support
Fig. A.1. Cross section of the embossed sheet.
802
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Substituting the above relations in Eq. (B.2), value of Nc can be calculated as Nc −(h t − e p ) ± =
r e −e 0.5 (h t − e p )2 − 4 × A pp f yp − b×0.85 f cm × (M pr − M) e −e 0.5 2 × A pp f yp − b×0.85 f cm
(B.6) The degree of shear connection µ can be worked out as µ= Fig. A.2. View of embossments in the sheet.
Nc Nc f
(B.7)
Appendix B
from the µ value the longitudinal shear stress may be calculated as
B.1. Longitudinal shear calculation by τv method of Eurocode 4 [1, Annex E]
τu =
The tensile force in the sheeting may be written as N p = µNc f which is equal to concrete compressive force Nc . Therefore: Nc = µNc f
(B.1)
where µ is the factor used to indicate the degree of connection between steel sheet and the concrete. µ = 100% for full connection and µ = 0% for no interaction between the steel sheet and concrete in which case, the moment carrying capacity is equal to the moment capacity of the sheeting. If the degree of connection is between 0 and 100, the slab is said to have partial shear connection. The moment M due to applied loads at a particular section may be calculated as: (B.2)
M = Nc z + M pr z = h t − 0.5 × x − e p + (e p − e) x=
Nc ≤ hc b(0.85 f cm )
Nc A p f yp
(B.3) (B.4)
e = distance from the centroid of the effective area of sheeting to its underside. e p = distance of the plastic neutral axis of the effective area of the sheeting to its underside. h t = total depth of the slab. M pr is the reduced plastic moment resistance of the sheet. It may be calculated as Nc f M pr = 1.25M pa 1 − A f < M pa (B.5) p yp
γap
where Nc f = h c b (0.85 f ck /γc ), M pa = plastic moment resistance of the effective area of the sheeting.
µ × Nc f b × (L s + L 0 )
(B.8)
where L s = shear span L 0 = length of overhang. The characteristic shear strength should be taken as τu =
0.9 × µ × Nc f . b × (L s + L 0 )
(B.9)
Also design shear strength of the sheeting may be calculated as τu =
0.9 × µ × Nc f . b × (L s + L 0 ) × 1.25
(B.10)
References [1] Eurocode 4. Design of composite steel and concrete structures. Part 1.1. General rules and rules for buildings. [2] Crisinel M, Marimon F. A new simplified method for the design of composite slabs. Journal of Constructional Steel Research 2004;60: 481–91. [3] Chen S. Load carrying capacity of composite slabs with various end constraints. Journal of Constructional Steel Research 2003;59:385–403. [4] Burnet MJ, Oehlers DJ. Rib shear connectors in composite profiled slabs. Journal of Constructional Steel Research 2001;57:1267–87. [5] Makelainen P, Sun Y. The longitudinal behaviour of a new steel sheeting profile for composite floor slabs. Journal of Constructional Steel Research 1999;49:117–28. [6] Tenhovuori AI, Leskela MV. Longitudinal shear resistance of composite slab. Journal of Constructional Steel Research 1998;46(1–3):228. [7] Calixto JM, Lavall AC, et al. Behaviour and strength of composite slabs with ribbed decking. Journal of Constructional Steel Research 1998; 46(1–3):211–2. [8] Evans HR, Wright HD. Steel–concrete composite flooring deck structures. In: Narayanan R, editor. Steel–concrete composite structures, stability and strength. London: Elsevier Applied Science; 1988. p. 21–52 [chapter 2]. [9] Wright HD, Evans HR, Harding PW. The use of profiled steel sheeting in floor construction. Journal of Constructional Steel Research 1987;7: 279–95. [10] Porter ML, Ekberg CE. Design recommendations for steel deck floor slabs. Journal of the Structural Division, ASCE 1976;102(ST 11): 2121–35.
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803 [11] Porter ML, Ekberg CE, Greimann LF, Elleby HA. Shear bond analysis of steel deck reinforced slabs. Journal of the Structural Division, ASCE 1976;102(ST 12):2255–68. [12] Porter ML, Ekberg Jr CE. Investigation of cold-formed steel-deckreinforced concrete floor slabs. In: Yu W-W, editor. First specialty conference on cold-formed steel structures. Rolla: University of
803
Missouri-Rolla; 1971. p. 179–85. [13] IS 10262. IS recommended guidelines for mix design. Bureau of Indian Standards; 1982. [14] IS: 875 (Part 2). IS Code of practice for design loads (other than earthquake) for buildings and structures: Pt.2 — Imposed loads. Bureau of Indian Standards; 1987.
Experimental studies on composite deck slabs to determine the shear-bond characteristic (m–k) values of the embossed profiled sheet V. Marimuthu a,∗ , S. Seetharaman a , S. Arul Jayachandran a , A. Chellappan a , T.K. Bandyopadhyay b , D. Dutta b a Structural Engineering Research Centre, CSIR Campus, Chennai - 600 113, India b Institute for Steel Development And Growth (INSDAG), Ispat Niketan, 52/1A, Ballygunge Circular Road, Kolkata - 700 019, India
Received 15 March 2006; accepted 19 July 2006
Abstract Composite deck slab floors are gaining wide acceptance in many countries as they lend themselves to faster, lighter and economical construction in buildings. The cold formed profile sheeting which is an integral part of the deck slab is provided with embossments to improve their shear bond characteristics. However, the shear behaviour of composite deck slab is a complex phenomenon and therefore experimental methods are often resorted to establish their shear strength under flexural loads. An experimental study has been carried out to investigate primarily the shear bond behaviour of the embossed composite deck slab under simulated imposed loads and to evaluate the m–k values. Totally 18 composite slab specimens were cast using M20 grade concrete. The 18 numbers of specimens were split into six sets of three specimens each in which three sets were tested for shorter shear span loading and the other three sets for longer shear span loading. The specimens were tested as per the general provisions in Eurocode 4 [Eurocode 4. Design of composite steel and concrete structures. Part 1.1. General rules and rules for buildings]. This paper presents details of the experimental investigations conducted on the composite deck slabs and the evaluation of m–k values for the embossed profiled sheet. c 2006 Elsevier Ltd. All rights reserved.
Keywords: Profiled sheet; Embossments; Composite deck slab; Shear bond failure; m–k value
1. Introduction Cold formed profiled steel sheets with embossments are widely used for composite floor decking wherein they remain permanently in place as an integral part of the floor system. They perform two functions: they act as the formwork while concreting and in the composite slab, they act as the tension reinforcement. The only additional steel that needs to be provided is for taking care of shrinkage and temperature. In the case of continuous slabs, reinforcing steel is required to resist the negative bending moment at the supports. This type of flooring results in faster construction, lighter floors and rational use of construction materials. They also provide certain other advantages such as easy handling, a good ceiling surface and convenient ducting for routing utility services. Finally, the ∗ Corresponding author. Tel.: +91 044 22549142; fax: +91 044 2254 1508.
E-mail addresses: [email protected], mari [email protected] (V. Marimuthu). c 2006 Elsevier Ltd. All rights reserved. 0143-974X/$ - see front matter doi:10.1016/j.jcsr.2006.07.009
thin sheeting is extremely light and hence can be transported conveniently, and handled and placed easily by the construction personnel. Some of the disadvantages pointed out for the system are inadequate fire rating, the need for proper bonding between the steel deck and the concrete and also the protection needed against damage from high local loads. Even though the steel deck is galvanized, it is advisable to apply anticorrosive paints on the exposed side of the sheet. Effects of ponding and edge deformation also need to be taken care of during design. The term ‘composite steel deck floor slab’ means that there is a provision in the system for bonding between the steel deck and the concrete by some mechanical means. In other words, for the steel deck and concrete to act compositely, a mechanical interlocking is needed. This is provided essentially by various ‘shear transferring devices’ such as rolled embossments, transverse wires, holes etc. Examples of composite steel deck floor slab systems are illustrated in Fig. 1. One of the efficient ways of achieving the interlocking between the steel deck and concrete is by means of embossments on the profiled
792
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
List of symbols As b d Vs Ls Lo
Cross sectional area of the profiled sheet (mm2 ) Width of the profiled sheet (mm) Average depth of the composite deck slab (mm) Shear force (N) Shear span (mm) Length of overhang of the composite slab from centre line of support (mm)
ρ
As bd
f c0 , f cm Cube compressive strength of the concrete (N/mm2 ) m slope of the ultimate shear bond regression line k Intercept of the ultimate shear bond regression line Φ Capacity reduction factor (0.8) s Parameter denoting supporting condition during casting. Nc , Nc f Compressive force in concrete (N) Np Tensile force in sheeting (N)
steel sheeting. The deck profile must provide the resistance to vertical separation and horizontal slippage between the contact surface of the steel and the concrete. Additional composite action may be achieved by attaching studs or similar shear devices. The shear bond characteristic of the embossed sheeting is rated by two empirical parameters ‘m’ and ‘k’, where ‘m’ represents the mechanical interlocking between steel and concrete and ‘k’ stands for friction between them. This paper deals with the experimental evaluation of m–k values for composite deck slabs using cold formed profiled
sheets with rectangular dishing type embossments. Totally 18 composite slab specimens were cast using M 20 grade concrete. Using the values of ‘m’ and ‘k’ as determined by parametric tests, ultimate load carrying capacity of the composite deck could be calculated. The longitudinal shear strength of the composite slab calculated using m–k method is verified with the results obtained by partial shear connection method in Eurocode 4 [1, Annex E]. 2. Load carrying mechanism of composite profiled sheet deck floors In a composite steel floor deck, the hardened concrete slab, acting compositely with the profiled steel decks, spans the supporting beams and carries the imposed live loads. The composite action depends upon adequate transfer of horizontal shear forces between the concrete slab and the steel deck to enable the deck to act as the tensile reinforcement. In addition to horizontal shearing forces, the bending action also leads to vertical separation between the steel and the concrete. The profiled sheet, therefore, has to be designed to resist vertical separation, in addition to transferring the horizontal shears. Resistance to vertical separation is achieved by suitable shape in trapezoidal profile and also by the embossments. There are three distinct phases in the structural action of a composite deck system [3]. In the first phase i.e. during the construction phase, the steel sheeting must rigidly support the wet concrete during casting. In the composite slab action phase, the composite steel concrete slab should support the imposed loads on the slab and in the composite beam action phase, the steel beams, which act compositely with concrete through the stud shear connectors, must support the imposed loads in the transverse direction. This paper deals with the study
Fig. 1. Composite deck slabs using different types of profiled sheets.
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
of composite slab action phase, wherein the behaviour of the composite action of the steel sheet and the overlying concrete is focused. The three primary failure modes important for design of a composite deck slab are: (1) flexure, (2) shear at support and (3) shear bond mode. Failure of the slab is said to be ductile if the failure load exceeds the load causing first recorded end slip by more than 10%. The failure load is taken as the load at midspan deflection of L/50 unless failure has already taken place. One of the principal modes of failure of steel deck slabs is by the shear bond. The shear bond mode of failure is characterized by the formation of diagonal tension crack in the concrete at or near the load points, followed by a loss of bond between the steel deck and the concrete. There is a slippage between the steel and concrete causing a loss of composite action in the shear span region, which lies between the support reactions and the concentrated load. Slippage usually occurs when the load reaches its ultimate value and this is followed by a significant drop in loading. 3. Review of literature Crisinel and Marimon [2] have presented a new design approach for the prediction of composite slab behaviour. This new approach combines results from standard materials tests and small-scale tests with a simple calculation model (referred to here as the “New Simplified Method”) to obtain the moment–curvature relationship at the critical cross-section of a composite slab. The New Simplified Method facilitates the calculation of the load-carrying capacity of composite slabs by considering three phases of the M–θ behaviour observed in composite slab critical cross-sections. It requires knowledge of the geometric dimensions of the slab, the material properties (steel and concrete) and the characteristic behaviour of the steel–concrete connection as determined based on tests on small-scale specimen. In order to study the shear-bond action in composite slabs, Chen [3] tested seven simply supported one-span composite slabs and two continuous composite slabs using different end restraints in the simply supported slabs. The slabs with end anchorage of steel shear connectors were found to bear higher shear-bond strength than that of slabs without end anchorage. To enable an effective end anchorage, however, it is the shearbond slip rather than the strength of anchored studs that governs the contribution of the end restraints to the shear-bond resistance in composite slabs. Burnet and Oehlers [4] presented a new form of push-test that simulates the bond characteristics more accurately and which is used in 33 tests to determine the main parameters that affect both the chemical bond and mechanical bond strengths of dovetailed and trapezoidal rib shear connectors. The effects of the geometry of the cross-section, embossments, sheet thickness and surface treatment on the bond strengths are presented in a form that can be used as guidelines in the development of new forms of profiled sheets for slabs, beams and walls.
793
Makelainen and Sun [5] studied the shear-connection behaviour of composite slabs with a particular profiled steel sheeting having a depth of 153 mm. Twenty-seven push-out test specimens of different shapes, sizes, locations of embossments and different steel sheeting thicknesses are carried out in two test series. The embossments are first made on the slant faces based on standard design norms. Thereafter, the embossed sheets are profiled as per the requirement suiting to the standards of the manufacturer. It is found that the shear-connection behaviour of composite slabs is significantly affected by the depth of embossments. For the profiled steel sheeting with indented embossments, the reduction of Young’s modulus caused by the penetrated embossments is an important factor that affects the determination of the depth and width of the embossments. Finally, a new type of profiled steel sheeting, which can offer longitudinal shear strength in composite slabs up to 0.6 N/mm2 , is proposed for further research. Tenhovuori and Leskela [6] studied the behaviour of composite slabs with profiled steel sheeting as affected by the bond failure in the longitudinal shear connection. The effect of various important parameters is considered and the critical factors are reviewed on the basis of numerical data obtained from non-linear calculation by the method of finite elements. A thorough study is carried out to compare the present methods of analysis for the bond failures in the ENV Eurocode 4 [1] Part l-l, and it is shown that they can be improved and simplified and finally unified so as to get a clearly comprehensible system describing when the bond failure in a composite slab is possible in the design and what is to be done for it. Calixto et al. [7] carried out an experimental investigation on the behaviour and strength of full-scale one-way single span composite slabs with ribbed decking. Several aspects were studied, including different steel deck thickness, total slab height, as well as shear span length. The effect of connectors (stud bolt type) on the end anchorage was also investigated. Normal procedures for batching and mixing of the concrete were used. Throughout the monotonic loading tests, midspan deflections, end slips and strains in steel decking were measured. The test results indicate clearly the better performance of the composite slabs built with stud bolt connectors. In this study the slabs fabricated with plain sheeting and stud bolts attained in all cases a higher ultimate load when compared to the respective specimens built with ribbed decking only. The floors constructed with ribbed decking and stud bolts showed a different behaviour characterized by no drop in the load during the entire monotonic loading procedure. In all cases the failure mode was by shear bond even in the slabs fabricated with end anchorage and ribbed sheeting. The experimental results are also compared with the partial interaction design method specified in Eurocode 4 [1]. The current design equations do not separate explicitly the resistance of the mechanical interlocking from the friction at the concrete-decking interface over the supports. Depending on the position and shape of the embossments on the ribbed decking (AXE type II decks for instance), the contribution of each resistance mechanism plays a different role. Therefore a procedure which explicitly takes into consideration the
794
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Fig. 2. Shape, size and frequency of embossments.
effects of the mechanical interlocking and friction separately is presented. The proposed method is compared with the current test results and those obtained in other investigations. The comparisons show good correlation. Evans and Wright [8] and Wright et al. [9] have carried out more than 200 tests on composite deck slab elements and compared the results with the available design methods. They have studied the aspects of construction phase, composite slab action phase and the composite beam action phase in detail. The studies have shown that the variation in concrete strength has little effect on the ultimate load capacity. The crucial parameter that has significant effect on the ultimate strength is the height of the embossment. They have concluded that though the present design procedures are safe, they are very conservative in certain cases. They have recommended that the system as a whole, namely, slab span, beam span and stud connectors, should be considered for obtaining maximum economy in design. Porter and Ekberg [10] have carried out a large number of experimental studies on cold formed steel deck floor slabs. The work primarily involved one way full scale slab elements and tested up to failure, emphasizing the ultimate strength design concept. Porter et al. [11] have further conducted experimental studies on the shear bond failure characteristics of one-way slab elements and reported several observations on the significant parameters influencing the behaviour. They have also reported √ a linear regression relationship between Vu s/bd f c0 and √ 0 ρd/L s f c to determine the slope and intercept constants needed for design. Design procedures for the design of composite steel deck floor slabs based on the ultimate strength concepts have been recommended by Porter and Ekberg [12]. The capacity is based on the shear bond strength of the deck slab. The design equations for the shear bond capacity are derived from the data collected from a series of performance tests on the slabs and establishing the linear regression relationship as mentioned above by Porter et al. [11]. A separate regression is recommended for each deck profile, each gauge thickness of the sheeting, steel surface coating and concrete strength. In the construction phase the sheeting is designed for the loads due to the wet concrete and its self weight. The review of literature shows that analysis of the composite deck slab behaviour is complex. The extent of shear bond achieved depends upon many parameters, like the height, shape,
orientation and frequency of the embossment pattern and the geometry and flexibility of the profiled sheet itself. Currently an accurate determination of strength is possible only by performance testing. Performance tests need to be carried out as each steel deck profile has its own unique shear transferring mechanism. The purpose of the tests is to provide data for the ultimate strength design equations. Particularly, a series of tests is needed for getting ultimate experimental shears for a linear regression analysis of the parameters that affect the shear bond capacity. 4. Experimental studies on composite steel deck floor slabs The specimens were split into six sets of three specimens each in which three sets were tested for shorter shear span loading and the other three sets for longer shear span loading. The shape, size and frequency of the embossment are given in Fig. 2. In the shorter shear span loading, shear spans of 320 mm, 350 mm and 380 mm were chosen, while in the longer shear span loading, 850 mm, 950 mm and 1150 mm were adopted. For each set of three specimens, one specimen was tested to failure under monotonic loading, test duration being not less than one hour; the other two specimens were tested for cyclic loading for 5000 cycles each for duration of three hours, followed by a static test. The details of the profiled sheet and the moment carrying capacity of the composite slab worked out as per Eurocode 4 [1] are given in Appendix A. 4.1. Preparation of the composite slab specimen 4.1.1. Casting of slab The composite slab was cast with the profiled sheet as the base. The sheet was thoroughly cleaned before concreting. The casting was carried out in a fully supported condition. Mild steel reinforcing bar meshes (using 6 mm diameter bars) of the required size with the spacing of bars at 250 mm c/c in both directions were prepared. These meshes were placed 25 mm from the top surface of the profiled sheet (Fig. 3). The concrete mix of M 20 (28 day cube compressive strength of 20 MPa) grade designed as per the relevant Indian Standard [13] code was chosen for concreting. The coarse aggregate size used in the concrete was 20 mm down. The slabs were cast and cured for 14 days. Figs. 3 and 4 show the view of embossed sheet with the steel bar mesh and detailing of the composite deck
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Fig. 3. Embossed sheet with the 6 mm diameter reinforcement mesh.
795
Roller and hinge supports were specially fabricated for the study. The schematic view of the roller and hinge supports is shown in Fig. 6. The photographs of the hinged and roller supports are shown in Fig. 7. Fig. 8 shows the complete experimental set up. The midspan deflection and the end slip in the concrete at the hinged support were measured using LVDTs (Fig. 9). The LVDTs were connected to a computer which automatically stored the data for the given interval of time. The slip of the steel sheet and the concrete at the roller support end and steel sheet at the hinged support end were measured using electronic dial gauges (Fig. 10). The readings were noted down at specific load intervals up to the maximum load. The load was applied using a computer-controlled servohydraulic 25 ton MTS actuator under displacement control. The loading history was recorded automatically in the computer for the given interval of time. The load was applied as two line loads distributed across the width of the slab by transferring the actuator load through a distribution beam section (ISMB 300, I beam of depth 300 mm) to two smaller sections (ISMB 150, I beam of depth 150 mm) placed across the width of the slab. 4.3. Shear span
Fig. 4. Detailing of the composite deck slab.
slab. After 28 days, the slabs were transferred from the casting yard to the testing laboratory using proper supports so as not to transfer any flexural load to the slab. 4.2. Experimental set up The schematic view of the experimental setup is shown in Fig. 5.
The shear span is defined as the distance between the centres of support at either end to the point of application of the line load in the slab. Two schemes of loading were chosen, namely, one with shorter shear spans and the other with longer shear spans forming the two ends of the shear span zone. The tests were conducted by varying the shear span with three sets of shorter shear span loadings and three sets of longer shear span loadings.
Fig. 5. Schematic view of the experimental set-up.
Fig. 6. Schematic view of the supporting arrangements.
796
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Fig. 7. Actual view of roller and hinged support. Fig. 10. Dial gauges to measure the end slip.
Fig. 11. View of a loading point in a shorter shear span.
Fig. 8. View of test in progress.
thickness of the composite slab. Hence shorter shear spans of 320 mm, 350 mm and 380 mm were chosen (>315 mm, three times the slab thickness). Fig. 11 shows shear span of 380 mm. For each shear span, three numbers of composite slab specimens were tested, one monotonically till failure and the other two by initial cyclic loading for 5000 cycles and then loaded to failure by monotonic loading. 4.5. Longer shear span The criterion for the longer shear span is that the shear span should be as long as possible while still providing failure in longitudinal shear. And hence for longer shear spans, values of 850 mm, 950 mm and 1150 mm were chosen. Here too, for each shear span, one monotonic test, and two cyclic tests followed by monotonic test to failure were carried out. Fig. 12 shows the details of the set up for longer shear span loading.
Fig. 9. LVDT at the mid span of the slab.
4.4. Shorter shear span Three sets of shorter shear spans were chosen in such a way that the shear span was greater than three times the total
4.6. Static test The specimen was placed over the supporting roller-hinge arrangements and the shear span and loading points were marked. The load was applied incrementally by displacement control. The rate of loading was adjusted in such a way that
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
797
(a) Shear span L s = 320 mm. Fig. 12. View of longer span loading points.
(b) Shear span L s = 350 mm.
Fig. 13. Sequence of cyclic loading.
failure did not occur in less than one hour. The rate of loading adopted for static test was 0.1 mm/s. For data acquisition in the computer, the time interval was set as 5 s. The server computer automatically records the mid span deflection and the slip in the concrete at the hinged end at every 5 s. A maximum mid span deflection limit was set to shut down the actuator to avoid sudden collapse of the specimen.
(c) Shear span L s = 380 mm. Fig. 14. Load vs central deflection for shorter shear span specimens.
5. Results and discussion 4.7. Cyclic test 5.1. Static test Two specimens under each shear span were subjected to preliminary cyclic loading. This preliminary cycling ensures that any kind of chemical adhesion formed between steel and concrete is removed and the static load that is later applied would provide the true indication of the mechanical bond formed by the embossment. For cyclic loading, the slab was subjected to 5000 cycles of loading applied in a time span of three hours. Fig. 13 shows the loading pattern for cyclic loading. The slabs were subjected to cyclic load ranging from 0.5Wq to 1.5Wq, where Wq is the anticipated value of the characteristic load which will be acting on the slab. For the present study, Wq was assumed as 3 kN/m2 (as per IS-875 (Part 2)) [14], the uniformly distributed load value recommended for commercial buildings. This uniformly distributed load was converted in to a concentrated total central load and this load was applied as per specification. For the present case, the load range applied was 3.69–11.07 kN. This load range was applied with the frequency of 0.5 Hz.
5.1.1. Shorter shear span specimens 5.1.1.1. Load deflection behaviour. Two stages of load deflection behaviour were observed in the case of shorter shear span specimens. Fig. 14(a)–(c) show the load–deflection behaviour for shorter shear span specimens. At first, the shear cracks formed near the loading points and a load drop was observed (Region A–B in Fig. 14). Secondly, there was a load pick-up and subsequent flexural failure of the specimen (Region B–C). The rate of deflection was high in the second stage of behaviour. Fig. 15 shows typical crack formation in the shorter shear span specimens. Table 1 shows the capacity and behaviour characteristics of the slab for shorter shear span loading. 5.1.1.2. Slip behaviour of slabs. The slip was observed from the early stage of loading. In 320 mm shear span slab, the slip in the initial loading was very minimal. The zigzag portion in the
798
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
(a) 320 mm shear span.
Fig. 15. Crack formation in shorter shear span loading. Table 1 Details of shorter shear span loading and its behaviour No
Shear span L s (mm)
Failure load (kN)
Behaviour
1
320
55.625
First stage: Shear cracks were formed near the loading point and sudden drop in the capacity. Region A–B in Fig. 14
2
350
52.191
Second stage: Carried additional load by reinforcement mesh provided at the mid-depth of the concrete. Also flexural cracks were formed in between the loading points. Region B–C in Fig. 14
3
380
47.340
Slip: Slip was observed from the early stage of loading and the rate of slip was higher after first stage.
curves depicts gradual debonding of the slab. In all shorter span loadings, the slip up to the first crack appearance is gradual and then the rate of slip increases. After the first crack appears, the slip was in reverse direction (see Fig. 16(a) and (b)). This shows that the bond between the profiled sheeting and the concrete slab is removed and both the sheeting and concrete slab acts independently. Fig. 17 shows the differential movement of the concrete slab and profiled sheet.
(b) 350 mm shear span.
(c) 380 mm shear span. Fig. 16. Load slip curve for shorter shear span specimens.
5.1.2. Longer shear span specimens 5.1.2.1. Load deflection behaviour. As in the case of shorter shear span loading, visible cracks were formed in between the loading points and drop in load carrying capacity. A load pickup with the aid of nominal reinforcement mesh provided at the mid-depth of the concrete was observed and the flexure cracks widened with a higher rate of central deflection. In Fig. 18(a)–(c), the point A denotes when visible flexural cracks start forming. The portion A–B shows the drop of the load and the region B–C shows regaining of load to some extent. Table 2 shows the capacity and behaviour of longer shear span specimens. Fig. 19 shows the crack patterns formed in a longer shear span loaded specimen.
Fig. 17. End slip in shorter shear span loading.
5.1.2.2. Slip behaviour of slabs. In all the longer shear span specimens, slip was more compared to the shorter shear span specimens. Here also the trends of the load-mid span
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
(a) Shear span L s = 850 mm.
799
Fig. 19. Crack formation in longer shear span specimens.
(b) Shear span L s = 950 mm.
(a) 850 mm shear span.
(c) Shear span L s = 1150 mm. (b) 950 mm shear span. Fig. 18. Load deflection curve for longer shear span specimens.
Table 2 Details of longer shear span loading and its behaviour No
Shear span L s (mm)
Failure load (kN)
Behaviour
1
850
22.612
Flexure cracks were formed in between the loading points accompanied by a sudden drop in the capacity.
2
950
26.920
Additional load was resisted by nominal reinforcement mesh provided at the mid-depth of the concrete.
3
1150
16.391
Slip: Rate of slip is comparatively higher.
deflection and load-end slip curves are same as those for the short shear span loading. After the first stage, the rate of slip is more (Fig. 20). As in the case of shorter shear span specimens, the zigzag pattern of the curve shows the differential movements of the slab and the profiled sheet. In this case, the reverse slip is not observed due to the
(c) 1150 mm shear span. Fig. 20. Load end slip behaviour for longer shear span specimens.
longer shear span. As the shear span is larger, the load position moves towards the midspan and the load will try to merge the slab with the profiled sheet. The load carrying capacity of the composite slab increased with the help of the profiled sheet. Fig. 20 shows the load–slip behaviour of the slab. Tables 1 and 2 present the capacities and behaviour of the specimens in static test for shorter shear span and longer shear span loading respectively.
800
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Table 3 Parameters to plot m–k curve No
Shear span, L s (mm)
Load, P (kN)
Shear, Vu (kN)
Vu /bd (N/mm2 )
As /bL s
1 2 3 4 5 6
320 350 380 850 950 1150
55.625 52.191 47.340 22.612 26.920 16.391
22.250 20.876 18.936 9.045 10.768 6.556
0.3501 0.3285 0.2979 0.1423 0.1694 0.1031
0.0036 0.0033 0.0030 0.0013 0.0012 0.0010
5.2. Cyclic test Two specimens from each set were subjected to preliminary cyclic loading. All the shorter shear span specimens withstood 5000 cycles of loading. After three hours of cyclic loading, the specimens were subjected to static load in such a way that the specimens did not fail in less than one hour time. The behaviour and the capacity were more or less the same as obtained in the case of the static loading. In the case of longer shear span specimens, especially the specimen with 1150 mm shear span could not withstand cyclic loading, because its ultimate static load carrying capacity itself was only 8.20 kN. Hence, it failed before reaching the upper limit of cyclic loading. The other two shear span specimens, namely, 850 mm and 950 mm withstood 5000 cycles of loading for three hours and behaved in the same way in the subsequent static tests as in the case of pure static loading.
(2)
loaded) is calculated for each set of specimen. For calculating Vu , a capacity reduction factor, Φ = 0.8 is applied to the average failure load. Table 3 shows the calculation of necessary parameters for plotting the curve. A further reduction of 10% is applied to obtain the reduced regression line based on which the values of the regression constants m and k are computed. The values are compared with those for other profiled sheets with chevron type embossments @ 0◦ and 90◦ configuration reported in literature (Table 4). It was inferred that each profiled sheet has its own unique properties. As specified in Eurocode 4 [1], the m and k values may be determined as shown in Fig. 21. Using the m and k values obtained, the longitudinal shear strength of the composite slab is found out and compared with the τv method. This method is an alternate method to m–k method to calculate the longitudinal shear resistance of the composite slab. This method is given in Annex E of Eurocode 4 [1]. The comparison is given in Table 5 and the detailed calculations are given in Appendix B. It was found that the values obtained by τv method were slightly higher compared to the values obtained by the m–k method.
(3)
6. Conclusions
5.3. Evaluation of m–k value The main objective of the present testing programme is to determine the m–k values which define the shear transferring capacity of the profiled sheet. The recommended design equation for shear bond capacity of composite deck slabs is given by Porter et al. [12]. p bd mρd 0 Vu = + k fc (1) s Ls where s is the parameter depending on the type of shoring during the casting of the composite deck slabs. Eq. (1) may be written as follows p Vu s ρd =m + k f c0 bd Ls p Vu As =m + k f c0 bd bL s
where ρ =
As bd
Fig. 21. m–k curve.
which is in the form of an equation for a straight line y = mx + c. As the casting of the slab has been carried out in a continuously supported condition, the weight of concrete and other dead loads were added to the applied load [2]. After completing all the static and cyclic tests, the total load at failure is calculated by adding the values of self-weight of the slab and weight of the distribution beams to the applied load at failure for each specimen. The average value of the total load at failure (average of one statically loaded and two cyclically
From the experimental investigations conducted on 18 specimens with varying shear spans, the following conclusions are derived: • The behaviour of the embossed profiled composite steel deck slab depends mainly on the shear span; • For the shorter shear spans, strength of the slab is governed by shear bond failure; • If the shear span is large enough but not greater than 1.25 m, the behaviour of the slab is governed by flexural failure;
801
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803 Table 4 Comparison of m–k values with other profile sheets Author
Type of profile
Embossment type
m
k
H.D. Wright et al. (1987)
Trapezoidal
Chevron embossements @90◦
107.527
0.0401
S. Chen (2003)
Trapezoidal
Chevron embossements @0◦
84.665
0.0221
Present work
Trapezoidal
Rectangular dishing type of embossments
87.956
0.0322
Table 5 Comparison of longitudinal shear strength (τu ) Shear span, L s (mm)
Longitudinal shear strength, τu (N/mm2 ) m–k method τv method
320 350 380 850 950 1150
0.281 0.285 0.241 0.122 0.112 0.097
0.318 0.303 0.284 0.156 0.167 0.118
during experimental investigations. They express their grateful thanks to Institute for Steel Development And Growth (INSDAG), Kolkata, for sponsoring the project. The paper is published with the kind permission of Director, Structural Engineering Research Centre, Chennai. Appendix A A.1. Properties of the embossed profiled sheet (approximate) The following are the properties of the embossed sheet:
• In all the specimens, the slip is observed from an early stage of loading. This inadequacy may be improved by varying the embossment details such as depth, width and position of embossment. This may also be improved by increasing the depth of concrete portion and providing stud connectors at the ends. Their efficacy can be verified by carrying out experimental investigations further; • The cyclic loading carried out as per the provisions in Eurocode [1] does not affect the load carrying capacity of the slabs; • The linear regression constants ‘m’ and ‘k’ for rectangular dishing type embossments are found to be: m = 87.956 k = 0.03 • As the m and k value differs for each profiled sheet, experimental verifications are necessary for each type of profiled sheet. • The longitudinal shear carrying capacities calculated by m–k method and partial shear connection method differ by about 26% in the average.
Length of the sheet = 3000 mm Width of the sheet, b = 820 mm Spacing of embossments = 65 mm Width of embossments = 21 mm Shape of embossments = Rectangular dishing type Length of embossments = 25 mm Thickness of the sheet = 0.8 mm (20 gauge) Area of cross section of the sheet, A p = 9.5382 cm2 Weight of the sheet = 6.91 kg/m Modulus of section, Z e = 16.79 cm3 Plastic section modulus, Z p = 17.350 cm3 Yield stress of the sheet, f y = 250 N/mm2 Effective area of sheet, Aeff = 4.20 cm2 Moment of inertia, I x x = 28.8 cm4 . Fig. A.1 shows the cross sectional view of the embossed sheet and Fig. A.2 shows the view of embossments provided in the sheet: Flexural resistance of the composite slab = 14.52 kN m.
Acknowledgements The first four authors thank Dr N. Lakshmanan, Director, and Dr C.V. Vaidyanathan, Advisor Management, Structural Engineering Research Centre, Chennai, for their kind support
Fig. A.1. Cross section of the embossed sheet.
802
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803
Substituting the above relations in Eq. (B.2), value of Nc can be calculated as Nc −(h t − e p ) ± =
r e −e 0.5 (h t − e p )2 − 4 × A pp f yp − b×0.85 f cm × (M pr − M) e −e 0.5 2 × A pp f yp − b×0.85 f cm
(B.6) The degree of shear connection µ can be worked out as µ= Fig. A.2. View of embossments in the sheet.
Nc Nc f
(B.7)
Appendix B
from the µ value the longitudinal shear stress may be calculated as
B.1. Longitudinal shear calculation by τv method of Eurocode 4 [1, Annex E]
τu =
The tensile force in the sheeting may be written as N p = µNc f which is equal to concrete compressive force Nc . Therefore: Nc = µNc f
(B.1)
where µ is the factor used to indicate the degree of connection between steel sheet and the concrete. µ = 100% for full connection and µ = 0% for no interaction between the steel sheet and concrete in which case, the moment carrying capacity is equal to the moment capacity of the sheeting. If the degree of connection is between 0 and 100, the slab is said to have partial shear connection. The moment M due to applied loads at a particular section may be calculated as: (B.2)
M = Nc z + M pr z = h t − 0.5 × x − e p + (e p − e) x=
Nc ≤ hc b(0.85 f cm )
Nc A p f yp
(B.3) (B.4)
e = distance from the centroid of the effective area of sheeting to its underside. e p = distance of the plastic neutral axis of the effective area of the sheeting to its underside. h t = total depth of the slab. M pr is the reduced plastic moment resistance of the sheet. It may be calculated as Nc f M pr = 1.25M pa 1 − A f < M pa (B.5) p yp
γap
where Nc f = h c b (0.85 f ck /γc ), M pa = plastic moment resistance of the effective area of the sheeting.
µ × Nc f b × (L s + L 0 )
(B.8)
where L s = shear span L 0 = length of overhang. The characteristic shear strength should be taken as τu =
0.9 × µ × Nc f . b × (L s + L 0 )
(B.9)
Also design shear strength of the sheeting may be calculated as τu =
0.9 × µ × Nc f . b × (L s + L 0 ) × 1.25
(B.10)
References [1] Eurocode 4. Design of composite steel and concrete structures. Part 1.1. General rules and rules for buildings. [2] Crisinel M, Marimon F. A new simplified method for the design of composite slabs. Journal of Constructional Steel Research 2004;60: 481–91. [3] Chen S. Load carrying capacity of composite slabs with various end constraints. Journal of Constructional Steel Research 2003;59:385–403. [4] Burnet MJ, Oehlers DJ. Rib shear connectors in composite profiled slabs. Journal of Constructional Steel Research 2001;57:1267–87. [5] Makelainen P, Sun Y. The longitudinal behaviour of a new steel sheeting profile for composite floor slabs. Journal of Constructional Steel Research 1999;49:117–28. [6] Tenhovuori AI, Leskela MV. Longitudinal shear resistance of composite slab. Journal of Constructional Steel Research 1998;46(1–3):228. [7] Calixto JM, Lavall AC, et al. Behaviour and strength of composite slabs with ribbed decking. Journal of Constructional Steel Research 1998; 46(1–3):211–2. [8] Evans HR, Wright HD. Steel–concrete composite flooring deck structures. In: Narayanan R, editor. Steel–concrete composite structures, stability and strength. London: Elsevier Applied Science; 1988. p. 21–52 [chapter 2]. [9] Wright HD, Evans HR, Harding PW. The use of profiled steel sheeting in floor construction. Journal of Constructional Steel Research 1987;7: 279–95. [10] Porter ML, Ekberg CE. Design recommendations for steel deck floor slabs. Journal of the Structural Division, ASCE 1976;102(ST 11): 2121–35.
V. Marimuthu et al. / Journal of Constructional Steel Research 63 (2007) 791–803 [11] Porter ML, Ekberg CE, Greimann LF, Elleby HA. Shear bond analysis of steel deck reinforced slabs. Journal of the Structural Division, ASCE 1976;102(ST 12):2255–68. [12] Porter ML, Ekberg Jr CE. Investigation of cold-formed steel-deckreinforced concrete floor slabs. In: Yu W-W, editor. First specialty conference on cold-formed steel structures. Rolla: University of
803
Missouri-Rolla; 1971. p. 179–85. [13] IS 10262. IS recommended guidelines for mix design. Bureau of Indian Standards; 1982. [14] IS: 875 (Part 2). IS Code of practice for design loads (other than earthquake) for buildings and structures: Pt.2 — Imposed loads. Bureau of Indian Standards; 1987.