ctbuh.org/papers A Study of Flat Plate Slab Column Connections with Shear Plate in Tall Concrete Building using Experimental and Numerical Analysis

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1 ctbuh.org/papers Title: Authors: Subject: Keywords: A Study of Flat Plate Slab Column Connections with Shear Plate in Tall Concrete Building using Experimental and Numerical Analysis Young Sang Cho, Professor, Hanyang University Cheol J. Seo, PhD Candidate, Hanyang University Eun S. Kang, Masters Degree Candidate, Hanyang University Structural Engineering Concrete Shear Structure Publication Date: 2004 Original Publication: Paper Type: CTBUH 2004 Seoul Conference 1. Book chapter/part chapter 2. Journal paper 3. Conference proceeding 4. Unpublished conference paper 5. Magazine article 6. Unpublished Council on Tall Buildings and Urban Habitat / Young Sang Cho; Cheol J. Seo; Eun S. Kang

2 A Study of Flat Plate Slab column Connections with Shear Plate in Tall Concrete Building using Experimental and Numerical Analysis Young S. Cho 1, Cheol J. Seo 2, Eun S. Kang 3 1 Professor, Dept. of Architectural Engineering, Hanyang University 2 Ph.D. Student, Dept. of Architectural Engineering, Hanyang University 3 M.S. Student, Dept. of Architectural Engineering, Hanyang University Abstract The trend of urban building structures shows a type of high rise building due to the concentration and increase of population into urban cities, rapid increase of land cost, and limited availability of land. Such a trend is not only a worldly demand but also no exception in Asian cities. To facilitate the demand of high rise structures, various types of structural system have been used domestically. Among those structural systems, there has been a high demand in using a flat plate slab system specially in the mix use high rise buildings including commercial and residential uses because the systems have various following advantages. The system can minimize the ceiling height which can reduce a total building height and reduce the corresponding material cost. The system also provides a room for flexible spatial planning. It has an advantage of time saving because it makes easy form installation and stripping because there are no beams. It also reduces noise transmission between floors due to the thick concrete slab. However, the system carries a disadvantage of punching shear due to the absence of beams. Reinforcing method of flat plate-column connection has been studied in this paper. This research is to study the response of flat plate slab-column connections consisting of various types of shear reinforcement and steel plate subjected to gravity loadings using the structural experiments and numerical analysis in tall buildings. The four test specimens were prepared to model the interior bay flat plate slab-column connections in a typical flat plate slab building. The base specimen has no shear reinforcement, and the other specimens have shear studs with strip base, shear studs with short steel plates under the slab and on the top portion of the column, and shear studs with long steel plates under the slab and on top portion of the column. The base specimen failed due to punching shear generated from the gravity loading. The three other types of slab shear reinforcement and steel plate showed effective in resisting punching shear for these types of connections under gravity loading. The specimen containing studs with steel plate showed the most ductile behavior under the gravity loading. The structural behavior of the four specimens has been tested using the structural experiment and the numerical method. Keywords : Punching Shear capacity, Flat Plate Slab, Shear Studs, Stud with Steel Plate. 1. Introduction In the most of the urban cities throughout the world, urban building structures have the trends of becoming high rise building, massive buildings, and high-tech integrated buildings using the least lot areas, building floor areas, and building volumes due to the expansion Corresponding Author: Young S. Cho, professor, Dept. of Architectural Engineering, Hanyang University, 1271 Sa 1-dong, Ansan-Si, Kyunggi-Do, , Republic of Korea Tel: , Fax : ycho@hanyang.ac.kr of urban population and the lack of available land. Recent construction industry in a large urban city demands high rise buildings such as apartments, hotels, office buildings and specially mixed use buildings consisting of commercial and residential uses. In such buildings, there have been high demands of flat plate slab systems because the systems have numerous advantages. First, efficient formwork reduces construction period and cost. Secondly, low ceiling height due to the deletion of beam throughout the entire super structures can save structural materials, cladding materials, air conditioning loads, electrical materials, etc. because of reduction in building height. 120 CTBUH 2004 October 10~13, Seoul, Korea

3 The system can produce maximum number of apartment units, and maximum building floor area under a given zoning regulation. The flat plate slab systems also have disadvantages. The systems are sensitive to the punching shear resisting capacity. The punching shear at the slab column connection is affected by the size of columns, the span length of slab, compressive strength of concrete, etc. To enhance the punching shear capacity at the slab column connection, researchers studied the various types of shear reinforcing methods [1,2] such as closed hoop stirrups or open stirrups with a clip closure and single-stirrups, single leg stirrups, headed-stud shear reinforcement, etc. However, bracket type steel plate with headed-stud shear reinforcement has not been studied elsewhere. Four specimens were prepared including a base specimen without shear reinforcement, a specimen with headed shear studs on steel strip, a specimen with headed shear studs on optimum bracket type shear plates, and a specimen with headed shear studs on larger bracket type shear plates. The four specimens were tested using a 1000 ton U.T.M. Fig. 1a. Floor framing plan of base floor in a 32 story building 2. Research significance Punching shear resistance capacities at slab column connections are important specially under gravity and lateral loads. There were numerous damages and collapses under gravity loads and lateral loads such as seismic force[3,4]. This research focused on the gravity loads. Usage of different types of shear reinforcement such as closed hoop stirrups or open stirrups with a clip closure and single-stirrups[5], single leg stirrups, lattice girder, headed-stud shear[6] reinforcement has improved the punching shear resistance capacity [7]. But the performance of shear studs on bracket type shear plates has not been studied. The ready made shear studs on bracket type shear plates are relatively easy to install on the concrete forms. This shear plates will function as clip angles as they are in the connections of steel beams and columns in the steel structures. This study is to explore technology how to effectively resist the punching shear. This research also concerns how to improve structural safety, convenience of construction, saving of structural material, construction cost. This study took samples of specimens at the interior slab column connection in the 25th floor of 32 story building as shown in Fig. 1a and Fig. 1b. Four specimens were prepared including a base specimen without shear reinforcement, a specimen with headed shear studs on steel strip, a specimen with headed shear studs on optimum bracket type shear plates, and a specimen with headed shear studs on larger bracket type shear plates. Fig. 1b. Detail of floor framing plan 3. Experimental Program Material and Shear Reinforcements Concrete was specified to have a 28 day compressive strength of 350 kg/cm 2. Deformed bars were used to make four specimens. Steel grade of reinforcement bar was SD40 (f y = 4000 kg/cm 2 ). Bar sizes were 10 mm and 16 mm in diameters. Shear studs, steel strips and bracket type shear plate were made of SS400 (f y = 2400 kg/cm 2 ). The configuration of shear reinforcement in each specimen is shown in Fig 2. T1c T2css T3csspl Fig. 2 Diagrams of shear reinforcement in the flat plate slab specimens T4csslpl Test Specimens Four specimens were prepared using 1/2 scale of actual size. The specimen size consisted of a 180x CTBUH 2004 October 10~13, Seoul, Korea 121

4 Table 1 Material properties of test specimens Specimen Concrete compress strength f c, Mpa Modulus of rupture f r, Mpa Steel yield strength, Mpa Flexible, f y Shear, f yr Reinforced ratio in c2+ 3h width, ρ₁,% Reinforced ratio in column strip, ρ₂, % Reinforced ratio outside column strip, ρ₃, % Top Bottom Top Bottom Top Bottom T1c T2css T3csspl T4csslpl (a) T1c specimen top bars and section (b) T2css specimen top bars and section (c) T3csspl specimen top bars and section (d) T4csslpl specimen top bars and section 180x10cm slab and a 25x25x130cm column on center. In first specimen (T1c) as shown in Fig. 3(a), D10 bars were used in the top and bottom bars each way. Longitudinal bars of the column were placed with 12-D16 Table 1 shows the reinforcement and material properties of each specimen. Reinforcement ratio in the range of c 2 + 3h, where c 2 is the column dimension transverse to the loading direction and h is the slab thickness, was 0.65% on the top (tension side) and 0.4% on the bottom (compression side). Reinforcement ratio in the range of column strip was 0.45% on the top (tension side) and 0.35% on the bottom (compression side), and in the range of outside column strip 0.3% on the top and bottom. D10 bars were used in the top and bottom bars each way. Hoop bars of column are placed at 20 cm on center. Concrete covers were 9.5 mm. Second specimen (T2css) is basically the same specimen T1c in terms Fig. 3. Specimen diagrams of reinforcement and material properties except shear reinforcement mm diameter shear studs are welded on a 3x20.4x0.64 cm metal strip as shown in Fig. 3(b). Two pieces of steel strips on each side of column are placed leaving 9.5 mm concrete cover. Third specimen (T3csspl) is basically the same as specimen T1c in terms of reinforcement and material properties except shear reinforcement as shown in Fig. 3(c) mm diameter shear studs in a row are welded on two 17.4x20.4x0.64 cm bracket type shear plate(l shape shear plate) as shown in Fig. 4 and Fig. 5. L shape, bracket type shear plate on each side of column is placed. Fourth specimen (T4csslpl) is basically the same as third specimen in terms of reinforcement and material properties except bracket type shear plate on the bottom of slab near column edge as shown in Fig. 3(d) mm diameter shear studs in a row are welded on the top portion of bracket type shear plate (L shape) as shown in Fig. 4 and Fig CTBUH 2004 October 10~13, Seoul, Korea

5 Fig. 4. Test specimens photo Fig. 7. Experimental Set-up diagram Fig. 5. Test specimens photo(t3csspl) Fig. 8. Experimental set-up photo Test results of compressive strength The cylinder tests of concrete were performed at the ages of 7, 14, 21 and 28 days after pouring concrete. 400 Fig. 6. Test specimens photo(t4csslpl) Compreesive Strength(kg/cm 2 ) Age(days) Test procedure Each specimen was placed upside down as shown in Fig. 7 and Fig. 8. The slab of specimen was simply connected to three hinge supports on each side. Monotonic static load was applied downwards on the center of column using 1000 U.T.M under deflection control. A load cell was placed on the top of column to measure forces. Deflections related to shear force were measured on the compression side of the slab at both sides of column about 2.5 cm away from the column faces using LVDT. The total deflection was measured at the bottom of column using LVDT. Strains in shear and flexural reinforcing were measured at various locations. The strain measurement at various locations provided the stress distribution in the reinforcement and the yielding information. Fig. 9. Result of cylinder Test Concrete compressive strength of 350 kg/cm 2 in mix design was evaluated as 379 kg/cm 2 in the result of cylinder test using the average of 3 cylinders as shown in Fig Experimental test results Crack and failure pattern In specimen T1c as shown in Fig. 10(a), flexural cracks occurred first. Cracks generated radialy from the column faces toward the edges of slabs. Inclined shear cracks started to occur from the column faces in the compression side and propagated to the tension side of slabs with the angles less than 45 degrees CTBUH 2004 October 10~13, Seoul, Korea 123

6 measuring from the horizontal line of slab surface. Shear cracks were developed around 25cm radius along column faces in the tension side. Punching shear cracks were developed right along the column faces within less than d/2, where d is the average depth of longitudinal reinforcement bar(8.25cm) and flat plate slab has been failed by punching shear. In specimen T2css where shear studs with steel strips are placed, similar crack patterns were generated as shown in Fig. 10(b). Flexural cracks occurred first. Inclined shear cracks started to occur from the column faces in the compression side and propagated to the tension side of slabs beyond the shear studs with steel strips, with the angles less than 45 degrees measuring from the horizontal line of slab surface. Shear cracks were developed around 30cm radius along column faces in the tension side. Punching shear cracks were developed right along the column faces within less than d/2, where d is the average depth of longitudinal reinforcement bar(8.25cm) and flat plate slab has been failed by punching shear. In specimen T3csspl where shear studs with bracket type shear plates are placed, punching shear cracks and failure have occurred beyond the edge of the studs with bracket type shear plates on the compression and tension sides as shown in Fig. 10(c). Cracks on the compression side showed the shape of punching shear failure. Cracks on the compression side showed the shape of punching shear failure and cracks in the tension side showed punching shear failure along the edge of bracket type shear plates. Cracks along the column faces have not occurred because the shear Studs with steel plates are embedded inside the column and the vertical plates are welded to the horizontal plates. In specimen T4csslpl where shear studs with larger bracket type shear plates are placed, punching shear cracks and failure have occurred beyond the edge of the studs with larger bracket type shear plates on the compression and tension sides as shown in Fig. 10(d). Cracks on the compression side showed the shape of punching shear failure and cracks in the tension side showed punching shear failure along the edge of bracket type shear plates and the radial cracks have been developed. Table. 2 Experimental result Cracks along the column faces have not occurred because the shear studs with steel plates are embedded inside the column and the vertical plates are welded to the horizontal plates. (a) T1c (c) T3csspl (b) T2css (d) T4csslpl Fig. 10. Crack development diagram of specimens Load-displacement analysis As the result of experiment, yield load (Py), ultimate load(pmax), yield displacement(δy), ultimate displacement (δmax), strength ratio (Pmax/Py), ductility (δmax/δy) and shear efficiency are shown in Table. 2. By observing the load displacement curve of Fig. 11, it is likely to have advantages that the combination of steel plates and studs increases the capacity of displacement and makes it possible to maintain the stable structural behavior by not only increasing shear capacity but also increasing the ductility of structure. By observing the load displacement curve of Fig. 11 before failure, it was 124 CTBUH 2004 October 10~13, Seoul, Korea

7 found that the specimen without shear reinforcement showed a sudden failure, and the specimen with shear reinforcement did not show the brittle failure by showing the smooth curve. With reviewing the above graphs, studs show the role that it prevents from a sudden brittle failure of structures, if the steel plates are reinforced together, the effects of reinforcement magnify further. Load (tonf) T1-C T2-CSS T3-CSSPL T4-CSSLPL Dis place me nt ( mm) In the specimens of T1c and T2css, the increase in the ultimate load capacity and the maximum displacement are similar, but in the specimens of T2css and T3csspl, the increase in maximum displacement rather than ultimate load capacity is larger. It shows that the reinforcement of studs contributes to the increase of shear capacity, but the reinforcement of steel plate contributes the increase of shear resisting capacity and displacement capacity. It was also found that the studs with plates increased the ductility so that those contribute to the stable structural behavior. In the case of T4csslpl which increased the plate length and the number of studs, the result showed the same increase in the shear capacity and ductility. Fig. 11. Load-Displacement curve As shown in Fig. 12, ultimate load capacity and yielding capacity in the specimens show relatively a linear increase, but in Fig. 13, it can be noticed that the trend is quite different from the previous graph. The ultimate force shows a smooth increase due to the shear reinforcement, but the maximum displacement due to the shear reinforcement shows a sharp increase based on the specimen characteristics. Fig. 14. Ductility comparison of each specimen Load (tonf) Pmax T1C T2CSS T3CSSPL T4CSSLPL Fig. 12. Ultimate and yield load of each specimen Fig. 13. Ultimate and yield displacement curve of each specimen Py P/Py T1C T2CSS T3CSSPL T4CSSLPL Ductility (δmax/δy) Fig. 15. Strength ratio vs. ductility curve In the ductility comparison based on yielding displacement as shown in Fig. 14, the difference between T2css and T3csspl shows a substantial increase, meanwhile the difference between T1c and T2css, T3csspl and T4csslpl showed relatively a small increase in the ductility. It was found that the steel plate played a major role in increasing ductility in flat plate slab rather than studs. In Fig. 15, the strength ratio versus ductility ratio of shear reinforced specimen can be observed smaller based on the strength ratio of 1. In the case of specimen that is not reinforced in shear capacity, the strength ratio corresponding to the related ductility CTBUH 2004 October 10~13, Seoul, Korea 125

8 ratio is high, which causes a brittle nature, and unstable shape in the structure. The effect of shear reinforcement To obtain the shear capacity of studs and steel plates, using equation (1), V s have been computed by substituting shear capacity of concrete and the ultimate load values in experiment into V c and V n. Such values are tabulated in table 3. Fig. 16 shows the shear efficiency of each specimen based on the concrete shear capacity of specimen T2css. In Fig. 16, it was found that studs takes 60% of shear capacity, which shows an excellent shear performance. With the addition of steel plate, shear capacity increases further. In specimen T4csslpl, 75% increase in studs and steel plates comparing with T3csspl cased 12.5% increase in shear efficiency. In Table 3, shear capacity of specimen T2css was derived based on Vc, which is concrete shear capacity, and Vs, which includes the shear capacity of shear studs, and re-bars. Meanwhile, in the specimen of T3csspl, shear capacity is contributed by the Vc, re-bars, shear studs and steel plates. The shear capacity of the steel plates using the specimens of T2css and T3csspl can be obtained by subtracting the shear capacity of T2css from the shear capacity of T3csspl. Therefore, the equation of the shear capacity in the specimen with shear studs and shear plates can be proposed as shown in equation (2) V Av f yd Vc + Vs = 0.53 f ck b d (1) s n = 0 + Av f yd n = Vc + Vs + Vspl = 0.53 fck b0 d + + α 0. Aspl f (2) y V 4 s A v = Area of shear bar (cm 2 ) s = Spacing of placed bar A spl = Area of steel plate F y = Yield stress of steel α = Shear reduction coefficient of steel plate Fig. 16. Shear efficiency of each specimens 5. Numerical Analysis Modeling The geometry of numerical modeling used in the numerical analysis is the same size of slab as the experimental specimens, which was 180x180x10cm. The model has been discretized into small meshes ranging from 5mm to 150mm. The rectangular 8-node solid elements have been used in the modeling[5]. The fundamental material properties used in the numerical analysis are as follows. The compressive strength of the concrete obtained using the cylinder test was 34.3 Mpa, the yield stress and the Young s modulus of the re-bar used in the experiment was 345 MPa, 200GPa respectively. The tension stiffening coefficient of concrete was assumed as 3.0e-3. In terms of loading method, the flat plate slab was supported using 12 pin supports around slab perimeters, and the gravity load has been applied on the top of column. However, in the numerical analysis, the bottom of column was fixed, and the loads have applied at the very spots where the pin supports were located. Table. 4. Comparison results of experiment & FE-analysis Table. 3. Ultimate load & the shear strength of concrete and reinforcement 126 CTBUH 2004 October 10~13, Seoul, Korea

9 Fig. 17. computed load-displacement curves of specimens Results of numerical analysis The result of numerical analysis is shown and compared in Table 4, which was in the range of yield load. Fig. 17 shows the load-displacement curves derived from the finite element analysis of flat plate slab specimens. The computed displacements under yield loads of numerical modeling were less than those of structural experiments. The numerical analysis has been conducted under the same condition as structural experiment. The value of numerical analysis result was about 85% of experimental value or higher, which indicates the difference is less than 15%, and the result is reasonably close each other. Specially, in the case of specimen T1c, the most accurate results have been obtained, which have been caused by the inexistence of shear reinforcement. In the case of specimen T3csspl, the result shows 95% because shear studs and steel plates were designed optimally, and the meshing was properly performed. In specimen T4csslpl, the difference between experimental and numerical values was about 14%, of which steel plates and shear studs were excessive in sizes and numbers. The result of numerical analysis can be improved further by refining the meshes, input parameters, etc. that will be close to the actual physical condition. 6. Conclusion The result of this study can be summarized as follows. 1. Specimen T2css which contains steel studs with stud-rail increased about 8% in shear capacity comparing to the base specimen, T1c, which was not reinforced in shear capacity. In specimen T4csslpl using steel studs and plates, shear capacity has been increased by about 21%. 2. In the specimens using steel studs and steel plate, those specimens were evaluated as more superior in ductile capacity than those that contain studs only. The reason is that the steel plate contributed to prevent from brittle failure due to the punching shear after ultimate load. 3. In specimens with steel studs and plates, studs take about 60% of shear force which plays an important role, but it is more efficient to use steel plate with studs to extend the ductile capacity for delaying failure time. 4. Efficiency in shear capacity may diminish when excessive steel studs and plates are placed. 5. In flat plate structure, the use of steel studs can increase the shear capacity of the slab which can reduce the slab thickness. But the failure mode shows a brittle failure due to the lack of ductility. The addition of steel plates on the shear studs contributed to stable structural behavior because both of shear capacity and ductility have been increased. 6. Based on yield load, the difference in displacement between the structural experiment and numerical analysis came out less than 14%. Using the result of structural experiment, numerical results can be further improved by considering the behavior of inelastic region in numerical analysis. 7. The results from this study can be used to improve the current design method. Acknowledgements Authors express a sincere gratitude to Korea Science & Engineering Foundation(KOSEF) for their partial financial support under R on this study. References 1) Menetrey, P. (2002) Synthesis of punching failure in reinforced concrete. Cement & concrete Composites. (24), ) Yamada, T., Nanni A., et al,,(1992) Punching shear resistance of flat slabs : Influence of reinforcement type and ratio, ACI Structural Journal, (88), No.4, ) Gardner N. J., Shao X., Punching shear of continuous flat reinforced concrete slabs, ACI Structural Journal, V.93, No. 2, March-April 1996, pp ) Robertson, I.N., Kawai, T., et al, Cyclic testing of slab-column connections with shear reinforcement, ACI Structural Journal, V99, No.5,September-October 2002, pp ) Beutel, R. and Hegger, J. (2002) The effect of anchorage on the effectives of the shear reinforcement in the punching zone. Cement & concrete Composites. (24), ) Campi E, Eligehausen R, Bertero VV, Popov, EP. (1982) Analytical model for concrete anchorage of reinforcing bars under generalized exitations. Earthqake Engineering Research Center, Report No. UCB/EERC 82/23, University of California, Berkley 7) Hammill N., Ghali A., (1994) Punching shear resistance of corner slab-column connections, ACI Structural Journal, (91), No. 6, CTBUH 2004 October 10~13, Seoul, Korea 127