STRENGTH OF EXTERIOR SLAB-COLUMN CONNECTIONS

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1 STRENGTH OF EXTERIOR SLAB-COLUMN CONNECTIONS S. Teng, Nanyang Technological University, Singapore J.Z. Geng*, Nanyang Technological University, Singapore H.K. Cheong, Nanyang Technological University, Singapore 29th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2004, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 29 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2004, Singapore STRENGTH OF EXTERIOR SLAB-COLUMN CONNECTIONS S. Teng, Nanyang Technological University, Singapore J.Z. Geng*, Nanyang Technological University, Singapore H.K. Cheong, Nanyang Technological University, Singapore Abstract Punching shear strength of exterior slab-column connections, including edge and corner connections is discussed in detail based on the analysis of available data in t he literature according to the ACI It is found that the interaction between moment and shear for exterior connections is not as strong as represented in the ACI The interaction is weak for edge connections, and even weaker for corner connections. A reduction 0 f Yv is proposed based on the analysis. For edge connections, the reduced Yv is equal to 60 percent of the ACI defined value; for corner connections, it equals to 10 percent of the ACI defined value only. Once this reduction of Yv is considered in the ACI , the accuracy of prediction can be improved greatly for the collected data. Keywords: punching shear strength, slab-column connections, moment transfer, design code. 1. Introduction The ACI presents an eccentric shear stress model for predicting punching shear strength of slab-column connections with moment transfer. It assumes that the shear stresses due to unbalanced moment can be added directly to shear stresses due to shear force. The shear stresses due to unbalanced moment vary linearly along the critical section. The interaction between shear and moment transfer is represented by a coefficient Yv' which defines the fraction of unbalanced moment resisted by eccentric shear. This paper begins with a summary of data obtained from numerous experiments on exterior slabcolumn connections, including edge and corner connections. The eccentric shear stress model in the ACI is reviewed. The predictions according to the ACI for the collected data are analyzed and compared with the experimental results. Detailed discussions are provided and the interaction between shear and moment is studied and emphasized. 2. Research significance The present study provides a fresh review of previous experimental data on exterior slab-column connections, including edge and corner connections. The punching strength of experimental data is checked based on the ACI It is found that the interaction between shear and moment is weak for edge connections, and even weaker for corner connections. Reductions of y v are both proposed based on the ACI defined value for edge and corner connections. 3. Review of experimental data Numerous experimental data on slab-column connections are available in the literature. Included in this study are seventy-four exterior slab-column connections subjected to combined shear and moment transfer, tested by over 15 research centers around the world. Of the 74 connection specimens, 46 are edge slab-column connections and 28 are corner connections. Some details of each group of specimens are described below. 529

3 3.1 Edge slab-column connections Forty-six data involving edge slab-column connections were collected in this study. The respective detailed experimental information can be found in Ref. (2) through [17]. For all included test slabs, the outer faces of the columns were flush with the slab edge. Most of specimens had columns extended from both above and below the slab, while some other specimens had columns extended from below the slab only. Those data included both isolated single slab-column edge connections (Ref [2] through [9]) and non-single specimens (Ref [10] through [17]). The specimens tested by Scavuzzo [10] and Sherif and Dilger [14] comprised both an interior and exterior connections. The specimens tested by Regan [11) comprised a slab spanning two edge connections. The subassemblies tested by Robertson and Durrani [15] consisted of two exterior connections and one interior connection each. Falamaki and Loo [16) tested a series of nine half-scale models representing two adjacent edge and corner panels of a building floor. Each specimen contained six columns, including three slab-column connections with spandrel beam or torsion strip. Only the connections without spandrel beam were collected in this study. Typical test specimens are shown in Fig.1. Roller-supported edge ~ Lateral load Reactions' ~" ~ Vertical loads Lateral Load Restrained against rotation about edge t Reactions Vertical reaction (a) Scavuzzo test specimens [10] (b) Specimens tested by Regan [11] Fig. 1 Typical non-single specimens of edge connections None of the connections had slab shear reinforcement or edge beams, and no moment transferred parallel to the slab edge. The specimens tested by Hawkins et al [5) were subjected to inelastic load reversals simulating earthquake effects. The subassemblies tested by Robertson and Durrani [15) were applied cyclic lateral load on the top of columns to study the load-drift response and interaction between interior and exterior connections. All other specimens collected herein were tested under static loading. Load Compression surface Simply supported r-.-\---i Transverse 10;10 Load plate Reaction! Tension surface ". Transverse load (a) Test specimen by Zaghlool et al [19] (b) Test specimen by Zaghlool et al [22] Fig. 2 Typical specimens of corner connections 530

4 3.2 Corner slab-column connections Twenty-eight data were collected from the literature (Ref [16], and Ref [19] through [23]) involving corner slab-column connections. The detailed experimental information can be found in the corresponding references. Normally, gravity-induced biaxial unbalanced moments are transferred from the slab to the column plus horizontal loads from wind or earthquake forces for corner slab-column connections. The specimens involved in Zaghlool et al [19], Walker and Regan [20], were corner bays of flat plate floors supported on four corner columns. The specimens tested by Ingvarsson [21], Zaghlool et al [22], and Hammill and Ghali [23], were isolated single corner connections. Typical test specimens are shown in Fig ACI for punching strength with moment transfer According to the ACI , the punching shear strength of slabs without shear reinforcement can be determined from the lowest of the following expressions (in SI units) Vc=0.083Xl2+;}ft; (MPa) (1) Vc = 0.083X( ~od + 2].Jf7 (MPa) (2) Vc = x 4.JC' (MPa) (3) where fj is the ratio of the longer side to the shorter side of the concentrated load (or columns), a sis 40 for interior column, 30 for edge columns, and 20 for corner columns. b o is the length of critical shear perimeter taken at a distance of 0.5d away from the column face and has square corners for square columns and round shapes for circular columns. d is the effective depth of slabs. ( is specific concrete cylinder strength, in MPa unit. r c 2 +O.5d=b A IZ r C i g Column centroid Critical section Vul ;" t.lm -V g ~ u u Bi,..-_C.:..=AB==---.-:-.it---'cC""D=---.I!z I Fig. 3 Eccentric shear stress model for edge connections A Column centroid -.2;" Shear stress Fig. 4 Eccentric shear stress model for corner connections 531

5 The ACI presents an analytical method (eccentric shear stress model) to calculate the shear stress when both shear force and unbalanced moment are transferred. It assumes that the shear stresses on the critical section due to the direct shear force can be added to the shear stresses on the same section due to moment transfer. The shear stress due to unbalanced moment is distributed linearly on the critical section. The critical ratio between measured and computed strength for edge connections is the maximum value of three ratios : v AB / v c' VCD / vc' and (1- Yv XM u- Vug}/M r ' where, v AS is the shear stress along critical section AB as shown in Fig. 3; v CD is the shear stress along critical section CD; y v is the fraction of unbalanced moment resisted by shear; (Mu - Vug) is the ultimate unbalanced moment acting at the centroid of the slab critical section; g is the distance between centroids of the slab critical section and the column critical section; M r is the flexural strength of slab reinforcement with a transfer width of c 1 + 3h. The critical ratio between measured and computed strength for corner connections is the maximum value of three ratios: vb/v e ' vc / ve ' and flexural strength ratio, similar to that for edge connections, where, VB is the shear stress at Point B; v c is the shear stress at Point C as shown in Fig Data analyses for all collected specimens 5.1 Edge slab-column connections: Table 1 lists the overall prediction for the forty-six collected data. Note that due to the space limit, the respective prediction of each specimen collected herein is not listed in this paper. The overall prediction includes the average strength ratio, the standard deviation (Stdev) of the strength ratio, and the coefficient of variation (COV). Table 1 Overall prediction according to AC and that with a reduction of Yv for edqe connections (46 data) Method Minimum of Maximum of Average of strength ratio strength ratio strength ratio Stdev COV AC ACI with the proposed reduction of y v According to ACI , analysis of the data collected reveals that calculated strength is governed by limiting shear stresses on the slab critical section rather than flexural yield for nearly all the test specimens, except two specimens (Specimen 5A tested by Hall and Rangan [12]. and Specimen C by Rangan [13]). The calculated strengths of those two specimens are governed by flexural yield. Calculated strengths are almost in all cases conservative, with ratios between measured and calculated strengths ranging from to 2.546, except four specimens, having a mean of and a coefficient of variation of It is interesting to note that even for the two specimens with moment transfer only (Specimen M/E/2 tested by Stamenkovic and Chapmen [3] and Specimen Z-V(4) tested by Zaghlool [4]) the calculated strengths are still governed by the limiting shear stress on the critical section, not by the flexural yielding. Moehle [18] suggested that there is no interaction between shear and moment for edge connections based on the analysis of 27 data he collected. The strong interaction between shear and moment embodied in the ACI is the coefficient of Yv (the fraction of unbalanced moment transferred by shear). Analytical work has been done herein to see how the predictions go by reducing this coefficient y v step by step. The criterion is the value of COV for the data collected in this study. Fig. 5 shows the relationship between C OV and the percent 0 f y v This figure clearly shows that when reducing Yv from 100 percent of ACI defined value, the value of COV becomes smaller and smaller until Yv was reduced to 60 percent of ACI value. After that, the value of COV becomes larger if we continue to reduce this Yv' This behavior suggests that the relationship between shear and moment for edge connections is neither as strong as that represented in the ACI (100 percent of y v should be used), nor as zero as that proposed by Moehle [18]. A 60 percent of ACI defined Yv value should be used for edge connections. Table 1 also lists the overall predictions when considering the reduction of Yv. 532

6 0.300 c: ~.Q.;:: m ~ / m > / "" 0 C / Q) '(3 "!E ~ / Q) U ~ / / Percent of y v Fig. 5 Relationship of value of COY and percent of yv for edge connections It shows that the predictions have been improved much after we use a reduced value of Yv (60 percent of ACI defined value), meaning that a less interaction between shear and moment exists for edge connections. The average strength ratio is 1.236, having a value of COY of Note that there are nine specimens which had strength ratios less than unity when we reduce this yv' This problem can be easily solved by using a slightly larger strength reduction factor, which will not be discussed in this paper. 5.2 Corner slab-column connections: Table 2 lists the overall prediction for the twenty-eight collected data. The overall prediction also includes the average strength ratio, the standard deviation (Stdev) of the strength ratio, and the coefficient of variation (COV). Table 2 Overall prediction according to ACI and that with a reduction of Yv for corner connections (28 data Method Minimum of Maximum of Average of strength ratio strength ratio strength ratio Stdev COy AC ACI with the proposed reduction of Yv According to ACI , analysis of the data collected reveals that calculated strength is governed by limiting shear stresses on the slab critical section rather than flexural yield for all the test specimens. Calculated strengths are in all cases conservative, with ratios between measured and calculated strengths ranging from to 3.441, and having a mean of and COY of The over-conservativeness and scattered trend of the data in Table 2 occurs in part because the analytical model assumes a significant interaction between shear and moment as we discussed in the previous section, which is embodied by the coefficient Yv as defined in the ACI Analytical work has been done similar to that for edge connections to see how the predictions go by reducing this coefficient yv step by step. The criterion is still the value of COY for the data collected for corner connections. Fig. 6 shows the relationship between COY and the percent of Yv' This figure obviously shows that when reducing yv from 100 percent of ACI value, the value of COY becomes smaller and smaller until Yv was reduced to 10 percent of ACI value. After that, the value of COY becomes larger if we continue to reduce this Yv' This behavior suggests that the relationship between shear and moment for corner connections is even less than that for edge connections. In the previous discussion part for edge connection, Yv was reduced to 60 percent of ACI defined value. 533

7 c: o iii ~.;:: rn.:: 0.2BO / o ~ / '(3!EO / (!) o U / ~ / O.BO 1.00 Percent of Yv Fig. 6 Relationship of value of COY and percent of Yv for corner connections The overall predictions following a reduction of Yv (10 percent of ACI defined value) are also listed Table 2. The strength ratio ranges from to 1.687, having a mean of and a value of COY about The accuracy of prediction is highly improved by reducing Yv only. However, there are nine specimens which had strength ratios less than unity, meaning that their strengths are overestimated. This problem can also be easily solved by applying for a larger strength reduction factor in the ACI , which is not discussed in detail in this paper. 6. Conclusions Based on the analysis of available data for exterior connections, including edge and corner connections, the following conclusions may be drawn. For exterior connections the interaction between shear and moment is not as strong as expected. The interaction between shear and moment is even weaker for corner connections than for edge connections. A 60 percent of ACI defined Yv value should be used for edge connections, and 10 percent of that value should be used for corner connections only. Once the reduced value of Yv is used in the ACI , the accuracy of the strength prediction for exterior slab-column connections can be improved greatly. 7. Acknowledgements This research is part of the joint BCA-NTU research on fiat plate structures. Research grants from the Building and Construction Authority - Singapore, and the Nanyang Technological University are gratefully acknowledged. 8. References [1J ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI ) and Commentary (318R-02)," American Concrete Institute, Farmington Hills, Mich., 2002, 443pp. [2J Hanson, N.W. and Hanson, J.M., "Shear and Moment Transfer between Concrete Slabs and Columns," Journal of the Research and Development Laboratories, Portland Cement Association, Vo1.10, NO.1, Jan. 1968, pp [3] Stamenkovic, A. and Chapman, J.C., "Local Strength at Column Heads in Flat Slabs Subjected to A Combined Vertical and Horizontal Loading," Proceedings, Institution of Civil Engineers (London), Part 2, V. 57, June 1974, pp [4] Zaghlool, E.R.F., "Strength and Behavior of Corner and Edge Column-Slab Connections in Reinforced Concrete Flat Plates, " PhD Thesis, Department of Civil Engineering, The University of Calgary, Calgary, 1971, 366pp. [5] Hawkins, N.M.; Wong, C.F.; Yang, C.H., "Slab-Edge Column Connections Transferring High Intensity Reversing Moments Normal to the Edge of the Slab," Structures and Mechanics Report No. SM78-1, Department of Civil Engineering, University of Washington, Seattle, May [6J Kane, K.A., "Some Model Tests on the Punching Action of Reinforced Concrete Slabs at Edge Columns," Honors Project, The Queen's University of Belfast, [7] Lim, F.K. and Rangan, B.v., "Studies on Concrete Slabs with Stud Shear Reinforcement in Vicinity of Edge and Corner Columns," ACI Structural Journal, V. 92, No. 5, Sep.-Oct. 1995, pp

8 [8] Mortin, J.D. and Ghali, A., "Connection of Flat Plates to Edge Columns," ACI Structural Journal, Vol. 88, No.2, 1991, pp [9] EI-Salakawy, E.F.; Polak, M.A.; Soliman, M.H., "Reinforced Concrete Slab-Column Edge Connections with Openings," ACI Structural Journal, Vol. 96, No. 1, January-February 1999, pp [10] Scavuzzo, L., "Shear Reinforcement at Slab-Column Connections in a Reinforced Concrete Flat Plate Structure," The Royal Military College, Kingston, [11] Regan, P.E., "Behavior of Reinforced Concrete Flat Slabs," CIRIA Report No.89, Construction Industry Research and Information Association, London, United Kingdom, [12] Hall, A.S. and Rangan, B.v., "Forces in the Vicinity of Edge Columns in Flat-Slab Floors," Magazine of Concrete Research, Vol. 35, No. 122, Mar. 1983, pp [13] Rangan, B.v., "Tests on Slabs in the Vicinity of Edge Columns," ACI Structural Journal, Vol. 87, No.6, Nov.-Dec. 1990, pp [14] Sherif, A.G. and Dilger, W.H., "Tests of Full-Scale Continuous Reinforced Concrete Flat Slabs," ACI Structural Journal, Vol. 97, No. 3, May-June 2000, pp [15] Robertson, LN. and Durrani, A.J., "Gravity Load Effect on Seismic Behavior of Exterior Slab Column Connections," ACI Structural Journal, Vol. 88, No. 3, May-June 1991, pp [16] Falamaki, M. and Loo, Y.C., "Punching Shear Tests of Half-Scale Reinforced Concrete Flat-Plate Models with Spandrel Beams," ACI Structural Journal, Vol. 89, No.3, May-June 1992, pp [17] Gardner, N.J. and Shao, X.Y., "Punching Shear of Continuous Flat Reinforced Concrete Slabs," ACI Structural Journal, Vol. 93, No. 2, March-April 1996, pp [18] Moehle, J.P., "Strength of Slab-Column Edge Connections," ACI Structural Journal, Vol.85, No.1, Jan.-Feb. 1988, pp [19] Zaghlool, E.R.F.; Paiva, H. A. R.; Glockner, P.G., "Tests of Reinforced Concrete Flat Plate Floors," Journal of the Structural Division, Proceedings of the American Society of Civil Engineers, Vol. 96, No. ST3, Mar 1970, pp [20] Walker, P.R. and Regan, P.E., "Corner Column-Slab Connections in Concrete Flat Plates," Journal of Structural Engineering, Vol. 113, No.4, April 1987, pp [21] Ingvarsson, H., "Experimentellt Studium Av Betongplattor Understodda Av Hornpelare, (Experimental Study of Concrete Slabs Supported by Corner Columns)" Meddelande Nr.111, Institutionen For Byggnadsstatik, Kungliga Tekniska Hogskolon, Stockholm, 1974, 26pp. [22] Zaghlool, E.R.F.; Paiva, H.A.R.; Glockner, P.G., "Tests of Flat-Plate Corner-Slab Connections," Journal of the Structural Division, Proceedings of the American Society of Civil Engineers, Vol. 99, No. ST3, Mar (2), pp [23] Hammill, N. and Ghali, A., "Punching Shear Resistance of Corner Slab-Column Connections," ACI Structural Journal, Vol.91, No.6, Nov.-Dec. 1994, pp