Keywords: theoretical reinforced concrete models, interior columns, floors, slab load, aspect ratio.

Interior column loads transmission through floors were first viewed in the early 60's [3]. Three ACI code [1] design strategies were devised for tackling the load transfer mechanism problem with a difference in the column and floor concretes strengths. The third strategy is related to interior column loads transmission, approaching the column-slab joint design using an "effective" concrete strength. Because the joint is confined to some degree by the surrounding slab, the effective joint concrete strength is higher than its cylinder strength and expected to be lower than the column concrete strength. Following the ACI code provisions [1], the Canadian Standards [2] have developed a model for interior column-slab joint effective strength estimation. However, the ACI code [1] provisions have been found unconservative, while CSA A23.3-94 [2] conservative [4]. The above design provisions ([1,2,4]) are based on the laboratory test results with unloaded slabs ([3,4]) that do not properly model the joint confinement conditions. It has been observed recently ([5,6,7]), that applying a slab load reduces the effective strength values. If any realistic amount of the slab load have been applied to the specimens ([3,4]), the measured effective strength values would have been smaller than those reported.

This paper explains the tests results of eight RC columns with modeled confinement conditions. The effects on high performance concrete (HPC) columns due to the normal strength concrete (NSC) floor layer in between were investigated. The specimens consisted of six interior columns, one sandwich column, and one isolated column built completely with HPC. Interior and sandwich columns had slab layers of NSC with a thickness of 120-240 mm at mid height while the isolated column had no such layer. The current experimental data combined with the previous studies was analyzed to find the effective concrete strength estimation parameters for an interior column simulating extreme loading on the slab with service axial load on the column. The effects of slab load intensity, joint aspect ratio, slab reinforcement ratio, and slab portions surrounding the column-slab connection confinement on the interior column-slab joint strength were profoundly investigated.

A relationship between effective strength ratio and joint strength ratio was established in this study. The results show that the effective strength decreases as slab load increases. The effective strength values have also decreased upon increasing the joint aspect ratio. Increase in the slab steel ratio and presence of surrounding slab confinement enormously enhance the column-slab connection strength and ductility.

It was confirmed that the ACI code [1] provisions overestimate the effective strength for high joint strength and aspect ratios. The Canadian Standards [2] are excessively conservative, for low aspect ratios. The design equation [5] has predicted the most appropriate and reliable effective strength for loaded slabs. However, this equation has been derived for relatively low aspect ratio, while during the current investigation it was observed that increase in the aspect ratio increases the slab load and enhances more effectively the interior column joint confinement conditions.

It is recommended to develop the design equation for high joint aspect ratios. The design relationship should not only be a function of joint aspect ratio, column concrete strength, and floor concrete strength but the effects of slab reinforcement ratio and surrounding slab confinement should also be considered as essential parameters in estimating the interior column joint effective strength.

1

ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-95) and Commentary (318R-95)", American Concrete Institute, Farmington Hills, Mich., 1995.

2

Canadian Standards Association, CSA A23.3-94, "Design of Concrete Structures, CSA", Rexdale, Ontario, 1994.

3

A.C. Bianchini, R.E. Woods, and C.E. Kesler, "Effect of Floor Concrete Strength on Column Strength", ACI Journal, 31(11), 1149-1169, 1960.

4

W.L. Gamble, and J.D. Klinar, "Tests of High Strength Concrete Columns with Intervening Floor Slabs", Journal of Structural Engineering, ASCE, 117(5), 1462-1476, 1991. doi:10.1061/(ASCE)0733-9445(1991)117:5(1462)

5

C.E. Ospina. and S.D.B. Alexander, "Transmission of Interior Concrete Column Loads Through Floors", Journal of Structural Engineering, ASCE, 124(6), 602-610, 1998. doi:10.1061/(ASCE)0733-9445(1998)124:6(602)

6

F. Jungwirth, "Knotenpunkt: normalfeste Decke-hochfeste Ortbetonstütze", Leipzig Annual Civil Engineering Report, 3, 165-174, 1998.

7

P.J. McHarg, W.D. Cook, D.Mitchell, and Y.-S. Yoon, "Improved Transmission of High-Strength Concrete Column Loads through Normal Strength Concrete Slabs", ACI Structural Journal, 97(1), 2000, 157-165.

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