Lee, Kim, and Song 2008

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This paper summarizes full-scale test results on CFT (concrete filled tube) column to RC (reinforced concrete) flat plate connections subjected to gravity loading. Several connection scheme based on strategies that maximize economical field construction were proposed and tested in this study. A semi-analytical procedure is presented to model the behavior of CFT column to RC flat plate connections. The paper focuses on punching behavior as part of on-going research concerning CFT column to RC flat plate connections.

Experimental Study, Results, and Discussions

Structural integrity and ease of construction were the primary concerns in connection detail design; this included exploring designs that did not require field welding. Reliable transfer of shear forces between column faces and slabs was attempted through use of a shear key. Full scale specimens were tested to obtain reliable results.

Each specimen represented a slab-column joint of a parking structure with a span length of 236 in. long in each direction, 7.87-in-deep flat plates and 15.75-in-square columns. The slab was designed using the ACI direct design method to resist live and dead loads for an indoor parking area according to AIK 2000 code. Design compressive strength of concrete for the specimens was assumed as 3.92 ksi. The column strip of the flat plate was reinforced using D16 bars with a reinforcement ratio of 0.64%. Two types of shear keys were tested, a wide flange shear key and two tee shear keys. The wide flange key was sized following the ACI design provisions for shear heads with a section of H100 x 100 x 6 x 8. The tee section was built using 0.354-in-thick steel plates. Three types of connection details were also tested. A full penetration type, in which longitudinal bars completely pass the CFT column through the holes on both sides of the steel box; a half penetration type, where separate longitudinal bars pass through the holes on each side of the steel box but stop at the inner face on the other side of the steel box such that the lap splice between the bars from each side is formed; and a hook anchorage type, where longitudinal bars are hook-anchored into the hole of the tension tab which is shop-welded to the outside of the column were the connection details considered. The wall thickness of each column was 1.575 in. in all specimens except one, which had a 0.787 in. thick wall.

The column-slab specimens were placed upside down and monotonic downward vertical loading was applied to the top of the column to simulate gravity load acting on the slabs. Four edges of the slab were simply supported and lateral movement was restricted to simulate the inflection lines. Tests demonstrated that all specimens showed satisfactory punching shear strength and connection stiffness which generally exceeded those of reinforced concrete benchmark specimens, demonstrating that the design provisions for strength and stiffness in the ACI code can be used for the proposed connections. Specimens with post-punching bars exhibited higher strength; a combination of full penetration of reinforcing bars and wide flange shear key exhibited best strength. Joint stiffness of all CFT specimens using the proposed details was higher than that of the benchmark specimens. It was found that the tension bars almost equally participate in resisting external load after punching failure.

Semi-Analytical Study

A model to describe the local catenary action after punching failure is proposed and its relevant parameters are calibrated based on the test data of this study. It is presumed that, after punching failure, tension bars and post-punching bars become kinked at the damaged regions around the column and the gravity loads are resisted by the vertical force components of these kinked bars. The kink angles can be back-calculated using test results from the study.

Generally, it is very difficult to analytically describe the behavior of CFT column-flat plates from the elastic phase to punching and subsequent local catenary range since the complicated behavior of the connection, and highly nonlinear and large deformation should be accounted for. Thus, a semi-analytical model was used. The model parameters include: punching strength, initial secant stiffness until punching failure, a negative stiffness right after punching failure, a positive stiffness for a region some time after punching failure and before the formation of local catenary mechanism, and residual strength.

References

Lee, C.-H., Kim, J.-W., and Song, J.-G. (2008). “Punching Shear Strength and Post-Punching Behavior of CFT Column to RC Flat Plate Connections.” Journal of Constructional Steel Research, Vol. 64, No. 4, pp. 418-428. doi:10.1016/j.jcsr.2007.08.003