Punching shear strength of RC flat slabs at interior connections to columns

Abstract

A method to evaluate the punching shear strength of reinforced concrete (RC) flat slabs without shear reinforcement at the connection to interior columns is proposed. The method is based on the assumption that the punching shear strength is controlled by the inclination of a unique punching shear crack that produces a conical failure surface.The inclination angle of the crack is variable. It follows that the location of the critical section is not established in advance, but changes with crack angle. Its location depends on the reinforcement ratio, material strengths and effective depth. According to the proposed method, the punching shear strength is a function of the inclination angle of the governing crack that controls the amount of shear carried by the compression zone and the flexural reinforcement crossing the potential punching cone by accounting for its slenderness and concrete size effect. The method describes the behaviour observed in tests and numerical and analytical investigations. The novel premise that the punching shear strength of flat slabs at connections to interior columns is controlled by the inclination of the failure surface shows remarkable agreement with the results of 209 tests on isolated specimens reported in the literature. This paper also assesses the adequacy of strength predictions obtained using the proposed method and the methods adopted in the codified provisions.

Structural performance of RC flat slabs connected to steel columns with shear heads

“…Before failure, the forces are transferred from the column to the slab through a tri-dimensional strut that develops at variable inclination angles from the root of the column to the tension reinforcement, as a function of the slab thickness, flexural characteristics and material strengths [30]. High stress levels in the strut activate inclined cracking that eventually leads to failure.…”

Experimental and numerical assessment of mixed RC beam and steel column systems

“…The dilation angles (ψ) may vary between ψ = 10° for RC corbels [26] to ψ = 40° for RC flat slabs [27,28] as a function of the structural configuration and modes of failure investigated. In this study, as illustrated in Figure 7b, a variation of ψ between 10° and 55°, shows that the best estimates for both peak strength and deformational behaviour are obtained for a dilation angle of ψ = 25°.…”

Failure assessment of flat slab-to-column members

“…In the case of slabs without transverse reinforcement, the punching shear failure is characterised by low stresses in the longitudinal reinforcement and the development of a diagonal crack with variable inclination, starting from the root of the column to the tension face of the slab (Figure 2(a)). The inclination of the failure surface is dependent on the reinforcement ratio, slab thickness and material strength and characteristics (Bompa and Oneţ, 2015). When transverse reinforcement is present, the critical crack might intersect the transverse bars and result in yielding of the transverse reinforcement.…”

“…For the specimens in the present study it was found that the ACI 318-14 design code gives conservative estimates, whereas EC2, Model Code 2010-LoA3 and the CSCT give similar results. A previous study on an extended test database of isolated slab specimens without transverse reinforcement indicated that the punching shear strength decreases with decreasing concrete strength and effective bending depth/isolated member moment span (d/l s ) ratio, and increases with the flexural reinforcement ratio (Bompa and Oneţ, 2015) (the moment span of an isolated slab specimen is the distance between the supports, and is a function of the load arrangement and test setup). In a similar manner, all the tests reported in this paper, regardless of the failure mode, were compared as a function of the varied structural parameters.…”

“…In a similar manner, all the tests reported in this paper, regardless of the failure mode, were compared as a function of the varied structural parameters. In Figure 12 the investigated parameters are plotted against the recorded test strength V test in relation to member effective depth d, concrete strength f c and critical crack length l 0 , which is dependent on the inclination of the punching shear crack (Bompa and Oneţ, 2015). The data for members that failed in punching shear are plotted as dots transverse reinforcement DB5 495 421 1·18 504 0·98 498 0·99 485 1·02 AG1 570 500 1·14 395 1·44 542 1·05 521 1·09 AG2 872 687 1·27 526 1·66 744 1·17 706 1·24 With transverse reinforcement AG4 727 687 1·06 495 1·47 734 0·99 751 0·97 AG5 1008 900 1·12 621 1·62 884 1·14 882 1·14 AG6 1328 1097 1·21 733 1·81 1006 1·32 1020 1·30 Average 1·16 1·50 1·11 1·13 COV 0·06 0·19 0·11 0·11 Table 3.…”