Page 101 - Riem-Vol6_nº2

272 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Experimental and numerical analysis of reinforced concrete mushroom slabs
plexity. Also, the quality of predicted results didn’t justified its use
instead of the simpler empirical methods presented by codes of
practice. Yet this method is still one of the most significant contribu-
tions ever made to the subject.
1.2 Research significance
Many experimental researches have been developed, mostly fo-
cused at evaluating the contribution of different types of shear rein-
forcement in the punching resistance of flat slabs. Few experimen-
tal results are available about the contribution of column capitals
or drop panels (Wey [8] and Hueste at al. [9]) and those available
are more focused in evaluating the ductility of slab-column con-
nections in case of earthquakes (cyclic loading). Design recom-
mendations presented by codes of practice for mushroom slabs
are superficial and strongly influenced by tests on footings. Thus, it
is evident the importance of providing experimental results on the
behavior and strength of mushroom slabs.
1.3 Methodology
This paper presents the results of four experimental tests, one of
a flat slab tested as reference and three tests in mushroom slabs
with circular column capitals. Experimental results are compared
with theoretical values obtained using recommendations of NBR
6118 [10], EUROCODE 2 [11] and of the Critical Shear Crack
Theory as presented by Muttoni [12]. Results of a computational
analysis using nonlinear FEA commercial software MIDAS are also
presented in order to better understand the behavior and failure
mechanism of mushroom slabs.
2. Theoretical methods for estimation
of punching resistance
2.1 NBR 6118 (2007)
NBR 6118 [10] recommends that the punching resistance of
mushroom slabs must be checked in three regions of the slab-
any beams. It was common to observe significant variations in the
flexural reinforcement ratio between different patented systems.
According to Melo [4], in 1911 the first structural accident was reg-
istered in a building with flat slabs. It was the collapse of Prest-o-
Lite building in the U.S.A., where a local punching failure led the
whole building to ruin, evidencing the need for research on the
behavior and resistance of slab-column connections.
The first experimental study that provided information for the de-
sign of slab-column connections was performed by Talbot [5],
which in fact tested concrete footings and observed that in many
cases they failed by punching forming a truncated failure cone with
faces tilted around 45 degrees(influenced by the high thickness
of the footings). He also noticed at that time that the flexural rein-
forcement ratio could significantly influence the ultimate punching
strength, which was later observed by Richart [6].
Only in 1960 it was presented the first theoretical model to explain
the punching failure mechanism and predict the ultimate strength of
slab-column connections. This model was presented by Kinnunen
and Nylander [7] and was based on experimental observations ob-
tained after conducting an extensive experimental program. The
authors observed that the portion of the slab outside the failure
surface showed rigid body rotations and created a model aiming
to satisfy the forces equilibrium (see Figure 3a). In this model, the
slab segments are treated as rigid bodies supposedly supported
on a conical imaginary shell confined between the column and the
shear crack.
Under load, each segment rotates around a point of rotation (CR)
and is supported by the forces shown in Figure 3b, with the internal
forces being a function of rotation (ψ ) of the slab. According to the
authors failure occurs when a point in the bottom surface of the
slab, located vertically below the root of the shear crack reaches a
critical radial strain (εcto) at the same time that the tangential strains
in concrete and in the imaginary conical shell reach characteristic
ultimate strain for concrete. This model was initially developed for
the case of axisymmetric reinforcement, but as in practice flexural
reinforcement are arranged orthogonally, Kinnunen and Nylander
[7] presented changes to the model. The method was always con-
sidered difficult to use in practice, due to its high level of com-
Figure 2 – Mushroom slab with a combination of drop panel and capital in a garage building in Brazil