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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5
Punching strength of reinforced concrete flat slabs without shear reinforcement
1. Introduction
Flat slabs are those which are directly supported on columns wi-
thout capitals. They can be considered as a good option for con-
crete buildings since they may reduce the construction time due to
the simplification of forms and rebars and especially by attributing
greater flexibility in layout of floors. The design of slab-column con-
nection is the most critical point in the design of flat slabs, because
of the concentration of shear stresses in this region that can lead
to punching, which is a localized failure mode that can occur wi-
thout significant warnings and may lead the whole structure to ruin
through the progressive collapse. Figure 1a shows an example of
punching failure recorded by Ferreira [1]. One way to ensure local
ductility and prevent progressive collapse of flat slabs is through
the use of post-punching reinforcement as those shown in Figure
1b, which must be designed to carry the vertical reaction in the
column, and must be detailed in order to ensure that they are suffi-
ciently anchored beyond the region of the possible punching cone.
Since tests carried by Elstner and Hognestad [2] many other stu-
dies have been conducted aiming to understand the behavior and
strength of flat slabs. Some theoretical methods were proposed
but none was generally accepted because they were not able to
accurately estimate the punching resistance of slab-column con-
nections and at the same time explains the phenomenon with all its
variables. Thus, the design of flat slabs to the punching is normally
done using recommendations presented by codes of practice for
design of concrete structures, which are essentially empirical.
Recently Muttoni [3] presented a new theoretical approach called Cri-
tical Shear Crack Theory (CSCT), which is able not only of predicting
the bearing capacity of slab-column connection, but also of estimating
their behavior in service (rotation, displacements and strains). This the-
ory is based on the idea that the punching resistance decreases with
increasing rotation of the slab, and has recently been embodied in the
first draft of the new fib Model Code [4,5], which was presented in 2010,
and has come to replace the old CEB-FIP MC90 [6]. This paper aims
to evaluate this method by comparing its theoretical results with experi-
mental results of tests of 74 reinforced concrete flat slabs without shear
reinforcement carefully selected (see section 6 of article) to form a lar-
ge database, with specimens with a significant variation of parameters
such as the effective depth, flexural reinforcement ratio and compressi-
ve strength of concrete. These experimental results were also compa-
red with the theoretical results obtained by using the recommendations
of ACI 318 [7], EUROCODE 2 [8] and NBR 6118 [9].
2. Historical development of flat slabs
There is controversy about who idealized the flat slabs structural
system. Gasparini [10] states that the credit for the development of
this system should be given to George M. Hill, an engineer who re-
portedly designed and built constructions like filtration plants and
storehouses in different regions of the United States between 1899
and 1901. He emphasizes, however, that C A. P. Turner, an Ameri-
can inventor and engineer, was the one responsible for demonstra-
ting that these slabs were reliable with numerous buildings construc-
ted, the first being Johnson-Bovey building in the city of Minneapolis
in 1906. Turner’s “mushroom” slab were characterized by the pre-
sence of capitals in the slab-column connection and also by the use
a cage comprising bars of 32 mm diameter, working as shearheads.
Furst and Marti [11] attributed the invention of this system to the
Swiss engineer Robert Maillart, most famous for his works with
bridges than the development of such structural system. According
to these researchers, Maillart would have designed the system in
1900, but had only completed his tests in 1908, coming to get the
patent of the system in 1909. Kierdorf [12] points out that while
the system was developed independently in the United States and
Switzerland, with the prohibition of the use of reinforced concre-
te in Russia in 1905, the engineer Arthur F. Loleit designed and
implemented a factory nearby at the Moscow in 1907 in flat sla-
bs, have been the first of several buildings in slabs without beams
made by him in Russia. The author further comments that if his
presentations of “beamless” construction at the regular meeting of
cement specialists in Moscow (1912) and to the Russian Society
for Materials Research (1913) had been documented, and also if
WWI (1914-1918) had not happened, Loleit would certainly have
presented his work to a broader public. Some details on the deve-
lopment of flat slabs can be seen in Figure 2.
Figure 1 � Punching shear failure in slabs without shear reinforcement (Ferreira [1])
Punching shear failure
Without
post-punching
reinforcement
With
post-punching
reinforcement
A
B
C