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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5
P. V. P. SACRAMENTO | M. P. FERREIRA | D. R. C. OLIVEIRA | G. S. S. A. MELO
the results are against safety. One of these results, represented by
point without filling in the graphs, is below of the design strength
estimated by ACI. It refers to slab HSC 9 from Hallgren [17], in
which a small flexural reinforcement ratio was used (0.3%) and,
although not specified by the author, is possibly a slab that failed
by flexural.
Figure 12 and Figure 13 show comparisons of experimental re-
sults with those obtained using recommendations of NBR 6118
and Eurocode 2, respectively. It is possible to perceive that Euro-
code, which presents recommendations similar to NB1, but with
limitations on the value of size effect (
ξ
≤ 2.0) and of the flexural
reinforcement ratio (
ρ
≤ 2%) shows about 11% of unsafe results,
but no results below the line of the design strength. However, NB1
presents average nominal strength close to the experimental re-
sults, with no results below the design strength, but is far from me-
eting the limit of only 5% of unsafe results. In Figure 14 are shown
comparisons with results obtained according to CSCT. It may be
noted that 11% of results are below the nominal strength, but no
result is below the design strength. Figure 15 shows graphs with
the tendency of the results of codes and CSCT compared with
experimental results of 74 slabs from the database. It can be seen
that the dispersion of these results, when using the recommenda-
tions of NBR 6118, is very small.
Table 2 summarizes comparisons between the experimental and
theoretical results. It is possible to perceive that the recommenda-
tions of ACI are conservative and show a high coefficient of varia-
tion if compared to the other methods due to the fact that the only
parameter used to estimate the punching strength of flat slabs is
the compression strength of concrete. However, this code presen-
ted only 5% of unsafe results, which is suitable for a code of prac-
tice. Both Eurocode and CSCT showed satisfactory accuracy with
CSCT presenting results slightly more accurate. By correlating the
punching resistance with the flexural behavior, CSCT was more
sensitive to variables, presenting a lower coefficient of variation.
Results from the Brazilian code indicate that its recommendations must
be reviewed. At the same time that it showed the smallest average (1.01)
and lower coefficient of variation (0.11), the Brazilian code presented
about 47% of results below the nominal strength, indicating that its equa-
tions need some adjustment. Many proposals could be, but undoubtedly
the one that requires lowest level of changes and that could eliminate this
Figure �� � Com�arison of results from database with those using recommendations of EUROCODE 2 [8]