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553
IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
J. M. CALIXTO | T. H. SOUZA | E. V. MAIA
Analysis of the effects of concrete compressive strength
f
c
, of the
column slenderness ratio
l
and of the dimensionless axial force
u
on the ratio
M test
/
M pred
were also done in this case for both
approximate design procedures. For both design methods, the re-
sults (Figure 2) showed a slight reduction in the
M test
/
M pred
ratio
with increasing values of the dimensionless axial force
u
. For the
other two parameters no trend was found.
The analysis of these results also indicates that NBR 6118 [1] ap-
proximate stiffness criterion provides better predicting results in
terms of safety, precision and economy for columns made with
concrete of
f
c
> 55 MPa
.
4. Concluding remarks
The goal of this paper was to analyze with respect to safety, preci-
sion and economy NBR 6118 [1] approximate design criteria for
slender rectangular RC columns subjected to eccentric loads by
comparing to experimental results. The study includes columns
built with conventional concrete (
f
c
≤ 55 MPa) and with concrete
having compressive strength above 55 MPa. The simplified meth-
ods prescribed by NBR 6118 [1] analyzed herein were -
approxi-
mate curvature method and approximate stiffness procedure
. For
the comparative study a database was created containing the re-
sults of columns tested in the laboratory.
The overall analysis of the comparative study indicates that the ap-
proximate stiffness procedure provides better results than the ap-
proximate curvature method in relation to the experimental results
of columns built with conventional concrete (
f
c
≤ 55 MPa) and fab-
ricated with concrete having compressive strength above 55 MPa.
5. Acknowledgements
The authors would like to thank Universidade Federal de Minas
Gerais (UFMG) and Fundação de Amparo à Pesquisa de Minas
Gerais (FAPEMIG) for the financial support.
6. Notation
A c
=
gross area of column cross-section.
b
=
side dimension of column cross section.
f c
=
concrete cylindrical compressive strength at time of tests.
f cd
=
design concrete compressive strength.
h
=
overall column height.
l e
=
column unsupported length.
M d,tot
=
total bending moment in midheight section.
M 1d,A
=
column maximum factored end moment.
N d
=
factored axial force.
a b
=
factor relating actual moment diagram to an equivalent uni-
form moment diagram.
l
=
slenderness ratio.
n
=
dimensionless axial force.
r
l
=
longitudinal steel ratio.
7. References
[01] Associação Brasileira de Normas Técnicas,
NBR 6118 – Projeto de Estruturas de Concreto -
Procedimento, Rio de Janeiro, 2007.
[02] SCADELAI, M. A., Dimensionamento de Pilares de
acordo com a NBR 6118:2003, Master’s Dissertation,
Escola de Engenharia de São Carlos, Universidade de
São Paulo, São Carlos, 2004, 124 p.
[03] SOUZA, T.H
.,
Projeto de Pilares Retangulares
Esbeltos de Concreto Armado pelos Métodos
Aproximados da NBR 6118, Final Civil Engineering
Undergraduate Report, Escola de Engenharia,
Figure 2 – M /M versus dimensionless axial force
n
test
pred
Approximate curvature method
Approximate stiffness procedure
A
B
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
0,00
0,20
0,40
0,60
0,80
n
M
test
/ M
pred
0,0
0,3
0,6
0,9
1,2
1,5
1,8
0,00
0,20
0,40
0,60
0,80
n
M
test
/ M
pr ed