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IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 1
Considerations about the determination of
g
z
coefficient
beams. In this way, in this model the average slab plane coincides
with the beam axis, figure [6], since the “beam 4” element does not
allow eccentricities to be considered.
In the third model, the columns and beams are depicted by
means of the “beam 4” element and the slabs are treated as rigid
diaphragms, that is, it is accepted that they have infinite stiffness
on their own plane and nil stiffness crosswise. In the ANSYS-9.0
software [1], the hypothesis of a rigid diaphragm is embodied in
the model by means of a specific command which relates the
degrees of freedom of the nodes making up the slab plane. Thus,
a “master” node, corresponding to the point representing all the
storey’s nodes is defined. The remaining nodes, called “slaves”,
have their own degrees of freedom and those represented by the
“master” node.
The fourth model, like the previous one, is also made up
of bars (depicting the columns and beams by means of the
“beam4” element), but without considering the hypothesis of a
rigid diaphragm.
Finally, the last model only differs from the previous one because
the “beam4” element is replaced by the “beam44” element to de-
pict the beams, whereby the eccentricity existing between the
beam axis and the average slab plane can be considered.
It can be seen, then, that in models 3,4, and 5 the structural sys-
tem just consists of bars, since the slabs are not modelled (unlike
models 1 and 2 in which the slabs are depicted by means of shell
elements). In all the models, the beams’ torsional stiffness was
reduced, by reproducing the cracking effect.
Table [1] sums up the main characteristics of the models employed.
5.2 Design considerations
The actions working on the buildings are divided into two groups:
vertical actions and horizontal actions.
Vertical actions consist of permanent loads and the accidental
load. The permanent loads considered were the own weights of
structures, the masonry loads and the slab coatings and finishings.
The accidental loads were determined in accordance with the pre-
cepts of NBR 6120:1980 [12].
The chief horizontal actions that must be taken into account in the
structural project are the forces due to the wind and those relating
to geometric imperfections (out-of-plumb). However, according to
NBR 6118:2007 [2], these loadings do not need to be overlapped
and only the most unfavorable (the one causing the greatest total
moment at the structure base) may be considered. According to
Rodrigues Junior [13], for tall buildings, just as with the main vari-
able load choice, it is possible to prove that, in most practical cas-
es, the wind corresponds to the most unfavorable situation. In this
way, in this paper, the horizontal loading applied to the structures
Figure 2 – Typical storey of building I (adapted from Costa [10])
Beams - V1 to V8: 20/60
Columns - P1 to P15
Measurements in cm
Slabs - L1 to L8: h = 10