Abstract:
Frame structure is most common in tall buildings due to simplicity in design and
construction. In Bangladesh brick walls are used in frame structures as exterior walls
and partitions. Present code of practice does not include provision of taking into
consideration the structural effect of infill in multistoried building construction. The
presence of infill increases the lateral stiffness of the structure and reduces the
amount of deflection. If this effect is taken into consideration, the resulting structure
will be economic.
This study has been undertaken to investigate the sway characteristics of
common reinforced concrete frames in presence of brick masonry infill subjected to
lateral loads. Here, a seven story and an eleven story building frames are taken as
reference structures. Finite element modeling is used to model the frame. Common
three dimcnsional frame elements are used to model the beams and columns. The inplane
stiffness of brick wall contributing to the stiffness of the frame against lateral
load is modelled by the equivalent strut model proposed by Saneinejad and Hobbs
(1995). Their model consisted of a bi-linear force-displacement relationship with an
initial stiffness called Ko. In this study, instead of the bi-linear relationship, a nonlinear
constitutive relation is proposed and used to simulate the force vs.
displacement behavior of the diagonal strut as infill. In the FE model the struts are
incorporated using special link element having only axial stiffness. Lateral loads
(wind load) are calculated as described in BNBC, 1993.
A parametric study was performed using various parameters of frame and
infill to see their effects on stiffness properties of infill and deflection characteristics
of the frame. A few parametcrs are found to have significant effect on the infill
characteristics while the other parameters do not have any appreciable effect. While studying the deflection characteristics, it is observed that deflection in frames with
infill is much smaller than that with frames without infil!. On the average, lateral
sway is reduced by about thirty percent if only one third of the panels of the frames
contain infil!. It proves that the sway predicted by ordinary frame analysis
overestimates the sway by a significant amount. A suggestion is thus made that after
computing sway by a ordinary frame analysis, it may be reduced by at least thirty
percent before it is compared with the maximum allowable sway specified by code.
