Civil Engineering Reference
In-Depth Information
Spandrel
l
b
l
o
l
b
l
b
l
o
l
b
L
L
w
w
Figure A.16
Walls with small (
left
) and large (
right
) openings
Favourable locations in plan of SWs, with or without openings, are along the perimeter of the plan
layout of the structure, as discussed in Section A.1.1. Arrangements of SWs in plan, which lead to
closed cross sections (or thin-walled sections) for wall systems, possess high torsional rigidity and are
desirable for adequate earthquake response. Cores employing SWs are also suitable for services, such
as stairwells and lifts. Lateral force-resisting systems employing SWs are generally cost-effective for
RC, steel and composite buildings with a number of storeys up to 25-30 (Di Sarno, 2002 ).
(iv) Hybrid Frames
Rigid moment -resisting frames are ductile systems with high resistance, but their lateral stiffness is
often inadequate to prevent large drifts under earthquake forces. To reduce storey and roof drifts, MRFs
are often connected to bracing systems or structural walls (also known as 'hybrid frames or dual
systems'). It is generally cost-effective for hybrid frames (HFs) to employ frames that are designed for
gravity loads only, while horizontal forces are resisted by bracing systems, e.g. braced frames, or
structural walls. However, under lateral earthquake loads, frames and bracing systems and frames and
walls interact to withstand seismic actions (Ghoubhir, 1984). This interaction varies along the height
of the structure; it also depends upon the type and the stiffness of structural components used to connect
the two components of the HFs, e.g. struts or beam with rigid or semi-rigid connections. For example,
if beams with rigid joints are used, bending moments, shear and axial loads can be transferred from
the bracing system or wall to the frame and vice versa. Under lateral forces, beams and joints connect-
ing the components of the HFs undergo large deformations: high ductile details are required to achieve
satisfactory seismic performance, especially in areas of high seismicity. Columns of systems employing
moment-resisting and braced frames are subjected to high axial loads, which are caused by overturning
