Civil Engineering Reference
In-Depth Information
render torsional effects more likely to occur. Asynchronous motions at the foundation of build-
ing structures may also be caused by high width- to - depth ratios.
(iii)
Distribution and concentration:
vertical and plan distribution of stiffness and mass is important
to achieve adequate seismic performance. In tall and slender buildings, lateral deformability
reduces the earthquake-induced forces. Problems related to defl ection control may arise,
however, in earthquake and wind response of high-rise structures. Low-rise buildings should be
fl exible to reduce the shear forces due to ground motions. Tall buildings should be stiff to control
the lateral deformations. Seismic motions are multi-dimensional, thus structures need to be able
to resist the imposed loads and deformations in any direction. Adequate distributions of struc-
tural systems to resist loads (vertical and lateral) can prevent concentrations of inelastic demands.
Structural elements can be arranged in orthogonal directions to ensure similar stiffness and
resistance characteristics in both main directions, i.e. they should possess bidirectional resistance
and stiffness.
(iv)
Perimeter resistance:
torsional motion tends to stress lateral resisting systems non- uniformly.
High earthquake-induced torsional moments can be withstood by lateral resisting components
located along the perimeter of the structure as displayed in Figure A.1. Perimeter columns and
walls create, for instance, structural confi gurations with high rigidity and strength (also referred
to as 'torsional stiffness and resistance'). The location in plan of systems for earthquake resis-
tance signifi cantly infl uences the dynamic response. The higher the radius of gyration of the
plan layout of the structure, the higher the lever arm to resist overturning moments. In framed
systems, the bending stiffness is signifi cantly affected by the layout of columns in plan and
elevation. Frames employing perimeter columns possess high bending stiffness and resistance;
this is also true for frame- wall systems.
B
B
B
B
Figure A.1
Confi gurations with different perimeter resistance: low (
left
) and high (
right
) torsional resistance
