Civil Engineering Reference
In-Depth Information
(v) System Properties
The lateral stiffness of a structure depends on the type of system utilized to withstand horizontal earth-
quake loads, the distribution of the member stiffness and the type of horizontal diaphragms connecting
vertical members. For example, moment-resisting frames (MRFs) are generally more fl exible than
braced frames. The latter class includes concentrically (CBFs) and eccentrically (EBFs) braced frames.
Structural walls are stiffer than all types of frames. Frames with rigid connections exhibit higher stiff-
ness than those with semi-rigid connections (
see
also Figure 2.13). A detailed description of horizontal
and vertical structural systems for earthquake resistance is provided in Appendix A. It suffi ces here to
state that uniform distribution of stiffness in plan and elevation is necessary to prevent localization of
high seismic demand. Soil-structure interaction should also be accounted for in the evaluation of the
global system stiffness. This type of interaction reduces the stiffness of the superstructure and may alter
the distribution of seismic actions and deformations under earthquake ground motions (e.g. Mylonakis
and Gazetas, 2000, among others).
2.3.1.2 Effects on Action and Deformation Distributions
Inertial forces caused by earthquake motion are distributed among lateral resisting systems in the
elastic range as a function of their relative stiffness and mass. The higher the stiffness, the higher the
load attracted for a given target deformation. Stiffer elements and structural systems will reach their
capacity earlier than their fl exible counterparts. Signifi cant reductions of the initial (elastic) stiffness
may occur in construction materials, structural members and connections, when they are subjected to
increasing loads. Repeated and reversed loading also reduces effective stiffness; an observation termed
' stiffness degradation '. Effects of stiffness on the distribution of actions and deformations are discussed
below.
The lateral deformability of structural systems is measured through the horizontal drift. In buildings,
storey drifts Δ are the absolute displacements of any fl oor relative to the base, while inter- storey drifts
δ
defi ne the relative lateral displacements between two consecutive fl oors (Figure 2.14). The inter- storey
drifts are generally expressed as ratios
δ
/
h
of displacement
δ
to storey height
h
. Drifts of the roof Δ
normalized by the total height
H
of the building (roof drifts, Δ /
H
) are also used to quantify the lateral
stiffness of structural systems. The roof drift ratio Δ /
H
may be considered
δ
/
h
averaged along the height
and hence is not suitable for quantifying variations of stiffness in the earthquake- resisting system. In
Δ
6
δ
i
Uniform distribution
Non-uniform distribution
5
Storey drift (d /
h
)
4
Roof drift (
Δ
/
h
)
3
2
1
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
L
L
L
Lateral drift (%)
Figure 2.14
Lateral drifts of multi-storey buildings under earthquake loads: defi nition of inter-storey and roof drift
(
left
) and their relationship for uniform and non-uniform lateral stiffness distribution along the frame height
(
right
)
