Response of Structures - Fundamentals of Earthquake Engineering - page 72

2.0

Elastic
Inelastic (D)
Inelastic (2D)

1.6

1.2

0.8

0.4

0.0

H030 M030 M015 L015 H030 M030 M015 L015 H030 M030 M015 L015

IF RF FW

Figure 2.22 Elastic and inelastic (at the design, D, and twice the design, 2D, ground acceleration) predominant
response periods of the buildings - average for the eight seismic actions
Key : IF = irregular frames; RF = regular frames; FW = frame - wall structures

may be used to control deformations up to and beyond the strength limit. Structural systems tend to
behave linearly under low-magnitude earthquakes; at this stage, values of drifts vary between 0.5% and
1.0%. Analytical work by Ascheim (2002) has shown that the yield horizontal displacement of steel
MRFs may vary between 1.0% and 1.2% of the height of the structure, while RC frames often yield at
about half of the above values. Modern seismic design codes also include stringent drift limits to ensure
adequate lateral stiffness of the structure and hence reduce the extent of non- structural damage. Sharp
variations of stiffness in plan and elevation can cause damage concentrations and should be avoided.
Infi ll panels and brick walls infl uence the response of frames with low lateral stiffness, e.g. multi-
storey steel frames. The more fl exible the basic structural system is, the more signifi cant the effects of
non-structural components are (Moghaddam, 1990). Masonry and precast concrete infi lls are frequently
used as interior partitions and exterior walls in steel, composite and RC structures. Their interaction
with the bounding frame should be accounted for in the assessment of the seismic performance. Studies
have demonstrated that the seismic behaviour of infi lled structures may be superior to that of bare
systems (Shing and Mehrabi, 2002); enhanced lateral stiffness and strength are readily achieved. While
their capacity for gravity loads may be low, infi lls often act as shear walls and affect the seismic struc-
tural response in the following respects:

(i) Stiffening of the structure: the fundamental period of vibration of the bare system is

shortened. Consequently, the dynamic amplifi cation characteristics vary. The importance of this
effect depends on the characteristics of the ground motion, which are discussed in detail in
Section 3.4 ;

(ii) Load path: infi lls alter the lateral stiffness distribution of the structure and hence change the

load fl ow illustrated in Section 2.3.2.2. Unexpected stress concentrations may also arise from
the interaction of wall panels and bounding frames;