Damage to Structures - Fundamentals of Earthquake Engineering - page 317

Figure B.35 Damage to beam-to-column connections in the 1994 Northridge (California) earthquake: fracture
through web and fl ange in column ( left ) and causing a column divot fracture ( right ) ( after Naeim, 2001 )
Figure B.36 Fracture in bolted brace connections during the 1995 Kobe (Japan) earthquake: web tear-out ( left )
and net fracture at bolt holes ( right ) ( after FEMA, 2000 )
(iii) System Failures
Steel framed structures typically fail under earthquakes because of their low lateral stiffness as illus-
trated in Section A.2.2. Inadequate horizontal storey stiffnesses generate extensive damage in non-
structural elements (infi lls) and signifi cantly increase P - Δ effects. The latter may, in turn, cause partial
or total collapse of the structural system. For instance, the damage in more than 200 residential steel
buildings during the devastating 21 June 1990 Manjil earthquake in Northern Iran ( M S = 7.7) was due
to excessive lateral deformability (Nateghi, 1995). Slender braces buckled out- of - plane about weak
axes, thus causing extensive non-structural damage (Nateghi, 1997 ).
Failure in columns due to buckling and excessive yielding may give rise to soft storeys, which, as
discussed for RC frames, should be avoided. Global mechanisms of failure characterized by formation
of plastic hinges in beams are desirable because associated with enhanced energy dissipation capacity
of the system as discussed in Sections 2.3.3.2 and 2.3.3.3. Figure B.37 provides two examples of frames
with low lateral stiffness surveyed in the 1995 Kobe and the 1999 Chi-Chi earthquakes. These old steel
constructions built in the outskirts of large cities did not comply with modern seismic code require-
ments. Generally, new steel multi-storey frames exhibit adequate structural performance even under
intense earthquakes. Figure B.37 demonstrates the ineffi cacy of very slender diagonal braces (frame on