Civil Engineering Reference
In-Depth Information
energy absorption; these parts are termed 'dissipative components'. For MRFs designed in compliance
with SCWB philosophy, beams are dissipative members. The remainder of the structure, e.g. columns
and joints, is provided with the strength to ensure that no other yielding zones are likely to occur; these
are 'non-dissipative components'. The design actions for the latter are derived from capacity design
principles. Elements carrying vertical loads are designed with added strength. The capacity design
factors that are used to defi ne the design actions for the non-dissipative components are referred to as
protection factors or overdesign factors. The overdesign factors should not be confused with the over-
design factor in assessment, as opposed to design. Overdesign in assessment is the ratio between the
intended and the actual strength of a structure or a component. Protection factors may also be employed
in the design of structural components where signifi cant shear effects, compressive/tensile forces or
brittle failure is expected. In steel and composite structures, spreading of inelasticity in column panel
zones is often allowed. However, there is no general agreement among earthquake engineers on this
issue; research is still ongoing (e.g. Di Sarno and Elnashai, 2002 ).
Bertero and Bertero (1992) compared the ductility required for two different failure modes of multi-
storey MRFs, i.e. SCWB and WCSB. Framed systems with WCSBs are characterized by high values
of imposed ductility, especially for fl exible structures, e.g. with fundamental period of vibration
T
> 1.5-2.0 seconds. Experimental and numerical investigations have demonstrated that WCSB designs
are not desirable in seismic regions (e.g. Schneider
et al
., 1993 , among others).
Failure mode control is signifi cantly affected by material randomness, presence of non- structural
components and quality control. Variations of mechanical properties depend on the construction mate-
rial utilized, as discussed in Section 2.3.2.1. Values of COVs for material properties are generally lower
than 15-20% and are often negligible compared to the randomness of both seismic input and quality
control (Kwon and Elnashai, 2006 ).
Infi lled walls, claddings and internal partitions can play an important role in the seismic response of
structural systems and may alter the hierarchy in the failure mode sequence, e.g. beam before connec-
tions and columns in MRFs or braces before beams, connections and columns in CBFs. While not
normally considered in the design, non-structural elements interact with the structural system and infl u-
ence its performance. To achieve an adequate control of the failure mode, non- structural components
should be accounted for in the analysis of the dynamic behaviour and in the seismic detailing of the
dissipative components. Infi lled systems were discussed in detail in preceding sections.
Failure modes that should be avoided are those involving sudden failure (e.g. brittle or buckling
modes) and those involving total collapse due to failure of vertical load- carrying members. Common
brittle failure modes are summarized in Table 2.5 categorized according to the material of
construction.
Table 2.5
Typical brittle failure modes as function of common materials of construction.
Material of construction
Brittle failure modes
Reinforced Concrete
Buckling of reinforcement bars
Bond or anchorage failure
Member shear failure
Masonry
Out - of - plane bending failure
Global buckling of walls
Sliding shear
Structural Steel
Fracture of welds and/or parent material
Bolt shear or tension failure
Member buckling
Member tension failure
Member shear failure
