Civil Engineering Reference
In-Depth Information
(i) Structural members of lateral resisting systems used for buildings and bridges should be
detailed so that they exhibit ductile response under severe earthquake ground motions. All
other elements should be designed elastically. Dissipative zones, e.g. plastic hinges, require
adequate concrete confi nements. Buckling of longitudinal steel rebars impairs the anchorage,
while splicing of bars should not be carried out in regions of high stress concentration.
(ii) Likely sources of overstrength, e.g. material mechanical properties and presence of slabs,
should be accounted for in the design of dissipative elements in ductile systems.
(iii) Values of compressive axial loads in bridge piers and building columns should not exceed
25-30% of the squashing capacity. High values of axial loads signifi cantly reduce the dissipa-
tion capacity of piers and columns as also discussed in Section 2.3.3.1. High axial loads lower
the maximum plastic rotations and increase the likelihood of buckling of longitudinal steel
reinforcement bars. Columns and piers should be designed to exhibit elastic response. Tensile
forces should be prevented; the latter give rise to brittle failure modes, e.g. under high vertical
components of earthquakes.
(iv) Short - column effects caused by partial infi lls in framed systems may be prevented by adopting
adequate separation gaps. This detail does not increase the shear stiffness of column
members.
(v) Failure modes involving shear and bond deteriorations should be avoided. These are brittle
failure modes and hence lower the energy dissipation of the structural system. Consequently,
fl exural failure should anticipate that of shear. Columns with shear span ratios α
s
greater than
4.0 are preferable to short columns (α
s
< 2.0). Close -spaced transverse stirrups or truss rein-
forcement may be adopted to prevent the degradation of shear resistance.
(vi) Confi guration irregularities in plan and elevation should be avoided as also illustrated in
Section A.1. Soft-storey mechanisms at the ground fl oor of buildings are, for example, often
caused by infi lls only in the upper storeys. Structural irregularities may also give rise to sig-
nifi cant torsional effects. Eccentricities between centre of mass (point of application of seismic
- inertial - forces) and centre of rigidity (point of application of reaction of the structure)
should be minimized.
(vii) Continuity in load path is an essential requirement for both gravity (vertical) and earthquake
(horizontal and vertical) loads as also shown in Sections 2.3.2.2 and A.1 .
(viii) A high degree of structural redundancy should be guaranteed so that as many zones of inelas-
ticity as possible are developed before a failure mechanism is created. Redundant structures
can accommodate large plastic redistributions.
(ix) Openings in slabs should be minimized because they detrimentally affect the in- plane strength
and rigidity of horizontal diaphragms. To prevent punching, additional steel reinforcement
should be located at connection between fl at slabs and columns, and between structural walls
and slabs.
(x) Joints should be provided at discontinuities between adjacent structures or part of them. Sepa-
ration gaps should employ adequate provisions for movements so that pounding and unseating,
e.g. of bridge spans, is avoided. In multi- span bridges, suffi cient gaps should be used both at
abutments and between adjacent spans. Overstressing of seismic restrainers should be
avoided.
(xi) Uplift and sliding of foundation systems due to high overturning moments and shear forces
often have detrimental effects on global structural response.
(xii) Large permanent ground displacements due to soil liquefaction and pile deformations should
be accounted for in the design of buildings and bridges.
Several of the above requisites are also applicable to masonry, steel and composite structures. There-
fore, the following sections focus only on the design solutions specifi c to each material.
