Civil Engineering Reference
In-Depth Information
OPENING IN
FLOOR SLAB
EARTHQUAKE
FORCE
FAILURE
WEB / SLAB
Shear
FLANGE / CHORD
Bending Moment
EARTHQUAKE
FORCE
Figure A.12
Beam analogy for horizontal diaphragms: load distribution (
left
) and common failure (
right
)
A.11. Diaphragms should possess adequate shear and bending resistance to withstand in-plane seismic
loads and out- of - plane gravity loads.
Inappropriate locations of large openings, due for example to stairs or elevator cores, can create
problems similar to those openings in the web of a beam (also known as ' notch effects ' ). These open-
ings signifi cantly reduce the diaphragm action and can lead to failure. Reinforcement around the
weakened regions helps to redistribute the actions in the slab around the opening.
A.2.2 Vertical Systems
Structural and non-structural damage under earthquakes is caused by inadequate stiffness and/or
strength of vertical components of lateral structural systems used for buildings, bridges and other types
of construction. Vertical components may also fail because of insuffi ciency or absence of ductility. To
achieve satisfactory seismic performance, vertical components of lateral resisting systems should
comply with the structural requirements discussed in Section A.1. Seismic behaviour depends on
materials of construction, system confi gurations and failure modes.
Earthquake resistance can be achieved through a wide range of vertical systems, which can range
from free-standing columns to complex three-dimensional framed tubes and/or cores. Figure A.13
shows basic structural systems, which have been ranked according to their lateral stiffness. Columns
are the simplest structural elements with lateral stiffness and strength. The relationship between applied
actions and lateral deformations depends on their geometric and mechanical properties, as discussed in
Section 2.3.1.2 .
The deformed shape of columns is generally characterized by double curvature, thus inelastic demand
can be concentrated at both ends. Frames show higher stiffness, strength and ductility than free- standing
columns because of their defl ected shape. Frame behaviour signifi cantly depends on the relative rigidity
of structural members (beams and columns) and connections (beam- to - columns and base columns).
Frames with diagonal braces exhibit higher lateral stiffness and strength than moment frames; the
ductility of braced systems is generally endangered by the occurrence of member (diagonal) buckling.
Moment frames can be stiffened by infi ll panels. Infi lled frames exhibit higher stiffness, strength and
ductility than bare frames. Under lateral seismic loads, infi lls behave like one diagonal compression
brace. Infi ll panels are often made of brittle materials, such as masonry or concrete, which crack due
to their low tensile strength. Lateral stiffness of braced and infi lled frames can be enhanced by employ-
ing structural walls. These elements usually exhibit high in-plane stiffness and resistance; their ductility
