Civil Engineering Reference
In-Depth Information
bridges because of their low lateral stiffness and restrainer stiffness. Moderate-to-strong earthquakes
may lead to out-of-phase motion of bridge frames due to the variability of ground motion, travelling
wave effects and structural characteristics affecting the dynamic response, especially stiffness (Des-
Roches and Fenves, 1997; DesRoches and Muthukumar, 2002). Inertial forces may exceed those
assumed for the design earthquake and in order to prevent damage at column bents, abutments and
bearings, adequate lateral stiffness of piers and restrainers is essential (Kim
et al
., 2000). It is generally
suffi cient to employ separation joints between adjacent buildings and in multi-span bridges. The estima-
tion of such joints requires thorough assessment of the seismic response including soil- structure
interaction.
A.2 Structural Systems
The dynamic behaviour of structures under earthquake actions is dependent upon the lateral resisting
system employed. Construction materials and structural confi gurations differ widely in stiffness,
strength and ductility; thus, different systems deform, resist actions and dissipate energy in various
ways. To achieve satisfactory seismic performance, structural systems should possess:
(i) Adequate stiffness;
(ii) Adequate strength;
(iii) High ductility;
(iv) High damping;
(v) High stability;
(vi) High redundancy.
The importance of the above attributes in the seismic response of structures has been discussed in
Section 2.3. Several lateral force-resisting systems, however, possess only a few of the above properties.
In these cases, different structural components or systems may be combined to improve the global
seismic response. For example, dual (or hybrid) systems, which combine frames with bracing
components such as structural walls, are more effective than either of the components on their own.
Structures suitable for earthquake resistance include horizontal and vertical systems. Those employed
in structures for buildings and bridges are outlined below. Design details can be found in the literature
for the various construction materials (e.g. Dowrick, 1987; Paulay and Priestley, 1992; Priestley
et al
.,
1996 ; Foliente, 1997; Bruneau
et al
., 1998 ).
A.2.1 Horizontal Systems
Horizontal bracing in buildings and bridges is provided by fl oor and deck framing systems (also known
as 'horizontal diaphragms'), respectively. Floor and deck systems have two functions. They carry
gravity loads and transfer them to vertical structural elements as described in Section A.2.2. They also
collect and distribute inertial forces among lateral load- resisting components. Force - resisting mecha-
nisms in horizontal diaphragms are very complex because of the interaction between in-plane and out-
of - plane behaviour. Figure A.11 compares the structural response of rigid and fl exible diaphragms under
horizontal loads for a simple box system. If the in-plane stiffness of the fl oor is high (
rigid diaphragm
),
horizontal actions (F in the fi gure) are distributed to vertical elements in proportion to their relative
stiffness, as also illustrated in Section 2.3.1.2. Floor deformations are negligible compared to those of
vertical resisting systems. Conversely, a fl exible diaphragm distributes horizontal inertial actions to
vertical components as a series of simple supported beams spanning between these components. In this
case, the action distribution is governed by equilibrium conditions. Floor defl ections may exceed those
of lateral resisting systems. Diaphragms, especially in existing buildings, are neither perfectly rigid nor
