Civil Engineering Reference
In-Depth Information
struction quality. Thorough control is imposed for materials, members and connections which are
manufactured in part or totally in fabrication yards. Therefore, the uncertainty in the evaluation of the
capacity of prefabricated structures is often lower than that of systems built
in situ
. In general, the ran-
domness of physical properties (mechanical and geometric) is small in metal structures (coeffi cients of
variation, or COVs, are typically between 4% and 6%) but can be signifi cant for RC and masonry
(COVs greater than 10-15%). Moreover, the construction quality of metal structures is frequently higher
than that for RC and masonry. Reliable estimation of section, member and structure capacities require
low values of COVs for both geometrical and mechanical properties.
Attainment of shear, axial and fl exural capacities in gravity and earthquake-resistant systems can
cause damage in structural components. Damage is related to the safety of the system but it does not
necessarily lead to structural collapse. Collapse prevention is the behaviour limit state controlled by
ductility, as illustrated in Section 2.3.3.3 .
Horizontal seismic loads usually exceed wind loads, especially for low- and medium - rise structures
in areas of medium-to-high seismic hazard. Earthquakes produce lateral forces proportional to the
weight of the structure and its fi xed contents; the resultant of seismic force is known as ' base shear ' .
Adequate shear, axial and fl exural capacity is required to withstand storey and base shear forces.
Bending effects, such as uplift and rocking, may be caused by horizontal forces due to masses located
throughout the height of the structure; these effects are also referred to as 'overturning moment' . Com-
bined horizontal and vertical loads in the event of ground motions increase the stress level in members
and connections. If total stresses exceed the capacity, failure of structural components occurs; this
corresponds to the structural damage limit state. Local damage, however, does not impair the integrity
of the structure as a whole. Correlations between strength and structural damage are presented in
Section 2.3.2.3 .
As with lateral stiffness, the strength of a structure depends signifi cantly on the properties of materi-
als, sections, elements, connections and systems (reference is made to the hierarchical link shown in
Figure 2.4) as discussed below.
2.3.2.1 Factors Infl uencing Strength
(i) Material Properties
The effi cient use of material strength may be quantifi ed by the ' specifi c strength', i.e. the strength- to -
weight ratio
σ
/
γ
. Values of
σ
/
γ
for materials commonly used in structural earthquake engineering are
provided in Table 2.3 . Specifi c elasticity
E
/
γ
was also included for purposes of comparison. Fibre
composites, wood and metals possess the highest values of specifi c strength; this renders them suitable
for earthquake structural engineering applications. In the case of wood, the drawback is that member
sizes required to achieve high levels of strength may be very large.
Construction materials may be isotropic, orthotropic or anisotropic, depending on the distribution of
properties along the three principle axes. Some materials, such as structural steel and unreinforced
concrete may be treated as isotropic. Laminated materials are usually orthotropic. Examples of aniso-
tropic materials include masonry, wood and fi bre-reinforced composites. Strength of materials is infl u-
enced by strain hardening and softening as well as strain rate effects (e.g. Paulay and Priestley, 1992;
Bruneau
et al
., 1998; Matos and Dodds, 2002, among others).
A loss of both strength and stiffness takes place in concrete as the strain increases; this is
referred to as strain softening or strength and stiffness degradation. Strain softening can be reduced
in RC systems by providing transverse confi nement of concrete by either hoops or spirals. Circular
hoops are more effi cient than those with rectangular shapes because they uniformly confi ne the
core concrete. The loss of bond between concrete and steel in RC structures under large alternating
loads reduces strength and stiffness. Conversely, structural steel exhibits higher strength at large defor-
mations, generally at strains
ε
greater than 10- 15
ε
y
, with
ε
y
being the strain at yield; this is known
