Civil Engineering Reference
In-Depth Information
of actions in accidental and seismic situations, each with a different set of partial
factors, but these are not of concern for simplified design. The design value of
resistance is given by Eurocode and its value is determined from characteristic
resistance values divided by partial factors on material strength
g
M
.
states is governed by the following equation:
E
d
C
d
28
Þ
where
E
d
is the design value of the effects of actions in the serviceability limit
state criterion and
C
d
is the limiting design value of the relevant criterion. At
serviceability limit state, there are in principle three combinations of actions
to consider: characteristic, frequent, and quasi-permanent. For bridges, the
characteristic combination is used for checking that no inelastic response
occurs; the frequent combination is used if deflection needs to be checked
(this includes evaluation of dynamic response). The quasi-permanent com-
bination relates to long-term effects; for bridges, provided that the appropri-
ate modulus of elasticity is used for long-term actions, this combination only
needs to be considered when determining crack widths in concrete. Only
the characteristic combination is relevant to simplified design. For the char-
acteristic loading combination, the same characteristic values of actions are
used at ultimate limit states but all the
g
factors are taken as unity. Thus, the
expression becomes
ð
3
:
!
E
X
G
k
,
j
+
P
+
Q
k
,
1
+
X
i
c
0
,
i
Q
k
,
i
ð
3
:
29
Þ
>
1
The serviceability limit state criterion that might need to be considered
in simplified design is the limitation that stresses in steel should not exceed
the yield stress. This limitation would need to be considered if the ultimate
limit state design resistance were based on plastic bending resistance. It must
then be verified that the stress calculated elastically at serviceability limit
states does not exceed yield. No partial factor is applied to yield stress
(strictly,
g
M
¼
1).
In the United States, AASHTO [1.23] also specifies that bridges shall be
designed for specialized limit states to achieve the objectives of construct-
ability, safety, and serviceability. AASHTO adopts the load and resistance
factor design, which is based on limit state design approach. Each compo-
nent and connection in the bridge shall satisfy the following condition,
which assumes that all limit states shall be considered of equal importance:
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