Civil Engineering Reference
In-Depth Information
Therefore, analysis based on such temperature changes during erection may
be required as well. The erection of the steel framing, whether the bridge is
straight or curved, is one of the most critical stages with regard to ensuring
stability, and these factors may need to be considered in the models during
construction.
Deck placement effects must be considered in the design of steel bridges.
When a portion of the deck slab is pouring, deck concrete casted in previ-
ous stages may be cured enough to form a composite action. Therefore, the
moment of inertia in the previously poured sections has to be so adjusted to
reflect the stiffness changes. The deck placement sequence also has an effect
on other aspects of bridge behavior including uplift, deflections, and bearing
rotations. Staging analysis process due to deck placement based on ACI209
(2008) are shown here.
1. Creep coefficient (φ[
t
,
t
0
]): The general form of the creep equation is
ψ
(
−
+ −
t
t
)
0
ϕ
( ,
t t
)
=
ϕ
(7.8)
0
u
ψ
d
(
t
t
)
0
where:
(
t
-
t
0
) is the time since application of load
ψ and
d
(in days) are constants
φ
u
is the ultimate creep coefficient
ϕ
=
(
ϕ
)
avg
⋅
γ
(7.9)
u
u
c
where:
(
)
ϕ
u
avg
= 2.35
γ
c
is the cumulative product of six applicable correction factors for
loading age, relative humidity, volume-surface ratio, and concrete
composition (slump, aggregate, and air content)
2. Strength at age
t
(
f
cmt
). The general form of the strength equation is
t
a bt
f
f
=
(7.10)
cmt
cm28
+
where:
f
cm28
(in MPa or psi) is the strength of concrete at an age of 28 days
a
(in days) and
b
are constants depending on the concrete type
3. Modulus of elasticity at loading age
t
0
and age
t
(
E
mcto
and
E
cmt
):
E
33
1 5
.
f
(psi) or
E
0 043
.
1 5
.
f
(
MPa
)
(7.11a)
=
γ
=
γ
mcto
c
cmto
mcto
c
cmto
E
cmto
E
=
)
(7.11b)
cmt
1
+
ϕ
( ,
t t
0
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