Civil Engineering Reference
In-Depth Information
to bending moments) and shearing stresses. Members and connections with inter-
nal stresses not greater than the allowable tension, compression, or shear allowable
stresses recommended by AREMA (2008) are considered to be of safe and reliable
design.
5.3.2.2
Serviceability Design
Serviceability criteria (or limit states) of deflection and fatigue are important aspects
of the structural design of steel railway superstructures.
5.3.2.2.1 Deflection Criteria
Flexural deflections are calculated at the location of the maximum live load bending
momentinaspan.AREMA(2008)recommendsthatthemaximumflexuraldeflection
from live load including impact not exceed 1/640 of the span. Railroad companies
and designers may further limit deflections based on span types (trusses, girders, and
composite girder/beam spans
∗
) and other operating practices.
The maximum flexural deflection in an ordinary simply supported span from live
load including impact,
Δ
LL
+
I
, can be estimated considering an equivalent uniform
load,
w
e
Δ
,as
L
2
w
e
Δ
a(L
−
a)
w
e
Δ
M
LL
+
I
=
=
at
a
=
L/
2.
(5.45a)
2
8
Therefore,
8
M
LL
+
I
L
2
w
e
Δ
=
,
(5.45b)
maximum bending moment in the span due to live load including impact (at
a
L/
2),
and
L
is the length of the simply supported span. Substitution of Equation 5.45b into
the equation for the maximum deflection from a uniformly distributed load on a
simple beam provides an estimate of the maximum flexural deflection due to live
load including impact as
=
5
w
e
Δ
L
4
384
EI
=
0.104
M
LL
+
I
L
2
EI
Δ
LL
+
I
=
,
(5.46a)
where
E
is the modulus of elasticity and
I
is the gross moment of inertia (used for
flexural member deflection calculations).
AREMA (2008) recommends, as do many other guidelines, codes, and specifi-
cations, that the maximum flexural deflection from live load including impact not
exceed
L/f
Δ
is an integer established based on structural behav-
ior and experience. Therefore, the minimum gross moment of inertia,
I
, of a simple
of the span, where
f
Δ
∗
To limit cracking and improve behavior of concrete decks.
