Civil Engineering Reference
In-Depth Information
Live load influence line
—An influence line is defined as the variation of
function, such as reaction, shear, bending moment, or stress, when a unit
load is moving over the structure. Figure 2.11 shows examples of influence
lines for a three-span continuous bridge.
Usually, influence line results from analyzing a beam model, with
x
as
the distance and
y
as the ordinate. The influence lines of moment, shear, or
reaction are recorded at a small interval. Having an influence line defined,
a standard live loading process can determine the positions of a live load
specified by a specification and thus the extreme live load results. Different
specifications have different live load definitions and combinations. For
example, AASHTO LRFD Specifications (2013) define the following load-
ing combinations:
• Design tandem with design lane load
• One design truck with variable axle spacing with design lane load
• 90% of two design trucks with axles from two trucks spaced mini-
mum 15,000 mm (two 145-kN axles spaced 4300 mm) with 90% of
the design lane load
The illustrations of the live loading application and combinations are
shown in Figure 2.12 with loads positioned longitudinally for extreme
effect.
(a) A three-span continuous bridge
(b) Moment influence line
(c) Shear influence line
(d) Reaction influence line
Figure 2.11
(a-d) Examples of influence lines for a three-span continuous bridge.
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