Numerical methods in bridge structure analysis - Computational Analysis and Design of Bridge Structures - page 89

close to what it should be when the bridge was built in one time. More
detailed 2D and 3D illustrated examples, including creep and shrinkage,
are shown in Chapter 5 for PC bridges.
3.4 INFlueNce lINe/SurFAce
lIve loAdING method
Live load analysis is a unique problem to bridge analysis and design. As
some bridge design specifications define that many vehicles with minimum
spacing are allowed to present in a lane, the simple search of maximum or
minimum positions by moving axles along an influence line will not work
well in general. A generic and effective influence line loading method that is
suitable for any type of live load definition is important in bridge analysis
and design. The traffic lane layouts in many bridges, such as interchanges
or curved bridges, can be complicated, and, therefore, spatial live loads
analysis becomes inevitable. Based on influence line loading, influence sur-
face loading with multiple traffic areas is another important topic in live
loading analysis, especially nowadays with advanced computer technolo-
gies, spatial analyses become essential to bridge designs.
In this section, the application of dynamic planning method in influence
line loading and the principle of multiple traffic areas in influence surface
loading will be introduced.
3.4.1 dynamic planning method and its application
in searching extreme live loads
Live loads usually contain a single concentrated load, uniformed (or
called lane) loads, and vehicle loads. Searching for extreme positions of
vehicle loads is complicated in live load analyses. Locating the positions
or areas where a single concentrated load or uniformed loads reach the
extreme is simple. In this section, vehicle loads are used as examples to
illustrate the principle of dynamic planning method in search of extreme
positions.
Different bridge specifications define different vehicle loads, and these
definitions may be changed per traffic demands. Figure 3.15 shows a single
vehicle model and two typical vehicle processions. As shown in Figure 3.15a,
a vehicle can be described as a number of axles with constant axle weights
and spacing. Because both axle weights and spacing are fixed, given only
the location of its front axle on the influence line, its influence value can be
obtained. Therefore, it can be simplified as a concentrated load as shown in
Figure 3.15b. Figure 3.15c shows a typical vehicle procession that contains
identical vehicles as illustrated in Figure 3.15a with a minimum leading