Approximate and refined analysis methods - Computational Analysis and Design of Bridge Structures - page 40

(a)

Z

Y

X

Girder 4 Girder 3 Girder 2 Girder 1

(b)
Figure 2.23 Live load on an influence surface: (a) moment influence surface; (b) place-
ment of vehicles on an influence surface.

technique of using influence surface directly projects the ordinates of the
axles' footprints, as shown in Figure 2.23b. A technique called compos-
ite influence line can be used in influence surface loading of 2D grillage
model discussed in Chapter 7 (Fu 1994, 2013). Composite influence line
is used to suppress the associated influence lines of adjacent girders to the
primary girder. Before suppression, distribution factors are multiplied by
their respective girder influence ordinates. Here, distribution factor is
defined as the fraction of the wheel load, not from the S/D method defined
by AASHTO. The advantages of using composite influence lines are in
the saving of computer memory for influence surface and in easy access
for future use (Fu 1994, 2013). Other than the composite influence lines
method, a more sophisticated 3D FEA influence surface method will be
discussed in Chapter 3 in detail.