Civil Engineering Reference
In-Depth Information
TABLE 4.4
Mean Impact Loads for Fatigue Design
Percentage of
Member
Total Impact Load
Beams (stringers, floorbeams) and girders
35
Members with loaded lengths less than or
equal to 10 ft and no load sharing
capabilities
65
Truss members (except hangers)
65
Hangers in through trusses
40
Source:
From American Railway Engineering and Maintenance-of-Way
Association (AREMA), 2008,
Manual for Railway Engineering
,
Chapter 15. Lanham, MD. With permission.
to the bridge, or where track discontinuities exist (i.e., movable bridge joints), can
be very large and may require refined dynamic analyses and special design consid-
erations for damping. Example 4.6 outlines the calculation of impact for an ordinary
simple span steel railway bridge.
Example 4.6
The governing Cooper's E80 or alternate live load maximum dynamic live load
bending moment is required for each track of the open deck steel multibeam
simple span railway bridge shown in
Figures E4.1
and
E4.5.
The maximum bending moment, shear forces, and pier reaction for each
track of a 45 ft span due to Cooper's E80 and alternate live load (Figure 4.1)
are given in Table E4.1 (see Chapter 5).
The appropriate values for determination of the maximum dynamic live
load bending moment are
• The maximum static bending moment
=
3420.0 ft-kips (alternate live load
governs in Table E4.1).
• The rocking effect RE
16.67% (Example 4.1).
• The vertical impact factor
I
V
=
=
36.2% (Example 4.5).
• The mean impact percentage for fatigue design
=
35%
(Table 4.4).
TABLE E4.1
Static Force from Moving Load
Maximum E80 bending moment
3202.4 ft-kips
Maximum E80 shear force
326.8 kips
Maximum E80 pier reaction
474.5 kips
Maximum alternate live load bending moment
3420.0 ft-kips
Maximum alternate live load shear force
328.9 kips
