Civil Engineering Reference
In-Depth Information
Table 13.1
Assumed mean impact load percentages (from AREMA 2009 Table 15-1-8)
Member Percentage (%)
Member with loaded lengths equal or less than 10 ft (3 m) and no load sharing 65
Hangers 40
Other truss members 65
Beams, Stringers, Girders and Floor beams 35
Note: Where bridges are designed for operation of trains handling a large percentage of cars with flat or out of round wheels
which increase impact and/or poor track which increases impact, and the loaded length of the member is less than 80 ft
(24 m), the mean impact should be 100 % of the design impact
Distance from support (
ft
)
0
80
160
240
320
400
480
560
Static
Dynamic
30
4
25
σ
σ
3
σ
20
,
dyn f
dyn
stat
15
2
10
1
5
0
0
-5
0
40
80
120
160
Distance from support (
m
)
Fig. 13.1
Example of stress time history
codes define the impact factor for fatigue as percentages of the static values of the dynamic impact factor. For example, the
Korean Railway Bridge Design Code [
2
] specifies the fatigue factor as 65% for bridges longer than 9 m, and the Japanese
Railway Standards for Steel Bridge [
3
] specifies 75%. Recently, Lee et al. [
4
] suggested that the impact factor for fatigue can
be only half of that used in ordinary static design of the present Korean code. The AREMA Specifications 2009 [
5
] defined
the impact factor for fatigue as the “Mean Impact Load”, calculated as a percentage of the dynamic impact factor used in
static design. AREMA has specified different values for different bridge members as shown in Table
13.1
.
The major factors governing fatigue of steel bridges are the numbers of stress cycles, the magnitude of the stress range,
and the relevant Fatigue Detail Category. Therefore, in fatigue design or evaluation, the maximum stress range rather than
the maximum stress is considered. Figure
13.1
shows typical dynamic and static stresses in a bridge member due to the
passage of a train passing over the bridge. For the normal static design, the impact factor (IM) is normally calculated using
(
13.1
). However, for steel railroad bridges, the impact factor for fatigue is defined as (
13.2
).
¼
σ
dyn
-
σ
stat
σ
stat
IM
(13.1)
¼
σ
dyn;f
-
σ
stat
σ
stat
IM
(13.2)
σ
dyn,f
is the maximum dynamic stress range.
In this paper, typical rail bridges on various New Jersey rail lines were reviewed to investigate the impact of the increased
railcar weight. A two-dimensional (2D) train-bridge dynamic model is developed and validated using results from field tests.
Using the validated model, the impact factor for fatigue of a typical steel plate girder bridge for different speeds of the train
was determined.
σ
stat
is the maximum static stress,
σ
dyn
is the maximum dynamic stress,
where,
