Civil Engineering Reference
In-Depth Information
bridgesarebestdevelopedforroutinebridgedesignusingempiricaldata. Theparam-
eters that affect the dynamic behavior of steel railway bridges are (Byers, 1970;Yang
et al., 1995; Taly, 1998; Heywood et al., 2001; Uppal et al., 2003)
• Dynamic characteristics of the live load (mass, vehicle suspension stiffness,
natural frequencies, and damping).
• Train speed (a significant parameter).
• Train handling (causing pitching acceleration).
• Dynamic characteristics of the bridge (mass, stiffness, natural frequencies,
and damping).
• Span length and continuity (increased impact due to higher natural frequen-
cies of short-span bridges).
• Deck and track geometry irregularities on the bridge (surface roughness) (a
significant parameter).
• Track geometry irregularities approaching the bridge.
• Rail joints and flat or out-of-round wheel conditions (a significant parameter
of particular importance for short spans).
• Bridge supports (alignment and elevation).
• Bridge layout (member arrangement, skewed, and curved).
• Probability of attaining the maximum dynamic effect concurrently with
maximum load.
Manyoftheseparametersarenondeterministicanddifficulttoassess.Therefore,as
with highway bridge design procedures, ordinary steel railway bridges are designed
for dynamic allowance based on empirical equations developed from service load
testing. AREMA (2008) provides deterministic values for design impact that are
considered large enough, with an estimated probability of exceeding 1% or less for
an 80 year service life, based on in-service railway bridge testing (Ruble, 1955;
AREMA, 2008). The AREMA (2008)-recommended impact due to vertical effects
for simply supported open deck steel bridges is shown in Figure 4.9 and Example 4.5.
The impact load for ballasted deck steel bridges may be reduced to 90% of the total
impact load determined for open deck steel bridges (AREA, 1966).
Example 4.5
The double track open deck steel multibeam railway bridge shown in
Figure E4.5 i s composed of two 45 ft simple spans.
The impact due to vertical effects, I V , on a 45 ft span is 36.2% (Figure 4.9).
A statistical investigation of steel railway bridge impact (Byers, 1970) revealed
that the test data (AREA, 1960) followed a normal frequency distribution with mean
values and standard deviation increasing with increasing speed and decreasing with
Although not often used in modern railway bridge design, impact equations for steam locomotives are
provided in AREMA (2008) in addition to those recommended for modern diesel and diesel-electric
powered locomotives.
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