Civil Engineering Reference
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Minimum simply supported span moment of inertia for deflection criteria
(note that rolling impact varies from 5% for long spans to
12.5% for short spans in these calculations)
3.00E+06
2.50E+06
2.00E+06
1.50E+06
L /640
L /800
L /1000
1.00E+06
5.00E+05
0.00E+00
0
20
40
60
80
100
120
140
160
180
200
Length of simple span, L (ft)
FIGURE 5.32 Stiffness design of simply supported spans for deflection criteria.
span to meet the deflection criteria is
M LL + I Lf Δ
1934
in. 4 ,
I
(5.46b)
where M LL + I istheliveloadincludingimpactbendingmomentforthespan, L ,ft-kips;
L is the length of span, ft.
If the AREMA (2008)-recommended f Δ =
640 is used in Equation 5.46b, the
0.33 M LL + I L , in. 4 , for a simply supported
beam or girder span. This relationship, and others using the deflection criteria L/ 800
and L/ 1000, are shown in Figure 5.32. It should be noted that the rolling impact used
in Figure 5.32 varies from 5% for long spans to 12.5% for short spans and may require
adjustment for particular span designs. Nevertheless, Figure 5.32 provides a ready
estimate of minimum gross moment of inertia required to meet various deflection
criteria for simply supported steel railway beam and girder spans.
The maximum deflection of a simply supported truss from live load including
impact,
minimum gross moment of inertia is I
Δ LL + I , can be determined by calculating truss joint horizontal and vertical
translations by the method of virtual work or through graphical means (Utku, 1976;
Armenakas, 1988). However, today the use of matrix methods (stiffness or flexibility)
and digital computer software enables routine calculation of truss deflections and
member forces. The analysis of trusses for joint translation should use the gross area
of truss members without perforated cover plates. If truss members are designed
with perforated cover plates the gross area should be reduced by the area determined
by dividing the volume of a perforation by the spacing of perforations.
Cover plates on truss members are usually used in compression chords and end posts.
 
 
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