Civil Engineering Reference
In-Depth Information
C
DT
/
C
D
2
h
1.7
s
<
h
1.15
0
0.25
0.6
f
1.0
FIGURE 4.20b
Typical relationship between the total drag coefficient on two girders or
trusses,
C
DT
, and the drag coefficient for a single girder or truss,
C
D
.
The drag force or total wind thrust on a bluff body, such as a bridge cross section,
created by the wind flow is of primary interest in the design of bridges. Therefore,
drag coefficients are established by wind tunnel tests, which incorporate the effects
of geometry and flow characteristics (as described by the Reynolds number), which
may be used for design purposes. Drag coefficients for a solid element, such as a plate
girder, are generally no greater than about 2.0 and drag coefficients for a truss are
typically about 1.70 (Simiu and Scanlon, 1986). The difference is related primarily to
the characteristic dimension of the effective area, generally taken as the area projected
onto a plane normal to the wind flow. The solidity ratio,
f
, is defined as the ratio of
the effective area to the gross area.
The effects of the usual pairing of girders, trusses, and arches in steel railway
bridges must also be considered. Figure 4.20b illustrates the typical relationship
between the drag coefficient relating to the total wind force on two girders or trusses,
C
DT
,tothedragcoefficientforasinglegirderortruss,
C
D
,intermsofthesolidityratio,
f
, for spans with girder or truss spacing,
s
, no greater than the girder or truss height,
h
.
Examples of the use of these drag coefficients are outlined in Examples 4.12 and 4.13.
Example 4.12
A125 ftlongballasteddecksteeldeckplategirderspanisshowninFigureE4.8.
Determine the design wind force for a wind speed of 75 mph.
3
'
15
'
10
'
FIGURE E4.8
