Civil Engineering Reference
In-Depth Information
Therefore, the following is determined for the shear, V LL+I , and bending
moment, M LL+I , live load forces:
V out =
V LL+I ( 1
0.097 ) =
0.903 V LL+I ,
V in =
V LL+I ( 1
+
0.097 ) =
1.097 V LL+I ,
M out =
M LL+I ( 1.013
0.097 ) =
0.916 M LL+I ,
M in =
M LL+I ( 0.987
+
0.097 ) =
1.084 M LL+I .
It should be noted that these shear and bending moment forces do not
include the effects of the centrifugal force. Section 4.3.2.3 and Example 4.10
in Chapter 4 outline the calculation for centrifugal forces.
The bridge deck must be superelevated as indicated in Section 3.2.4.1.1. The
required superelevation is easily accommodated in ballasted deck bridges (Figure 3.8 )
but may also be developed in open deck bridges by tie dapping, shimming, or
varying the elevation of the supporting superstructure or bearings ( Figure 3.9) .
Varying the elevation of the deck supporting members may be problematic from
a structural behavior, fabrication, and maintenance perspective and, generally, is not
recommended.
Curvedspansmustbedesignedforflexuralandtorsionaleffects.Dynamicbehavior
undermovingloadsisparticularlycomplexforcurvedgirdersasflexuralandtorsional
vibrations may be coupled. Even the static design of curved girder railway bridges
requirescarefulconsiderationoftorsionalanddistortional warpingstressesandshear
lag considerations. These analyses are complex and often carried out with general
purpose finite element or grillage computer programs. Curved girders are best suited
to continuous span construction and, therefore, not often used for freight railroad
bridges. Continuous construction is relatively rare for ordinary steel freight railway
bridges due to remote location erection requirements (field splicing, falsework, and
e
60
= tan -1
θ
e = superelevation,
inches
FIGURE 3.8 Track superelevation on ballasted deck bridges.
In the case of box girders.
The FHWA has also performed extensive research on steel curved girders at the Turner-Fairbanks
laboratory. A synthesis of the research is available from the FHWA.
 
 
Search WWH ::




Custom Search