AASHO Load Help

[jcox]

I'm getting ready to take the PE soon. I've never had an occasion to look at AASHTO's bridge design book, but I've just recently borrowed one. I was going throught some live load distribution examples that the NCEES puts out in there sample exam. The problem illustrates a 2-Lane, 85 ft span simply supported bridge with steel girders and concrete deck. The span between girders is 8'-6". You're asked to determine the Impact Moment + Live Load Moment for a HS20-44 truck load. The published solution is as follows with equations and referenced sections:

Impact = 50/(L+125) (3-1)
= 50/(85+125)=0.24
Distrbution = S/5.5 = 8.5/5.5 = 1.55 (3.23.1)

Mom. = (1.55)(1.24)(1254.7/2)<-Apendix A, AASHTO
Mom. = 1206k-ft

I'm OK with most of the solution. The manuals not to clear (to me at least) what the &quot;Distribution Factor&quot; is for or how it is supposed to be applied, but I can guess how it is used from the solution. Where I really don't understand the solution is where the 2 in the expression (1254.7/2) comes from. Is it from the number of lanes? Any help would be appreciated. I'm not to worried about being asked to many questions like this. If it's an easy question with an easy solution, I wouldn't mind knowing how it's done.

Thanks

 

[koo]

jcox,

Distribution factor is for estimating how load is distributed on one beam line. a girder line in a closer spaced girder system usually carries less load than that in a more seperated system. And acting as a system, beams usually share load thru slab and diaphragm action.

Next, S/5.5 is a wheel load distribution factor, and 1254.7 from Appendix A is derived from a truck load, implicitly a pair of wheel loads. You need to divide either of them by 2 to make them consistent to each other.

Hope this helps.