AASHTO Live 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

 

[wiktor]

jcox:
The distrbution factor represents the fraction of wheel load applied to the girder.
As you are using the moment produced by a lane load (see AASHTO - lane loading), you have to convert it to the wheel load, i.e. divide it by 2.

 

[FM1]

the moments given in the appendix are per lane per truck, that is 2 wheel line. so when you divide by 2 it gives you the moment per wheel line(wheel line= one front wheel , one middle wheel and one rear wheel for HS loads. wheels in one longitudinal line)
the distribution factor represents the fraction(percentage ?) of each wheel load carried by the each girder. the closer the girders are spaced the fraction will be less.
Good luck