AASHTO Distribution Factors for wheel spacing other than 6' wide

[TS59401]

I think I read somewhere in the AASHTO Code (we have the seventh, 2014 currently) that for wheel widths other than 6' the distribution factor should be calculated assuming the deck acts as simple spans between the girders. I cannot find it know and it's making me wonder if I made it up.

If simple spans is not correct, how should I be calculating the loads on the girders for wheel loads less or greater than 6'. This comes up when we evaluate existing bridges for supporting construction vehicles.

Thanks for the advice,
Tom

 

[Lomarandil]

TS --

You are right that the approximate distribution factors given in chapter 4 are not necessarily applicable to construction equipment (or anything that doesn't look like a design truck). How far off it is will depend on the equipment and bridge cross section. It can often be a decent approximation.

I deal with this all the time, and I've not heard of considering the deck as a simple span. You aren't perhaps thinking of the lever rule, are you? That might apply in some cases, although I wouldn't use it universally.

For tightly packed floor members (like 2' deep stringers at 5-6' spacing), we've seen some benefit from building a 2D model of a bridge cross section, using elements with the equivalent stiffness of a cracked deck and stringers, and developing our own LLDFs. But there are a lot of ways to go wrong with a method like that -- tread carefully.

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The name is a long story -- just call me Lo.

 

[TS59401]

Lo--

Thanks for the advice. The more I read through AASHTO the more I believe I was mistaken about the simple beam idea. I might have mixed it up with the lever rule. I was hoping that I wouldn't have to do a cross section stiffness model, but that might be the best bet here. I will play around with few different ideas and see how they compare. Take Care.