R/C slab Bridge overhang design 1

[KMA]

I have been given the task to analysis a R/C slab for a bridge deck overhang. The overhang will be fixed on one end
(beam flange interface) and free on the other. The top steel from the bridge slab is the main tension reinf. for the overhang. The major load is an HS25 type truck. I am using AASHTO spec. I believe that I can use membrane action and would like to distribute the wheel load longitudinally, more than that allowed by AASHTO (E=0.8*X +3.75). Any thoughts? I have heard of design charts (Broussard?), but am not familiar with these. The overhang will not exceed 5ft, and I would like to stay away from grillage or FE analysis.

 

[PEinc]

If you are designing a bridge deck for a govenrment project, you will have to use AASHTO or the government requirements if more stringent. I would be greatly surprised if you are allowed to distrubute a wheel load over a greater distance than that allowed by AASHTO.

 

[Qshake]

For one of the best references to the design of bridge overhangs as you described follow the link below to the MoDOT website, bridge design manual and click on section 3.30 PDF file for a design example. This is extremely useful for engineers new to bridge design.

In addition to the loads you've mentioned - the vertical HS25 load. You will also have to look at two more situations. First is the collision load acting on the safety barrier curb usually set at the edge of the overhang. The collision load is defined in section 3 of AASHTO. Secondly, you must check the interior positive and negative moment between girders. The latter probably won't control until you encounter some very large girder spacings, but it should be ruled out by beginners.

http://www.modot.state.mo.us/bridgemanual/general.htm

 

[KMA]

Many Thanks Qshake...kudos to the MODOT for an excellent chapter to their Manual.
Several thoughts....AASHTO specs based on the Westergaard theory assumes the cantilever slab to be a rigid support, such that the moments at midspan are the same as the moments at bent lines. But for modelling closer to actual behavior, the cantilevered slab is supported on a flexible girder, that may defect at midspan. This deflection allows for more distribution width than that allowed for by AASHTO. In Texas we normally see Prestress Girders Ty IV spanning at least 115ft. In this case distribution width is different at midspan and bentlines.....This question came up for a non standard slab overhang for a radial span and chorded girders, where the overhang at midspan may calculated out to be greater than current highway standards. Based on example calcs from MDOT chapter, AASHTO ok for my case.
Thanks