AASHTO Bridge Deck Design for 12' driveway bridge

[keyPitsimplE]

Can anyone comment on how conservative (or not) the old ASD method of designing a 12' span steel beam with timber deck bridge would be compared to the new AASHTO LRFD methods? The existing concrete abutment is in fantastic shape so will not need to be modified. We just want to slightly widen it and lower it as we replace the old rotted timber deck. The requirements are to use HS20-44 loading, so I am quickly able to come up with the steel beams required, using L/800 deflection limit, and a timber deck over it. My main questions are if AASHTO still allows a 24k axle load to be used to design the timber deck (instead of 16k), if the wheel load distribution of S/4 is still applicable for longitudinal beams, and if the tire contact area can still be figured using Area = 0.01*P, (with P = wheel load) and tire width/length ratio of 2.5 to 1. I would not attempt to design a highway bridge with the old code, but for such a simple project where light residential traffic will never exceed 5-10 mph I'm curious if the simplicity of the 1992 AASHTO Std Spec for Hwy Bridges will produce an adequate bridge. I can already see the new superstructure will be stronger than the existing one which has worked for 40 years with no signs of overstress.

 

[bridgebuster]

I'm not in the office so I don't have my AASHTOs in front of me. A few things: Your bridge is 12' long, since it's less than 20' it's not considered a bridge by AASHTO. You can use AASHTO, but at that length either spec will give you the same results. You can only get one axle of the HS truck on the bridge. HS20 or H20 won't matter; tandem axle probably won't control for moment; for such a short span shear may actually govern your stringer design.

Check with your municipality about the live load. You might have to design for a fire truck, which could have a higher axle load than the HS trucks.

There's nothing wrong with the Standard Specs. I just finished a rehab project using them.

 

[keyPitsimplE]

Thank you very much, bridgebuster. I suspected as much, but feel better having done my due diligence. I spoke with another older bridge engineer who confirmed the same thing. Long live the old simple way of doing things! (at least where it still make sense)

 

[bridgebuster]

No problem. Good luck.

 

[keyPitsimplE]

By the way, regarding my understanding of being under 20', that really doesn't mean anything with respect to loads. It still has to meet all of the same load criteria. It is really only there to give states a line in the sand to classify which structures have to be inspected every 2 years, and which might be considered "culverts". Do you agree?

 

[bridgebuster]

I agree.

 

[bridgebuster]

If gives you some "wiggle room" if you need it.