AASHTO LRFD "At-Grade" Box Culvert Loading

[TypeIV]

I am designing an "at grade" (Cover < 2'-0") RCB using AASHTO LRFD. The spans are < 15'-0" and the code says to design for a 32 kip axle load.

I have a three barrell box and I am wondering if that 32 kip load is acting alone and moved across the three spans for maximum forces or is it multiple 32 kip loads at some (14'?) spacing moved across the three cell spans?

[I would also be interested in any software that anyone has had success with.]

Thank you

 

[bridgebuster]

You have to position the truck in a way that provides maximum moment. You could have all three axles on the culvert.

Below is a link to CalTrans, go to Chapter 6 for a design example (it's LFD).

http://www.dot.ca.gov/hq/esc/techpubs/manual/bridgemanuals/bridge-design-practice/bdp.html

 

[TypeIV]

Thanks bridgebuster!

Oklahoma specifically requires us to use LRFD for RCB's. The LRFD Code only requires the full analysis for HL93 and Lane Load if the supporting walls are > 15 feet apart. I am designing a Triple 10'(W)x6'(H) reinforced concrete box and the code says "Where primary strips are transverse and their span does not exceed 15.0 ft.-the transverse strips shall be designed for the wheels of the 32.0-kip axle."

I am trying to understand how to apply this load, as stated above.

 

[bridgebuster]

Are you designing the top slab with the main reinforcement parallel or perpendicular to traffic?

 

[TypeIV]

The main reinforcement is generally parallel to the traffic (with a 60 degreee skew to the Highway).

 

[bridgebuster]

It sounds like you should be designing according to the 3rd bullet of 3.6.1.3.3 and positioning all axles of the truck for maximum effect.