AASHTO Buried Slab vs. Culvert Frame

[Dusni]

Hi,

Background:
I'm load rating a 3-sided culvert with about 3.5' of fill. There are provisions for slabs and culverts scattered throughout AASHTO LRFD Chapters 3, 4 and 5 as well as the MBE, and I find it confusing what is applicable to culvert frames vs culverts with simply supported top slabs. In my case there's only (1) layer of reinforcement going from the walls into the slab (near inside face of wall), so I was planning on treating it as a simply supported slab, and ignoring horizontal earth loads.

AASHTO LRFD Provisions:
LL distribution:
4.6.2.3 Slab-Type bridges with no fill
4.6.2.10 "Box Culverts" with fill <2'
3.6.1.2.6 Slabs with fill >2': From commentary: "This provision applies to relieving slabs below grade and to top slabs of box culverts".

Impact:
3.6.2.2 For "culverts and other buried structures covered by Section 12".

Questions:
1) Is 4.6.2.10 intended for rigid frames only? If so how do you handle fill <2' on slabs?
2) Is 3.6.2.2 applicable to simply supported slabs or intended more for rigid frames, pipes, arches etc, which is what Section 12 seems to be geared towards?
3) I found several LRFD DOT slab design examples (with not fill), as well as an example in the MBE, that do not check shear due to 5.14.4.1 which states that slabs using LL distribution from 4.6.2.3 are considered satisfactory for shear. 5.14.5.3 says simply supported slabs of box culverts with <2' fill use Vc = 0.0791sqrt(f'c)b*d. This seems to somewhat contradict 5.14.4.1. Slabs of box culverts >2' of fill (no mention of frame or simply supported) use eq 5.14.5.3-1 for Vc. Not sure if this is applicable to simply supported slabs or if its attended for rigid frames.
4) Section 6A.5.12 in the MBE has provisions (some very important, like using different LL factors) specific for Box Culverts. Is this section applicable to box shaped culverts where the top slab is simply supported?

 

[BridgeEI]

I don't have my code in front of me so I'll try my best to answer from memory. When you have fill, the wheel load is distributed by using a patch load. It's a function of the fill depth, more fill the more the wheel load is distributed as a uniform load.

When there is less than 2' of fill, the slab is designed like a deck. However, the equivalent strip width is calculated different than a deck and that makes you check the slab for shear also.

Article 5.14.4.1 is intended for concrete slab span bridges. Their methodology for the equivalent strip width is conservative enough in AASHTO's mind that shear is not going to control the design (or some similar reason).

In your case, I would use 3.6.1.2.6 and 5.14.