Round manhole resting on beams in a square arrangement

[Awre]

I have a round manhole, the base frame (round) is resting on 4 beams in a square arrangement. The bearing area on the concrete will be a segment of the circle on each side with maximum height of 6" from the edge of the beam (attached is a sketch).
Since the load is not spanning the entire beam surface, rather, just 6" max at the center of the load applied at the edge of the beam, and I need to check the capacity of the reinforced concrete beam, what should I check the beam for with respect to the shear and moment?

- Does this load cause torsion on the beam?
- could it cause splitting of the concrete edge? and how to check for that?

I am assuming an HS-20 load for one tire on top of the manhole (~16000 lb)

 

[JedClampett]

Check the beams for the concentrated load from the manhole like a point load. Technically there is a torsional component of this load, but it's usually not checked. I'd use closed stirrups and call it good. But you should prove that to yourself.
As far as splitting the concrete, that's a possibility. Try to make sure your stirrups cross the failure plane and still check the shear on a 20 degree surface on just the concrete. The PCI Handbook has an example. Note that because you have a circular loading, the failure plane varies in width. You might want to just assume a 6 inch wide load.
In situations like this (wheel load pounding on a small area), a belt and suspenders and another belt is the best approach. Have multiple load paths available, just in case.

 

[Awre]

Thanks JedClampett!..since I am not in the office, is the pic example available online?
Also, since the load will be distributed alone circular surface, any idea on whether there would be spots along the loaded area in the beam be weaker; specially those spots that are closer to the edge; that would trigger bigger concerns?
Thanks again

 

[JedClampett]

I'm not sure if it's online. You might have to wait in suspense until Monday.

 

[dhengr]

Awre:
Why not cast the entire area/volume around manhole frame and within your four shown beams solid and 24" deep or whatever depth the main beams are. Reinforce the two cross beams and add some 45̊ diag. rebar, and shear reinf’g., at the four corners and bent into (parallel with) the far sides of the beams. This will tend to make any load on the manhole cover a 3 or 4' long distributed load onto the two main beams. It will also support the manhole frame all around, and reinforce against any torsion on the main beams.