Manhole riser design with holes

[FeSE]

It seems that there is a huge hole in the practice of design and supply of manhole riser sections that have holes in them. Granted, when there are no holes in a manhole riser, the math shows that the riser sections have plenty of strength. Now then what really happens inside a riser when we put big holes in it? I briefly looked through my references and the internet and found no design standard or guidance on how we should check risers with holes. I see plenty of 'owner's guides' for supply of manholes such as the max size hole in a given size round section and the minimum distance between holes; e.g. 6ft riser can have a max hole in it's wall of 54". I have also seen that the minimum wall between holes is typically 6". So, here's my questions.
For a given size of manhole riser, how do we determine the maximum size, quantity and layout of holes in the wall of the riser?
I am looking for equations and structural engineering guidance. I am not seeking anything of the nature; we've always done such and such, or in the City of <your experience> we follow the city's manhole specification.

Thanks in advance for your input.

 

[JoelTXCive]

Unfortunately, I do not think there will be a straight analytical solution with equations to this question. I think I finite element model would be the route to go for a more exact answer.

But....Unless you have special circumstances or a really big design budget, this is an area where engineering judgement can be exercised.

A hole in the pipe is similar to hole in a slab where the moments, shear, and compressive loads 'flow around' the opening. Therefore you want enough room around the opening to fit some continuous steel. If you cut 4 bars for your penetration, then you want an additional 2 bars per side of the opening. (that probably falls into the "we've always done it this way" category...)

Maybe others will have good ideas for analysis.

 

[DoubleStud]

Are you talking about round riser? Usually these risers are just following the ASTM as far as reinforcement goes. It is a ring design so where you have an opening, the compression force from the ring will just transfer to the pipe attached to it. Typically I just have to make sure I can ship and handle the product (assuming the riser is precast).

 

[FeSE]

Yes, round risers.

 

[FeSE]

With the FAA requirement for loading on structures to be 100k it seems that there needs to be research done. It seems very much that the precast industry should be obliged to obtain this research on this issue. Can I get support on that? If I can get a few people to support this then I will write a letter to PCA. Thanks in advance.

 

[JedClampett]

I think you're kidding yourself. You have thousands, maybe millions of risers happily being loaded. And the precasters are to do reasearch that:
[ul]
[li]Will cost money.[/li]
[li]At best, not change their designs, but maybe cause them to be beefed up.[/li]
[/ul]
I don't see it happening. Maybe the FAA or FHA could fund it, but it's a solution for a non existent problem.

 

[rb1957]

is your "FAA" the same as my "FAA" ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[DaveAtkins]

I have designed a lot of "custom" sanitary and storm water junction structures. These are larger - sometimes much larger - than the typical round manhole structure, and are typically rectangular. For example, today I am working on one that is 6'-6" X 16'-2".

The points being brought up so far are all valid. On these larger structures, the 100 kip wheel load required by design for an airport runway or apron (per the Unified Facilities Criteria) will control the design of the top slab of the structure. The top slab on the structure I am designing today will be about 2 feet thick. But I don't think anything special is needed for the top of a typical round manhole. Keep in mind that for non-airport situations you would use an AASHTO wheel load (16 kips times 1.33 for impact).

And as DoubleStud mentioned, a round manhole will be in compression, with soil pushing in on all sides. There really is not much bending, mostly ring compression. That is why lightly reinforced round manholes work.

DaveAtkins