MANHOLE DESIGN 1

[Erasa]

Hi Engineers,
I have design a manhole using plates and have some doubt. I would be glad if someone could clarify this for me;
1) how do i find the maximum bending moment and shear forces in the plate for my rebar design?
4) I did considered the active case in my design using surcharge load and a backfilling load both applied separately to walls. Is am i doing the right thing?
3)my base pressure is more than my bearing capacity of the soil and i don't want to increase the base nor introduce piles. What will be the appropriate solution to this?

 

[thaidavid40]

You have not provided nearly enough information to be able to properly help you with answers to your questions. There are innumerable solutions to your problems, but they all depend on the specifics of your site conditions, and your design. Posting some drawings or sketches would help tremendously.
Dave

Thaidavid

 

[Erasa]

Hi Dave,
Please see attached. Will this help?

 

[thaidavid40]

That drawing helps somewhat, thanks.

1) The lift-out top cover plate can be treated as a free rectangular plate, pinned on all four sides. The formulas for stresses from various loads are in Roark's handbook of formulas. The relative proportions and thicknesses of your walls and base appear to be appropriate.

2) If the surcharge load is as large as you show (wheel loads, perhaps), then those will likely be a part of the governing load case equation. Their Boussinesq pressure will add to the active soil pressure case.

3) The answer to this depends on the specifics. If the difference between the actual and the allowable soil stresses is nominal, then site improvement of the in-place soil may be sufficient to get you to an allowable situation. If that is not going to be sufficient, then you must revise either the geometry (the base size), or the support system. As you mentioned, driven piles would be one way to do this. There are several options to think about (among others):

a) undercut and remove the existing soil, and replace it with good soil, properly compacted
b) install compaction piles
c) install drilled caissons
d) installed micro-piles


Thaidavid

 

[robyengIT]

external surface should be flat (without 83 mm offset) : much easier to manufacture and much cheaper the cost of the mold

 

[bridgebuster]

Regarding Q3: A few years back a co-worker had the same problem. One of the geotechs said if the weight of the basin was equal to or less than the removed soil don't worry about it. It was; he didn't worry about it; no one's complained since.

 

[Ron]

Agree with BB....check it.

 

[msquared48]

I agree too.

Mike McCann, PE, SE (WA)

 

[emmgjld]

In circumstances/areas which manholes have settled more than the roadway, I have recommended & supervised careful subgrade improvement (geogrid & angular structural fill), which proved to be sufficient. I have also placed a low capacity helical pier, with a lean concrete pilecap or geogrid & angular structural fill beneath a manhole. The only case of requiring a driven pile or similar element was a lift station.

 

[BUGGAR]

Can you show us how you calculated your base pressure? What is your allowable bearing?
Thanks.

 

[Erasa]

Hi Buggar,
This was done using StaadPro. Find attached the file.

 

[JedClampett]

kyekwa, you're on the right path if you're designing a large pump station or buried tank. For a small manhole like this, you want to design the bottom portion of the manhole as a strip spanning horizontally, and use that for entire height. It's just not worth the level of refinement that STAAD provides for a little manhole like this.
Do you have a supervisor? Really, they should be training or mentoring you on the right tools for the right job. You don't use dynamite to dig a hole for a rosebush and you don't use a finite element program for a little manhole. It's not necessarily wrong, it's just overkill.

 

[Erasa]

Thanks JedClampett,do you know of any software that i can use for small manhole?

 

[JedClampett]

kyekwa, that's my point. You need to do this by hand. It's a simple problem, but if you can't do it by hand, you're not ready to cut loose on any software.
Your model shows an understanding of structural engineering. Dust off that structural engineering handbook, find a simple/simple concrete beam analysis and design and go to town.

 

[Erasa]

Thanks to you all for your time.