Precast Concrete Manhole Design

[mmmbmf]

I have been asked to provide a price for structural calculations for typical standardized precast manholes by the precast manufacturer (30 manholes and 2 wet wells). Specifications were provided that require the contractor to provide signed and sealed calculations including shear, moment, buoyancy, and camber. I have been looking for any software that may be written specifically for this type of calculation. Does anyone know of any program that will accomplish these calculations? Also, most specifications I have seen, do not require these calculations due to the fact that the precast construction of most typical round manholes are indexed in the DOT standard indexes and are accepted design. Do most all Civil Engineers require calculations on these type of typical structures? I have entertained the idea of modeling a typical or each manhole in Risa, but the cost of doing this would most certainly make the bid noncompetitive. Any help would be greatly appreciated.

 

[ukengineer58]

I dont think you need to write software or use risa for this kind of design it would be wholly disproportionate. Just use hand calcs, it oculd be done it about 3 pages if that. If you cant, then your not the person for the job.

 

[mmmbmf]

Thank you for your opinion but the way things seem to be progressing regarding liability issues, the EOR are going beyond and above on documentation. I Agree a typical diameter manhole with no openings can be done very simply long hand, but with multiple size manholes, openings, thicknesses, I anticipate them requiring one in detail for each structure. I understand the requirement if it relates to a typical manhole in compression and bouyancy. I just am not sure of the requirement for moment, shear and camber if the design for these structures are based on a concentric loading with concrete being in compression. Any other thoughts would be helpful.

 

[ukengineer58]

I think you need to define your problem a little more precily to get any meaningful response on here . i mean that with no offence intended. simply stating that you do not know how to analyse it in a general sense, which is how the post is worded will mean the really useful and helpful guys on her will simply breeze past your post.

 

[fel3]

I had to do this once, years ago, when I was designing some storm drainage piping at a petroleum tank farm. The City of ######'s Development Department, in their less than infinite wisdom, would not accept their own Public Works Department's standard detail for manholes. They wanted calcs to prove that the MHs were going to be structurally adequate. IIRC, the deepest MH was less than 15 feet, so it's not like I was operating outside of any normal engineer's comfort zone. My appeals to reason fell on deaf and dumb ears, so I did the calcs, for which my client (thankfully) agreed to pay me extra for. He understood that I had not included such calcs in my fee proposal because it was self-evidently unreasonable to assume that I would ever need to do them. For goodness sakes, that's why we have standard details in the first place. My client later told me that the City of ###### and the City of ### #### were the two most insane agencies he had every had to deal with. This was not the first time, not the last, he and I butted heads with the idiots at the City of ######. On the other hand, when it came time to upgrade the site fire protection system, the City of ######'s Fire Department was a pleasure to work with.

Now to the problem at hand: If you look at a typical manhole installation--buried to the top with homogeneous backfill--the first thing you should notice is that the MH cylinders are subjected to only two main forces: [1] hoop compression from the backfill and [2] traffic loads. The traffic loads will [a] add to the lateral soil pressure when outside the cylinder (this only matters near the top, though, and even there it doesn't matter a whole lot) and create a vertical compression (with some eccentricity) when inside the cylinder (i.e. load on the manhole lid). If you take just the lateral soil load as an equivalent fluid pressure you will find that a standard MH section is good for hundreds of feet of burial. I can't remember what I came up with years ago, but I vaguely recall the answer is >1000' of burial. This result is unrealistic, of course, because I doubt the equivalent fluid model is good to that depth, which you would never reach anyway. Regardless, I also vaguely recall that this was true failure point for the MH: exceeding the compressive strength of the concrete due to deep burial.

When it comes to traffic loads on the manhole lid, it is useful to note that a typical wheel load (say 16,000 pounds) is easily handled by a only a few square inches of 3000-psi MH cross-section, whereas the cross-section for a 48" ID x 5" thick wall is 833 sq in. In my opinion, the only part of a MH that really merits any attention in the cone at the top. Table 30 of Roarks' Formulas for Strees and Strain, 6th Edition, has a bunch of loading cases for conical sections, some of which appear to apply. I don't think you have much to worry about with respect to moments because the individual MH cylinder sections are only fitted together rather than firmly attached. I also don't think you're going to find any shear problems either. Even without calculations, we know standard MH sections work.

Wetwells made from large diameter concrete pipe are handled the same except there are no conical sections to worry about. Cast-in-place wetwells need more attention.

Buoyancy is pretty straight-forward. Here is a link to an example provided by the USEPA:

http://www.epa.gov/opp00001/regulating/buoyant-force-calc.pdf

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[mmmbmf]

Thanks for your input. I anticipated just writing my own calculation program and priced it accordingly.