Mechanical Connection of Anti-Flotation Slab

[05dtaylor]

I'm trying to wrap my head around something and I can't quite figure it out.

Say I have an 8' diameter precast manhole that is 36' deep with 8" thick walls (Just pulled numbers out of the air). The manhole will be comprised of a 5'tall base section, six 5'tall riser barrel sections and a 12" slab on top. The joints between sections are of a standar joint & spigot design with a butyl-ruber sealant in the joint. It has been determined that this structure will need a 11' diameter by 12' thick anti-flotation slab. Due to state engineers, this anti-float slab will need to be cast-in-place instead of being integrally cast as part of the base section of the manhole. It's common practice to mechanically attach the base section of the manhole to the anti-float slab with some sort of angle bracket and hilti-stly anchors. Now my question is, why do I need this mechanical connection? If it is needed, how would i go about determining the required shear strength and pullout strength needed for my hilti-style anchors/angle bracket attachment? Also, why do the joints between other manhole sections in the same structure not need a mechanical connection?

Thank you very much for any help,
-Dan

 

[jdonville]

05dtaylor,

I suggest that you discuss this issue (politely) with a state engineer - somebody who reviews your designs may be initially more receptive/available than a very senior person, although the more senior, the better. If you can, approach them at a conference or invite the engineer in question to lunch (the latter ensures you some continued "face time" if they accept).

In the meanwhile, consider each piece of the structure as a free body. Except for pipes, the barrels will have the weight of the stack on top, self weight and will have some sort of skin friction with the surrounding backfill. The bottom slab will have a buoyant force acting up on the bottom, dead weight (possibly buoyant) and the buoyant or effective weight of soil on the projecting area.

Also consider that the groundwater elevation may vary from season to season.

My supposition is that the bottom barrel gets mechanically attached to the anti-flotation slab to ensure that the bottom barrel moves up and down with the slab. The assumption may be that the remainder of the manhole stack does not have enough skin friction to overcome self-weight, so that the rest of the stack will move vertically with the slab+bottom barrel.

J