Lift Station Buoyancy

[Trackfiend]

I've noticed on several details here in my office that for a concrete wet well (sewer lift station), the caisson section is keyed into the concrete base. There are no other notes or details for anchoring or connecting the sump to the base. Should there be some kind of anchoring to prevent uplift on the circular portion of the sump. I realize that the base is sized to resist the uplift forces through it's weight, but if the circular section is not attached with some sort of rebar or dowels, then wouldn't the sump float up with the base not moving? What would be a proper way to tie in to each other?

 

[bimr]

The base and first cylindrical section are typically combined into a single prcast component:

http://www.jensenprecast.com/produc...mpstations/96'' dia pump station_layout_b.pdf

The base typically extends outward at least 12" beyond the cylinder to prevent uplift of the lift station.

You can use the concrete-pipe.org brochure on manhole floatation to calculate the uplift forces:

http://www.concrete-pipe.org/pdfs1/2008-05-DD_41-manhole-flotation.pdf

 

[SteveWag]

There is no uplift force exerted on the vertical wall section, only compressive forces. The only uplift forces exerted are on the horizontal component of any surface, and these forces come in two kinds; first, the force exerted on a solid that is less dense than the liquid (a piece of wood in water, or concrete for that matter); second, the force exerted on a hollow or composite structure, like a boat or empty wet-well. In any event, the lifting force is only exerted on the horizontal component and the walls of the wet-well, if vertical, have no horizontal component and therefore no lifting force is exerted.
Steve

 

[rconner]

I suspect you have gotten some good answers thus far. e.g. I think it is true any uplift force of a low bulk density tank (or station) in a truly "buoyant" environment (in accordance with Archimedes)is indeed upward on the bottom.

However, I'm not sure you should necessarily want to immediately abandon your intuition of somehow tying walls to bases only on this revelation without further investigation. It has already been inferred that if it is desirable to handle a station as one piece, attachment would be helpful in that case. I also wonder a little what if the surrounding environment was not "fluid" (not buoyant conditions, but there was instead some tendency for whatever reason of the base to settle downward instead of float (and even relative to walls for whatever reason might be somehow held up by some sort of non-obvious inverse action, say with more firm/founded surrounding backfill?) Of course, with fluid in the station that fluid would exert a pressure DOWN on the bottom. I guess the odds or experience of that happening could be so low however that tying might not have been economically or otherwise justified/ in the design you are looking at?

As far as exactly how to tie the base in considering all, I alas don't know enough about that business to answer that question!