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PEMB on Retaining Walls

PEMB on Retaining Walls

PEMB on Retaining Walls

(OP)
I really haven't seen any PEBM (moment frame type) utilizing below grade floors / basements. Almost all are on SOG. Is this due to the lateral load at the pin ended column on a retaining wall? (Guess that depends on magnitude, but just wanted to ask in general practice).
Also, is there any benefit of using PEMB's typical moment frame over steel roof joist (with plywood shearwall) in terms of constructability, cost, performance? Any feedback is greatly appreciated!

RE: PEMB on Retaining Walls

We've done a few PEMBs on retaining walls to provide for additional height and to take advantage of existing topography. In some cases the columns bypassed the walls and went all the way to the slab, so those weren't really supported by the walls, but by the footings. In other cases, we formed reinforced pilasters integral with the wall stems, and founded the columns on these at the tops of the walls. The pilasters were flush on the inside face, and projected into the fill on the back side. This required us to slightly inset the girts to align properly, but that was no big deal cost-wise.
Hope this helps a bit,
Dave

Thaidavid

RE: PEMB on Retaining Walls

To answer your first question, typically PEMB are built on SOG's so that the thrust can be resisted through the slab. There are many other ways to do it, it just seems like in my area, it's the slab with hairpins or they find another engineer. I think you can do basement/grade floors, but I would say if you're gonna pay for a basement, go with conventional construction. If I was asked, I would never advise anyone to go with a PEMB over conventional, unless cost was the only variable, and I don't work for those types of clients (or at least try not to). With that said, I don't think much can compete with PEMB cost, at least in my area. But you have to remember that it is very difficult, if not impossible, to modify PEMB buildings once they are done. Want to add a rooftop unit, well, you better have accounted for it in design or it's not happening without a significant undertaking.

And I think it's Alexander Newman who has a great book on PEMB foundations. Goes through different types of foundations used, loads, etc. Great reference.

Lastly, I'm not all negative on PEMBs, they work and they're inexpensive, but the client needs to know that they will likely leak, won't be able to be modified much, and will be flexible when loaded. If it's a warehouse where no one cares, it might be the right solution.

RE: PEMB on Retaining Walls

@njlutzwe;

I'm with you on the PEMB's. Re-sale is also going to be lower which may make up some of the cost differential.
I had a client make a giant house and garage out of a new PEMB. It turned out pretty cool.

RE: PEMB on Retaining Walls

Are these cantilevered retaining walls or basement walls braced at the top by a concrete or steel floor diaphragm? If the latter, it might actually work out better than the SOG scenario.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: PEMB on Retaining Walls

Ask your PEMB about the permissible deflections at the base connection and you might find this to be a challenge. Zero movement is not an uncommon response. We struggle with this always because we use a lot of piles and our geotechs do not want us to dowel the slab to the foundation (high plastic clays).

RE: PEMB on Retaining Walls

I do not recommend that the PEMB columns be placed on top of a cantilevered retaining wall. As Brad805 said, the outward deflection at the top of the retaining wall (due to the PEMB thrust force) will essentially weaken the frame, which is designed based upon an assumed zero movement support.

You can easily set up a model of the PEMB with a horizontal spring support and COMPARE IT with a model of the PEMB with unyielding pinned supports. This will give you an idea of how the cantilever wall will weaken the frame. I suppose you can specify an increased load to make up for the weakening affect. I doubt that you could get the PEMB manufacturer to design their frame to accommodate to support movement.

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