PEMB over a basement

[XR250]

Got a client who wants to build a 2 story PEMB house over a basement. The basement will be concrete walls supporting bar joists and metal deck.
The outward frame thrust is about 13k - mainly due to gravity loads from a green roof on top. I was thinking about tying the opposing frame legs together with a 2 1/2" piece of tube steel running in the floor system cavity rather than trying to resolve the forces individually into the basement wall. Of course the net horizontal force will still have to be dealt with.
The PEMB folks seem to be on board.
Any things I should consider?

 

[hokie66]

Since the ties will be accessible, I would prefer providing some adjustment. Maybe use rods with rated turnbuckles.

 

[XR250]

Good idea. Thanks!

 

[BSVBD]

XR & hokie,

My gut is telling me to warn you to at least reconsider this adjustable tie rod.

Maybe i'm misunderstanding - I hope you are NOT considering providing a (unique) post-construction adjustment option.

Once the frame is set and tie-rod, of whatever material, is "set", why would you want to provide ANYbody, especially the tenant or ANY non-engineer or non-construction -minded individual the opportunity to "adjust" the tie rod?

I HOPE i'm misunderstanding.

Maybe i need enlightening?

Please advise...

 

[msquared48]

The adjustment would be for construction purposes.

Once completed, the threads could be destroyed by grinding or welding to prevent movement.

Mike McCann, PE, SE (WA)

 

[KootK]

1) Are the steel joists set out in such a way that a few of them could be used to resit the tension? If they need to become steel beams so be it.

2) I feel that tension tie elongation here will be more critical than strength so I'd keep an eye on that. Unless your tie is axially stiff relative to your floor deck, there's a pretty good chance that your floor deck will wind up taking the load and possibly cracking etc. This may lead you to a much larger tie member that you might design based on strength alone.

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.

 

[hokie66]

BSVBD,
The adjustment would be for the construction phase only in this case, but post construction adjustments make sense sometimes.

XR250 was considering a tie made of "tube steel". If that got fabricated incorrectly or if column erection tolerance dictated, there would be no good way to change the length on site.

I see little distinction between using tie rods with rated turnbuckles in this application as opposed to using similar rods for other elements of the building bracing.

 

[JAE]

Am I missing something here? Why wouldn't you just use rebar hairpins to tie in the column to the metal deck concrete slab?

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[hokie66]

JAE,

I would guess that most of the tendency of the legs to spread is due to the "green roof" dead load. This force would presumably be applied after the slab is cast. If you use hairpins (which I have an aversion to), the slab would likely crack. With the adjustable rods, you can monitor it and keep everything in equilibrium.

 

[XR250]

XR250 was considering a tie made of "tube steel". If that got fabricated incorrectly or if column erection tolerance dictated, there would be no good way to change the length on site.

In our area, fabrication for these small jobs is usually done on site so it is not an issue. After thinking about it, I'm not sure why the turnbuckle system would be needed. They are not needed on slab PEMB foundations and there is always the issue of it being under or over tightened.

1) Are the steel joists set out in such a way that a few of them could be used to resist the tension? If they need to become steel beams so be it.

The details have not been hashed out yet, but I was actually thinking about switching to i-beams as the building is small and it may be more economical. These is still the issue of getting the load into the bar joists or beams. It would have to go thru the pilaster/wall into the embeds and then into the bj's. The tube steel is a more direct path and less to worry about.

2) I feel that tension tie elongation here will be more critical than strength so I'd keep an eye on that. Unless your tie is axially stiff relative to your floor deck, there's a pretty good chance that your floor deck will wind up taking the load and possibly cracking etc. This may lead you to a much larger tie member that you might design based on strength alone.

I was going to isolate the columns from the floor deck even though they will still be connected thru the basement wall. I'm getting 0.1" elongation in the tube steel. I'm thinking that should be ok to limit the movement.

 

[KootK]

Balanced thrust aside, I still like the idea of connecting directly to the floor deck framing for unbalanced column lateral reactions rather than taking them out through the basement walls / retained soil.

Unable to control self...

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.

 

[XR250]

Balanced thrust aside, I still like the idea of connecting directly to the floor deck framing for unbalanced column lateral reactions rather than taking them out through the basement walls / retained soil.

Nice detail, (how do you have time to do this :>) but I feel mine is far simpler (don't have time to draw it right now.)
Less fab work. Just cut it and weld it. It could lap over the intermediate beams so length isn't even critical.
The tube steel tie would take the opposing thrust. The unbalanced loads (i assume you mean the net lateral) would go into the piers, into the wall , into the joists and into the diaphragm without using the retained soil. The net lateral is significantly less than the spreading thrust so this is not an issue.

 

[KootK]

Fine. I guess you can be trusted to detail your own project...

Nice detail, (how do you have time to do this :>

This time, multi-tasking while I eat breakfast. 2hr time zone difference between us I think.

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.

 

[XR250]

This time, multi-tasking while I eat breakfast. 2hr time zone difference between us I think.

You are certainly dedicated! Thanks for all of your time and energy.