Seismic Design: Detailing Load-Bearing Wall not to take Seismic Shear

[structural_AMZ]

Hello

I am designing a high rise building for which the Seismic-Force Resisting System will be Special RC Shear Walls.

Now I would like to have some of the load-bearing walls not to take seismic shear but still take down the gravity load. Hence such walls will need to act as load-bearing walls but not as shear walls.

My concern is about detailing the floorplate-wall connections accordingly. I believe that the continuity given by cast concrete and rebars at the joints will transfer the seismic load from the floors to the walls and therefore special connections may be required.

Looking forward to hearing your opinions on this.

Many thanks

 

[KootK]

I'm not sure that a practical solution exists to achieve what you're looking for. We sometimes use grouted dowels and polyethylene sheets to create slab/wall slip connections in post tensioned structures. Those are temporary slip joints, however, and I wouldn't trust their efficacy in a seismic event. In theory, you could put some PTFE slip material on the walls to make 'em really slippery. I'd think that would be ridiculously expensive though.

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.

 

[JAE]

I would agree with KootK here. Very difficult to provide vertical support while releasing any horizontal resistance. Lots of expensive teflon slide bearings? Too much money.

Try using simple posts and beams with "soft" wall infills to avoid the resulting shear wall stiffness that WILL attract seismic loads.

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[MrHershey]

Load bearing walls are tough. When we've got extremely low load like at a staircase where our trib width is only a couple to a few feet, we'll use a steel ledger angle with a post-installed epoxy anchor in horizontal long slots (usually longer than AISC's long slots) and have them either just hand tighten the nut or omit completely if it's not needed for load in other direction. Really only works in extremely low load areas though, once gravity load gets too high your connection will bind and your gravity wall turns into lateral. For heavier loads, we've used the bearing pads as JAE suggested. Just ends up getting expensive.

 

[sticksandtriangles]

As others have said, the condition you have described is difficult to achieve.

I often take solace in the fact that let's say this floor plate connection can take some lateral load.
It is not specifically detailed and more than likely under a seismic load event something will fail as it is not specifically detailed to transfer diaphragm forces (connection to wall etc). Hopefully this wall will give or slide past the load bearing wall, allowing lateral forces to move to your designated shearwalls/diaphragm attachments and the lateral load will distribute as you had assumed in your lateral analysis idealization of the building.

 

[Aaouatiffaisal]

I think that it's not realistic to design a wall as bearing one , and at the same time assume it's "non seismic".
It will always participate to lateral resitance system.
I guess the only way to proceed ( a work around ) is to realise a wall with thickness smaller that the minimum required by the seismic code, but should at the same support static loads as you whish.
Even with that you should also take a look to the pourcentage of effort attracted by this wall and compare it with other assumed shear walls and compare on what is stated on the code.
Difficult to achieve ... unless with small loads.
Hope it helps

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[dik]

wall on rollers/teflon with ropes to hold it in place? because of the stiffness, the wall will invariably take load.

Dik

 

[jdgengineer]

Forgive me, I don't do a lot of concrete design, but can't you just deem it not part of the lateral force resisting system and instead detail per the seismic requirements for elements not part of the seismic system (i.e. deflection compatibility)

 

[KootK]

You could and be technically code compliant I think. However:

1) The compatibility shears you'd be absorbing into the non-lateral walls would be enormous. You'd be better of just using them in most cases from an economy perspective.

2) I'd argue that, despite technical code compliance, you'd have drifted pretty far from realistic behavior.

More of a column thing.

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.