Wood Shear Wall 1

[Sanira]

Hi, I am a junior structural engineer. I want to know what is the difference between normal wood wall and wood shear wall. While modeling in RISA, do we consider all the walls as lateral walls even if we will be using only some walls as shear walls.

 

[msquared48]

A shear wall will have a physical connection to the framing above and below that are capable of transmitting the lateral force. A true partition wall will not.

It will also have plywood, GWB or GSB applied with special nailing and blocking and hold downs if needed.

In RISA you need only enter the structural elements. Partition walls are non structural.
Mike McCann, PE, SE (WA, HI)

 

[Sanira]

Thank you Mike tor your response. But I have a load bearing wall which I will not be considering as shear wall. What do I assign to it? Lateral or Gravity? Furthermore, do all exterior walls in RISA needs to be shear wall? What if we are only considering corrider wall for shear wall? I read somewhere that all the diaphragm edges must have lateral elements. But in my building, not all the exterior walls are shear walls.

 

[jayrod12]

Any wall you do not want to participate in the lateral system you would designate as gravity.

That being said, if it is a reasonable section of wall, you may want to envelope the design to make sure it doesn't attract a significant amount of load. Although, in my experience, as long as you ensure you've got enough shear capacity in each direction in some walls, nothing bad seems to happen to the other walls that we've just ignored. Even when they are reasonably large enough to participate.

 

[Sanira]

Thanks jayrod12

But what about having lateral elements in the diaphragm edges. RISA requires the perimeter of diaphragm edges need to have lateral elements. But in my building I m only using corridor wall as shear wall. My exterior walls are not shear walls.

 

[jayrod12]

Well then you can't consider it a flexible diaphragm.

I'm assuming that you are doing a long skinny 3/4 storey condo with jog filled, window filled exterior walls and long straight relatively unbroken corridor walls.

I've always ended up running those by hand, and usually I'm able to stick the odd wall in the exterior that works.

I've never used RISA for a wood project yet, especially for lateral with RisaFloor for the diaphragm calcs.

I usually just run my wood diaphragms by hand, rarely had something so convoluted it amounted to a significant amount of time.

 

[JoshPlumSE]

Santosh -

The first question is whether you are using RISA-3D or RISAFloor + RISA-3D?

If you are using RISAFloor, then you can model both the walls that only resist gravity loads and the ones that resist both gravity and lateral load. The difference is that the gravity wall will not be brought into RISA-3D for the lateral analysis.

If you are modeling this only in RISA-3D, then I would suggest only modeling the lateral walls. You can model the "effect" of gravity walls (the vertical reaction they impart on other elements or such). But, if you don't want them to participate in the lateral force resisting system then they probably shouldn't be modeled in RISA-3D.

 

[Sanira]

Thank you JoshPlum.
I am using RISA Floor + RISA3D. I am just confused in one thing. RISA Floor mentions that Diaphragm region should have a perimeter of lateral elements. However, what if we have gravity walls in the perimeter. I have one more question if you don't mind. How do you model offset wall in RISA Floor. For eg, upto 2nd story the shear walls are in same vertical plane, however from 3rd floor, there is a slight horizontal setback in the vertical plane. When I run this in RISA, it gives warning that there is no member to support the lateral element from 3rd floor

 

[JoshPlumSE]

RISAFloor should have three types of diaphragms.
1) Rigid: When using rigid, it doesn't matter whether you have perimeter lateral walls.
2) Semi-Rigid: When using semi-rigid, it doesn't matter whether you have perimeter lateral walls.
3) Flexible: When you use flexible diaphragms, the load attribution may not work correctly if you don't have perimeter shear walls. Most of the time, I believe, the lateral load attribution will still be okay. However, the diaphragm design / code checks cannot be trusted if you have anything other than a rectangular diaphragm supported by lateral elements on the perimeter.

The rigid and semi-rigid options don't offer diaphragm design / code checks. So, you would have to do that portion of the design by hand. However, the load attribution should still be pretty good.

 

[Aesur]

I am curious why you are using RISA for a wood structure? I have never used RISA or any other modeling program for wood and wouldn't even consider have a jr. engineer working for me using programs until they had done a few by hand first. By the time you figure out all the "gotchas" in RISA (or other programs) you could have already finished it by hand in most cases.