Wood Shearwall Question 2

[Lion06]

This may sound like a stupid question, but I never had a wood design class and I just bought the ASD/LRFD wood design book by Breyer.
I was doing some reading to familiarize myself with some of the niceties of wood design (beams and columns are relatively straight forward), and I am reading the shearwall chapter.
It says that for residential buildings it is common pratice to use interior walls as sheaarwalls, and that, generally, the longer walls and the walls that stack from floor to floor are selected first.
My question is this: no house I have ever lived in has had walls in the basement so how do you take an interior shearwall load and get it to the foundation? It seems like the floor system would need to be designed for these forces, which could be significant.

 

[UcfSE]

You check the diaphragm for the shear forces and add columns under the chords of the discontinuous shear walls, all of which is accompanied by proper detailing.

 

[akastud]

Structural EIT,

If it is a true basement (meaning retaining walls on all sides), then the diaphragm may not need to be designed for lateral point loads because the lateral loads will go into the soil behind the walls before the diaphragm deflects. In wood frame structures where the soil is at or near finished main floor level, I neglect lateral forces once they are in the floor diaphragm at the main level.

akastud

David S. Merrell, P.E.
TOR Engineering

http://www.TOREngineering.com

 

[whyun]

To supplement UcfSE's response, at the shear wall boundaries you may provide a beam (Glu-lam or other) in lieu of columns at the basement.

Diaphragm at the first floor need to be designed for the portion of the lateral force from the interior shear walls and transfer it out to the perimeter basement walls. But the vertical distribution of lateral force need not consider the basement level as a story. First floor level is used as the base of structure typically when all four sides of the basement walls are retaining.

 

[Lion06]

whyun-
I understand that the first floor is used as the base, that makes sense. What doesn't make sense is where is the support for the "interior" shear wall? It is fine to say the first floor is the base, but that is not providing a support for the shearwall chord loads. I see how the shear in the base of the wall would get shed into the first floor diaphragm ( I assume the first floor diaphragm needs to be designed for this as well as it's own lateral load), but the chord forces would just get picked up by a glulam?
Is this a new development? I've never seen a house with anything more than one "girder".

 

[COEngineeer]

Structural EIT, interior shear wall (load bearing wall) need to be picked up by a wall or a beam. So if you have an open basement, then you need to make sure you have a beam that is strong enough to support the main floor AND the linear load from the load bearing wall. There are a lot of houses that have a load bearing wall in the basement.

http://www.krmconsultants.com

 

[Lion06]

COEngineer-
What type of connection would you use to tie the shearwall chord to the beam?

All-
Thanks for the clarifications. I was very curious because I have rarely seen basements with walls in them (at least stacked walls that could be used for this purpose), and have little experience in this area.

 

[COEngineeer]

I just follow the typical IBC nailing schedule,
stud to sole plate: 4-8d toe nail.
sole plate to joist or blocking: 16d@16" o.c.

http://www.krmconsultants.com

 

[Lion06]

But assuming there is a net uplift on the tension chord of the interior shearwall - what sort of connection would you use to tie it to a steel beam acting as a girder that is supporting the wall?

 

[COEngineeer]

if the nailing is not enough for the uplift then go look up simpson ties to hold down the wall. I am no sure what you are asking.

 

[Lion06]

COEngineer-
My question is if you have an interior shearwall that is supported by a steel beam rather than a wall below, and it has a net uplift in the tension chord, that chord force must be taken into the supporting beam, correct? If the tension chord isn't connected to the beam, then what holds it down? For an exterior shearwall, you would use a Simpson Hold Down with an anchor bolt in the wall. What would you use for the interior connection?

 

[COEngineeer]

What you meant by tension chord is the column at wall ends? The column is connected to sole plate, sole plate is connected to floor plywood, plywood is connected to 2x nailer that is drive pinned into the steel beam. If for some reason that is not enough, then use simpson product. It is all connected.

 

[whyun]

One method of holding down a shear wall end post over a glued lam beam is to provide a metal strap from one side of the post down and wrap around the underside of the glued lam beam and go up the other side of the post. Plywood floor sheathing should be notched to let the strap through.

Other way is to have a through bolt and holdown(s) with a washer plate at the underside of glued lam beam.

Any fastener that partially penetrate the glued lam should be avoided to prevent delamination of the glued lam when connection is in tension.

Interior shear wall may be bearing or non-bearing. In any case, the beam supporting the interior shear wall and its end connections should be designed for the lateral uplift/down force combined with other gravity loads.

 

[tdsull]

StructuralEIT,
If I have a shearwall on the first floor that does not span from from one side of the foundation to the other, and the uplift loads are not that intense, I will run a threaded rod down thru a beam in the first floor framing to a deadman footing in the basement. Additionally I locate a lally column in that basement location as well so the rod doesn't look to out of place or get removed at a later date for 'extra' space.