Where will I put a Shear Wall?

[patrick526]

Hello! I am a newbie and I hope you guys will have some patience answering my questions. I tried hard looking for books that can tell me when and where am I gonna put a shear wall along the whole length of wall, but none of them really gives the rule when to provide a shearwall and where will I put a shearwall exactly on a specific location?

Let's say I have a 60 ft. span, 8' high masonry wall. Where will I put a shear wall? I remember an older engineer told me before that the rule of thumb for this provide a shear wall every "20 ft." of wall span, but there is no concrete basis for this. I would wonder if you could shed me light regarding this.

Thanks!

 

[jike]

There are no general hard and fast rules that I am aware of because each building can be different: depth of diaphragm, length of shear wall, height of structure, etc. You may be able to develop some general rules for a particular type of structure with similar dimensions and material but you have not given us that info.

 

[RARWOOD]

The problem generally is where will the architect allow you to put a shear wall. With openings in the walls and expansion joints normally the problem is finding enoungh wall to resist the building shear.

 

[whyun]

Total length of shear wall should be sufficient to resist the total lateral force along that particular shear line. Each individual shear wall length should be such that severe overturning is minimized. The layout of individual sheal walls along a shear line should be such that drag forces are minimized.

Usually masonry buildings have perimeter walls as shear walls which are pretty much the full length/width of building with openings. My comments above are general and not restricted to masonry shear wall buildings.

 

[csd72]

patrick, some questions for you.

Size and shape of building?

What type of building is it (e.g. domestic, commercial, industrial)?

What is the building framing- concrete,masonry or timber floors/walls/roof?

Where is it?

Give us some more information and we may be able to give you some better answers.

 

[patrick526]

Thanks for all the quick answers! CSD72, the shape of the "residential" structure is about 60'x 60' house (not perfect square because of how it was designed aesthetically). The structure is CMU on the first floor and frame on the 2nd floor.

Theoretically and analytically, I can support the fact that I don't need shearwall in this structure. However, the FBC (Florida Building Code) said that shearwalls are needed in a structure, but doesn't specifically specify "where" and "when" to put it.

 

[whyun]

In an area where wind governs the lateral design over seismic, you still have to provide a uniform windward and leeward reaction on the floor diaphragm. Then this load is used to calculate the reaction into the shear lines.

Unless you have a structure with wide exterior surface, drag forces tend to be negligible. Depending on the geometry and number of shear elements provided in the building, you still need to "design" the shear elements.

 

[csd72]

For the size and shape that you are talking about, you could the design the building with a shear wall along each line of the exterior walls. Your floor and roof diaphragms will then act as a simply supported beam spanning between them.

You then basically take half the wind load into each shear wall.

For larger and more complicated shapes you may want to break it up and add interior shear walls, but it is usually best to avoid this so that the resident has more flexibility in the future.

There is no particular limit as to the spacing of the shear walls as long as your diaphragm can take it.

Dont forget that your shear wall loads are cumulative, i.e. the lower floor takes the lateral force from the upper floor and from any floor/roof above.

 

[patrick526]

Thanks for all the advices. I'll keep that in mind.

 

[LGARG]

I think that is a good idea to provide shearwalls at the exterior walls.If the shearforces are big,and so the uplift, you should try adding interior shearwalls. if you provide exterior shearwalls only, check that diapraghm could take this load(check with plywood thickness, edge nailing and boundary nailing pattern the allowable shear capacity of your shearwall, there ara tables in the different codes showing this).Also the UBC says that you have to use a R= 4 when the diapragm is supported by masonary or concrete CMU walls. I hope that this could help you.