Shearwall location

[CEmonkee]

Hi - I'm doing a lateral wind/seismic analysis for a garage. The plans call for the garage doors to be positioned close to the west wall (please see attached .jpg file). I'm planning on using the wall segment east of the doors for the shearwall on this front wall. This wall is approximately 25% of the length of the front wall.

Typically I like to have the shearwall(s) more evenly distributed along the wall, but in this case this is the only wall segment I have to work with. I can show this wall good for the critical load. Are there any issues that I also need to consider since the wall is not symmetrically located along the wall?

Thanks for your input!

 

[frv]

you need to make sure you account for the additional shear due to eccentric loading as your center of rigidity will not coincide with your center of mass..

 

[ash060]

You are going to need a drag strut as well to get the load into the wall.

 

[JAE]

frv - unless it is a flexible diaphragm right?

ash060 - agree - there needs to be a collector along the full width to drag the roof diaphragm into the shearwall at the end.

 

[CEmonkee]

Hi everyone, thanks for your responses.

Regarding the diaphragm - ASCE 12.3.1.1 says the following: "Diaphragms of wood structural panels or untopped steel decks in one- and two-family residential buildings of light-frame construction shall also be permitted to be idealized as flexible." So I think I can consider it to be flexible, unless I'm misinterpreting this.

For the collector force, its maximum value would be the diaphragm unit shear times the length of the wall opening (= total length of the wall less the length of the shearwall), correct?

 

[JAE]

"Light framed construction" in the IBC is a strictly defined empirical method of design so that section may not really apply globally to all diaphragms of wood.

However, we usually assume wood diaphragms to be flexible...just not sure what that section is really aiming at without reading through the code logic.

For the collector force, its maximum value would be the diaphragm unit shear times the length of the wall opening (= total length of the wall less the length of the shearwall), correct?

Well - sort of. For wind the answer is yes. For seismic, it gets a bit more complicated as the connections between diaphragm and collector, the collector itself, and the collector-to-shearwall connections may have to be amplified by the overstrength factor.

 

[CEmonkee]

Thanks JAE - I haven't run any numbers yet, but I'm pretty sure wind will be the controlling load.

I looked up the overstrength factor in ASCE 7 - it looks like there is an exception to the requirement for the application of the overstrength factor (section 12.10.2.1) - if the structure is braced entirely by light frame shearwalls the collectors only have to be designed for the diaphragm forces.

Thanks again, I really appreciate your input!

 

[JAE]

I'm pretty sure wind will be the controlling load.

Keep in mind that you don't just check wind vs. seismic using the overall lateral forces from each, pick the higher one, and then discard the other.

Each element must be checked for both throughout the whole building. In some cases the seismic overstrength factor, seismic detailing requirements, etc. might control in a single element over the wind force for that element.

 

[CEmonkee]

But if the garage is constructed entirely of light-frame wood shearwalls, the seismic overstrength factor won't apply, correct?

 

[JAE]

If it is "light framed construction" per the IBC, then you aren't calculating wind loads anyway (per the code) - just providing braced wall lines per the code.

Since you said "wind controlled" I assumed you were calculating wind pressures. I'm referring to section 2308 in the IBC.

However, in ASCE 7, it doesn't appear that the term "light-framed" means the same thing as in IBC section 2308.

Thus, your reference to section 12.10.2.1 in ASCE 7 does appear to simply use the term as a descriptor of sheathed wood shearwalls and so the overstrength factor wouldn't apply as you suggest.

 

[CEmonkee]

Hi JAE - I think ASCE 7 and the IBC both define light-frame construction the same way:

IBC 2006, section 202, Light-frame construction: A type of construction whose vertical and horizontal structural elements are primarily formed by a system of repetitive wood or light gage steel framing members.

ASCE 7-05 Chapter 11, page 111, Light-frame construction: A method of construction where the structural assemblies (e.g. walls, floors, ceilings, and roofs) are primarily formed by a system of repetitive wood or cold-formed steel framing members or subassemblies of these members (e.g. trusses).

IBC 2308 is for Conventional Light-Frame Construction... which as I understand it is prescriptive and does not require design by an engineer. Light-framed construction that does not meet the limitations of this section must be engineered.

 

[JAE]

Sounds true.

 

[WillisV]

CEmonkee is correct in that regard. The term light frame construction in and of itself does not necessarily trigger the prescriptive provisions- its the "conventional" moniker that does that. So yes with a standard wood (light frame) building you don't have to increase chord forces with overstrength factors.

 

[CEmonkee]

Hi everyone - just wanted a quick opinion... First a brief review of my original question. I'm designing a 2- car garage (doors are 10' wide). As you look at the front wall of the garage, there is 2' of wall at the left corner, then a door, then 3' of wall, then another door, and finally an 8' wall segment. I was planning on using just the 8' wall segment for a shearwall (I can show it good for the load - approximately 2200#). As was discussed above, I will detail the collector etc. for that load.

My question is would it be better to incorporate the narrow walls between the doors in my design (i.e. a portal frame like the attached .jpg file)?

 

[JAE]

You could but each shearwall in line will take load proportionate to its stiffness...usually with wood shearwalls this is looked at as relative to their respective lengths.

So you might end up going to a lot of trouble to hold down tall narrow shearwalls for very little gain. If you main, 8 ft. segment can take the load OK I'd stick with that.

 

[CEmonkee]

Hi JAE - makes sense. Thanks for your advice, I really appreciate it!

 

[DHKpeWI]

Why not design this as a (3) sided structure? The north wall will have to resist all the east-west shear and the north south walls the torsion torsion created by the east-west loads.

 

[CEmonkee]

Hi DHK - that's an idea... I'll definitely consider it. Thanks for your input!

 

[CEmonkee]

Hi - I'm still working on this garage (trying not to make it a career, but hey...). Anyway, the client would like to use taller garage doors, and to accomodate this, I need to go with headers that are not as deep. I can get a 9" deep beam to work, but the only way I can get that to fit is to take out the double top plate (I need those 3" of depth).

So my question is - if I specify a glulam beam than runs the entire length of the front garage wall, can I eliminate the double top plate on this side and just make sure that the ends of the header are sufficiently strapped to the top plates coming in at the corners?

I didn't really see anything in the IBC that addressed this...

 

[JAE]

That would work.