Steel Stud vs. Wood Stud Brick Veneer Backup

[BadgerPE]

While researching options for a potential 16' brick veneer wall, we came across a deflection requirement in the Brick Institute Associations Tech Note 28 that only requires wood studs to be designed for L/240 for out-of-plane forces. This is significantly lower than the L/600 that we typically design for and in disagreement with their Tech Note 28B (Steel Studs) which requires L/600. I can't seem to find a reason for the difference other than wood design values are typically quite conservative. Has anyone run across this? Any ideas for the discrepancy? I have contacted BIA for their guidance in the matter and will follow up here if I hear back from them.

This project no longer has full height brick but it was a 16' tall building and was going to require a 2x10 wall (SPF No. 2) to meet the L/600 requirement that we normally follow.

Attached is Tech Note 28

 

[BadgerPE]

Tech Note 28B

 

[AELLC]

Was the L/600 only for in-plane deflection?
L/600 seems ridiculously ultra-conservative for out of plane.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[XR250]

IBC only requires L/240 for either. I do a lot of metal stud design and only use L/600 if the specs require it and L/360 if there are no specs. The BIA once recommended L/720.

A few things you can due to lessen the pain is to:

1) Take a 30% reduction in wind pressure for deflection

2) Use the tributary area of the stud as span^2/3

Both are permitted in ASCE-7

 

[BadgerPE]

AELLC,

I highlighted the important sections of the attachments that discuss the deflections. L/600 was given as the out-of-plane deflection of the steel backup studs.

 

[AELLC]

It doesn't make sense, considering there must be 1000's of ordinary houses with brick veneer where this wasn't even calculated.

The fact you have drywall fastened on the inside obviously helps a lot, but I know of no method to calculate it.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[CANPRO]

see the link below for some useful information.

A quote from the refrence --- "For comparison purposes, the crack width permitted for exterior exposure in reinforced concrete is given as 0.33 mm (CSA 2004e & CAC 2006). The previous issue of S304.1 (CSA 1994a) required a more stringent stud deflection limit of L/720 which resulted in average crack widths approximating those required for reinforced concrete. The current S304.1 (CSA 2004b) has liberalized the deflection limit to L/360 with the recognition that the wall as a system can accommodate any increase in water penetration that might result (CSSBI 2006b)"

I'm the same as Excel, I only design beyond the L/360 limit if required by the job specs.

Excel, point #2...should that be spacing^2/3?

http://www.cssbi.ca/Eng/_pdf/CSSBI-S19-08.pdf

 

[BadgerPE]

Thanks for the info!

CANPRO

I think Excel was referring to the definition in ASCE 7-05 in which the tributary area for a single component (i.e. stud) "is the span length multiplied by an effective width that need not be less than one-third the span length" which equals L*L/3. For C & C loading, the actual wind pressure goes down as the area increases so it is a fairly helpful provision.

EXCEL

Thanks for the reminder. I had used those provisions to get down to a 2x8 but forgot to post that in the original post.

 

[SteveGregory]

The "Effective Wind Area" is defined in ASCE 7-05 6.2 as H^2/3 and is used to determine the C&C wind pressure. The Tributary Area is just the height times the stud spacing and is still used for the actual bending stress and deflection calculations. For deflection calculations only, IBC Table 1604.3 says you can use 0.7 x the load.

I'm not trying to split hairs with the definition, but tributary and effective mean different things in this context.

 

[AELLC]

If L/600 is needed for prevention of water intrusion, and if your Archi already has a moisture barrier in the detail, then L/600 would be "legally" too conservative.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[XR250]

@CANPRO;

I am pretty sure it is (span^2)/3 for C&C members that have a long and narrow tributary area.