Point load on stick frame wall 1

[Robvh1984]

I have a 8' 2x4 @16" wall that is sheathed by 1/2" structural plywood. There is a very heavy mechanical unit being installed at the top of the wall (on the second floor). I am checking the compressive strength of the studs and I know the species and grade of the lumber but I am not sure whether to consider the studs to be continuously laterally braced for (Cp) adjustment factor. Is it likely that there is enough restraint by the plywood to develop continuous bracing? I do not know the type of fasteners or spacing used on the sheathing. Do you think it is better to calculate the required fasteners needed to develop the bracing, remove the exterior siding and install those fasteners (if not present) or to forgo using the plywood as bracing and install additional studs from the inside of the room. Cost is a limiting factor.

 

[Lion06]

If you have plywood on one side and gyp or some other kind of sheathing on the other side I would consider it fully braced for weak axis buckling and the unbraced for strong axis buckling.

 

[a2mfk]

Agree with Lion.

 

[MiketheEngineer]

Agree - but you might want to check the 3.5'' direction. Doubt it will come into play - BWTH - check it.

 

[Robvh1984]

Thanks guys, that was my initial assumption.

 

[sforesman]

My question: is the top plate capable of transferring the large point load to the studs. Have you checked the top plate for perp. to grain crushing, or bending about the weak axis?

 

[msquared48]

Very true the above post. Might need a metal bearing plate or additional studs to spread out the load.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jaypartain]

This is a sample of a wood stud design article I am writing. Tests have been performed to verify the bracing of wood studs by various materials. The reference article is very good and presents the practical issues related to this subject, as opposed to a typical journal article focusing on theory.

"Column stability of a rectangular compression member is analyzed for each axis of a member. A column stability factor of 1.0 applies when a member is supported throughout its length to prevent lateral displacement. Preventing weak axis buckling through lateral restraint of a stud is important in achieving full capacity. Attachment of wood sheathing panels to a stud wall is believed to adequately provide lateral restraint to limit weak axis buckling. Drywall, fiberboard, and laminated fibrous boards, such as Thermo-Ply®, are less obvious choices to prevent lateral displacement for studs. Building codes leave judgment of what adequately braces a wall for interpretation by engineers, which has led to varying opinions on bracing materials. Tests have shown that drywall, fiberboard, and laminated fibrous boards are capable of resisting weak axis stud buckling when properly attached (Marxhausen, 2009). Test specimens used minimum code requirements to fasten sheathing materials to studs."

Marxhausen, P. (2009). Axial Buckling Strength of Conventionally Sheathed Stud Walls. Wood Design Focus, 19(1): 12-18.

Jason A. Partain, P.E.

http://www.myconstructionfinds.blogspot.com

 

[sforesman]

So does anyone else check top plate weak axis bending, or just me. Most of the time when I check, it forces me to require that the studs be framed inline with the trusses/rafters. In my calcs I typically only count for one top plate since, who know where they'll be spliced.

 

[BAretired]

sforesman,

Sometimes studs are spaced at 16" o/c and trusses are spaced at 24" o/c which means there will be top plate bending on every second truss. Top plate weak axis bending needs to be checked, but it seems ultra conservative to count only one top plate. I would consider two top plates sharing a bending moment of PL/4 and a shear of P/2.



BA