Minimum Header bearing - IRC2003 2

[DarrenEmery]

Hello All,

I am a building inspector in the midwest, looking for some help understanding the engineering issues behind a code requirement. I searched the archives but didn't find anything that addressess my question.

The IRC and the IBC call for two jack studs (called trimmers around here) on header spans based upon header size and span, as well as loading factors. Framers around here have a rule of thumb that a single trimmer is good for anything under 6'. When I cite this violation, I get all kinds of arguement, much of it valid in my opinion.

The IRC/IBC chart jumps to two trimmers much earlier than the UBC did, and I'd like to have a good reasoning behind the intent of this code when I explain it.

I'm wondering if the trimmer requirement is more a function of compression on the stud itself, or a deflection issue on the header material, or a combination of both?

Any thoughts?

Thanks in advance for the input!

Darren Emery

 

[scottiesei]

Darren,
I am not familar with the IRC but the IBC (table 2308.9.6) allows single jacks to be used under certain loading conditions (one floor, 20' width) for up to 5'-9" spans. The jacks studs, or trimmers are used for bearing of the header, they do not help much in the way of deflection. What do the plans say? I always spec header sizes as well as # of jacks and bypass (kings) at my openings. The # of jacks required is based on the load applied to that header. Even if a header and jacks meet the table requirements, they may not be sufficient if a large load from a girder or beam is resting on them.

 

[DarrenEmery]

Thanks for the input, Scott.

So the issue is the ability of the jack stud to withstand the pressure from the header, and not crush or deflect... That's what I was thinking, and makes the most sense to me.

Plans? What plans? We're talking residential construction - all we get is a floor plan around here - no design work what so ever. Pretty frustrating for an inspector.

 

[scottiesei]

Well, here in Florida plans are required for even the simplest of structures. Sorry!

 

[HeberPE]

Darren:

Actually, the jack studs / trimmers are specified so as to prevent the header wood from crushng, not the jack stud. The ability of wood to withstand compression parallel to the grain (against the end of the jack stud) is greater than the ability of wood to withstand compression perpendicualr to the grain (against the bottom of the header, and also the sole plate beneath the jack stud.) If the jack stud (say a single 2x4) is not big enough, then the bottom of the header or the top of the sole plate might crush, causing cracks in the finish material (stucco, gypsum board, window trim) and binding of the door or window. Structural collapse in not the concern, but serviceability of the construction.

By adding a second or third jack stud, the compression against the header is cut by 50% or 66% - a great reduction for a very small cost of a stud.

In addition, if tall walls are being framed (say an 18' tall great room) then the ability of the jack studs to resist buckling may dictate the number or size of the jack studs intead of the compression. The taller the post/column/stud, the less load it can carry before it buckels.

I suspect that the difference between the two codes is that the IRC uses more updated and conservative values about wood species, compressive strength, etc., and maybe errs on the side of caution. I would go with the newer code requirements, and maybe add an info. sheet to the permit plans that explains the new methodology for sizing jack studs.

Hope this helps!

 

[boo1]

Additionally the header material may be a different species then the studs (SYP vs white woods).