WOOD KING STUD 1

[deereman]

I'VE GOT A 20'-0" HIGH WALL ON A PROJECT THAT HAS AN 8'-0" WIDE OPENING. BASED ON 20 PSF WIND LOAD THE KING STUDS WOULD HAVE A DISTRIBUTED LOAD OF 80 LB/FT AND NEED TO BE (4) 2X8'S. THE CONTRACTORS SAY THIS IS EXCESSIVE AND IT DOES SEEM TO BE, BUT I CAN'T WORK THE NUMBERS OUT ANY OTHER WAY. IS THERE ANYTHING I'M MISSING,DOES THE WINDOW FRAME ITSELF TRANSFER SOME OF THE LOAD?

THANKS,

 

[concretemasonry]

How many other openings on the wall and what is the wall finish?

I have seen drywall cracking problems due to wind loads on tall walls where the strength was OK, but the deflection caused the cracking. Extra openings caused points for the cracks to begin. Especially important if you are situated where you can get wind gusts that can cause momentary deflections.

Dick

 

[bylar]

Is the contractor the engineer?
if the opening is 8' what is the spacing to the next vertical member if its 16" then the kin g is carrying 1/2 x (8+1.33) and therefore greater than the 80psf. What grade and species is the lumber? I think you are too conservative.

 

[UcfSE]

They jamb would see half the window plus its own width plus half the distance to the next stud. I don't think you're all that unreasonable. It's a 20-foot span with a fairly wide opening. What you can do is go back to your calcs and sharpen your pencil and remove the simplifying conservative assumptions, such as uniform loading, and try to more accurately assess the loads. You'll have uniform load from the width made up of half the stud spacing plus jamb width. You'll have linearly varying loads and point loads from the window and header, and uniform loads for half the cripple stud spacing below the sill and above the header.

The problem is, you'll have to do that for every window instead of once for every 8-foot window. In the end, it is what it is.

 

[jike]

Are you able to refine the 20 psf wind load that you are using to something lower? What code are you using?

 

[structuralaggie]

20 feet is a tall wall. A rule of thumb is to add a stud for each one that was cut out (a little conservative). For 8', that would roughly be 5, so add three to each side to make it even. That means 4 - 2x6 studs should work. If it doesn't, then 1-2x6 @ 16" doesn't work either. My gut feel is you need 2-2x6 studs @ 16" for that height wall. The required king stud width will probably be getting rediculous, so a steel tube or channel may be in order in place of some of that wood.

 

[Lion06]

I agree with JIKE - double check the wind pressure. I don't know your location, building geometry, or anything else, but the component has a relatively large effective area and you may be able to get a smaller wind load.

 

[deereman]

thanks for all the responces. I was pretty confident my calcs were correct. I have in the past reduced wind load as much as I could and more accurately imposed the loads onto the studs but have always come up with large king studs on these two story walls. I just wanted some confirmation that I wasn't missing anything.

 

[COEngineeer]

If im not mistaken, your king stud should see 8/12 x 20 psf (13.3 plf) span of wind along the whole stud, and a point load at the header location of (4'x height of wall above the header x 20psf). I am not sure how you got 80 plf.

http://www.krmconsultants.com

 

[whyun]

The point load at the header/window sill end connections should also include the tributary wind load from the window itself.

 

[COEngineeer]

oh yes whyun.. doh.. Yeah, so the point load should be (4'x10'x20psf).

http://www.krmconsultants.com

 

[bylar]

I don't know how you fasten your windows but we fasten them around the perimeter and the king studs are takeing a load from the window along with the header and the sill. I'm in the mountains of NC and 20 psf windload on a window is not to code.

 

[GoBizon]

Assumed loading aside, the problem of (4) 2X8's still exists. Have you considered using LVL material, which should have better allowable bending and deflection values than dimensional lumber? I'm not sure of the cost difference and it may also be more readily available in 20ft lengths than the 2X8's. Either way, I would be sure to check the connection of the studs to the top and btm plates of the wall.

 

[DaveAtkins]

I find that 3-2 X 8 works for stress, but 5-2 X 8 are required to limit deflection to L/240.

DaveAtkins

 

[jhoulette]

I agree with stenger. I design houses in Colorado and most of them always have a 18ft-20ft wall in the great room with large windows. I have found using LVL or LSL studs is the best solution to prevent deflections, reduce amount of lumber, and to provide a nice straight piece of material. I don't know how musg framing experience you have, but I hate trying to find a 20ft long 2x8 thats worth using. And yes you will need something more than just a few nails per code minimum at the top and botom plates. From your 80plf x 20ft/2 gives you 800lbs at each end. I usually use a Simpson A35 or similiar.
Also, are you using the component and cladding pressure? You should be. Typically, I have to design for 110mph exposure C (3 sec gust, IRC) which depending on geometry and building height usually gives me close to 30psf wind load. I hope this helps.

 

[apsix]

In my neck of the woods (Australia) we would seriously consider steel instead of multiple large studs as proposed above, typically a square hollow section.

 

[nac125]

(4)2x8'S king post for a 20' high king supporting an 8'-0" opening is NOT excessive, contractors are always trying to cut costs. Using LVL's or steel would be more expensive, however it may be necessary to control deflection..

 

[dik]

Reasoning for Contractor...

With a 20' high wall, you have 2x8@16?... A 4' trib width of wall loading would require 3 studs plus the one in the wall...

I don't think that 4 is unreasonable or over conservative...

Dik

 

[twinnell]

Remember, in the IBC you can reduce the wind pressure 30% for deflection. It is shown as a footnote under the deflection table in chapter 16. Don't know the exact table number.

 

[epr]

Builders (and many times architects) have a problem understanding the wall thickneses required for higher walls.

I have found that the easiest way to explain it to them is as follows (tailored to your situation:

"Assume the wall is an attic floor, with a 20' span and a 20 psf live load. What joist size would you use? Now, create an opening through the floor for a stairway. What would your trimmers be?"

This puts the situation in a perspective that can be understood. I cannot think of anyone that could honestly say less than 2"x10"s for the floor joists and at least two, or three 2"x10"s for the trimmers.