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BadgerPE (Structural) (OP)
20 Oct 10 13:39
Hey all,

I have a post frame ag building that only has sheet steel for sheathing on top.  So I have designed the building with knee bracing from the post to the top and bottom chords (see attached drawing) of the truss to resist my lateral forces.  My question is what information should be provided to the truss designers?  I spoke with one today that said they typically do not design the trusses with knee braces figured in because there is enough reserve capacity in the chord material of the truss.  Being a young engineer, just saying it is ok does not sit well with me.  The designer did say that if I provided him with knee brace locations and forces that they could design the truss to include the knee braces.

If I am to provide the forces, what should I provide?  It seems like an axial tension force from the wind would be all I would be able to provide.  I know that the knee brace will see a compressive force due to snow loading, but the amount of force is relative to the overall stiffness of the truss.  Therefore, I feel like I can't provide and adequate compressive force.

Any help would be greatly appreciated!
Helpful Member!  msquared48 (Structural)
20 Oct 10 13:57
FYI, I could not open your sketch due to software conflicts, so please excuse any misinterpretations here.

If you use one Knee brace each side of the truss, through- bolted to the column, and throuth bolted to the top and bottom chords of the truss, and applied after the truss is erected, just show the range of reactions that the knee brace induces on the truss at the top and bottom chords.  You shuld be able to generate that from your computer program.

If you integrate the Knee brace with the truss, i.e., in the same plane as the truss members, that's a different situation as the knee brace will probably be placed at a panel point in the lower chord and a diagonal in the truss serve as an extension of that brace.  The connection of the Knee Brace to the truss could be a problem, so it would have to be coordinated with the truss supplier.  As for the force, just see what the reaction at the end of the knee brace is and supply that.  

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

Helpful Member!  BAretired (Structural)
20 Oct 10 14:08
Crackerjack,

Under wind force parallel to the trusses, one brace will see tension, the other compression.  Under snow load, they will both see compression.

I don't think you will be able to resist wind forces with knee braces alone and they do not help gravity loads.

My suggestion would be to select roof sheathing so it can act as a diaphragm, then transfer the wind to endwalls.

BA

BAretired (Structural)
20 Oct 10 14:45
Attached is a PDF of your drawing with suggested knee braces shown.  The green brace can be installed by the truss supplier, the red one by the contractor.  The bottom chord should be laterally braced at the knee brace.

If the trusses and studs are spaced at 2' centers, you will not generate much wind resistance due to the flexibility of the stud.  The brace should align with both truss and stud with a metal plate each side for a better connection.

Doubling the stud would make it a bit stiffer.  Then you could extend the lower end of the red brace to the outside face of the stud and bolt, nail or screw through three laminations.
 

BA

Helpful Member!  woodman88 (Structural)
20 Oct 10 14:47
Crackerjack47,
For your truss you should run the brace to a top chord panel point and have a diagonal web (I would spec out a 2x8 for this web, to give some leeway for the brace placement) parallel to the brace to connect to. Spec out your own wind cases for the truss with a (plus or minus, be sure to get the directions correct for the manufacturer) vertical and horizontal force at the bottom chord/brace joints to be applied, as part these cases.

Garth Dreger PE
AZ Phoenix area

BadgerPE (Structural) (OP)
20 Oct 10 15:08
Thanks for the help guys....will let you know what direction we head with this one
MiketheEngineer (Structural)
20 Oct 10 15:09
In general and at longer spans and taller columns - I have found these just do not work out. Either the connection is a problem or you blow out one or both chords or columns.

At lower wind speeds they are great as they do stiffen the frame and help keep things square.  But when you go 90 - 110 mph exposure "C" - things get out of hand.


I try to use shear walls and ceiling diaphragms as suggested.  Also - you might search through these forums - it has been discussed a few times.
Good luck
msquared48 (Structural)
20 Oct 10 15:25
What BA saus is true, but it depends on the eave height, bay spacing, wind loads and roof pitch as to whether the frames will work laterally or not.  You will have to run the numbers to find out.  

If you have problems, we are here...

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

Helpful Member!  Splitrings (Structural)
21 Oct 10 15:16
We are using knee braces for ag structures with heights less than 30' regularly. Attached is on structure that is designed pinned at the base of the post and fixed at the top. This is not typical, most have embedded posts. The forces are very significant in the knee brace and need to be designed for. It is not the wind loading that controls the knee brace design, it is typically snow or unbalanced snow. Axial forces in the neighborhood of 4-5k are not unusual. Obviously the bottom chord of a wood truss can't resist this force. We have had truss designers use 2x12 BC's between the post and first truss joint after the knee brace connection. We have also had trusses designed with a knee brace that extends to the TC.
Splitrings (Structural)
21 Oct 10 15:34
You might notice in the picture the knee brace closest to you extends to the TC and the far one only to the BC. It was shown this way on the drawings for the truss designer, to show that we could do it either way. The contractor followed the drawings....
iv63 (Structural)
21 Oct 10 15:37
Splitrings:
From your photo it looks like knee brace on the other end (far side) does not extend to the top chord. Maybe it is not done yet ...
IV
iv63 (Structural)
21 Oct 10 15:40
OOPS You type faster than me smile
IV
Splitrings (Structural)
21 Oct 10 15:46
I never answered CrackerJack's question. The axial force I supply is based on the truss stiffness (I use an approximate truss stiffness). The trusses we are seeing on these projects are fairly similar. In some cases I was able to get the preliminary truss plans before the design was complete.
 
BAretired (Structural)
21 Oct 10 16:03
Nice photo, Splitrings.  And a nice design too.

BA

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