Truss Design With Knee Bracing
Truss Design With Knee Bracing
(OP)
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!
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!






RE: Truss Design With Knee Bracing
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
RE: Truss Design With Knee Bracing
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
RE: Truss Design With Knee Bracing
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
RE: Truss Design With Knee Bracing
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
RE: Truss Design With Knee Bracing
RE: Truss Design With Knee Bracing
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
RE: Truss Design With Knee Bracing
If you have problems, we are here...
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Truss Design With Knee Bracing
RE: Truss Design With Knee Bracing
RE: Truss Design With Knee Bracing
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
RE: Truss Design With Knee Bracing
IV
RE: Truss Design With Knee Bracing
RE: Truss Design With Knee Bracing
BA