Sag rods bracing wind girts 1

[Karmanstuttgart]

What is the proper engineering approach to the design of wind girts with respect to sag rods? I have heard multiple versions of the ability of sag rods to laterally brace the girts, thereby reducing the unbraced length. I have heard of papers by Joe Yura and Jim Fisher on the subject, but can't find them. Can anyone help?

 

[dbuzz]

I would only rely on properly installed bridging to laterally restrain girts.

 

[DaveAtkins]

I went to a seminar several years ago where Jim Fisher was a presenter. His handouts say that sag rods do brace hot rolled girts for suction (the wall sheathing braces the girts for pressure).

DaveAtkins

 

[JAE]

We assume the same thing as what DaveAtkins states. Here's why:

The exterior flange of the wall girt is obviously braced laterally by the exterior skin. For suction, the rods hanging down from above have good tension support, but questionable compressive stiffness. In addition to that, the rods are usually bolted through the web, and not connected to the interior flange.

But if you assume that the purlin has just a bit of downward sag in it, then any suction on the wall would immediately initiate a compressive force in the interior flange which would try to buckle downward. The sag rod through the web, and the skin attachment to the exterior flange create a sort of couple that would resist rotation.

And resisting rotation is almost as good at bracing a beam as resisting lateral translation of the compression flange (according to Yura).

 

[twich]

I agree with JAE and Dave. I have considered sag rods as lateral support for my entire career. However, since the sag rod is generally placed in the middle of the web, I will use double sag rods (with the line of holes perpendicular to the girt) which puts a rod much nearer each flange, for C10s and larger.
Twich

 

[JAE]

Oh - also, we sometimes even dictate that the horizontal purlin should be installed with a slight downward bow to ensure that this behavior occurs.

 

[Taro]

JAE,
I agree with your torsional bracing approach. However, it should be mentioned that this requires an evaluation of the girt web distortional stiffness, etc. Section C3 of the AISC 1999 LRFD spec addresses this.

As for the idea of pre-deflecting the girt downward to preclude buckling in the upward direction...
Have you considered the possibility of snap-through buckling? I'm not sure whether the downward pre-deflection can eliminate upward buckling or not.

 

[JAE]

Taro - please elaborate a bit on "snap-through" bracing.

The weight of the girt also tends to direct the buckling direction downward - along with a little bit of resistance from the rod - it does have a small bit of initial compressive resistance.

 

[Taro]

Snap-through buckling is typically thought of in regards to arches and domes, but it can be applied to this situation as well.

Let's say you have an axial member that is pre-deflected in one direction and there is no bracing. Now when you apply a compressive force, the p-delta effects will cause the deflection to increase in the initial direction.

Next let's say you have another axial member that is pre-deflected in the same manner, but there is bracing to prevent further deflection in that direction. Now when you apply a compressive force, the member can actually "snap through" to the other direction and buckle the opposite way.

You can demonstrate this behavior with a standard business card. Hold it lengthwise between you thumb and fingers and squeeze it lightly so that it just starts to deflect. Now use your other hand as a brace to prevent it from moving further in the initial direction. As you apply more compression, the card will suddenly snap through and buckle in the opposite direction.