Sag Rod Support 1

[bdSE]

I am designing an industrial building to house a process plant, the frame is clad with C10 wind girts which span 20' at each bay. I have decided to use sag rods in my design.

Based on my investigations here I have found several excellent discussions regarding the use of Sag Rods for providing bracing to wind girts.

What seems to be missing from my research is information regarding the end support of the sag rods. Are they supported at the eve by a dedicated beam for the gravity and bracing loads? Are they also connected at the foundation? Or do they simply connect to adjacent steel members?

I am assuming it is supported at the eve level and I have some creative ideas for providing this support, but did not want to wander too far from typical practice.

Thank you for your time,
- bd

 

[JedClampett]

There is a special beam at the eave level that carries the vertical load from the sag rods (usually the DL of the girts and the stability load from the sag rods). The rods don't need to be attached to the foundation in my view.

 

[paddingtongreen]

Usually, they hang from the eaves beam, the same one that carries the wind load to the braced bays. It is okay if the top rod comes down at a small angle to get to the CL of the girt. I agree with Jed, I never attached to the foundation.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

With negative pressure on the wall, the inside flange is in compression. Do you regard the sag rods as providing lateral support to the compression flange?

BA

 

[bdSE]

BA,

Based on a report that offered photographs and numerical results of a full-scale test I am convinced of the bracing characteristics of a sag rod design for the wind-girts.

I'm pretty sure I found this information at Eng-Tips on another thread.

"Behavior and Design of Girts and Purlins for Negative Pressure" by Peter C. Birkemoe. Found in the Canadian Structural Engineering Conference of 1976.

 

[BAretired]

damronb,
You're probably correct, but I've always been a bit leery about considering the inside flange braced, particularly if the sag rod is fastened to the middle of the girt. Maybe it would be better to fasten the sag rods nearer the inside flange. The outside flange is normally braced continuously by the exterior sheeting.

BA

 

[bdSE]

BA, I think you are correct about doing my best to optimize the detail by placing the rod toward the inside flange. I will show this setup in my details.

Now I just need to decide where to place the eve strut to act both as my collector element for lateral forces and as support for the sag rod. I was hoping to place it on the center line of the building frame columns at the knee of my bent, but that's pretty "far away" from the edge where the girts are located. I think it would generate a 10 degree angle from vertical to connect the first rod from eve strut to wind girt at their current locations.

 

[hokie66]

Not sure where you are located, but why not use proprietary bridging sections rather than sag rods? They brace the inside flange and provide sag support. With bridging sections, you can prop from the floor, while sag rods are normally tension only.

In my experience, the bridging or sag rods are not hung from the eave strut, but rather from the eave purlin or purlin bridging, or in the case of the gable ends, from the barge.

Why would you use C girts rather than Z's? Maybe it is just local practice, but lapped Z's provide better economy.

http://www.lysaght.com/files/dmfile/LysaghtZedCeesPart1Sep08.pdf

 

[paddingtongreen]

If the girt is considered to support the vertical weight of the siding, an eccentric placement will put a torque on the girt.

We used to make a composite eaves strut by connecting a channel of the same size as the girt to the bottom of the wide flange. This started before we had wide flanges and used I beams that needed the extra lateral capability. They, the I beams, often had a channel on the top flange as well.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

paddington,

Correct me if I'm wrong, but if the girt supports the siding, even a concentric placement of sag rods will put a torque on the girt. In the case of 10" channels, the eccentricity would be 5" plus half the depth of siding.

I like hokie's idea of proprietary bridging sections supporting the siding from the bottom, but common practice in my area is to use sag rods. I don't think I have ever seen bridging sections.

BA

 

[JAE]

Usually the siding is vertically supported by the eave beam or strut on most buildings. I've never relied on the horizontal girts for vertical load.

On the previous posts - I mentioned our method - installing the girts with a very small downward sag at the midspan. The sag rods, whether at the centroid or near the inside flange, then act as tension members coupled with the siding to form a rotational couple. Since the girt is downward sagging, any wind suction that induces compression on the inside flange will develop lateral torsional buckling downward (due to the initial sag). The sag rods are then in tension resisting the LTB.

 

[BAretired]

Here is a previous discussion on this topic:

http://www.eng-tips.com/viewthread.cfm?qid=115545

BA

 

[hokie66]

And another one. Some of the same characters, with mostly unchanged opinions.

thread507-236394

 

[BAretired]

Another point...if sag rods are expected to provide compressive as well as tensile resistance, double nuts are required at each girt. I don't think a nut above the girt has been standard practice in my area, but it could be adopted at little extra cost.

BA

 

[BAretired]

On second thought, it would make more sense to tie the bottom girt down to the foundation. Then the sag rods could work in tension to prevent either upward or downward movement.

BA

 

[JAE]

That is a good idea.

 

[ToadJones]

I have detailed sag rods such that they were "X" shaped between girt for this very reason. Basically amounting to tension only "X" bracing between girts parallel to the wind loading.

 

[ToadJones]

...like this

 

[ToadJones]

With regard to industrial buildings, it is poor practice in my opinion to count on wall sheathing for strength. Plants cut holes int he sides of their building all the time.

 

[bdSE]

The input has been great for this question, thank you everyone.

Hokie66 asked where I was located. Our design firm is in the States, but the project is in mid-Oaxaca Mexico, not the middle of nowhere, but a lot of options either. We've been given a limited list of what they have available as far as structural shapes are concerned. They may have access to proprietary shapes or wind girt bridging, but I am unaware of it if they do.

ToadJones - I too had the design intent to rely little on the building skin for strength due to unknown holes in the future, but I had applied this philosophy mainly to the design of the frame bents, not to the wind girts, this is an insight that I will need to investigate further. Thank you.

JAE - It sounds like you've not relied on the wind girts for supporting the dead load of the wall skin, this is an interesting idea. How do you typically support the eve beam over the wind girts?