Sag Rods 1

[vmirat]

A contractor installed a new HVAC unit in an existing building, which hangs from the inside roof structure. The roof structure has horizontal cross bracing and each set of cross braces has a sag rod in the middle. The cross bracing bay size is 16'x16'. The new HVAC unit will prevent re-installation of the sag rod for this cross bracing. The cross bracing is L3x3x1/4 connected to the bottom chord of the open web steel joists.

At what point do members need sag rods? I'm thinking of ignoring this sag rod instead of trying to attach something to the bottom of the HVAC unit.

 

[KootK]

Can you simply relocate the sag rod to a neigboring joist? Or two neighbouring joists? In tension only bracing, which I assume this is, the need for the sag rods is mostly a judgment call. Some reasons for using it to limit gross deflections include:

1) Not frightening the villagers

2) Avoiding damage to connections that can't handle the associated end rotations.

3) Avoiding slack in the bracing that might need to be taken up before fully engaging brace stiffness.

4) Preventing long slender rods from being highly stressed in bending under their own self weight.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[vmirat]

The HVAC unit takes up pretty much the entire horizontal bay where this cross bracing is located, so relocation is not possible. As I said, I could connect a sag rod to the bottom of the HVAC unit, but I'd rather not.

 

[KootK]

I'm confused about the spatial setup here. If the rods are in the plenum space and the HVAC is below the joists, there should be no interference. Can you post a sketch of your arrangement?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[vmirat]

Let's just forget the specific example. If you have an L3x3x1/4 angle spanning 25 feet horizontally as a tension member, do you need a sag rod? If so, why?

 

[KootK]

Alright, we'll keep it general then. I listed some of the reasons WHY a sag rod may be necessary above. Investigate those things and then decide IF the sag rod is necessary.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

Vmirat:
It’s payback time. Those electrical, mechanical and plumbing guys never give a second thought to cutting our joists, beams, columns, etc. in half to put their stuff in after the fact. Tell them to make room for the sag rod which would not have been their if someone didn’t think it was needed for the bldg. structure. Alternatively, you are gonna get a cutting torch and make room for the sag rod, and gonna back charge them for your time and effort. We assume, of course, that that location was the only place in the whole roof system where that HVAC unit would fit, not just the most convenient for them, everyone else be damned.

 

[vmirat]

dhengr,

I share your sense of ire. Unfortunately, the mechanical engineer won't take kindly to me running a rod up through the middle of his HVAC unit, so I'm stuck leaving it out.

However, do you have any advice from a purely academic standpoint as to when a sag rod is needed for something like this?

 

[Triangled]

"running a rod up through the middle of his HVAC unit"

pour me another...

 

[BAretired]

An L3x3x1/4 has a radius of gyration of 0.59" about the Z axis, so the slenderness ratio, L/r on a 25' span is 254 with a sag rod at midspan and 508 without a sag rod.

The slenderness ratio L/r of a tension member cannot exceed 300 by code but that limit may be waived if other means are provided to control flexibility, sag, vibration and slack in a manner commensurate with the service conditions of the structure, or if it can be shown that such factors are not detrimental to the performance of the structure of which the member is a part.

BA

 

[Lnewqban]

It seems that the new unit has been installed between the horizontal cross bracing and the roof.
Can the HVAC unit be accessed for service and completed removed without removing any member of the horizontal cross bracing or any other permanent structure?
Mechanical codes forbid such a situation, unless the obstruction or structure below the unit is made removable.

If the new HVAC unit hangs from the inside roof structure, and the weight supported by the sag rod is small, I would run a uni-strut underneath the unit (and overflow condensate pan, if any), between two of the rods supporting the unit and would hang the removed sag rod from there.

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

 

[dik]

BART: There are qualifications on the KL/r < 300. KL/r = 500 is a bit extreme, but, is doable. Check Pre-Eng buildings with the x-bracing. The 300 value is often exceeded.

Dik

 

[vmirat]

BAretired,
By code, the 14th Edition AISC Steel Manual, Chapter D, does not limit slenderness ratio for tension members. It has a suggestion to do so, but the commentary states that it's for erection and other non-structural conditions (i.e. slapping). The commentary states, "This slenderness limit is not essential to the structural integrity of tension members..." Just wanted to clarify that point.

So far, no one has been able to give me a definitive code or construction maxim, other than KootK's advice.

If you assume that the angle is a simple span beam with fixed ends, the deflection at mid-span due to self-weight would be 0.6 inches (for Z-axis value of I). To me, that's not enough to worry about. So I'm not sure why the engineer back in 1963 felt it was necessary to add sag rods to these cross braces.

 

[jayrod12]

Because sag rods are cheap and it meant that he met the recommended value of l/r<300 by putting it in.

When I design new bracing, I make sure I keep it within those recommendations. When I check existing bracing, then I'm a little more casual about them.

 

[KootK]

The sag rods really only help to improve the slenderness ratio about one axis anyhow. For me, it's more about self weight flexural stress.

@vmirat: it may well not be enough to worry about but, at a span to depth ratio of 90, it's worth a quick check. I'd run the defection and bending stress as a pin ended member as I doubt that end fixity is appropriate for these checks. Then see how much of your 30-ish ksi tension leg tip stress is used up that way.

23' is a fair ways for an unbraced L3x3. Lateral torsional buckling under self weight might be a possibility. I think that the sag rod was a wise choice originally and that reconstituting it would be a wise choice now.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Hokie93]

Can you remove the L3x3 cross bracing in this particular bay and replace them with a larger size, such as an L6x3-1/2x1/4?

 

[vmirat]

Hokie93,

I could, but I'd rather not incur the additional cost to the project. However, from an academic standpoint, what is your take on when you need a sag rod?

 

[Hokie93]

Like KootK, I question the degree of fixity at the end of each L3x3 brace. Taking the span as 23'-0 and assuming pinned ends, I calculate a selfweight vertical deflection equal to 1.35 inches. In theory, there should be a lateral deflection of approximately 0.8 inches. Are the two L3x3 braces bolted together where the cross?

My first choice would be to replace the angle bracing with a larger size that does not require a sag rod. I would hope the cost to replace two angles would be roundoff error in the overall construction cost. Or does this replacement happen numerous times and/or access to the angles is severely limited?

Is there adequate space to "piggy back" an angle (or channel or small tube) on top of the L3x3 so as to increase the section properties?

My second choice would be to live with the deflection, and no sag rod, if the bending stresses were okay and I could convince myself the boundary conditions were more favorable than pinned.

 

[BAretired]

Maybe the angle could be suspended from the underside of the HVAC unit. The weight of the angle is pretty much negligible.

Some of the metal building manufacturers use tension tie rods instead of angles. I don't think they worry too much about slenderness ratios.

BA

 

[hokie66]

Sag rods for bracing in the horizontal plane are to control unsightly sagging of tension members. Nothing more, nothing less. If the cross bracing is intended to be tension/compression, sag rods won't do what you want.