Overstresssed Bracing for D+S

[Lesali]

I've designed an OCBF equipment platform in the Tahoe area where the snow load is 230 psf. While the "X" bracing utilizing L3x3's works well for the lateral design (as tension only or Euler buckling members), if I consider the compression they would "see" in the D+S case they are overstressed by 80%. I had an idea that by using short slotted holes (and slip critical connections) in one end of the braces, the dead load stress component could be removed after the platform is loaded by releasing the bolts and retightening them. Then, only the snow load induces compression in one brace at 100.2% d/c (for the max design snow condition and I can accept that).

Does this seem a reasonable approach?
Is there a safety concern that should be addressed when the braces are released and tightened after the dead load is applied?
Should the structure be analyzed for the increased drift and P-d effect under a seismic event if the bolts slip to the end of the slotted hole?

 

[KootK]

I understand what you're trying to accomplish with the slotted holes but I don't think that you need to go that far. To be honest, this is usually ignored in practice. Some folks claim that the bracing should be the last thing to get installed for exactly this reason but I'm not sure how practical that is. As an erection contractor, I think that it would be quite advantageous to have the permanent bracing installed right away. Here are some tips that you may find helpful:

1) This usually comes up when brace cross sections are of the same order as column cross sections from an area perspective. You could always bump up the column cross section.
2) Are you considering the compression brace to be laterally braced where the two braces cross? You might be able to make an argument for that.
3) Maybe you just don't care if the compression brace buckles out of the way under gravity loads. Estimate the lateral "kick" you're likely to see. If it's small, let it go.

Is there a safety concern that should be addressed when the braces are released and tightened after the dead load is applied?

I doubt it.

Should the structure be analyzed for the increased drift and P-d effect under a seismic event if the bolts slip to the end of the slotted hole?

Technically yes, if it adds up to a significant amount. I wouldn't bother.


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.

 

[SteelPE]

Maybe I don't get what is going on, but why are you considering compression loads in the bracing system if it is tension only? Is this because both column will be "shortened" by their gravity loads causing slack in the bracing making it difficult for the bracing to act properly?

 

[Lesali]

Thank you for your thoughts and advise.
Yes, I agree the braces should be installed before the equipment dead loads are applied otherwise it's just a house of cards. A little further reading and I see I cannot use short slotted holes in the braces - only oversized holes.

[ol 1]
[li]I ran the D+L without the braces and determined the column size to be sufficient.[/li]
[li]I have considered connecting the braces with one bolt mid span thinking the tension brace would brace the compression brace cutting down the Kl/r factor to half and therein increasing the compression capacity on the one brace and reducing the vibration tendency of the tension brace. But in addition, I'm thinking this joint should be allowed to slip and an oversized hole in each brace using a finger tight bolted connection.[/li]
[li]Perhaps I should not care if the compression flange is at 180% capacity under D+S load when it is at 480% capacity under seismic load - it will obviously buckle then.[/li]
[/ol]

 

[Lesali]

To StevePE,
Yes. While tension only bracing is a design approach where the dynamics of a seismic event will impose a reversing transient load, the reality under the D+S condition is the columns have an axial deflection of -0.127" and all braces will go into a constant overstressed compression for days or weeks. Does the time involved really matter? Of the equipment platforms I've designed in the past the snow load was not so great has to have this impact.

 

[Lesali]

Oops, I mean SteelPE - sorry

 

[SteelPE]

From what I understand the timing of the loads is considered in the load combinations.

The building that I have designed using tension only members I have never really considered axial shortening of the columns. I also cant say if I have ever end up with .127" of shortening in the columns either (never really ran that calculation on a column). Also most of the members I have used would be tube columns with angle bracing, you might have something different.

In my instance, if the bracing was never considered to take any axial load, if it did the kl/r>200 and it was expected to buckle out of plain slightly. In reality assuming the bracing did not take any axial load is not true as you need to force compatibility at the joints.... but I can't say that I have ever worried about it in my structures.

If it really a problem you need to solve I suggest following the advice given by KootK.