Cross Bracing Deflection
Cross Bracing Deflection
(OP)
My office was having a discussion about the design of rod bracing as a lateral resising system. The modeling software we are using (RAM Advanse) checks for bending + axial load stresses, which indicate the member is overstressed. The deflection in the model is over 10 inches, which I'm assuming is the cause of the high bending stresses. However, I have heard that braces are typically pretensioned to eliminate sag (and tensioned even further when lateral load is applied) and the brace straightens out so in reality there is no bending.
It is a light seismic zone and the rods are assumed to be tension only. The building is single story. Does anyone have any experience with this or have a reference we can check? I posed the question to AISC steel solution center but that didn't get me anywhere.
Thank you,
Graeme Sharpe
It is a light seismic zone and the rods are assumed to be tension only. The building is single story. Does anyone have any experience with this or have a reference we can check? I posed the question to AISC steel solution center but that didn't get me anywhere.
Thank you,
Graeme Sharpe






RE: Cross Bracing Deflection
If you apply any bending to a thin rod it will be overstressed in bending because the Sx is so small. As mentioned above, however, there shouldn't be any bending in these diagonal braces.
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
A common problem with analysis programs with straight matrix solutions is that they do not resolve tension straightening as it is a second order effect.
What happens is the program thinks that the rod is a simple spanning member (pins at the ends of course) and there is some small, but significant, self-weight to the rod that, over long distances of an X-brace, create simple, positive moment due to that self-weight.
What you should do, since the rod weight is insignificant to the analysis, is assign to that rod a special material property whereby it has very small, or zero, density. This eliminates the bending in the rod.
Also - I agree with kslee1000, that you should ensure that the rod is a tension-only member in the analysis.
RE: Cross Bracing Deflection
There will de some additional drift due to overturning tension or compression axial deformations in the columns also, but the additional drift contribution of these will be minimal in a one story structure. A simple virtual work analysis can determine the column's contribution to drift if you are curious.
Give your computer results the "ho ho test" by HAND...
RE: Cross Bracing Deflection
I assume he means a high moment in the columns due to the 10" of drift. Could happen if the beams have fixed connections into the columns.
TxGraeme,
The rods do get tensioned up but it is nothing like the tension on a pretensioned bolt, I wouldnt really count on it. If the deflection is a problem, then I would suggest using anle bracing instead. 10" sounds like a lot of drift to me.
csd
RE: Cross Bracing Deflection
Basically the brace is trying to resist self-weight by going into bending, but in reality this will resisted by catenary action. I agree that a second-order analysis would be required to model this correctly, but I was hoping for some rules of thumb to (hopefully) disregard bending in rod braces.
Thanks for all the replies so far,
Graeme Sharpe
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
One firm I worked at would not detail "tension only" rod braces because on some previous project they had gotten noise complaints from the occupants when the compression brace buckled and "whapped" the drywall studs under just average type wind events.
RE: Cross Bracing Deflection
DaveAtkins
RE: Cross Bracing Deflection
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: Cross Bracing Deflection
The comments about the braces "whacking" the wall seems important, has anyone else had problems like this when using tension-only braces?
The AISC manual's chapter on braces says that sag problems can be avoided by "drawing" or pretensioning the brace and lists the length each brace should be shortened by. Has anyone ever specified this or had problems reported by installers?
Graeme Sharpe
Indianapolis, IN
Thanks for all the help!
RE: Cross Bracing Deflection
If you simply ignored the bending results from the output and took the axial load then this would have negligible difference from reanalysing as stated above(I would expect much less than 1%).
Regarding pretensionining to take out the sag, as this is mostly a visual requirement then visual inspection and common sense should be sufficient. If you shake it and it moves around then it is too loose, if you can easily see the sag then a bit more tension should be applied.
Be careful using rods with light fraing as the pretensioning can sometimes prematurely fail the surrounding light members (although this doesnt seem to apply to this case).
csd
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
But I believe it doesn't make a difference with total load--you will get the same tension in the rod under total load whether or not the rod is pretensioned.
Someone correct me if I am wrong.
DaveAtkins
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
I have seen some engineers using circular hollow sections instead of cable bracing (X-bracing). Any pros and cons?
KC
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
I tried breaking up the rod and it does straighten out if it is under enough tension and a second order analysis is performed - a very good suggestion.
Graeme Sharpe
Indianapolis, IN
Thanks for all the help!
RE: Cross Bracing Deflection
The pretension doesnt make any difference for the load, but it does for deflection.
clefcon,
I would be concerned about using circular hollow sections for cross bracing unless they are designed for compression. The stiffness means that they will behave more like a beam and will have bending stress, it also means thatt hey are more likely to have a column type buckling failure rather than just buckling as a rod does.
It is also much more expensive to use circular tubes as these need welded connections each end rather than a rod that can just be threaded.
I have seen neoprene sleeves specified to minimise any banging of the rods.
Otherwise I would suggest angles as noted above.
csd
RE: Cross Bracing Deflection
But for most rod X-braces, this is a lot of work and I'm not sure of the benefits of all this effort when simply turning off its individual self-weight provides essentially the same analysis results.
Turnbuckles typically used with rods are used to provide just enough tension to initially straighten the rods and for most conditions this tension isn't all that significant in terms of the design of surrounding beams and columns.
With light gage framing and rod/turnbuckles it may be necessary to consider the tension but I've not had to deal with that before.
Besides rods, we've used square tubes - but not pipes.
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
1 or 2 ksi of prestress should be plenty to pretension a rod brace. Over tightening and relaxing will help assure the rods ends are set. You should specify either double nutting the rod ends and turnbuckles or damaging the threads or tackwelding the nuts to prevent loosening over time. The big thing to watch for is over-tensioning the system - it does not take much tightening torque to generate high rod stresses.
Specifying how many nut turns after snug is the most controlled way to specify - 1 full turn = 1/thread pitch shortening and PL/AE will tell you the internal stress. for a 30' long 3/4" rod with coarse thread ends that equates to about 3.5 ksi internal stress.
Prestressing will help prevent compression rod buckling. You should reduce the allowable stress of the rod by at least the specified prestress amount.
sennafan
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
The computer was analysing the rod as a simply supported beam, the 10" was a beam deflection of the rod with no pretension.
What relevance is kl/r for tension members?
csd
RE: Cross Bracing Deflection
The AISC spec says that kl/r for tension members should preferably not exceed 200. I believe that it has something to do with vibrations.
RE: Cross Bracing Deflection
DaveAtkins
RE: Cross Bracing Deflection
Many, many, many times I've seen PEB braces bent, or cut, or missing.
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
...then by taking out the false bending you can then design the rod for what its true purpose is.
RE: Cross Bracing Deflection
I am not sure if I follow. If the bending is taken out by pretensioning, then you still have the bending stress due to the self weight without the deflection, plus you now have added pretensioning tensile stress. I am confused about the 'false bending'. Isn't this a tension overstress situation?
RE: Cross Bracing Deflection
No, a rod which is sagging is not stressed in bending. It is a catenary, and any stress in it is tension. As JAE said, the bending given by the program is false. Just goes to show, engineers should be smarter than computer programs, and structures are always smarter than both.
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
There is no bending stress, it is all tension. While you may need to account for the pre-tensioning in the design, the load is still all tension.
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
It's still sagging when tensioned up, it just isn't sagging by much.
It still has some bending stress too, but you just ignore it.
RE: Cross Bracing Deflection
I think this long, thin rod more closely approximates a cable than a beam.
RE: Cross Bracing Deflection
I agree. I was just commenting that there will be *some* bending stress, albeit a fairly negligible amount. You can't have zero bending stresss unless the rod is tensioned perfectly straight, and that's impossible.
RE: Cross Bracing Deflection
RE: Cross Bracing Deflection
The 10" sagging is not accept to me, I guess your client wouldn't either. Suggest to provide turn buckle or connection plate in the middle to control the deflection due to selfweight (assuming tension capacity of the rod is adequate).
RE: Cross Bracing Deflection