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Cross Bracing Deflection

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Cross Bracing Deflection

Cross Bracing Deflection

TxGraeme

(Structural)

(OP)

18 Jul 07 15:24

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

Replies continue below

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RE: Cross Bracing Deflection

Lion06

(Structural)

18 Jul 07 15:39

I am picturing that you are using rods for diagonal bracing from top of one column to bottom of the other.  This being the case, you should make sure the node at the junction of the rod and column is pinned, not fixed.  You shouldn't be getting any moment in there as the braced frame system you are talking about is designed to carry everything in axial load (as far as the lateral forces go anyway).
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

kslee

(Civil/Environmental)

18 Jul 07 15:46

Have you release the joint when the rod is in compression?

RE: Cross Bracing Deflection

JAE

(Structural)

18 Jul 07 16:35

TxGraeme,

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 kslee, that you should ensure that the rod is a tension-only member in the analysis.

RE: Cross Bracing Deflection

sundale

(Structural)

18 Jul 07 16:44

It is very easy to calculate the PL/AE elongation of a tension rod and then compute via simple trigonmetry the lateral drift that results.

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

csd72

(Structural)

18 Jul 07 17:08

StructuralEIT,

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

TxGraeme

(Structural)

(OP)

18 Jul 07 17:22

Sorry guys, I might have led you astray.  Let me clarify.  The member under consideration is the rod brace itself.  The bending is from dead load / self weight of steel.  Drift is not an issue, nor are moments in the columns.  The brace member is pinned at both ends (relative brace from top of one column to base of adjacent column).  There is no nodal connection at the intersection of X-braces, so no interaction exists there.  

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

JAE

(Structural)

18 Jul 07 18:13

...which is pretty much what I said above.  Simply assign the rods zero density and the sag goes away.

RE: Cross Bracing Deflection

hokie66

(Structural)

18 Jul 07 18:42

Not for analysis, but for detailing, if your rods are roof bracing, sag rods or straps from the purlins should be used to control the sag.

RE: Cross Bracing Deflection

sundale

(Structural)

18 Jul 07 19:02

I was thinking you must have a very tall and flexible structure for 10" of drift!

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

(Structural)

19 Jul 07 06:56

Do what JAE said, or set the selfweight to zero (e.g., on RISA 3D, don't put a -1 in the Y Gravity column in the Basic Load Cases spreadsheet).

DaveAtkins

RE: Cross Bracing Deflection

swearingen

(Civil/Environmental)

19 Jul 07 08:37

For grins, break the rod into 20 pieces and see what happens. You should see it straighten out...

If you "heard" it on the internet, it's guilty until proven innocent. - DCS

RE: Cross Bracing Deflection

TxGraeme

(Structural)

(OP)

19 Jul 07 08:40

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?  

It seems that the consensus opinion on this issue is that sag won't be a problem, so it can be disregarded in the analysis by either setting the density to zero or ignoring the self-weight deflection. Sounds good to me.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

csd72

(Structural)

19 Jul 07 10:35

TxGraeme,

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

Lion06

(Structural)

19 Jul 07 10:36

I would you need to account for the pretensioning in the design of the rods?  Is this correct?

RE: Cross Bracing Deflection

DaveAtkins

(Structural)

19 Jul 07 10:53

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.

I recently learned a neat trick--you can induce a pretensioning in a rod, in RISA 3D, by giving that member a negative temperature load (this contracts the member, putting it into tension).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

UcfSE

(Structural)

19 Jul 07 11:03

I would account for the pretension in the rods.

RE: Cross Bracing Deflection

KCRatnayake

(Structural)

19 Jul 07 11:07

JAE,
I have seen some engineers using circular hollow sections instead of cable bracing (X-bracing). Any pros and cons?

KC

RE: Cross Bracing Deflection

Lion06

(Structural)

19 Jul 07 11:26

I would think that (depending on size) the hollow circular sections are less likely to buckle.  That being the case, care should be taken to not necessarily treat the bracing as tension only and account for the compression in the column attached to the compression brace.  

RE: Cross Bracing Deflection

TxGraeme

(Structural)

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(OP)

19 Jul 07 11:27

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.  

swearingen,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

csd72

(Structural)

19 Jul 07 12:41

Dave,

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

JAE

(Structural)

19 Jul 07 12:44

Yes, by adding nodes along the length you introduce stiffness terms at each node and the second order analysis will respond to the pretensioning (in RISA using temperature per DaveAtkins above) by straightening out.

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

UcfSE

(Structural)

19 Jul 07 12:55

I took the question about the pretension in the rod to mean accounting for the extra tension in the rod itself when checking it along with the applied tension from the lateral load.  

RE: Cross Bracing Deflection

Lion06

(Structural)

19 Jul 07 13:00

UcfSE, that was the way I took it also.

RE: Cross Bracing Deflection

sennafan

(Structural)

19 Jul 07 17:06

TxGraeme,

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

OUe

(Structural)

19 Jul 07 19:16

I'll chime in on one more thing:  it seems to me that the braced-bay underconsideration would have to be pretty darn long in order for 10 inches of deflection to occur.  I realize that the kl/r can be ignored for rods, but this seems to be a very, very long rod for there to be 10" of deflection on a single story!!!  

RE: Cross Bracing Deflection

csd72

(Structural)

19 Jul 07 20:44

OUe,

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

Lion06

(Structural)

19 Jul 07 20:51

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

(Structural)

19 Jul 07 22:12

I believe the kl/r for tension members, which is NOT a requirement, is to prevent the "slapping" that was mentioned above.

DaveAtkins

RE: Cross Bracing Deflection

JLNJ

(Structural)

20 Jul 07 08:01

Providing a kl/r less than 200 also gives some capacity in compression.

Many, many, many times I've seen PEB braces bent, or cut, or missing.

RE: Cross Bracing Deflection

Lion06

(Structural)

20 Jul 07 08:24

The 13th edition has this value at 300.  It also states that this recommendation does not apply to rods or hangers in tension.

RE: Cross Bracing Deflection

UcfSE

(Structural)

20 Jul 07 08:40

The limit for tension members has always been 300, at least since the 9th edition.

RE: Cross Bracing Deflection

haynewp

(Structural)

25 Jul 07 19:36

If the rod is overstressed with bending from self weight plus lateral induced tension, how can it not be still overstressed if the bending deflection is taken out by pretensioning and then adding the lateral load? I don't think that removing the density of the rod is solving the tensile overstressed problem you have.

RE: Cross Bracing Deflection

JAE

(Structural)

25 Jul 07 19:57

...well...if the stress from bending via self weight is 99.99% of the total stress

...then by taking out the false bending you can then design the rod for what its true purpose is.

RE: Cross Bracing Deflection

haynewp

(Structural)

25 Jul 07 20:16

JAE,

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

hokie66

(Structural)

25 Jul 07 21:20

tunacan,

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

haynewp

(Structural)

25 Jul 07 21:33

It is not sagging when it is pretensioned or when it is under tension from lateral, so I don't think it is acting like a catenary in those cases.

RE: Cross Bracing Deflection

Lion06

(Structural)

25 Jul 07 22:02

I agree with JAE.
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

haynewp

(Structural)

25 Jul 07 22:24

I would love to see the proof of it. A rod that is under a uniform transverse load combined with an axial tensile force, high enough to cause an immeasurable deflection, is in pure tension. I would just bump the rod up one size. But I usually disagree with others on this forum anyway so maybe you shouldn't listen to me.

RE: Cross Bracing Deflection

Tomfh

(Structural)

25 Jul 07 22:43

tunacan,

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

Lion06

(Structural)

25 Jul 07 23:04

Where do you draw the line between a thin rod and a cable?  A cable certainly can't support any bending stresses.
I think this long, thin rod more closely approximates a cable than a beam.

RE: Cross Bracing Deflection

Tomfh

(Structural)

26 Jul 07 01:15

Quote:

I think this long, thin rod more closely approximates a cable than a beam.

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.

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

haynewp

(Structural)

26 Jul 07 18:48

A rod is not a zero rigidity bending member. The ends are also being forced against the natural slope of the hanging rod by the tensioning attachment at each end. I wouldn't go so far to say negligible bending, though I agree somewhere between catenary and bending. I don't know what effect the  pretensioning of the rod (stiffening against lateral movement) has on its catenary ability.

RE: Cross Bracing Deflection

kslee

(Civil/Environmental)

27 Jul 07 10:55

For 3/8" dia. steel rod with 20' horizontal span length, it yields 10" deflection in the mid-span.

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

kslee

(Civil/Environmental)

27 Jul 07 13:17

Correction: 13.5' span, not 20'.

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