Design X brace

[Staed79]

I needed information to design X -braces.
Help would be appreciated.

 

[JAE]

If you have a specific question, please provide more detail in your question.

If you are looking to simply "learn" how do design X braces, it would be better if you consulted with an experienced engineer.

 

[Staed79]

I have designed X-braces in past, not done for a long Time, I needed if some one had an example, or links, to Design X -braces, which would be beneficial.

Thanks for quick response.

 

[MiketheEngineer]

Analyze the structure and determine the X-brace load assuming it will ONLY see tension forces. Assume only one brace in frame or structure.

Then design for that. If the forces reverse - the other part of the X-brace will see the same load.

Make sense??

 

[Staed79]

No that doesnot, ring bells....

 

[JAE]

Designing an X-brace based on some textbook examples isn't the way to go if you don't know how to perform the calculations, include wind and seismic forces, detail for seismic requirements, transfer diaphragm forces into the brace, and so on.

I think, based on your request, that you should consult with an engineer who can help you directly.

X brace design isn't a plug-in-the-variables-and-go effort.

 

[msquared48]

I thought tension only X bracing was restricted in it's useage now. Mainly in light gage Metal buildings - agricultural - for sidewall and endwall bracing.

In larger or commercial applications, I believe that they must be designed as T/C to avoid slapping of the members, IE, min kl/r....

Mike McCann
MMC Engineering

 

[abusementpark]

Is X-bracing the most economical choice??

 

[Staed79]

Yes, it's good to consult an experienced engineer to design such things, but it will be benificial too, if you knew something ahead, so it goes easy on him/her...
So they too take interest on the topic

 

[slickdeals]

X bracing is the most efficient. However, architects generally have issues with them especially with where they can be placed if you have door openings to deal with

 

[gyroman10]

X-bracings are members that take load applied perpendicular to the main frame, although mostly they are Tension only member but not necessarily has to be tension only! In large multistory buildings where Wide flange beams and HSS are used they are also designed to take compression forces.
Anyhow for more clarification model a frame of 30ft span and 10ft height in any structural software (staad, sap, risa etc.) make sure it is modeled in a Space frame structure (3 axes) environment and not Plane frame environment.
(Coordinates of frame: (0, 0) (0, 10) (30, 0) (30, 10))
Apply a UDL of say 2Kip/ft and then design the frame; select a suitable section, and run a code check and section search till it passes with flying colors.

Now apply a point load of 5kip on any one column's top (eave) in Z direction. Say on coordinate (30, 10). Run analysis and code check again, you will see that the section fails with flying colors now. Go to model again and from the point where you applied the nodal load model another member (say angle L3x3x1/2) in -Z direction from (30, 10) to (30, 0,-10). Assign it a pin support at (30, 0,-and 10) make it a tension member only or truss. Now you will see that your section passes again.

Therefore x bracing acts a pull only rope in the perpendicular to framing direction.

I hope you get something out of this.

 

[Vinny7]

I agree with Miketheengineer.

If you have a point load from wind etc. on say a horizontal eaves beam this must be resisted by the brace which will go into tension. Simply resolve the forces to gat an axial load in the brace member and check the axial capacity of that member at the connections (rember to reduce the cross section area for any bolt holes). There will also be a tension and compression force in the posta at either side of the bracing.


------- <-- F
|\ |
| \ |
C | \T |T
| \ |
| \|

F = FORCE
T = TENSION
C = COMPRESSION

Ignore the other diagional member as it will not be able to take any axial compression - assuming flat bar is used.

 

[Staed79]

Thanks Guy's..
That Helps a lot...

 

[honat]

The main reason for tension design is economic. In AISC 5-37 B7, slenderness requirement there is a 50% difference between the compression and tension.

As your brace element get longer as is the case for large bay and heigh floor requirement (industrial plant), a compression brace element will require very large section properties (Area, Modulus of elasticity, Rx, Ry). Such members in large project become very uneconomical. Simply desgning them as Tension only element will drastically reduce the size required for your purpose.

In a pure number crunching the only difference between a tension and a compression element is with the AISC reference stated above. The rest is only a P/A calculation compared to and allowable Fa(compression)AISC chapter E2, Ft(tension)AISC chapter D section 1. Here again you will see that Ft can be substancial larger than Fa.

Hope this will help making your design decision. Good luck