X roof bracing (Accidental compression)

[palk7 EIT]

Hi,

When having a X roof bracing designed only to be a Tension only bracing. With that member being an L angle section it's possible that due to stiffer member it can take a Compressive axial load. If it takes a Compressive load then the lateral displacements of the portal frames are enormously huge when compared to modelling those X braces as tension only braces. Has any one experienced this before?. If so how to ensure that the L angle cross bracing works only as a tension only member and doesn't take Compressive force. Cuz of a longer span I had to go with A little heavier angle to avoid too much deflection under its own weight and since it's heavy it could be susceptible to take Compressive load as well creating huge lateral displacement.

Thank you

 

[driftLimiter]

How does adding more braces (i.e. the compression braces) increase displacement? Intuitively I would think you have more elements resisting (tension and compression) not sure how this leads to more deflection.

I am also curious what people here are going to say about this. For seismic braced frames special detailing is required in order to provide sufficicient rotational ductility to the brace connection to allow the brace to buckle . The buckled brace does not resist compression loads and upon load reversal takes up the tension.

Not sure how this might be handled for a horizontal brace.

 

[palk7 EIT]

There is a Uniqe situation in this case I have the roof bracing placed to take out the loads in the direction of the portal frame cuz I don't have a portal frame column in one of the location so take out that, have the roof bracing to re distribute the Transverse lateral loads into the Adjacent frames

 

[human909]

I'd say it is an issue with the way your computer model handles the bracing. I've had some odd behaviour of a similar fashion to what you describe. What software are you using.

Sometimes I have to add dummy lateral loads to ensure some braces remain active when in 'tension only mode'. I've also seen circumstances where too many braces buckle an remove themselves from the model.

 

[SteelPE]

Sorry, I read your post wrong, you are talking about roof X bracing.

What are you using for roof decking?

I don't quite understand how adding bracing would induce more deflection, but it sounds very suspicious.

 

[KootK]

I think that it would be helpful if you could post a screen capture of your structural model.

 

[palk7 EIT]

I see, yeah I get a picture that if it's X braces or braced by other means in roof it's acting like a one piece and tigthens the system to displace by locking everything together and transfer them to the Vertical braced bay, this was shocking when I applied the braces as a tension and compressor member. The software is robot structural, if it's modelled as tension only bracing what I see for displacement makes sense

 

[palk7 EIT]

KootK, I will post it shortly of the model pic.

 

[palk7 EIT]

Steel Pe, roof deck type will be for those ones of pre Engineered building, I don't know if I can do a Canam roof metal deck. I am unsure whether if Canam roof decks and the ones used for pre Engineered building standard stuff are the same

 

[SteelPE]

So it's a screw down roof or a standing seam roof then? That's depressing. Looks like you are stuck with the horizontal bracing. Metal building manufacturers often use rods and I'm not sure how much the care about sag of the members, if you care concerned, couldn't you attach it to your roof members?