Portal frame bracing question

[John1967778]

Hello guys,

I've recently just made the transition from a geotechnical role into a structural role so my knowledge in structural engineering design is quite limited at the moment.

Quite a simple question i believe for some of you.

Just how important is wall bay bracing in the longitudinal direction when it comes to the design of portal frames. The stiffness of each column combined with the girts and purlins is surely enough to resist any force in this direction if i have a high number of bays should it not?

Any help much appreciated.

 

[Toby43]

Google "argyle building collapse video" and importance of bracing in the longitudinal direction will become apparent.

Toby

 

[hokie66]

And don't forget the bracing in the roof plane.

 

[12345abc6ttyui67]

"resist any force".

I doubt it.

 

[Mjkkb2]

agreed, girts will brace the columns fine but won't do much for you for overall building stability.

 

[CANPRO]

I've seen many, many, (too many) pre-eng buildings where the occupants have removed bracing for one reason or another - building expansion, need room to install some equipment, didn't like the looks of it...the list of reasons go on. All of the those buildings managed to not fall over because of the girts, sheathing, and a bit of luck. That does not mean that I would eliminate bracing from the design of a new building.

Its the difference between "it should work" and "it will work". The guy who doesn't know any better and cuts the bracing gets to stand back and say "this should work". As structural engineers, we don't get to say "it should work"...the bracing makes it a sure thing.

Keep in mind that tension bracing isn't the only way to stabilize the building in the longitudinal direction - you can use moment frames in both directions (maybe you have large openings in every bay). There are probably other options as well, but I'd say about 100% of the pre-eng buildings I've seen have been tension bracing and/or moment frames.

 

[Mjkkb2]

I don't disagree with anything you say. Any material that is mechanically attached will provide some stiffness/bracing but as structural engineers we don't have a luxury to just say: "I thing it should be ok". It has to be ok and we have to be able to back it up with numbers.

 

[JAE]

All of the those buildings managed to not fall over because of the girts, sheathing, and a bit of luck.

...until a straight-line wind storm of about 63 mph comes through and we see those tin foil buildings have been blown away....

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[CANPRO]

JAE - thats the "bit of luck" part. Sometimes its hard to convince people that just because something hasn't fallen over yet, that doesn't necessarily mean that it is structurally sound.

 

[dik]

...until a straight-line wind storm of about 63 mph comes through and we see those tin foil buildings have been blown away....

think of it as nature, recycling...

Dik

 

[JAE]

...or the official "Building Sciences Darwin Awards"

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[canwesteng]

If you're willing to crunch some numbers on the cladding and fastening to use it as a diaphragm, by all means don't use bracing. But the low cost of rod bracing combined with the fact all the PEMB guys still use it tells me it isn't worth anyone's time.

 

[John1967778]

Thanks for the responses blokes. Let's just say the all of the bays are open ( no cladding to hide the braces), a moment connection to the base would provide stability would it not? Thanks gents

 

[hokie66]

Sure, you can provide the longitudinal capacity necessary with moment frames, even cantilevered columns, adequately designed. But this is not an efficient way of doing it.