Purlins

[leisure17]

I'm interested in the system of purlins/side rails and ancillary items like stays & braces. where can I get info from.
Most manufacturers only give their sizes/loadings but don't say why or where you would use them.
For example - why are the short braces from a purlin to a beam compression flange used, is it to stop the beam rotating or is it to support the purlin. If it's to stop the beam rotating does the point of attachment count as one end of the length for calculating twist?
Cheers
Mick

 

[dpcoon]

Good question. The braces you're refering to are common in pre-engineered steel buidlings. To the untrained eye, they could easily be mistaken for knee braces meant to augment the purlins. Not the case - the purlins are designed to be structurally adequate without these braces. Another clue: the fact that these braces are often not on every purlin. Their actual purpose is to laterally brace the compression flanges of the supporting frame. In a continuous moment frame (unlike a simple span beam), the compression flange isn't always the top flange. Along significant portions of the frame, the bottom (or in the case of vertical members, the inner) flange is in compression, although which portions are in compression will shift depending upon the nature of the loading. The braces are used in order to provide lateral support to the compression flange, the upper (or outer) flange being fully braced by the purlins (or girts) themselves. Hope this helps...

Dave

 

[csd72]

They are normally called fly braces.

dpcoon covers the details more than adequately.