Difference between Purlins, Joists and beams. 2

[dashter]

I get really confused as these terms are used interchangeably. I know that joists are kind of beams that transfer loads to girders. But, are all beams transferring loads to girders joists? And, what exactly is a purlin. As far as my knowledge goes, it does the same job.

Thanks in advance!

 

[JAE]

I don't think there is any real specific definitions - in general this is how I see them:

Purlins - Fairly small, repetitive members holding up deck directly. Always referring to roof members.
Joists - Same as Purlins but perhaps larger members with longer spans. Either roof or floor members.
Beams - Similar to joists but usually spaced further apart (5 to 14 feet). Either roof or floor but usually floor members.



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[dashter]

Hmm, makes sense. Thanks!

 

[Guastavino]

A lot of times purlins are placed on TOP of girders, think metal buildings and some wood framed structures. And JAE is right on.

 

[apsix]

In my locale purlins are repetitive roof support members perpendicular to the slope as opposed to rafters which are parallel.

 

[IDS]

My Irish mates tell me that the difference is that Girder wrote Faust, whilst Joist wrote Ulysses.

They weren't sure about Purlin, but they looked him up and apparently he wrote The Hedgehog and the Fox.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Triangled]

pretty much like JAE said with regional and local variants all over. Therefore I usually spend first paragraph of conversation with new collaborators just getting terminology synchronized, especially so with contractors, and caution my EITs not to place too much stock in such terms until they know that who they're talking to is using a similar dictionary.

 

[JAE]

apsix - that is a good point about orientation with regard to the slope - I agree.

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[Triangled]

in the southwest US I see "purlin" used on daily basis without regard to slope... shrugs....

 

[msquared48]

Beams support joists and purlins.

Rafters are sloping joists at the roof, that are parallel to the slope direction.

Mike McCann, PE, SE (WA)