Roof Purlin Eccentricity
Roof Purlin Eccentricity
(OP)
Should I be worried (i.e. take into account) eccentric loading on a steel roof purlin of about a 1.5 degrees.
The roof has a 0.25 to 12 slope. The steel roof purlin spans ~22ft. Or is 1.5 degrees considered negligible once the steel decking distributes the load.
Your thoughts?
The roof has a 0.25 to 12 slope. The steel roof purlin spans ~22ft. Or is 1.5 degrees considered negligible once the steel decking distributes the load.
Your thoughts?






RE: Roof Purlin Eccentricity
RE: Roof Purlin Eccentricity
RE: Roof Purlin Eccentricity
From what I found in CISC the requirement for bottom chord (tension) bridging is a spacing of 240r where 'r' is the applicable chord radius of gyration about its axis in the plane of the web. It also says at least one line of bridging is required for joist spans exceeding 4m.
I am using Z purlins. What is the industry standard for bridging the bottom chord? Angles?
RE: Roof Purlin Eccentricity
The gravity load on purlins further down the slope can be resolved into two components, one parallel to the web, the other parallel to the roof. In this way, each purlin carries only load normal to the roof. The other component goes back to the double purlin at the ridge. The horizontal component each side of the ridge is balanced by a similar force on the other side.
If it is a single slope, you should provide bracing members at the high end to resist in-plane forces and again, the purlins down the slope carry only load normal to the plane of the roof.
For a slope as gentle as 1/24 you probably could get away without doing that, but personally, I would do it anyway.
BA
RE: Roof Purlin Eccentricity
RE: Roof Purlin Eccentricity
If uplift does not produce stress reversal in the purlin, you do not need bridging, otherwise you should bridge as required to control the slenderness ratio of the purlin.
BA
RE: Roof Purlin Eccentricity
That was what I was thinking, but I couldn't find anything in CISC to back this up. Any idea where bracing requirements are noted? As best I can tell the code states you must have bridging at max 4m o.c. no matter what.
RE: Roof Purlin Eccentricity
The clause reads:
"In no case shall there be less than one line of horizontal or diagonal bridging attached to each joist spanning 4 m or more."
RE: Roof Purlin Eccentricity
I don't think the metal building suppliers use bridging between purlins, but I could be mistaken. Does any one else know for sure?
BA
RE: Roof Purlin Eccentricity