Roof Purlin Eccentricity 2

[JrStructuralEng]

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?

 

[hokie66]

Not a problem. One advantage of Z purlins is that when the top flange is placed up the slope, gravity load gets closer to shear centre than if the purlin is horizontal. If C purlin, also make sure flanges are up the slope for same reason. Remember, deck can not brace the bottom flange. You need bridging for that.

 

[civeng80]

Agree 100% with hokie66. This would be ok for up to 5 to 6 degree roof pitch.

 

[JrStructuralEng]

Hokie, Re: bridging.

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?

 

[BAretired]

If the roof has a ridge line, it is usual to use a double purlin, i.e. one each side of the ridge line. These members are tied together so they can take vertical load. For significant slope, sag rods are sometimes used at third or quarter points.

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

 

[JrStructuralEng]

It is a single slope. Do I still need bridging to the bottom chord if the compression flange is continually braced?

 

[BAretired]

I read your first post again. The slope is only 1/48, even more gentle than I stated in my previous post. It is virtually a flat roof with just enough slope for drainage. If the roof deck provides diaphragm action, you should not require any additional in-plane bracing.

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

 

[JrStructuralEng]

BA,
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.

 

[JrStructuralEng]

whoops that should have read "must have bridging when span exceeds 4m no matter what".

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."

 

[BAretired]

I think you are reading clause 16.7.9 "Spacing of Bridging" which applies to open web steel joists. I am not aware of any requirement which requires minimum bridging for purlins, although it might not be a bad idea to provide midpoint bridging.

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

 

[hokie66]

I can't really advise you on the Canadian practice for purlin bridging, as I am in Australia. Here, almost all of our buildings use steel purlins for roof framing, and the slopes vary a lot, starting at 2 degrees. Whether or not on a slope, the design here is usually controlled by wind uplift, as we only have snow in a few areas. Our purlin manufacturers have proprietary bridging systems which essentially are short lengths of cold rolled channel, with end clips bolted to the purlins. All fabricated by the purlin supplier. I suppose to an extent our use of purlins mirrors your use of bar joists, which are not used in Australia.