Coefficient of Friction on Roof

[peinma]

I'm checking sliding (due to wind) of a temp. guardrail system on a membrane roof. The bases are coated with rubber, so it's rubber on rubber, but could be wet. I'm looking for opinions on a value for the coefficient of friction. Thanks.

 

[EngineeringEric]

A Temporary guardrail does not need to be designed for wind.... it is temporary.

And i would be more worried about the fact it is a guardrail that is not connected to anything... doesn't seem like a guardrail.

 

[dhengr]

I would think you would pick those CoF’s out of a handbook or ask a few rubber and roofing manufactures for their opinions of a range. And, if you apply much friction (in plane) loading to the roofing mat’l. what damage can you do to the roofing? Regarding wind loading, you don’t really have very much face area to the wind do you? Don’t you have to design the railing and its support bases for some significant lateral loading at the top rail, for example 200lbs. anyplace or 50lbs/lf? Doesn’t this control the base design? Doesn’t this mean you have to load the bases down to prevent tipping?

 

[peinma]

It's a pre-engineered system designed per OSHA for a 200 lb. load. The wind load on 10' section is less than the 200 lbs. (around 120 lbs.), but it appears that the railing section counts on adjacent sections to support it when subjected to a 200 lb. load. The wind load acts on all sections at once. The client asked us to look at the systems performance with regard to wind loading. As of now, I'm using 0.7, but looking for opinions.

Eric - Depends on duration of temporary, but, in my opinion, winding loading should be looked at for temporary structures. In this case, it will be approaching permanent installation.

 

[JStephen]

If you can get samples of the material, it's not too hard to run a test on it. Specifically, set one piece on top of the other, tilt the bottom piece until the top piece slides, and you can deduce the coefficient of friction from that and a little statics (I think it's the tangent of the angle from horizontal, but it's been a while). The test could easily be done wet, for that matter.

I would consider that approximate at best. Actual sliding could involve rocking back and forth moving a bit at a time or something like that rather than just a smooth motion.

A little sand thrown in there may reduce the COF as well.

 

[EngineeringEric]

I agree that wind loading should be looked but the question of what wind loading are you looking at comes up? Are you designing straight on ASCE 7? wind chance equal to that of the parapet on a permanent building? 12plf seems high for a rail.

I again reiterate my earlier statement that the overturning effect of any lateral load (OSHA 200# or your 12plf wind) will negate the friction factor unless your railing is so heavy that (200*3.5' = 700 ft-lb) 700 ft-lb of moment can be resisted without permanent anchors. At which point you will crush all the insulation below the EPDM/TPO/PVC membrane and possibly compromise the structure.

But maybe this is a system that i cannot imagine and you are 100% correct, Very possible!