U-Bolt rotational resistance

[Eldorado]

I have been asked to design a solar panel support for two (2) to three (3) panels approximately 3'x5' each (see attached picture). The panel assembly is to be attached to a horizontal galvanized steel pipe with u-bolts. I am not comfortable with relying on the u-bolts alone to resist rotation of the panel frame, so I want to add a brace to the support post to brace against rotation. This is less desirable than designing without a brace because it limits the degrees of freedom for adjusting the plane of exposure for the solar panels.

Can anyone guide me to literature or design guides for u-bolts that provide industry accepted design limits for u-bolts to resist rotation. I would think that this is done all the time for basic traffic signs, etc.

 

[MiketheEngineer]

Never heard of it - but good luck!!

I would think it depends on how "tight" you make the U-bolt??

 

[RFreund]

I've been looking for the same thing. Let us know if you find something. I'm thinking maybe contact a manufacture? I'd like to know if you can assume a friction factor to find the required clamping force then see if tension yeilding (tension due to offset mount bending?) is ok?

random info:

http://www.truckcomponentsonline.com/U-Bolt-Torque-Specifications_df_53.html

EIT

http://www.HowToEngineer.com

 

[msquared48]

Why don't you fabricate a "V" shape strut there the open part of the "V" is attached to the bottom of the angles at the edge of the panel, and the closed part of the "V" to the round vertical shaft? I would not depend on the U-Bolt as the force couple is way too small to limit torsional deflection to a manageable degree.

More direct approach, and it could be adjustable depending on the angle required of the solar panel. Do this all the time in the telecom business at the tops of poles and towers to support antenna arrays.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[chicopee]

If you google U bolt clamping analysis, there will be explanations about the formula: Clamping force= Torque/(K x D). I don't know how strong your U bolts are and the size of the support piping is, however I doubt the panels will rotate if the U bots and piping are properly rated with an adequate factor of safety and installed. Your design show that the panels can be adjusted to any angular position on the horizontal cross bar which seems OK but I do have reservations about the single vertical pole holding up the whole contraption.

 

[IFRs]

I would not count on the u-bolts - they will loosen with temperature swings, they will be lubricated with rain, the pipe may spring inwards, the panel is probably not that stiff, the wind can be pretty strong and it will induce vibration. You need a bracket.

 

[RFreund]

Is there a typical coefficient of friction to use?

EIT

http://www.HowToEngineer.com

 

[ATSE]

U-bolts are used extensively in the telecom and utility industries for axial and twist restraint, where exterior-exposed structures with lots of pipe and round tube members exist.
Always use lock washers or locking nuts. Don't oversize or undersize the clamping plate. You want a little bit of plate bend during U-bolt tightening.
U-bolts are typically A36, HD galvanized.
Use U-bolt pairs, as you get beneficial binding action (not counted for in calculation, but real).
A reasonable approach is to use 30% of yield stress x net section of threads, and a low friction coefficient (like 0.3). This gives clamping (normal to pipe axis). For torque (twisting), F*d.
Provide installation notes on your dwgs regarding range of installation torque. Structural engineers don't usually specify installation torque and threaded fasteners, but this is a good exception.

 

[msquared48]

ATSE:

Yes, U-Bolts are used extensively in Telecom for that, but where the structure gets too large, as in T-Arms of various sizes, a stabilizing arm is installed from one tower leg to one end of the T. Otherwise, in a high wind, antenna performance is too spotty due to the continuous movement affecting the sector angle.

The solar panels, I believe, are too large for that application, and should have stabilizing arm(s) installed. In all probablilty, if the solar panel frame is stiff enough, one diagonal brace should be sufficient.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RacingAZ]

I've done a similar assembly for PV panels but with at least 2 horizontal pipes, so no rotation issue at the U-bolts.

I think the flat plate that clamps into the pipe, you can find a different one with teeth. Also check the Uni-strut catalog page 109 for a similar U-bolt assembly that might work.

 

[Eldorado]

Thanks to all for your input. I need to digest all that has been posted.

 

[RFreund]

Thanks from me as well. I was mostly interested in regards to usage in telecom.
So when designing these you would have:
Torque = T
Coef. of friction = u
diameter of u-bolt= d
required friction force = F = T/(d/2)
required Clamping force = C = F/u
This must be less than the bolt tension capacity = 0.3*Fy*Anet (as suggested above).
So in this process you would not consider shear in either direction, because of the clamping force and the 0.3 factor used in the tensile calc, correct?

The clamping force required would be

EIT

http://www.HowToEngineer.com

 

[ATSE]

Agree with msquared48, as component gets wider, strut is required.
For a 2' microwave or 3' long Tee on a 4" pipe, you can calc a double 5/8" U-bolt to work. And I've seen hundreds in the field that have stood the test of time and high winds.