U-bolt Design

Hi--
I'm trying to calculate a minimum diameter for a U-bolt resisting only a lateral wind force. An 18 ft span of 24" pipe is sitting on top of an S beam and is clamped down by a round-bend U-bolt. Will both the windward and leeward sides of the U-bolt have equal reactions or is there a ratio of the two as a rule of thumb? Are there any references that cover this topic? Thanks,

ER

 

[csd72]

licensetochill,

this is a difficult one.

If you think about it in detail you can get some reasonable parameter.

I assume that the bolt forms an upside down U and that it is sitting on a flat platform?

If the wind blows laterally on the pipe it will tend to move/roll away from one side and towards the other it will therefore give a lateral load onto the side of the leeward u-bolt at the center of the pipe. This will then induce shear and bending in the one bolt.

The pipe tendency to roll will be resisted by tension in the windward bolt with the lever arm being to the center of the pipe.

Hope this helps

csd

 

Yes, the U-bolt is upside down and fastened to the platform.

I think the main concern here is calculating the bending stress. For the leeward side, if I assume that that bolt takes the whole bending load applied at the center of the pipe (i.e. cantilevered), then the min. diameter is 1-5/8" which is huge.

So, any help on how to calculate the bending stress would be greatly appreciated.

 

[csd72]

Stress = M/S

M = applied moment
S = section capacity = (pi*d^3)/32

where d = diameter to inside of threads.

You may want to apply a bit of a stress concentration factor to that.

Also make sure you have thick washers at the ends to take the bending into the plate, and a plate that can take it.

csd

 

What would the applied moment be? Would it just be the lateral force * pipe radius for leeward? Doesn't the other portion of the U-bolt resist the moment too?

 

[vonlueke]

licensetochill: If the U-bolt ends are simply supported, the maximum bending moment on the U-bolt occurs on the unthreaded shank at the midpoint of the leeward leg and is M = 0.62*r*F, where r = U-bolt mean radius, and F = lateral wind force on the pipe. If the U-bolt ends are fixed (clamped), then the maximum bending moment on the U-bolt occurs on the leeward thread and is M = 0.45*r*F. The above answers assume the U-bolt is not preloaded. If the U-bolt is preloaded, add the tensile stress due to preload to the bending stress. A rough estimate for tensile force in the U-bolt due to preload is P = T/(0.20*D), if the threads are unlubricated, where T = nut installation torque, and D = bolt nominal diameter.