Supporting HDPE Pipe accross a trench

[arij001]

I need your expertise. I have a 36" HDPE pipe SDR 17 (assume completelly filled with water-unit weight of just pipe is 8.21 lb/in). the pipe needs to go across an open trench 50-55 feet in width. I was thinking of using a W30x235 or similar, lay it flat and put my pipe on the web. questions; Is it going to support? how much will be the defelction and how do I calculate? I am pretty sure this a case of simply supported beam with uniform loading, am I right? I don't know all the same formulae are applicable if web of beam is loaded instead flange? how far beam needs to go past the support or ends?

Any input is greatly appreciated

 

[HeavyCivil]

1. Calculate allowable moment by multiplying allowable bending stress by section modulus (ABOUT Y AXIS). Compare that to actual moment. Not sure what the Zx is.. but you can find your Sx in your steel book, or figure it out the old fashioned way.

2. Calculate deflection by 22.5*[(wL^4)/(E*I), w in plf,L in ft, E in psi, I in in^4.

3. Its simply supported but your end support requirements depend on how you're supporting it. Don't just plan on laying it down on earth.

4. Buckling of the compression flangeS might play a role here as a failure mode. I'd be careful and hire a licensed structural engineer to provide a design. That beam is going to run you 5 figures, an engineer could probably provide a design for a small percentage of that.

 

[JedClampett]

HDPE is very flexible, so a steel beam is going to be stiffer than the pipe. But you're loading the beam in a very inefficient manner. I'd lace together two W27's and use that to support the pipe. For less weight, you could reduce the stress and deflection.
Also, HDPE has a very large thermal coefficient of expansion. If you don't accomodate that, you're going to be in a world of hurt. If it's exposed, I've seen it laid in a serpentine manner to allow for expansion and contraction.
Ditto VTEIT's advice. If you don't have a strucutral enginner on staff, you need to hire one, at least for this.

 

[beton1]

You can do as VT or Jed have suggested. Another option, use the beam you mention as a saddle and then have a proper designed beam
under it, conventionally placed(forming a Tee so to speak). You could then, for efficiency use pieces for the saddle (instead of whole length) if you are comfortable with the seating and support. Personally, I would use the approach by Jed, this gives a box type section, which is probably needed at this span.