CSP as a structural member 5

[rsmathew]

I'm a junior water guy -- does anyone have experience calculating failure modes of CSP as a structural member? I'm using CSP to protect/support a watermain across a bridge with piers spaced 4m apart. I wish to verify deflection in that span, and yield condition.

Do I use wl^2/8 for max moment and 5wl^4/384EI for deflection? Then how do I get my ultimate values?

Thanks,
Stan

 

[JStephen]

And what exactly does CSP stand for? Is there a particular standard for it?

 

[rsmathew]

Sorry -- Corrugate Steel Pipe -- the steel code doesn't cover it, as it is not typically used as a structural member. It is very convenient to use it though!

The CSP I'd like to use is found at:

http://www.big-o.com/catalogue/Culverts Drainage and Stormwater/Hel Cor.pdf

They give a moment of inertia, I'm just not confident in the process of calculation (Ultimate/Allowed).

 

[SlideRuleEra]

I have not looked thru this information, but perhaps the "Corrugated Steel Pipe Institute" offers guidance. Here is a link

http://www.cspi.ca/english/technical_publications.html

http://www.SlideRuleEra.net

 

[SlideRuleEra]

...or maybe the "National Corrugated Steel Pipe Association"

http://www.ncspa.org/shop.asp

http://www.SlideRuleEra.net

 

[cjd97]

With a moment of Inertia you could get an elastic section modulas Sx by dividing "I" by the radius of the pipe. Then you could check a service load condition, you will just have to assume some low allowable stress. Maybe 20ksi or something unless the corrugated steel pipe institute tells you the steel type.

Ultimate loads are a little tougher since you need Zx or the plastic section modulus. This value is derived through integration.

I would suggest a service check as they are normally more conservative.

 

[swearingen]

The bending and especially deflection formulas will be skewed because of the corrugations. You are not actually just tensile yielding the steel, but trying to elongate the corrugations. This puts the "outer fibers" in local bending as well. I would expect deflections to be much greater than with standard pipe of the same moment of inertia. One way to get around this is to obtain an E that is adjusted for this effect. The problem is, I would expect that E to be non-linear...

 

[UcfSE]

In addition to what swearingen stated, the corrugations will definitely affect the compression side of the pipe, I expect adversely. Even though you can find material strength and section properties, you won't know how the corrugations will affect the local buckling of the portion of the pipe under compression. It won't be as simple as sigma = M/S.

 

[cjd97]

What about trying to elongate or crush a finite piece of a corrugation like a plate of the same thickness? Those two forces could be used as a couple moment. No quick check is going to yield accurate results. Hopefully, conservative though.

There is always finite element analysis but I doubt you are wanting to put that kind of time in.

 

[csd72]

This is effectively pre-buckled pipe that you are talking about. it is not appropriate for use as a longutudinal member.

If you calculated the bending due to the eccentricity of the corrugations you would find this would be excessive, and that the pipe would fail at almost no load at all.

Have you ever seen those straws that bend at half way down their length? The only difference between the straight and the bendy bits is that the bendy bits have corrugations!

 

[Tmoose]

I think the water main will be supporting the CSP

 

[rsmathew]

If you check the link I supplied, it is manufactured in a spiral formation, hence it's not quite as "pre-buckled" as you think.

The supplier gives a moment of inertia in mm^4/mm, which I'm told is called the pipe wall moment of inertia. I'm additionally unclear of how to use this type of inertia (how to get just mm^4).

Thanks to all for responses.

 

[csd72]

I still would recommend against it.

How about using a circular hollow section as the actual water main?