Scaffold loading on pipeline casing

[benbern125]

Hello all,
We have a pair of pipeline canal crossings that need to be recoated. There are two casings, one 20" and one 24", approximately 5 feet apart. The plan is to hang scaffolding between the piping sections to complete the coating. The scaffolding weighs ~5000lbsmass all things included.
The 24" casing houses 3 pipes, a 5" (wt 0.258"), a 6" (wt 0.280"), and a 6" (wt 0.280"). The casing has a YS of 35000psi and the wall thickness is 0.375".
The 20" casing houses 2 pipes, a 6" (Wt 0.280") and an 8" (0.322"). The casing has a YS of 35000psi and the wall thickness is 0.375".
The span length between supports is 90ft.
I'm trying to perform stress calcs on the loading to determine maximum stress as well as maximum deflection. I initially calculated the stress and deflection for the 20" casing considering it is empty inside, but it causes a large deflection (order of ~5ft). I know there will be interaction between the pipes inside, but what I can't wrap my mind around is how to calculate the section modulus or the moment of interia for this scenario?
I'm following the general methods found in "Shigley's Mechanical Engineering Design" Chapter 3. Does anyone have any tips that could be used to determine the correct section modulus or moment of inertia?
Thanks in advance. I've gotten some great advice from these forums but always been a google lurker.


Ben

 

[JedClampett]

If you're trying to get additional strength from internal pipes, you're barking up the wrong tree. They'll be of very little help. It wouldn't be hard to calculate the preperties, but it would be tedious, and not add a whole lot of section properties.
I'd check you numbers for a unit mistake. It's easy to make (we've all done it) when doing deflection calculations. I did a rough calculation for a 24 inch pipe filled with water spanning 90 ft and got 7 inches. You got about 12 times that much, and that indicates maybe using inches when you wanted feet.
On the otehr hand, 90 feet is a long span for a 24 inch deep member.

 

[benbern125]

Jed, thanks for such a quick reply. I reviewed and didn't see any major mistakes, but technical toolbox is agreeing with you. My concern with it was I'll watch them while they're open laying 20" pipelines, and it looks like spaghetti when the excavator picks it up.
Bad part about this is I work for a company that is light on engineers, so there's not much senior help available.
Wish they would just accept my first answer of using a big man lift instead of scaffolding.
Thanks again Jed!

 

[JedClampett]

Scan in you calculation and we'll review it.
The good news is that the casing will slip off the supports and fail way before it deflects 9'-0".