horizontal tank support

[JJEL]

Hi guys, not sure if this is the right forum or not but...

I'm designing the foundations to support the saddles for a 30,000 gallon carbon steel horizontal storage tank (30' between saddles). I'm told that we (my company) typically design these supports utilizing teflon slide bearing pads to allow for expansion/contraction of the tanks. This particular tank is not under pressure and the contents are at ambient temperature. Here in Mich we get swings of 100 deg +/- over the course of the year.

expansion will be .0000065*100*(30'x12)=.234".

1) Is the above formula for expansion of free ended members applicable to a closed end tank (I picture the heads at each end somewhat restraining the expansion)?

2) If so, I don't think this amount of movement is substantial enough to warrant providing slide bearings.

Thoughts?

 

[kslee1000]

How much is the saving?

 

[doct9960]

JJEL,

0.0000065 in/in/°F is the mean coefficient of thermal expansion in going from 70°F to indicated temperature. You should deduct 70°F from 100°F in your calculation...

expansion = 0.0000065*(100-70)*(30'x12) = 0.0702"

 

[jike]

I don't agree with a 30 degree swing. It may be installed at 70 but then it could get to be 30 degrees below zero some winter. I would still use 100 degrees.

 

[doct9960]

Sorry for the confusion. I misread the "swing" in the original post.

Yeah for winter the tank will surely contract. For -30°F, the contraction will be -0.234"

 

[JJEL]

Wow. You guys are quick!!

Doct-you might be right, but I've always used the overall change in temperature. Conservative if nothing else....

Not sure what these things cost, I've only used them once before on a beam bearing plate. Here, I've got 3 tanks. So 3 saddles would have the slides. Saddle base plate is 11" wide x 11'-0" long. It's probably not a huge add in cost, in fact it would probably be miniscule relative to the overall project cost. I'll probably end up using them becuase may boss says so. I've done similar tanks in the past at my previous job and never used slides and never heard of any problems.

I just know I'm right .....

 

[kslee1000]

The other option is to pay the price to design the saddles as cantilever supports, with H = V tan(phi). What is the friction coefficient for steel on steel, steel on concrete? Go figure it out.

 

[sundale]

Design your support piers for the LARGER longitudinal force of:
- static friction at slide bearing at the free end
- seismic base shear at the fixed end
Friction at the free end will likely control in Michigan.

Teflon can work outside ($$$) and so can greased steel slide plates. I have also used elastomeric bearing pads, as used in precast concrete bearings and bridge bearings. These pads have a known shear modulus and the shear force can be calculated if you know the thermal elongation.

I would suggest adding 30 degrees for solar gain on bare steel if your tank is not insulated. I concur with your pessimistic (and conservative) delta T assumption; you never know for certain if the tank will be set in the middle of winter or the summer.

 

[kslee1000]

H = V tan(phi) maybe is questionable for this case (temp effect). For 3 saddles, assume there is no movement in the center, divide the elongation/contraction equally to each end saddle. Then the saddle would have to prevent the free movement by friction. Wow, my head starts to spin

 

[JStephen]

"Is the above formula for expansion of free ended members applicable to a closed end tank"- Yes, or to an open-ended pipe; it's just the expansion in the shell itself. If the tank was pressurized, you'd get additional movement due to that.

Wouldn't the supports also be cooler if the temperature drops 100 degrees? That would offset much of the differential change.

 

[kslee1000]

JS:

I don't know if this comparison valid or not - it is similar to piping system with restrains. When temp changes, the pipe moves, but the supports move less. Then we hear noises due to expansion, or contraction, and find damaged supports with passing time.