Support for ductwork

[aalmeter]

I am a beginning civil/structural engineer (in training) and have been given the task of designing supports for some large ductwork.

The ductwork will be supported by a pier foundation and a coulumn extending a few feet above grade to the duct itself.

My question is how do I calculate the lateral forces from the material moving through the pipe? I have no data on hand regarding the material properties (its a grain by-product) or the velocity or flowrate, but Im sure I can pick some of that info up from the equipment manufacturers (I'm off today).

The ductwork ranges from 24" to 36" in dia.

Thank you.

 

[BigInch]

If its moving in the direction of the pipe, all forces from acceleration and decelarations will not be in the lateral direction. I'd say don't worry about that and concentrate on weight of pipe, its contents (during possible hydrotest and during operation), weight of pipe coating (if something other than paint), maintenance loads, snow, ice, wind and earthquake and whatever else might hang itself up there.

If the pipe is just sitting on top of the "T" support, then consider that a thermal load might occur when the pipe changes temperature. When that happens, look to have a horizontal load applied to the top of the "T" support (in the pipe's axial directions) = friction coefficient between pipe and support materials X maximum gravity load from any case above.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[aalmeter]

Thanks BigInch.

I believe it was in one of your posts regarding a pipe rack design that I read of a typical factor of 0.3 being used as a ballpark for any lateral forces a pipe may generate. I am assuming this is 0.3 x (downward force).

The reason I am asking is that if I consider a lateral load of 0.3 x (downward) force in designing my supports, it adds a fair amount of concrete to the piers.

I'd rather be safe than sorry, but Id also rather not be using an unnecessary amount of concrete.

The supports are, essentially, a W6 (we have a lot of them on hand) or tube steel t post bolted to a concrete pier. Obviously, these are exterior supports.

 

[RossABQ]

I think the .3x is meant to cover seismic loads as well.

 

[BigInch]

Yes. 0.3 is a default friction coefficient between steel on steel, so it is true that 0.3 x Max Weight serves for setting a maximum possible lateral load due to thermal, wind and seismic loads for any pipe resting on its support (no u-bolts, guides clamps, etc.). It is true it could actually be less of a load than the friction coefficient x weight, but then you'd have to calculate the exact load to know rather than using the default maximum.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[JohnBreen]

Aalmeter,

I think that you are misinterpreting some of what you read in previous threads. We have used a 0.3 coefficient times the weight at the support point to calculate the lateral sliding static friction which RESISTS THE MOVEMENT of the piping/ducting that is supported on sliding supports (for example, the sliding friction resists movement due to thermal expansion of the piping/ducting). The lateral "friction forces" would be transferred into our supports and the supports would have to be designed to accommodate these loadings as well as all other loadings.

Do not confuse this with lateral forces on the ducting that would be caused by internal flow impinging on the inside of the ducting at changes in direction (that would also tend to cause the ducting to slide against the resisting friction). As you point out, you will have to have a lot more information to calculate the flow impingement forces acting on the inside of the ducting.

Another important issue that we often discuss here is the loadings on the pipe/duct wall. Ducting is typically thin wall relative to standard piping and transferring the loadings (dead and live weight, lateral forces, etc.) from the duct to the support slide plates and on into the ground must be considered as a whole. At the point of support you may need to have "stiffening rings" to prevent the ducting from deforming under loading.

The first thing you will need to do is get a copy of the ASCE publication "The Structural Design of Air and Gas Ducts for Power Stations and Industrial Boiler Applications". It may seem to "not quite be your application" but it is full of good duct design information. Similarly, look at the SMACNA duct design manuals.

None of this addresses your initial question of "...how do I calculate the lateral forces from the material moving through the pipe?". In that you are asking about "Fluid Structure Interaction" and this might help you get some background:

http://www.win.tue.nl/fsi/

Regards, John



Regards, John.

 

[JohnBreen]

.....By the way. The piping Codes DO NOT allow "taking credit" for friction damping in seismic analyses. This is due to the considerable vagaries in the way friction acts - support friction is not a nice neat continuous effect. The forces build to the point where the static friction is overcome and then the pipe moves in a "jump". Once the support is sliding it will move to a point at which the force has decayed to be less than needed to move the pipe. So the pipe moves in "fits and starts" and there is some "black art" involved in this. That "highest" force needed to initially move the pipe support against the resistance of friction is you "design force".

You might also get some interesting views on support from the vintage papers by L.C. Peng:

http://www.pipestress.com/Pages/PEpapers.html

Regards, John.

 

[DrRTU]

The firm designing the duct should have the SMACNA guides Round Industrial Duct Construction Standards and Rectangular Industrial Duct Construction Standards. Section 13 in the round standard should help.

http://www.smacna.org/bookstore/ind...sults&topicID=4&cfid=6060787&cftoken=42380747

They also offer RIDCS software.