Is Frost Depth Always Required? 1

[JAX91]

A co-worker and I got into a debate yesterday about one of the foundations he designed. The foundation acts as a sleeper type pipe rack. It consists of an 8" thick by 2'-0" deep grade beam with a length of 7’-6”. At its worst case, the concrete sticks 1'-0" out of ground, which leaves only 1'-0" into the ground. Frost depth in the local area is 3'-0".

First off, I am concerned with stability issues. The foundation is only buried 1'-0", which leaves very little depth for passive pressure to develop. During its lifetime, it will have to withstand dynamic loads from wind, seismic, and thermal expansion. There is some compressor piping on the rack, so there may be some vibratory pressure loads as well. In reality, the pipe will likely keep the sleeper from overturning, but I don't like relying on pipe to brace a structure. The pipe is not a structural element, and we should not add any additional stress to it.

Second, the foundations come nowhere near frost depth, leaving them susceptible to frost heave. I'm concerned the foundation will heave and put additional stress on some of the flanges and valves. If they ever need to remove a valve, they may take it out without realizing the pipe has heaved. When they go to replace it, they will have a hard time getting it back in because the pipe ends are no longer aligned. There is also the issue of pipes moving and damaging equipment or causing leaks in a natural gas facility. I approached my co-worker with my concerns, and he said he is aware the foundations may heave. He thinks with the sleepers at a 15'-0" or 20'-0" spacing, heave will not cause any issues with the pipe. What he is failing to see is that the rack does not consist of just long, straight runs of pipe. Pipes are jumping on and off of the rack at various locations to attach to equipment. There are very rigid areas of pipe that will not have the flexibility of long, straight runs of pipe. My co-worker also made the argument that transformer foundations are typically not given frost protection, to which I responded that they are able to float because they are connected with flexible wire and not rigid pipe. He also made the argument that sidewalks are not extended down below frost level. I did not even respond to this one.

Am I out of line thinking that this foundation design is incorrect? Has anyone else every designed sleepers for pipe without giving them frost protection?

 

[msquared48]

Well, I think of one circumstance where I did not use it recently.

I had to design an extension for an outdoor equipment platform sitting on strip and spread footings that had to rest on the surface of the ground. The platform was within the copper wire gridwork of a large radio antenna, and the grid was placed just below the surface pf the ground. It could not be interrupted. The electronics grumped the structural here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

Is Frost Depth Always Required? No, not always. If you do not mind frost heaving, there is no need to bury foundations that deep. Retaining walls not attached to buildings, equipment pads, roads and sidewalks are examples of foundations which do not need to be taken to frost depth.

If frost heave would have an undesirable effect on the installation, then it would be prudent to extend the foundation to frost depth or to provide insulation to prevent frost from penetrating to the underside of foundation.

In my locale, frost depth adjacent to a heated building is usually taken to be four feet but in an open area, it can go as deep as eight feet or more. It is usually not practical to extend foundations to that depth, so insulation is often used to prevent heave.

BA

 

[msquared48]

Trumped...trumped...trumped...trumped.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

It depends on the ground water conditions and the soil types... Not always an issue, but, in these environs, can cause heave of several inches... we provide 6' for frost penetration...


Dik

 

[JAX91]

I have had cases in the past, too, where achieving full frost protection was not an option because of various circumstances. Sometimes you have to play the cards you're dealt. In this new case, however, there is no legitimate reason to not achieve frost depth. There is no underground, bedrock and water are deep, and a 3’-0” depth is not absurd. The only reason for not meeting frost depth is to dig a shallower hole and save a little money on concrete. On oil and gas jobs, the structural portion of the cost is small, so the money savings on the overall job is trivial. On this particular job, there are only eight sleepers total. We are saving digging an additional 2’-0” depth in eight trenches and less than three yards of concrete total! As a company, we would be taking on significant liability to save our client a negligible amount of money. I just don’t think it is worth the risk.

 

[ACtrafficengr]

Frost heave requires three things - cold, water, and frost susceptible soils.

Silts are the worst culprits when it comes to frost heave. Sands and gravels generally do not, since they don't have enough capilary action to draw water up from the water table. Clays have plenty of capillary action, but are also impervious enough that you usually don't get enough moisture to heave.

 

[SrVaro]

No, frost depth is not always required if what you are supporting can withstand the movement. However, for most pipe racks we have designed we have gone to frost depth. There have been a few short runs where the client did not want to go that deep. We informed the client of the potential heave and the possible issues (similar to you describing). However, I would never allow a 12" below grade bearing in an area of 36" frost depth. This is just asking for issues in the short term. 18" or 24" would allow for protection against all by the worst freezes.

Are you the Engineer of Record? If yes, demand the change. If no, is there a PM or other Senior Engineer that you two can use as a mediator? Our company policy is the E of R generally rules. Else, a third party is brought into the discussion. This works most of the time.

 

[ToadJones]

I have done some similar designs and we used two concrete piers with a Galvanized wideflange beam spanning just above.

http://www.eng-tips.com/viewthread.cfm?qid=336114

Seemed to make everything thing easier.

 

[BAretired]

Your reference leads back to this thread, Toad.

BA

 

[ToadJones]

Yea, I have no clue how that got inserted.

Sorry, I meant to say "I have done some similar designs and we used two concrete piers with a Galvanized wideflange beam spanning just above grade"

I'll add that in industrial applications, cutting corners on something like this is not a good idea.
The owners will want this thing up and running and without issues for as long as possible.
I have been at many chemical and petro plants during shutdowns for maintenance issues and I can tell that that money they lose while not operating far outweighs the initial costs to avert the shutdown.