Underground Mech'l Duct

[RobStruct]

I have a job in Dallas, TX. Interior slab is on grade. Some how the architect has brewed up the idea of running mech'l duct work under the building footprint to AC the building. This building is very much like a storage warehouse in that there will be large vehicles driving around the slab-on-grade. I've designed a cast-in-place below grade trench to host the duct work and a thickened slab over the top that will support the vehicle loading. The contractor is wanting to get rid of the cast-in-place trench and simply bury the mech'l ducts underground. My concern with this is the slab loads + soil loads will crush the conduit.

Are there any design guides out there speaking to load distribution under a slab-on-grade vs. depth below grade?

Does anyone have alternative solutions to protecting below grade mech'l ducts?

Thanks for your help.

 

[JedClampett]

There are tons (literally) of articles and literature available on buried pipes. You're going to have to break the news to your genius architect that these will be designed like buried pipes, not thin gage ducts. But if he or she thinks they can get away with 24 gage ductwork, you might as will fill them with dirt right now, because they will have dirt in them sooner rather than later.
It's not that bad to use pipe, but I suspect it will shock them.

 

[BAretired]

I have seen peripheral ducts encased in cast-in-place concrete. One problem which occurred in more than one occasion was a high moisture content in the air. The ducts behaved as large diameter weeping tiles but were not graded so water collected at the low points. Air flowing through the duct picked up water at these locations. Structurally, it worked fine but mechanically, it was a disaster.

BA

 

[JedClampett]

Does your architect know that cool air sinks? It seems that you want the AC up high to take advantage of this.

 

[dhengr]

To tag onto BA’s comments, you have to know that you can keep them dry or they become mold factories and a real health problem. Any ground water problems just add to the problem, they almost become a drain tile system. And, tipping the hat to JedC light duct work under an industrial slab on grade is a problem waiting to happen, without special detailing. At the same time it is pretty easy to design the roof structure for those mechanicals.

 

[hawkaz]

Don't forget about your expansive soils in Dallas. That slab is probably going to move all over the place- make sure you account for vertical movement in this duct and any other underground utilities that are tied in hard.

I would guess that if you have slab on grade construction, the geotechnical engineer probably warned of at least 2"-3" of vertical slab movement due to expansive soils


I had a north-east architect try that on a west coast job once. After that suggestion, he wasn't the architect on that project for long.

 

[hokie66]

The ducts either have to be pipework, precast trench sections, or cast in place trenches as designed. Sheet metal, no.

 

[RobStruct]

Thanks for the feed back. To touch on a few items brought up. The trench will be water proofed and possibly filled with a flowable fill to eliminate the risk of ponding water. The Texas soils are very expansive, however they will be conditioned appropriately resulting in a maximum PVR of 3/4" or 1", which still could be a problem where the duct penetrates the slab and enters the building. However the trench and slab are both on grade...if the trench is moving so is the slab...so the connection between duct and slab should not be a problem.

The flowable fill would also serve as formwork for the slab above, however we are back to who is going to design the duct for flowable fill loading.

I am recommending using non-composite deck as formwork for the slab and suspending the ductwork. Perhaps add a sump pit for ponding water issues?

Thoughts?