Spread Footings or Raft Slab ??? 2

[4wilmar]

We are currently designing and constructing several low rise (2-3 floor) commercial office buildings (all reinforced concrete construction). We have relatively good soils (5000 psf bearing capacity) and our geotechnical engineer has recommended conventional spread footing foundations. However, when considering the time and costs to excavate and construct individual footings, pedestals, grade walls and then backfilling and compacting the same with small equipment before placing a slab on grade, would it not be less expensive and much faster to just place a thick raft slab in one pour??? My old text book says a mat (or raft) foundation should be considered when conventional spread footings cover more than 50% of the building footprint, but does the same still hold true in today’s market, especially if the construction schedule is really tight? Any comments or advice would be greatly appreciated.

 

[SlideRuleEra]

Because of the tight schedule all of the footings would most likely have to be placed at one time - a lot of forming material & labor to expend, without being able to reuse any of it. I say that factor tips the economic balance in favor of a mat foundation.

http://www.SlideRuleEra.net

 

[BigH]

I once saw a side-by-side comparison of a Wendy's and a Pizza Hut construction(if I remember correctly the food shops). The latter used short drilled caissons (600mm dia or so) with grade beams on top which were formed below outside grade by a backhoe and then the concrete poured neat against the soil excavation wall - then they poured their floor on the grade beams. The former used standard footings with, as you described the excavation, forming, etc. The traditional way was some 6 weeks ahead of the short caissons - but all time difference was made up easily and Pizza Hut opened first. Contractor put all the caissons in in something like 1.5 days. This might be better than heavy slab - and the slab would need to be founded below frost line (or thickend to do so - or use of geofoam insulation).

 

[TDAA]

That is really going to depen on the contractors in your area, and the cost of concrete in your area. It has gone up quite a bit in this area.

Talk to you geotech about design parameters. Also, if you column loads are so high that you will be >50% coverage at 5000 psf, then look at the thickness of your slab to distribute those loads.

 

[Zambo]

Are the floor loadings for Wendy's and Pizza Hut the same? From my studies I would say that Wendy's customers are considerable heavier than the Pizza Hut customers (although these are by no means to be considered as light weights) It's likely that by serving salads and healthy snacks a significant saving could be made on rebar and concrete - a further loading reduction/cost saving could be gained by the reduced numbers of customers attracted by this kind of low fat fare.

 

[4wilmar]

Thanks for your comment Zambo
We do indeed have a few heavy weight clients who are apparently junk food junkies, however most of our clients have big egos and small brains.

 

[oldestguy]

What about plumbing under the floor? When is is scheduled? Are the plumbers going to mess up your support? Getting them to compact to your bearing requirements may not be easy.

Will you ever make changes to that piping system, requiring getting under the floor to change the layout for new equipment? or fix problems?

Is there any parking underground and is there a drain system under the floor? What if it malfunctions?

Once that raft is in, you have major problems getting under it.

Is there any "stupid" local code requiring "frost walls"? Are the inspectors inflexible?

Is there a potential problem for frost heave for parking areas left open to the cold? If so, do you insulate? I'd rather insulate under a slab than under a raft. Not as fussy.

Look down the road.

 

[dik]

You might discuss this with your geotechnical engineer... with a little tweaking, you should be able to coordinate your footing pour...

Dik

 

[4wilmar]

We are in the tropics here, so frost heave is not an issue, but I have given some thought to plumbing and utilities which would have to be placed below the raft slab and come up thru conduit tubes or box-outs. I thought we might trench below slab level and backfill around the utilities with gravel. All piping would have to be tested before casting the raft slab as there would be no 2cd chances.

 

[slipstick50]

spread individual footings and foundations can be designed to be accurately place bearing loads on the soil and control footing/foundation flexing..

With raft foundations, the foundation mat areas are so far from loads between walls and columns that is difficult to control cracking, deflection, and differential movement unless the raft is very thick at times with lots of reinforcing,, even then load distribution is difficult..

 

[kslee1000]

For mat foundations, utility trench equipped with sump or duck bank should be considered. Place bare piping beneath the concrete slab seems inviting trouble ahead.

 

[4wilmar]

Kslee
Do you mean an open trench large enough for personnel access?

 

[kslee1000]

4wilmar:
Large enough room for ease of utility replacement, repair..etc. You might consider placing grate or cover plate on top of trench.