Spread Footings or Raft Slab ???
Spread Footings or Raft Slab ???
(OP)
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.





RE: Spread Footings or Raft Slab ???
www.SlideRuleEra.net
RE: Spread Footings or Raft Slab ???
RE: Spread Footings or Raft Slab ???
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.
RE: Spread Footings or Raft Slab ???
RE: Spread Footings or Raft Slab ???
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.
RE: Spread Footings or Raft Slab ???
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.
RE: Spread Footings or Raft Slab ???
Dik
RE: Spread Footings or Raft Slab ???
RE: Spread Footings or Raft Slab ???
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..
RE: Spread Footings or Raft Slab ???
RE: Spread Footings or Raft Slab ???
Do you mean an open trench large enough for personnel access?
RE: Spread Footings or Raft Slab ???
Large enough room for ease of utility replacement, repair..etc. You might consider placing grate or cover plate on top of trench.