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Bottom Reinforcment in Footing Under Load Bearing Wall ???

Bottom Reinforcment in Footing Under Load Bearing Wall ???

Bottom Reinforcment in Footing Under Load Bearing Wall ???

(OP)
I am designing a raft footing for light gauge steel building (thickness 220 mm). I am placing the top reinforcement continuously in whole slab but I want to place the bottom reinforcement only where i have the load bearing wall (as shown in below sketch) so my question is that to calculate the bottom reinforcement I designed this as strip footing and 750mm width of strip is enough according to my calculation , so accordingly I am placing the bottom reinforcement. Does this way is correct to calculate the bottom reinforcement in raft footing???

RE: Bottom Reinforcment in Footing Under Load Bearing Wall ???

You should design it as slab on grade. See Ringo book, us army corps of engineers manual or many many previous threads here on this topic

RE: Bottom Reinforcment in Footing Under Load Bearing Wall ???

The slab-on-grade Ringo book probably doesn't deal with concentrated linear loads from walls...only forklift/wheel loads and rack post loads as I recall.

You can do what you suggest but I would say that your 750 mm length might be a bit too short. The compressibility of the soils will affect how much negative or positive bending you might have and how far the inflection point in the slab will occur away from the wall. Just looking at the drawing you presented the bottom bars appear too short to me.

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RE: Bottom Reinforcment in Footing Under Load Bearing Wall ???

Thoughts:

1) Decide what you want this thing to be structurally. Is it a slab on grade with compacted fill etc? Is it a raft slab poured directly on a bearing soil stratum? Is it just strip footings with SOG between? What drove the decision to use this particular system in the first place?

2) From a purely strength perspective, I'd say that you're probably fine with what you've suggested. You may, however, see some significant bottom side cracking due to the stuff that JAE mentioned. Could that cause corrosion/durability problems in the environment in which the raft will be installed?

3) If it were me, at a minimum, I would detail the bottom steel extensions to reflect how we typically detail one way suspended slabs. With soil pressure dominating your loading, you'll never be able nail down the inflection points with much accuracy but, at least this way, you've got a fighting chance of having it covered.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bottom Reinforcment in Footing Under Load Bearing Wall ???

Agree with above - and per 3) above be sure to extend your reinforcement beyond the inflection point per your applicable concrete code/specification.

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