Spread footer or pad under footer for wall point loads 2

[LJordan316]

I am working with an architect here in central Florida who insists on using large reinforced concrete pads under standard 12"x20" monolithic footers for high point loads on exterior CMU walls. Vertical reinforcement is provided between the bond beam and footer - 3,000 PSI concrete filled cells with(1)#5 rebar for each 5,000 lbs of reaction load. The vertical reinforcement is extended down into the pad.

I am accustomed to using spread/bell footers under the point loads with the width determined by the magnitude of the point load and the depth at the standard 20". The load bearing capacity of our sandy soils is typically 1,500 to 2,000 PSF, but tests are seldom done to verify the capacity for residential construction.

Any thoughts that you have regarding the need for a pad under the standard footer vs just spreading the width of the footer would be appreciated.

 

[msquared48]

Normally, the strip footing is enlarged to take care of the additional load if the strip footing cannot handle it. What the Architect is describing is used in an underpinning situation where the strip footing is already poured and the capacity of the strip footing is insufficient to take the load. Such is not the case here.

What he wants to do is also requiring two separate pours - more expensive and time consuming for the contractor and owner.

I would politely tell the Architect to do the architecture and let you do the structural engineering. That's the expertise he is paying you for. He should listen to the advice if he is smart. I know. I'm asking for the moon.

Mike McCann
McCann Engineering

 

[hokie66]

With strip footings and spread footings in sandy soils, it is generally better to have them all founded in pretty much the same horizontal plane. So I think it preferable both for structural reasons and for constructability to cast the enlargement monolithically with the strip footing.

 

[shin25]

Larryjordan,

You have not mentioned, why the architect is asking you to provide this enlarged pad?

 

[csd72]

Tell them that is okay as long as you get some input into their architectural decisions!

csd

 

[civilperson]

Spend more effort in compacting the sands under your foundations. Couple of hours with vibrating plate or vibrating roller will be more effective than adding a reinforced mud slab for sub-foundation. Do your really use concrete to fill the cells? What methods do you use for concrete filling?

 

[LJordan316]

Regarding soil compaction: we do call for 95% compaction beneath footers as well as slabs on grade. However, the engineer is not involved in construction of the homes. I doubt the builders do anything other than drive their trucks and equipment over the soil before they start to dig the footers. The county inspectors don't do their first inspection until the footers are dug.

Regarding filled cells: In the 110 MPH wind load areas (most of central Florida), we call for: Vertical reinforcement 6’ o.c. max, 3000 psi conc, (1) #5 w/10” std hook, 25” min overlap from footer to bond beam and located at all corners, openings, beams, and girders.

 

[UcfSE]

I wasn't too clear on just what you're saying. You're saying the architect wants you to use a concentric spread footing under the thickened edge monolithic footing, top of the spread ftg at the bottom of the thickened edge? What is it you normally spec? The bell footing sounds normal for central Florida (I'm there too) but I'm not clear on what you are describing for the other case.

 

[LJordan316]

See attached diagram (Uploaded)of the pad-under-footer design at:

http://files.engineering.com/getfile.aspx?folder=7757f5c5-2dd3-4b2e-8d50-1aa00d6cda83&file=Footer

- Pad Under Footer.jpg

My concern is that a lot of the point load is distributed to the regular 12"x20" footer rather than the pad beneath. I like the pad design when it is used as a stiffner under a floor slab to aupport a column because there is no footer that absorbs the load.