Quick Rebar Size Selection 1

[Bedrock1977]

I am looking for an easy way to figure out rebar sizes for footing pads supporting large columns in an underground parking ramp. The dimensions of the pads have been determined - some are combined and others are individual. This is for a budget price. I need to make sure I have the proper bar size figured in.

There are two conditions

24" x 24" Columns supporting 500 to 2,000 kips.
32" x 32" Columns supporting 4,000 to 5,000 kips.

The net allowable bearing capacity is 15,000 psf

 

[jike]

Use CRSI tables for isolated footings.

For combined footings, check beam and punching shear and determine thickness required. Conservatively use M+ = wL1^2/8 for moment between columns and wL2^2/2 for cantilevered moment. L1 = distance between columns and L2 = largest cantilever. As approx. = Mu/4d. Use continuous top and bottom layers.

 

[csd72]

Allow for a lever arm of 0.8d, moment is easy to figure out.

2 minute calculation.

 

[JLNJ]

5000 kips with 15 ksf? Your footing is nearing 20' square and you're at 500'-k per foot of width.

Maybe you will need to consider something other than an isolated footing.

 

[RacingAZ]

Star for JLNJ. Check the soils report first. Geotech normally puts a limit to the amount of load on their isolated footing recommendations. You'll probably need to consider something else.

 

[Bedrock1977]

I was able to obtain the required bar sizes. Thank you for everybody's input. For those who are interested, here are comments from the GeoTech report for the project.

Spread Footings and Mat Foundations are recommended. The maximum net allowable soil pressure of 15 ksf bearing on medium compact to extremely compact fine to coarse sands may be increased by 1/3 for temporary transient loads such as wind and seismic loads.

Deep foundation systems utilizing drilled shafts are possible but difficult to design and construct due to the frequent cobbles and boulders present at the site and the highly variable relative densities of the deep sand and silt deposits. They would have to be constructed with full-length temporary casings to a depth of at least 100-ft.

The use of H-pile or auger cast piles are not considered to be feasible for the project.

 

[slickdeals]

Typically in order for a geotech engineer to make recommendations we provide anticipated column loads at a few locations on the project. Were such column loads provided for the study? As JLNJ indicates a 32x32 column on a 20' square footing is going to create considerable shear and bending moments.

 

[Bedrock1977]

Yes. The proposed underground parking garage supporting the future buildings will have typical interior column loads (service loads) of 4,000 to 5,000 kips and interior column loads of 1,500 to 3,700 kips. For the portion of the parking garage supporting the new street, typical exterior column loads are anticipated to be 1,000 to 1,400 kips. It is quite an impressive structure!

 

[hokie66]

I would look at using a mat rather than individual spread footings. Your footings will in some cases almost join anyway. In an underground application, you can use a working slab, waterproof membrane, then your mat.

 

[Ron]

15ksf allowable bearing capacity in sand is pretty generous. With such large footings and stress overlap, I agree with hokie66....consider a mat.

 

[Bedrock1977]

The design ended up with mat foundations. All of the larger interior and exterior loads are now supported by mats that are roughly 190-0 x 60-0 x 6-0 deep.

Thanks for everyone's input.