Topic: Challenge: Foundation Pad

[CAB]

Create a LISP or other language program to calculate the size of foundation pad needed for a given load.
Extra credit for calculation the weight of the 12" thick pad.
You may also do this using metric if you like.

PS I'll be out of touch for the next 4-5 hours.

[mkweaver]

Code: [Select]

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;  Procedure:    FoundationPad ;;;
;;;  Purpose:    Calculates the required area of a spread footing and the width ;;;
;;;    of a square footing with that area. ;;;
;;;  Arguments:    AppliedLoad - real, the load applied to the footing, lbs ;;;
;;;    SoilCapacity - allowable soil bearing capacity, lbs per square ;;;
;;;    foot ;;;
;;;  Returns:    List with the following: ;;;
;;;      car = the required area for the footing, square feet ;;;
;;;      cadr = the required dimension for a square footing to produce ;;;
;;;        the required bearing area, feet ;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(defun foundationpad(AppliedLoad SoilCapacity / )
  (setq
    NetSoilCapacity (- soilcapacity 150)
    ReqArea (/ AppliedLoad NetSoilCapacity)
    ReqSquare (sqrt ReqArea)
    )
  (list ReqArea ReqSquare)
  )

[CAB]

Looks like you are going to win first place.

I see you use 150, I use 20% as the safety factor which works out to 400# in this example.

[Keith™]

Well, I was going to post my solution, but it went way beyond figuring a simple footing ...
The spreadsheet I use (sorry but I am not allowed to post it) uses allows the user to specify these loads in live/dead PSF:

• Tributary roof loads
• Tributary floor loads
• Snow loads
• Point loads

Then it allows the user to enter the following for the footing:

• Soil Bearing Capacity
• Concrete strength PSI
• Steel Tensile Strength PSI
• Depth of rebar cover

The user can specify various items by default as well

• Footing size (x,y)
• Footing depth (z)
• Rebar size
• Number of rebar
• Smallest bearing plate
• Depth of footing below grade

The footing is calculated based on the input parameters for size and number of rebar, size and depth of footing, depth of rebar cover.

Loads are increased if the user selects a checkbox to do so. Liveloads are increased by 1.7 and dead loads by 1.4

Loads are increased/decreased based on the depth of the footing taking into consideration the backfill on the pad.

I have been working on converting it to lisp and vba over the last year or so so that I could enter the information (or better yet select the location in the drawing) and have it automatically calculate and draw it for me.

If I get it completed, I'll post it.

[CAB]

Looking forward to your routine Keith.

[Joe Burke]

Hi Alan,

FWIW, my first thought after looking at your example was similar to Keith's. The routine should at least ask for live and dead load values. Live load meaning pounds per square foot added to the structure. Dead load meaning the weight of the structure itself.

I use these terms in a simplistic fashion since I'm not a structural engineer.

[CAB]

Could you give me an example of how breaking out the live load & dead load would change the footprint of the Foundation Pad?
I'm not an engineer of any kind. 

[Joe Burke]

The typical live load given residential construction is 40 pound per sf at the first floor and 30 psf at the second floor.

I think this points to the fact what you are trying to do in your example is too simplistic. That's not meant to imply the problem cannot be solved codewise. It just needs more user input variables to solve the problem in a meaningful fashion.

[CAB]

Thanks Joe.
Where I live the FBCR separates out the live loads as shown in the table below.

This was meant to be a Calculation Challenge or LISP exercise and not an engineering solution.
I just choose this example because it was somewhat simple. That's why I used Total Load and
not breaking them out. If you wish to follow the load path in a structure you will quickly see
that there are too many variables to incorporate in a simple lisp as opposed to a complex routine.

For example a two story home might have a load path that included tributary area from the roof,
plus tributary area from the 2nd floor, plus tributary area from the 1st floor, plus point load from
a beam. All ending up on a CC Pad. I find that calculation the loads on each level with a load path
 to transfer the load down like a wall or column works for me. Then calculating the next levels load with the
load from above if applicable and again finding a path to take the load down. Eventually ending up
at the foundation or Footing Pad. So I use a some lisp to calculate tributary type loads & hand calculate
column & wall loads with all information placed on a non-plotting layer.

So you see I like to work level by level as opposed to Keith who apparently like to work vertically.

Wow, somehow I got carried away here. Sorry.

My point was that this was a simple lisp exercise.

My question to you is: Does applying the Live & Dead Load separately result in an appreciably different size Pad?

[Keith™]

Live load is live load period .. unless you are including a roof live load. Typical duration calculations for a standard living area live load is 100%, live loads (and snow loads) for a roof is 115%, meaning that a footing designed entirely for a roof support theoretically can be smaller while supporting the same amount of load as a floor. Almost nobody does this for footings, although they do it for beams and framing members regularly.

[Joe Burke]

Alan,

Given it's just a LISP exercise, I agree there's no reason to consider live and dead loads