# calculate square feet formula

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```					Estimating Foundation Loads

It doesn't have to be a guessing
game anymore…
As you know, the purpose of the foundation is to provide a stable base for the
building. It does this in several ways. The foundation anchors the building, keeping it
from moving. It also eliminates heaving from ground movement. Finally, it controls
settling that can lead to structural damage. As we design foundations, we are most
interested in the third factor.
In order to determine the characteristics of the needed foundation, we first need to
be aware of the load that the building will place on it. The load that a building places on
following example.
The building pictured at
the left is a simple shed supported
by one footing at each corner. To
engineer footings for this
building, we first need to
determine how much weight it
will apply to the ground. We can
do this by filling in the following
chart.

(Area of roof)   200    sq. ft. X (weight per square foot) 30 lbs. Per sq. ft. = (Total weight of ceiling) 6000 lbs.
+
(Area of 2nd Floor)    0 sq. ft. X (weight per square foot) 50 lbs. Per sq. ft. = (Total weight of 2nd Floor)   0   lbs.
+
(Area of 1st Floor)    0 sq. ft. X (weight per square foot) 50 lbs. Per sq. ft. = (Total weight of 1st Floor)   0   lbs.
=

In this case, the structure does not have a second floor, and the first floor is slab
on grade, so it does not bear on the footings. The only weight is from the ceiling and roof
framing. This can be estimated at 30 pounds per square foot. Taking the dimensions of
the building 20' x 10' we find that the roof has an area of 200 square feet. Multiplying
this by 30 pounds per square foot gives us 6000 pounds. This means that the roof of the
building weighs 6000 pounds.
Now we need to calculate the weight of the walls. Walls are commonly estimated
to weigh 10 pounds per square foot. To determine the area of a wall, multiply its length
by its height. Once the areas of all of the walls in the structure have been added up,
multiply by 10 pounds per square foot. This will give you the weight of the walls. In our
example:
(Length of walls) 20'+10'+20'+10' = 60 linear feet of walls

(Area of walls) 60 linear feet X 8 feet tall = 480 square feet

(Weight of walls) 480 sq. ft. X (weight per square foot) 10 lbs. Per sq. ft. = (Total weight of walls) 4800        lbs.

The Wisconsin Uniform Dwelling Code states
"Roof structural members subject to snow
accumulations shall be designed for all of the following
load distributions: 40 pounds per square foot in zone 1,
and 30 pounds per square foot in zone 2" Observing
the map to the right, you can see that we are clearly in
zone 2. Using this information and the square footage
roof.

(Area of Roof)    200    sq. ft. X (weight per square foot) 30 lbs. Per sq. ft. = (Total weight of snow)    6000     lbs.

Live Snow Load on Foundation           6000 lbs.

Putting it all Together - To engineer our foundation, we now need to total all of the live
and dead loads acting on the building. In this case, it would look as follows.

Total Load on Foundation          16,800 pounds
There is one last step as we prepare to specify footings. We need to take the foundation
load and divide it by the number of footings we will be using. This is making the
assumption that all of the footings will bear the weight evenly, which is acceptable for
simple structures. For our shed, our calculation would be:

(Total Load on Foundation) 16,800 pounds      (Number of Footings) 4   = (Load on Each Footing) 4,200 pounds

Load on Each Footing        4,200 pounds

NAME__________________
Directions: Use the formulae below to calculate the total load that your building will
place on each footing.

(Area of roof)      ____sq. ft. X (weight per square foot) 30 lbs. Per sq. ft. = (Total weight of ceiling) _____     lbs.
+
(Area of 2nd Floor) ____sq. ft. X (weight per square foot) 50 lbs. Per sq. ft. = (Total weight of 2nd Floor) _____   lbs.
+
(Area of 1st Floor)____ sq. ft. X (weight per square foot) 50 lbs. Per sq. ft. = (Total weight of 1st Floor) _____   lbs.
=

(Length of walls) ____' + ____' + ____' + ____' = _____ linear feet of walls

(Area of walls) ____ linear feet X ____ feet tall = _____ square feet

(Weight of walls) _____ sq. ft. X (weight per square foot) 10 lbs. Per sq. ft. = (Total weight of walls) _____lbs.

(Area of Roof)      _____   sq. ft. X (weight per square foot) _____ lbs. Per sq. ft. = (Total weight of snow) _____lbs.

Live Snow Load on Foundation         _____ lbs.

4. Total combined load on the foundation.

Total Load on Foundation         _____ pounds

(Total Load on Foundation) _____ pounds    (Number of Footings) _____   = (Load on Each Footing) ______ pounds

Load on Each Footing        _______ pounds

```
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