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					Estimating Foundation Loads




It doesn't have to be a guessing
       game anymore…
                                   Estimating Foundation Loads
                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
        the foundation is composed of dead load and live load components. Lets look at the
        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.
                                                                                                              =
                                                                                Dead Load on Foundation        6000 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.

                                                                              Dead Load on Foundation           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
          of your building, calculate the live snow load on your
          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.

(Dead Load of Roof)    6000 pounds + (Dead Load of Walls) 4800 pounds + (Live Snow Load) 6000 pounds = 16,800 pounds

                                                                                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

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

            1. Dead load from floors, and roofs.

      (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.
                                                                                                                     =
                                                                                       Dead Load on Foundation _____       lbs.




            2. Dead load from walls.

      (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.

                                                                              Dead Load on Foundation         _____ lbs.




            3. Live load from snow.

   (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.

(Dead Load of Roof) _____ pounds + (Dead Load of Walls) _____ pounds + (Live Snow Load) _____ pounds = _____ pounds

                                                                                Total Load on Foundation         _____ pounds
   5. Load per footing

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

                                                                    Load on Each Footing        _______ pounds

				
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