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- Measure how far the porch roof sticks out from the wall, D = _________
ft.
- Measure the width of the porch parallel to the house wall, W = _________
ft.
- Count the number of columns supporting the roof N = _________
(whole number).
- Multiply the distance the porch sticks out from the wall by the width
of the porch. D x W = _________ (this is the area of the porch roof exposed
beyond the wall of the house).
- Divide this number by two and by the number of columns along the
edge of the porch roof that is farthest away from the wall. This gives
the average number of square feet of porch roof that apply uplift loads
to a typical edge column. (D x W) / (2 x N) = A = _________.
- Select the appropriate net uplift pressure (wind pressure minus weight)
for the design wind speed at your house from the table below, P = _________ psf.
- Multiply the net uplift pressure times the typical area, P x A = _________ pounds.
This is the uplift on each column, on the connection at the top of the column,
and also on the connection at the bottom of the column. If the column is
heavy (e.g. concrete or masonry) then you can reduce the force on the connection
at the bottom of the column by the weight of the column. Make sure that you
have enough weight in the foundation or floor system that can be logically assigned
to the column to counteract the uplift forces. Otherwise, you may just fail
the foundation.
Uplift Pressure Table
Wind Speed
(mph) |
Uplift Pressure
(lbs per sq. ft. = psf) |
Weight of Roof
(psf) |
Net Uplift Pressure on Roof
(psf) |
| 110 |
32 |
10 |
22 |
| 120 |
38 |
10 |
28 |
| 130 |
44 |
10 |
34 |
| 140 |
51 |
10 |
41 |
| 150 |
59 |
10 |
49 |
Example Calculation and Observations: A porch is 8 feet deep and 25
feet wide with 4 columns along the outside edge of the porch. Consequently:
- D = 6 ft
- W = 25 ft
- N = 4
- D x W = 200 sq. ft.
- A = 100/4 = 25
If the design wind speed is 130 mph, the net uplift pressure on the roof is:
Then P x A (the average uplift on each column) is 850 pounds. You can pick an
appropriate strap or connectors from the manufacturer's literature. Generally,
the small metal straps are good for around 400 pounds each so two small straps
would get you pretty close and three would be more than enough. The larger
straps are generally good for 800 to 900 pounds so if a larger strap were
found, it would likely be adequate. The threaded rod, if properly anchored in
the foundation can resist several thousand pounds (provided you have a
big enough foundation - concrete weighs about 125 pounds per cubic foot).
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