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Loads and Load Paths:
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This picture is intentionally upside-down to demonstrate that keeping a house together in a hurricane requires a continuous load path from the roof to the foundation - think about it as trying to keep all the parts of the house connected if you turned the house upside-down and shook it.
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Wind resistant homes are not only built to hold up their roofs but to also
hold the roofs down. The weight of the roof is typically about 7 to 10
pounds per square foot (psf); but, the uplift pressures averaged over the
roof in a strong hurricane can be four to five times that amount. On some
areas, the uplift loads can be 10 to 15 times higher than the weight (70
to 130 psf). The uplift at the wall connections are magnified by the size
of the roof because the pressures act on every square foot of the roof.
For example, if the average uplift pressure on the roof is 50 psf, the weight
of the roof is 10 psf and the roof is 30 feet wide, the uplift at the wall
would be 600 pounds per foot of roof length [(50 psf - 10 psf) times one
foot wide, times 15 feet (half the roof width)]. If the trusses or rafters are
spaced every two feet, each connection of the rafter or truss to the wall
would have to be able to hold down with a force of 1200 pounds. A way to
visualize what is needed is to think in terms of how you would connect the
roof if you intended to turn the house upside down and shake it up and down.
Hurricane straps are used to anchor the roof trusses or rafters to the
tops of the walls. Recent storms have shown that roofs anchored using modern
wind resistant connections have not failed. However, it is not enough to
just connect the roof to the tops of the walls. The uplift loads have to
be carried far enough down into the house so that the weight of the house including
the floors becomes greater than the uplift forces caused by the wind. This
is called developing a continuous load path.
But, wind doesn't just apply uplift forces to the roof that try to rip it
off the top of the house, the wind also creates pressures that push and pull
on all exposed surfaces of the house. In general, it tries to lift it up,
tip it over, slide it sideways and suck the side walls away from the connection
to the windward and leeward walls. Consequently, the walls and roof have
to be tied together like a well built box and anchored to the ground with
enough weight to keep it in place. In addition to these "external" wind forces,
if windows, doors or garage doors fail on the side facing the wind, and
allow wind pressures to build up inside the house, these pressures will try
to push off the roof and push out the walls. This is another major reason it
is important to protect openings. There are enough pressures being applied to
the outside of the house, trying to tear it apart, without the added "help"
of wind pressures from the inside.
Sorting Through the Options - Sources of Design Help:
Each home has its own features and vulnerabilities. This tends to make it
difficult to prescribe a one-size-fits-all set of recommendations for retrofits,
particularly structural retrofits. Consequently, this discussion of structural
retrofits describes typical situations and outline the types of retrofits that
will tend to raise the wind resistance of your home and bring it closer to
the kinds of wind resistant construction features prescribed in modern building
codes and standards. However, we have also tried to focus on what may be
practical and beneficial, realizing that bringing the structure all the way
up to the latest high wind requirements could entail almost rebuilding the
house, which is neither cost effective nor practical unless you are rebuilding after
a major event and the house is essentially gutted. With structural retrofits it
is smart to work from the top of the walls downward as each retrofit will
engage more and more of the weight of the house. The parts of a house do
have a way of working together to help each other out when they are properly connected.
By starting at the top, each retrofit you accomplish will make a difference,
increasing the ability of your home to survive a hurricane.
If you do decide to take on structural retrofits that involve major changes inside
your walls, this guide may be of some assistance. However, you should review
some of the design guides that have been developed in recent years that have
proven effective in improving the wind resistance of the structural systems in
houses. Probably the least expensive and easiest guide to follow, thanks to
an abundance of sketches, is the Standard Building Code Congress guide SSTD
10 that was last published in 1999. This document is available as a free
download from the
International Code Council website.
The specific location of the download is
www.iccsafe.org/cs/standards/is-hrc/SSTD10/index.html.
The wind speeds used in SSTD 10 are fastest-mile wind speeds, not the gust
speeds used in this guide. You can add 20 mph to the speeds listed in SSTD
10 to convert to gust speeds. Consequently, the recommendations for 90
mph are for 110 mph gusts, 100 mph correlates to 120 mph and the highest wind
speeds listed in the guide, 110 mph, convert to 130 mph gust speeds. Furthermore,
if what you are looking at doing is time consuming, difficult or expensive,
you may benefit by having a knowledgeable structural engineer review your
particular situation and help you determine what makes the most sense for
your home.
Usually the easiest and least intrusive structural retrofit is strengthening a
gable end wall. If your house has a gable end, click on
Gable End Walls to look at some of the
options available for retrofitting this condition.
The rest of the structural retrofits are organized according to the type of
walls you have in your house. If you have wood frame walls (regardless of
the type of cladding from vinyl to brick) click on
Wood Frame Walls to explore how to assess
your wall's strength, connections and possible options for retrofitting.
If you have masonry walls (concrete block) click on
Masonry Walls to explore how to assess your
wall's strength, connections and possible options for retrofitting.
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Garages with narrow walls on either side of the garage door have little structure in that wall to resist wind forces acting on the side walls
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Garages that stick out from your house and have very short walls on both sides
of the garage door are a special case and area of concern because that wall
has very little capacity to resist wind pressures applied to the side of
the garage extension. Click on
Narrow Shear Walls Beside Garage Door Openings
to see what you can do for either wood frame or masonry wall garages that
have this condition.
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