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To complete the development of an effective load path that connects the
roof to the foundations, you need to make sure that the exterior walls
are well anchored to the foundation. Although this is not as critical as
some of the other tasks on your list, poor connections between the wall
and foundation can weaken your entire home. These vulnerabilities tend
to show up when the highest winds of a major hurricane (Category 3 or higher)
strikes your home. This is especially true if you live in a one-story
home with large roof truss or rafter spans and a low-sloped roof or a two
story home. The walls that are most important to helping to hold the roof
on are the exterior walls that the ends of rafters rest on. These walls
can have a lot of uplift applied to them. The dead weight of these walls
is only 5 to 10 pounds per square foot. So if a wall is 8' high its dead
weight can only restrain 40 to 80 pounds of the hundreds of pounds of uplift
that may be applied in a strong hurricane. This points to the necessity
of tying the bottoms of walls to the floor below and ultimately to the
foundation. Furthermore, one of the important factors that affects the
performance of the wall itself in resisting lateral loads as well as the
uplift is how well it is anchored to the foundation.
Generally, it will be pretty difficult to determine how well your walls
are built or anchored to the foundation unless you open up part of an exterior
wall. If you end up having to repair drywall on an exterior wall or are
having your home re-sided, you have an opportunity to check the wall construction
details. You may also be able to gain some insight into the likely wall
construction by talking with the building official in your community or
older well established builders and asking what was typically done at the
time your home was built. Other indirect clues include the absence of
hurricane straps connecting the trusses or rafters to the top of the wall
or straps that are installed on every other truss or rafter connection.
If either of these cases exists, chances are that you don't have a particularly
strong wall structure and that it may not be very well anchored to the
foundation.
Anchoring of the bottom of the wall to the foundations requires the uplift
loads to be transferred into the bottom plate of the wall and from that
plate into the foundation (slab on grade) or into the floor system and
then into the foundation (elevated wood frame floor). The transfer of
loads into the bottom plate can be accomplished either by the exterior
sheathing if it is plywood or OSB or through straps between the studs and
the bottom plate or a combination of the two methods. In areas where the
3-second gust design wind speed is greater than 120 mph, the latest high
wind construction guides for slab on grade construction require straps
embedded in the concrete and nailed to the bottom plate or the wall studs,
or 5/8-inch diameter anchors bolts spaced at 18 inches or less. They
also require 3-inch by 3-inch by 1/8-inch thick washers between the nuts
and the bottom plate if anchor bolts are used. If you have a wood frame
floor, anchor bolts don't work. Straps or other methods have to be used
to transfer loads from the wall to the band joists around the floor system
and from them into a sill plate which is anchored to the foundation. If
you have framed floors, you will be well advised to seek professional help
to come up with cost effective solutions.
Finally, laboratory testing and engineering analysis indicate that the
ends of the walls should have large anchors called hold downs (sometimes
threaded rods are used that run the full height of the wall). Except for
a few parts of the country, it is unlikely that you will find these if
your house was built before 1995.
Homes built after the mid- 1990's along the coasts of North Carolina and
Florida, most of the coast of Texas, some parts of coastal South Carolina
and a few coastal communities in Louisiana and Alabama should have well
built and connected walls. This may be true for some older homes in these
areas as well.
Inspection of Wall Anchorage:
If you have a wood frame house with an un-finished garage or remove drywall
from an exterior wall near one of the corners of the house or in the garage,
you can get a pretty good idea about how well your walls are built and
how well load paths are developed. Print out the
Wood Frame Wall Checklist
and use it to record observations about how your wood frame exterior walls
are anchored to the floor or foundation. This checklist also includes
some suggestions about the implications of what you find and what you might
consider doing. The questions in the checklist assume slab on grade construction.
For homes with wood frame elevated floors, the exterior walls have to
be connected by straps or sheathing to the rim joists (a band of wood members
that frame around the edge of the wood frame floor system) and the rim
joists have to be connected to the sill plate (pressure treated wood member
lying flat on the top of the foundation wall and anchored to the foundation).
You should be looking for anchor bolts that attach the sill plate to
the foundation. Because the floor framing helps tie the system together,
the anchor bolt spacing for the sill plate (below a wood frame floor)
can be about twice the spacing indicated in the questions in the checklist.
Look at the connection of the wall studs to the bottom plate and the
connection of the bottom plate to the foundation: The latest high
wind anchorage uses 5/8" anchors with 3" by 3" washers that are at least
1/8" thick. Older installations used 1/2" anchors that were frequently
shorter so that there was less embedment into the concrete and were frequently
installed without a great attention to detail. The washers were frequently
just regular round steel washers for 1/2 " bolts. Recent tests have demonstrated
the importance of the larger washers when winds approach design values.
Retrofit Suggestions for Wall Anchorage:
If you decide to retrofit the wall to foundation connection, you will need
access to the base of the wall studs and the bottom plate from either the
inside or outside of the wall. If you have brick veneer, your only option
is access from the inside of your home. The most cost effective time to
do this retrofit is when you are undertaking a major renovation that calls
for removing drywall, if you are replacing the siding on your home, or
if you are rebuilding after flood or storm damage. The retrofit involves
the following actions. Make sure the wall studs are adequately connected
to the bottom plate of the wall framing and the bottom plate is anchored
to the floor or foundation. If you have undamaged bottom plates and the
wall sheathing is either replaced or is in good condition, you may be able
to get much of the strength you need by improving the nailing of the sheathing.
Metal clips are available at your building supply store that also make
it easy to strap the wall studs to the bottom plate. A palm nailer or
a pneumatic nail gun that is designed to locate the nail holes in straps
can be a good investment if you have a lot of straps to install. For slab
on grade construction, install anchors by drilling holes through the existing
bottom plate into the concrete slab or foundation. Place the holes as
close to the center of the bottom plate as possible and space them every
16 to 18 inches along the exterior walls and within 12-inches of the ends
of the walls. You can either use the new large anchor bolts with threads
for screwing into concrete or you can install threaded rod anchors using
epoxy grout. In either case, use 3-inch by 3-inch washers that are at
least 1/8-inch thick. In many cases all these materials will be available
at your local building supply store.
Warning: If epoxy grout threaded rods are used, you will need to clean
the holes after you drill them or the epoxy will not develop a good bond
with the concrete. Carefully follow the epoxy manufacturer's instructions
about how to clean the holes. It would be a shame to go to all of the
effort to install rods only to have them perform at half of their potential.
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If you have an elevated wood frame floor, look at one of the high wind
guides for wood frame construction to get ideas for properly attaching
the walls to the floors and then the floor system to the foundation. If
you have this condition you should seek the assistance of a design professional.
High wind construction guidance for wood frame construction is available
in the AF&PA Wood Frame Construction Manual or the International Code Council's
SS TD 10-99. The AF&PA (American Forest and Products Association) is
in the process of releasing new guides with great illustrations of proper
wood frame construction for high wind areas. The SS TD 10 is available as a
free download
from the International Code Council.
Back to Wood Frame Walls
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