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Background and Assessment:
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Masonry walls with grey concrete grout filling sections of the wall where block was cut and steel has been added - The remaining steps include adding stucco and re-painting the walls.
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As is common for most structural retrofits, it is much easier and cheaper
to build in the strength when the home is first built than after it is
completed. Un-reinforced masonry has not fared well in strong hurricanes.
Most of the collapsed buildings in Hurricane Charley were un-reinforced
masonry buildings. If the walls are tall or if the roof structure is damaged
that is providing lateral support for the tops of the walls, un-reinforced
masonry walls will collapse.
If you have a concrete masonry block home there are a couple of techniques
for determining how well the exterior walls are reinforced and anchored
to the foundation. If you have installed permanent anchors for shutter
on your windows and found that the blocks on both sides of your windows
are hollow, you probably have un-reinforced or at least under-reinforced
walls and some additional checks would be a good idea. In addition, just
because a few holes you made indicated solid concrete does not mean that
the blocks are necessarily reinforced with steel and concrete. It is possible
that you hit a concrete partition within the concrete block. Some blocks
have two cells so they have one interior partition of block that goes from
front to back of the block while some blocks have three cells and have
2 partitions. Because concrete blocks are easier to drill into than solid
concrete, you may be able to distinguish between the partitions of a block
and solid concrete simply by how hard your drill has to work. Un- or
under-reinforced means that the cells of blocks did not have steel placed in
them and were not poured with concrete at corners and beside each door
and window. If you have not installed permanent anchors for shutters,
you can use a small masonry bit and drill holes in the wall on both sides
of the window opening, in an inconspicuous location about 3-inches outside
the line formed by the vertical edges of the window. In newer wind resistant
construction, you should find solid concrete on one side or the other of
the window. If you find hollow blocks on both sides, you probably have
un-reinforced or at least under-reinforced walls and additional checks
would be a good idea. You can patch your little exploratory holes using
a little dab of exterior caulk after you have blown off the concrete dust.
There are some stud finders with metal detection that are capable of determining
the location of reinforcing bars in masonry walls. Typically they have
a "deep penetrating" metal detector option. Be sure to follow the manufacturer
's instructions for calibration when you turn them on. These metal detectors
can be used to conduct additional checks to determine how well your masonry
walls are reinforced and anchored to the foundations. The checks outlined
below are intended to help you make this determination. In older masonry
homes, conventional practice was to grout "J" bolts into the top of the
walls and anchor a wood plate to these "J" bolts. The trusses or rafters
are then connected to this wood plate. In these walls, there may not be
any reinforcing steel installed that runs from the top of the wall to the
foundation. In some cases, the top blocks in the wall have a "U" shaped
cutout and reinforcing steel is laid in that "U" shaped cradle and the
top blocks are filled with concrete to form a bond beam at the top of the
wall. In some cases, a header beam (also known as a lintel) installed
above the window and door opening is the only part that is reinforced and
no reinforcing is installed along the remainder of the top of the wall.
To determine how well your wall is reinforced, use the deep penetrating
metal detector setting on the stud finder / metal detector to search steel
in your walls. Some of the better metal detectors will also indicate if
both metal and electricity are detected. Consequently, be careful that
you don't pick up a false positive from an electrical cable. Also don'
t be confused when using a metal detector near metal from window frames
or downspouts. Make an effort to distinguish between steel in the wall
and other indications by trying to track the indication. Click on the
Masonry Wall Checklist
to get more information about inspecting your masonry walls and some suggestions
about what the findings may mean to you.
Inspecting the Walls for Reinforcing:
Begin your search for reinforcing by checking for steel at the outside
corners of the house. Go to an outside corner of your house and with the
stud finder / metal detector in deep penetrate mode and placed against
the wall about head high at the corner, move the stud finder sideways across
the wall away from the corner. Repeat this on the other face of the corner.
If you detect metal, move the stud finder vertically along a line that
passes through the point where you detected the metal to make sure that
the metal runs from the top of the wall to the bottom. Note the results
on the Masonry Wall Checklist.
Then check along the top of the walls to determine if there is reinforcing
steel running along the top of the wall. Note: if the block wall extends
one or more block heights above the soffit panels, you will not be able
to check for this from the outside of your house without removing the soffit
in the area where you want to make a test. If you do remove soffit do
not be confused by the metal detector finding aluminum parts of the soffit
system. Next, move the metal detector horizontally along the outside wall
of the house, sliding it along over a long stretch of wall without openings
to see if reinforcing bars are found. Determine the distance between the
reinforcing, if any is detected, and note the results in the checklist
below. Finally, check the location of anchor bolts or straps embedded
in the masonry wall from the attic and estimate the distance of one of
these from a corner of the house. Then go to that location on the outside
of the wall and from a ladder determine how far down into the wall the
metal is indicated. Most "J" hooks are about 16-inches long and will engage
the top couple of blocks when they are grouted into the top of the wall.
If you get an indication of metal running all the way down through the
wall, then it is likely that the cell below the "J" hook contains reinforcing
steel and that these cells have hopefully been fully grouted throughout
the height of the wall. Note the results on the checklist.
Interpreting the Results:
What you want to find is steel in the top row of blocks, steel coming down
the wall at least at corners and openings for doors and windows over 5'
wide, and steel going down to the foundation. Older homes may have very
little or no vertical reinforcing. The evolution of vertical reinforcing
in masonry walls is from no vertical steel; to the use of steel in the
outside corners; to adding steel at one or both sides of wide openings
such as sliding glass doors or garage doors; to steel at one side of even
more openings and occasionally interspersed along long walls; to today's
standards of steel at every opening and regular spacing on all exterior
walls.
If you did not find any indications of reinforcing steel in your walls
(answered "no" to all of the wall questions on the Masonry Wall Checklist),
you have un-reinforced masonry walls and, if your home faces a large open
area or the water or your house is 30' or wider, your home may be at risk
of wall collapse or losing its roof in a Category 2 or stronger hurricane.
The least expensive thing you can do to reduce your risk of wall damage
or loss of the roof is to protect all large windows and doors. You should
make sure that your garage door is protected or replaced with an appropriate
wind rated one if you don't already have one. Protecting your windows
and doors could mean the difference between the home keeping its roof on
or losing it when the winds climb above category 1 strength. You should
definitely plan on evacuating if a category 2 or above storm is threatening
your area, even if you protect the windows. You may want to consider adding
reinforcing as described in the following sub-section when you undertake
a major re-modeling job. If you decide to try and reinforce your walls,
you should get a design professional to help you find the best solution
for your house.
If you found steel in all the right places (answered "Yes" to all
the Masonry Wall Checklist questions), you have reinforced masonry walls
and you do not need to worry about retrofitting your walls. Just to be
clear that you found steel at the top of the walls and down the full height
of the walls at the corners and at wide openings.
If you found some steel at the corners but that was about it (answered
"yes" to some but "no" to other questions), you probably have under-reinforced
masonry walls. You should put a high priority on protecting all large
windows and doors, including garage doors. You may also want to consider
adding reinforcing as described in the following sub-section when you undertake
a major re-modeling job. If you decide to try and reinforce your walls,
you should get a design professional to help you find the best solution
for your house.
Improving the Strength of Masonry Walls:
Reinforcing an existing masonry wall is probably the most difficult and
expensive retrofit you might consider undertaking. The International Existing
Building Code (Section 305.1) and the Florida Existing Building Code (Section
305.1), and perhaps other building codes require that the entire building
has to be brought up to current building codes when remodeling work progresses
past 50% of the building area within any 12-month period. The following
description is for illustrative purposes only, if you are considering undertaking
this type of retrofit, you should get a design professional to help you
find the best solution for your house.
The technique we have seen used for adding reinforcing to an existing masonry
wall house involves cutting out the face of the blocks in a vertical column
at the wall corner, on one side of each window or door opening, and at
a spacing of 4 to 8 feet along the wall when there is no opening. Steel
reinforcing rods are then epoxy grouted into the bond beam at the top and
the foundation with an overlap of at least 25 inches where they meet.
The cells are then filled with grout and the wall finish re-applied. An
idea that has been used in earthquake retrofits has been to install steel
plates on the inside and outside of the walls that are tied together with
bolts that pass through the wall and the two plates. The top bolt has
to go through the bond beam at the top of the wall. A third idea that
has been the subject of limited research is where a cutout is made at the
top and bottom of the wall and a cable is fed through the cells in between
the holes. The side of the block at the bond beam is also chipped out
and a rod is epoxy grouted horizontally into the bond beam. The bottom
of the cable is then formed into a loop with several cable ties and positioned
in the middle of the cell. Grout is poured in through the top hole and
once it begins to flow out of the bottom hole, the bottom hole is blocked
and grout is added until it flows out of the top hole. The top of the
cable is hooked around the rod in the bond beam and cable clamps are tightened
so that the cable is pulled tightly around the rod. Grout is than added
until both the top hole and the side of the bond beam are filled. This
hole is then blocked and the grout is allowed to harden.
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Masonry block cut out to allow steel reinforcing to be added - short pieces are epoxy grouted into the foundation and into the bond beam at the top of the wall.
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Closer shot of bottom of reinforcing added to masonry wall showing the short segment epoxy grouted into the foundation and the rod added through the wall - Note that the two pieces have to overlap a significant distance so that the forces can be transferred down through the walls.
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