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Frequently
Asked Questions |
All
construction on new and existing structures require several forms to be
filled out, certified, and submitted to the Texas Department of Insurance.
Windstorm Forms and their use:
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The
WPI-1.
Form is the Application for Windstorm Building Inspection Form. The WPI-1
form is used to set up the inspection for a structure and to initiate
a file. A WPI-1 form is required to be submitted for all structures that
are to participate in the Windstorm Inspection Program.
The
WPI-2
form is the Building Construction Compliance form. The WPI-2 is the form
submitted by the appointed Texas licensed professional engineer that certifies
that the structure has been inspected to meet the prescriptive requirements
of the Code or Guide, or has been designed and inspected to meet the wind
loads specified by the appropriate construction document. The WPI-2 form
is submitted to the TDI after the erection and inspection of the structure.
The
WPI-2D
is the Building Design Compliance form. The WPI-2D is submitted to the
TDI with plans and calculations by a Texas licensed professional engineer
for a structure that is to be inspected by a TDI windstorm field inspector.
The WPI-2D is submitted to the TDI Windstorm Section for review along
with the signed and sealed building plans. The building plans shall state
the standard used to calculate the wind loads, and the design wind speed.
Form
BC-1085
– Texas Windstorm Insurance Association form for Building Certification
(structures built prior to January 1, 1988)
The WPI-8 is the Certificate of Compliance form that is issued by TDI
to the owner of the structure following completion of all inspections
and payment of all fees.
Modifying or Renovating Existing Structures for TDI Windstorm Certification:
Modifying or renovating an existing structure is unique and requires an
examination of the structure by a registered professional engineer with
notice of appointment as a qualified inspector by the TDI to determine
if TDI certification is required.
Plumbing Leaks, Structural Problems, and Cosmetic Cracks in Residential
Homes Explained:
People who are experiencing difficulty with their homes ask some familiar
questions.
- Why is this happening?
- How do leaks cause these problems?
- Why do I have plumbing leaks?
- If the problems I see are not due to leaks, why is the brick and sheetrock cracking anyway?
- How do I fix my home?
- If I were going to build a new home, what would I do to ensure these types of problems do not happen there?
Below we will explain the basis for design of foundations, how the foundation
affects the other materials and framing, and some of the reasons for which
problems arise. We hope this will answer your questions. Please give us
a call if you have other questions or do not understand something.
Basis for the Design of Concrete Foundations and Why Problems Occur
Cracks are inherent in concrete construction because of shrinkage that
results from the curing process, and because of concrete's inability to
resist large tension forces. In the early stages of construction, cracking
is due to the shrinkage characteristics of concrete. As the concrete cures
and hardens, it shrinks, setting up stresses which induce tension. While
the concrete tends to want to shrink, ground friction tries to hold it
back. Cracks form because of this, but are held tightly together by the
post tensioned cable or rebar. Thus one might see some very tight cracks
in the concrete after it has cured for several days. This is not a problem
and is not a failure of the foundation.
Many homes in the area are constructed using post tensioned cable. Post
tensioned cable slab construction uses cable to hold the slab firmly together,
thus reducing the size of shrinkage and temperature cracks and giving
strength to the slab. Both post tensioned and rebar slabs have the ability
to restrain the effects of bending, tension, compression, and shear. While
rebar does this by bonding to the concrete, post tension cable does not
bond but is held firmly on each side of the slab by anchors. When properly
designed, both post tensioned cable and rebar reinforcing foundation methods
resist the forces that are induced by wind, differential settlement caused
by soil movement, and the weight of the walls, roof, etc. However, the
designer cannot design for leaks, poor drainage, or other variable forces
that should not be allowed to occur over long periods of time. While a
certain amount of these problems are dealt with in code formulas, the
designer does not know to what extent they will occur.
Most residential structures are floating slab designs and are subject
to seasonal moisture change, plumbing leaks, and poor drainage. A floating
slab is simply a slab installed on top of or near the top of the ground.
This is differentiated from pier or drilled footing designs. A floating
slab will move with changing moisture conditions in the soil. Often that
movement is differential across the slab's length. Unless the slab is
strong enough to remain level in relation to the structure's walls, cosmetic
or structural damage will occur.
Brick and sheetrock coverings are brittle elements with little ability
to resist shear and tension which depend on the ability of the foundation
to resist these forces. Slight tension stresses in these coverings set
up by foundation movement, and to a lesser degree wind, induce cracks.
The Coastal Bend has many areas where the top several feet of soil is
an expansive clay material often referred to as Gumbo. Swelling and shrinking
of expansive soil takes place as the seasons bring alternate dry and wet
conditions. This creates a phenomena known as "edge lift" and "dome lift"
modes. As moisture is depleted from the soil during dry periods, the soil
shrinks. The soil near the center of the slab retains more moisture than
does the soil near the edges of the slab. This results in the edges of
the slab deflecting down more than the center. In very wet periods, the
opposite situation occurs. The cycle through dome to edge lift modes causes
(often significant) tensile stresses in the slab. Deformation of the frame
is usually detectable in the areas of stress buildup or reversal. One
would expect to see the effects of frame deformation due to stress at
window edges and corners, at door frames, along the floor line, and in
roof framing elements. Also, cracking of the brick is usually seen with
head joints open and bed joints closed under these circumstances. Where
large vertical differential displacement of the slab takes place, bed
joints may be open as well.
Leaks often lead to the same types of problems as expansive clay soils.
Usually, however, the damage will be more localized. Leaks are investigated,
found, and repaired by plumbing contractors who specialize in such work.
The structural engineer still has the job of understanding what has been
damaged and how to repair it after the contractor completes his work.
It is imperative that a structural engineer watch and record the plumber's
work and design a structural fix.
As noted above, minor cracking will occur during the curing process. The
concrete tries to shrink in length but is restrained by the friction between
the ground and the slab. This results in hairline cracking that is non-structural
in nature. These cracks run perpendicular to the edge of the slab and
may be found anywhere in the surface of the slab.
During "dome lift" mode or when the soil falls away from beneath the edge
of the slab or the soil becomes partially liquidized, the slab begins
to bend. Stress becomes high at the point of bending, and if it continues,
a structural crack results. These cracks are parallel to the edge of the
slab and occur approximately six feet from the edge of the slab. However,
at re-entry corners where wings protrude from a basically rectangular
slab, the crack is often forced to begin at the slab's edge (at or near
the reentry corner) and travel parallel to the line of stress.
The Post-Tension Institute, the governing body for the method of design
and construction of foundations, describes this condition. Reference PTI
Technical Notes Issue 6, "Cracking in Ground-Supported Post-Tensioned
Slabs on Expansive Soils" by Ken Bondy. This is true for rebar constructed
slabs as well.
Problems generally occur because, (1) the slab is not stiff enough to
adequately resist deformation caused by soil movement; (2) the house has
wings or enclosed garden areas that trap water and present reentry corners
where stress buildup occurs; (3) of poor workmanship, and; (4) in some
instances, plumbing leaks lead to stress induced cosmetic damage, and
in rare instances, significant damage to structural elements.
What Can Be Done?
You may have seen problems in your home that you are worried about, or
heard other people talking about, or you may be in the process of buying
or selling a home. The first thing you should do is hire a professional
structural engineer to look at the property. He or she will submit a report
outlining a description of the property, findings about the exterior and
interior, the status of the foundation and framing, the roof, drainage
of the property, and an overall statement of the problem. He or she should
then note recommendations for repair if required, as well as notes for
future care.
Repair
If repairs are required, you should obtain the services of a structural
engineer to affect a design. In most cases, the engineer will want to
obtain the services of a soils engineer to obtain information about the
plasticity and the liquid limits of the soil and the depth to which piles
should be placed if they are indicated.
Most home owners do not have plans of the construction of their home.
If they do, there is no way for the engineer to know if the post tension
cable or rebar was placed as per the plan. The engineer then has to work
from minimums to affect a successful design. Let us explain. Most homes
are constructed on floating slabs reinforced with rebar or post tensioned
cable. In either case, the slab was designed to be supported by the soil
along the full length of the grade beams. If piers are placed at the corners
only, for example, on a wall line that is 20 feet long, those two corners
are restricted from movement, but the rest of the span between the piers
is not. As the soil drops away during a drought, the grade beam between
the piers is no longer supported and will begin to crack, causing considerable
damage to the home. Repairs require a structural engineer to make the
necessary determination of pier placement.
Some foundation repair companies use round, concrete cylinders about 1.5
to 2 feet long as pier material. These are placed under the exterior grade
beam at an interval that is determined by guesswork. These cylinders are
forced in the ground under pressure until a "no go" force is reached.
There are two problems with this method. First, the "no go" depth has
to do with the moisture level at the time the piers were placed, not necessarily
to a depth that will not move during changing moisture content levels.
The second problem is that when moisture content is high, the foundation
is lifted by the soil. When that happens, the foundation often moves up
and away from the piers. Soil gets between the cylinders and makes them
ineffective, particularly when the moisture content drops. There are several
other problems with this type of repair as well.
New Home Construction
You should make sure that both a soils engineer and a structural engineer
design the foundation with input from you. You need to ensure that the
rebar and/or post tension cable are strong enough to ensure that the foundation
will remain level throughout its length and without the formation of cracks
during seasonal moisture change, even though the slab may move differentially
with soil change. This does increase the cost of the foundation slightly,
but helps keep you from having problems in the future or losing significant
equity if the home is ever sold.
There are other methods of insuring that soil moisture change does not
affect the foundation. One is to construct the foundation on piers placed
at the correct depth. The grade beams and the slab are then placed on
cardboard boxes. In a short time the cardboard degrades and the slab is
left above the soil approximately 4 inches. As the soil moves up and down
with seasonal moisture change, the foundation is unaffected. The grade
beams are designed to span between piers without support from the soil
and without failing. Also, the interior grade beams are supported by piers
as well.
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