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Exterior Masonry Walls for Homes in High Wind Areas
It is not uncommon to see different building materials used in different areas of the United States. This applies to the exterior walls of homes. Wood framing (platform framing) is typically used in most areas of the United States. However, in Florida, especially in the southern half of the state, it is common to see homes built with concrete block exterior walls.

Concrete block provides some inherent wind-resistant benefits:

1. Using reinforced concrete block develops a good load path to resist uplift forces caused by wind. (See the article about Load Path for an explanation about the importance of maintaining a continuous structural load path.)
2. Concrete block is heavier than wood framing.
3. A reinforced concrete block wall system creates fewer connection points than wood framing does. This may translate to fewer construction errors at the connections.
4. Concrete block offers impact resistance to windborne debris.

In south Florida, concrete block exterior walls either sit on a thickened concrete slab or on a concrete foundation wall. The roof structure, typically wood trusses, rests on top of the exterior wall.

Concrete block exterior walls that are designed for high wind areas are not made of concrete block alone. They are actually a combination of poured concrete, steel reinforcing bars (sometimes referred to as rebar), and concrete block. The concrete blocks sit within a grid of concrete and rebar. The concrete and rebar provide the continuous load path that resists the uplift forces created by high winds.

Key structural elements of the wall are the tie beams and tie columns. (See Figure 1)

The tie beam is located at the top of the wall and at all floor levels that are elevated above the ground. The tie beam can either be poured concrete or masonry bond beams (U-shaped concrete block filled with concrete). The building code in south Florida requires a tie beam and its steel reinforcing bars to be sized by a design professional. The code also sets a minimum size of the tie beam to be at least 8" x 12" with a minimum of (4) 5/8" diameter reinforcing bars inside.

Many contractors in south Florida prefer to use a poured concrete tie beam instead of the masonry bond beam because of quality control. The poured concrete tie beam requires an extra step of setting formwork. The problem with masonry bond beams is that the quality of installation is frequently poor and the top of the wall is not always level. By using poured concrete at the top of the wall, the wet concrete naturally creates a level tie beam.

Within the tie beam are cast the hurricane straps that hold down the roof trusses. These must be placed in the concrete while it is still wet. One problem that can occur is when these hurricane straps are not correctly located where the truss will sit. In these cases special hurricane straps can be anchored into the tie beam after the concrete has hardened with truss anchors designed for retrofit applications.

It is very important to have the roof trusses securely tied to the supporting walls. Any hurricane strap that is not wrapped tightly around the truss or any truss that does not set firmly down on the top of the wall leave the potential for a connection failure in high wind situations. That is why the placement of the hurricane straps in the tie beam and the levelness of the top of the tie beam are so important.



The tie columns run vertically between the foundation and the tie beam at locations determined by the structural designer. The building code in south Florida requires tie columns to be sized and spaced by a design professional, but they should be spaced no greater than 16 feet apart with a minimum size of 8"x 12". The steel reinforcing bars run vertically inside the wall from inside the footing to inside the tie beam. This creates the continuous load path.

Between the tie columns are individual steel reinforcing bars running vertically and set inside the concrete block with the block cores filled with concrete. These reinforcing bars are spaced according to the design professional.

In south Florida the house structure, including the exterior walls, is required to be designed by a design professional. The building code also sets minimum limits to the design. If a house is located in a high wind area such as a coastline, the exterior walls, whether they are wood-frame or masonry, should be designed to withstand the uplift forces and the impacts of windborne debris associated with high wind. Addressing both of these is important to the survivability of the home during a hurricane or tornado.

State Farm® believes the information contained in the Good Neighbor House® is reliable and accurate. We cannot, however, guarantee the performance of all items demonstrated or described in all situations. Always consult an experienced contractor or other expert to determine the best application of these ideas or products in your home.

12-06-2000

Wind Design Concepts: Maintaining a Continuous Load Path
One key for a house to survive the strong winds from a hurricane or tornado is maintaining a continuous Load Path.

The concept of a continuous load path applies to resisting any type of force on a structure, not just winds. This includes forces from gravity, earthquakes, floods or snow. For the purposes of this article, we will concentrate on wind forces.
A load path is made up of a chain of structural components such as roof truss members, metal connectors, nails, anchor bolts, wall studs, and floor joists to name a few. When a home is hit by a hurricane or tornado, wind loads are applied to the exterior walls and roof.

These wind loads must be transferred from structural component to structural component until they are transferred into the earth. These various structural components make up a chain creating a continuous load path.

If the load path chain remains continuous and unbroken, no structural damage will occur. If there is a break in this chain, damage will occur.

In regards to gravity the load paths start at the top of the roof and work down to the foundation and finally, to the supporting soil. For the most part, these loads are supported by the element below them. Technically, gravity forces do not require a connection between structural components to support the load above if the load is applied directly on top of the supporting member below.

Wind creates forces on a house that work in different directions than the downward direction of gravity. Wind creates sliding, overturning, and uplift forces on a structure. Uplift forces, in particular, cause much of the damage seen in windstorms.




Damage can happen anywhere along the chain of structural components. All it takes is one "weak link." These "weak links" are more likely to occur at the connections between components, not within the components themselves. The design and construction of these connections must be considered carefully, keeping in mind the basic concepts of how the load path chain works.

There are several major connection points common to houses. There are:
• Foundation to Floor System
• Floor System to Exterior Bearing Wall System
• Connections within the Exterior Bearing Wall System
• Exterior Bearing Wall System to Roof Structure System
• Roof Structure System to Roof Deck

Each one of these connection points is a potential failure point. As can be seen from the previous graphic, there are many potential failure points in a typical house.

Constructing a home to resist the forces of gravity is simple and straightforward. If the structure is not built with a proper continuous load path, it probably won't remain standing for very long. This is not the case with constructing to resist wind uplift.
If a structural connection was not properly built, the contractor and building inspector may not catch this "weak link" in the continuous load path during construction. The structure may stand for many years, until a tornado or hurricane hits. Then, the "weak link" will fail and damage will occur.

Because its effects are not immediately seen, designing and building for wind uplift take more thought. The concept of a continuous load path must go into the design and construction of each and every connection when the structure is located in an area with the potential of high winds.

State Farm® believes the information contained in the Good Neighbor House® is reliable and accurate. We cannot, however, guarantee the performance of all items demonstrated or described in all situations. Always consult an experienced contractor or other expert to determine the best application of these ideas or products in your home.

01-24-2001 

 
Jahna Concrete - 104 S. Railroad Avenue, Avon Park, FL 33825
Phn: 863-453-4353 | Fax: 863-453-5156 | Email: info@jahnaconcrete.com


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