Bracing is one of the most important, integral parts of a roof structure. It is also one of the most overlooked parts of the roof structure. Your roof structure can be perfectly designed and manufactured, but if it is not correctly installed and braced, you will end up with an unsafe roof. In order to ensure the stability of a roof structure, it is necessary to properly brace the roof trusses and to ensure that the trusses and roof covering are securely tied down.
What is a Brace?
A roof truss is made of several pieces of wood that are joined together. Each piece of the truss is referred to as a "member". A brace is a member that prevents a structural element from buckling or racking. Members are generally subjected to tensile or compressive forces. As shown in illustration 1, tension forces tend to pull a member apart while compressive forces tend to push a member together.
When a compressive force becomes great enough, it will tend to buckle or bow a member as shown on the left side of illustration 2. This can be demonstrated by placing a ruler on end on the floor while holding the upper end. When you push down on the upper end, the ruler buckles in its narrow direction.
The right side of illustration 2 demonstrates the use of a brace at the middle of the structural member. Under the same compressive force, the brace prevents the member from buckling. This can be demonstrated by applying the same downward pressure on the ruler with one hand while restraining it in the middle with your other hand. The ruler will not buckle under the same pressure.
A brace can also be used to prevent structural elements from racking. The left side of illustration 3 shows a rectangular structural frame that is racked due to a force applied to its top corner. The right side of illustration 3 shows the same rectangular frame with a diagonal brace added to prevent racking when the same force is applied. The triangles formed by the addition of the diagonal brace are the fundamental basis for the structural design of trusses.
Truss bracing can be broken down into three categories; temporary bracing, permanent bracing of individual truss members, and permanent bracing for the overall stability of the structure.
Temporary bracing is used during the installation of roof trusses to prevent the trusses from bucking and falling over during the installation process. It is a series of continuous braces along the top and bottom chords of the truss and may include "X" bracing between vertical web members of the trusses. See illustration 4 for the nomenclature of truss members.
Permanent bracing of individual truss members
Permanent bracing of individual truss members prevents certain members of individual trusses from buckling under compressive loads. During the design process of the truss, the members are checked for buckling and for slenderness restrictions. If a member is found to buckle in the narrow direction, a brace is added. If a member is found to buckle in the wide direction, the size of the member is increased.
Under normal gravity loads, the top chord of a typical truss is in compression and tends to buckle in its narrow direction (sideways). The plywood roof sheathing prevents the top chord from bucking sideways. Other members of the truss, such as various web members and the bottom chord may also experience compressive forces under different load conditions. Some web members may be under compressive loads caused by gravity loads, while other web members or the bottom chord may be subjected to compressive forces due to uplift forces caused by high wind events such as hurricanes. Under certain combinations of member length and magnitude of the compressive force, the member may buckle in the narrow direction. When this combination is reached, bracing of the web member or bottom chord is required.
Permanent bracing for the overall stability of the structure
Permanent bracing for the overall stability of the structure is required by the major building codes and is required to brace the overall truss system and structure as an entire system. The design of this bracing is generally the responsibility of the building designer. An example of this bracing would be the bracing at the gable ends of a roof system to stabilize the gable ends. See illustration 6 for the various types of bracing that may be required at a gable end.
Some building codes also require specific bracing as a minimum requirement. This bracing is generally necessary for the overall stability of the structure.
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