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Roof Truss Bracing
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 figure 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 in the left
side of figure 2. This can be demonstrated by placing a
yardstick on end on the floor while holding the upper
end. When you push down on the upper end, the yardstick
buckles in its narrow direction. The right side of
figure 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 yardstick with one hand, while
restraining it at the middle with your other hand. The
yardstick will not buckle under the same pressure.
A brace can also be used to prevent structural
elements from racking. The left side of figure 3 shows a
rectangular structural frame that is racked due to a
force applied to its top corner. The right side of
figure 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
Temporary bracing is used during the erection of roof
trusses to prevent the trusses from bucking and falling
over during the erection 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 figure 4 for the
nomenclature of truss members.
Temporary bracing is extremely important to life
safety during the erection of trusses and is required by
all major building codes. This bracing is the
responsibility of the builder and the builder should
consult the literature described in the code, and
available from the truss industry, on the proper use of
temporary bracing during the erection of trusses.
Information on the use of temporary bracing of trusses
is generally provided by the truss manufacturer.
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.
The three most common methods of bracing individual
truss members are: (See in figure 5)
- Continuous lateral bracing is
generally used when the members of adjacent
trusses line up with each other. A 1x4, 2x4,
or larger brace is run across the truss
member and the adjacent members,
perpendicular to the trusses. If one brace
is required, it is placed at about the
midpoint of the webs to be braced as shown
in figure 5. If two braces are required due
to a higher compressive force, they are
placed at about the third points of the
webs. The continuous lateral bracing must be
restrained at the ends to prevent all the
webs from buckling in the same direction.
This is accomplished by fastening the ends
of the lateral bracing to an appropriate
point of the building structure, or by
adding additional diagonal bracing at the
ends, and intermediate intervals, to brace
the lateral bracing.
- "T" bracing is generally used
when the web requiring bracing does not line
up with the webs of adjacent trusses. It
consists of adding a 1x4, 2x4, or larger
brace along the length of the web to be
braced forming a "T" when looking at the
cross section of the web and brace.
- Scab bracing, like "T" bracing is
generally used when the web requiring
bracing does not line up with the webs of
adjacent trusses. It consists of adding a
member of the same size as the web, nailed
to the side of the web, effectively doubling
the thickness of the member.
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The design of permanent bracing of individual truss
members is the responsibility of the truss designer as
is generally shown on the individual truss engineering.
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 figure 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.
In 1992, Hurricane Andrew caused major damage to the
South Florida area. Gable ends being pushed in or pulled
off buildings caused a substantial amount of damage.
A hip roof slopes down to the walls on all four sides
of a building. A gable roof slopes down to two sides
that are opposite each other and forms a triangular wall
section at the other two sides. The triangular wall
sections above the eaves are called gable ends and
provide a large vertical surface, which must resist wind
forces. Some of the longer verticals of the gable end
trusses require bracing to keep the verticals from
bucking from sideward wind loads. The truss designer
generally designs this bracing.
Many gable ends that failed during in hurricane
Andrew had this bracing installed on the verticals. The
gable end failures resulted because bracing of the
overall gable system was not provided. This bracing is
generally the responsibility of the building designer.
State
FarmŽ believes the information contained in the Good
Neighbor HouseŽ 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.
2/22/1999
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