Roof Truss; Permanent Bracing
By John F Mann, PE
Prefabricated roof trusses are fabricated from relatively slender elements or members. For wood roof trusses, these members are most often 2x4s.
Members of trusses must resist tension and compression forces, parallel to length of the member. Compression tends to cause sideways buckling of slender members that can easily result in member failure long before the ultimate compression stress of the material is exceeded.
To prevent buckling failure, permanent lateral bracing must be provided.
Permanent bracing is different than temporary bracing required during construction to prevent trusses from falling over, even though some temporary bracing might be useable as permanent bracing.
When (if) the cost of proper permanent bracing is included, the overall cost of a roof truss installation will often be much greater than the "installed" cost quoted by builder or truss manufacturer. Many builders have not been properly installing permanent bracing for roof truss installations, especially for relatively complicated roof geometries.
Responsibility For Design
To be effective, permanent lateral bracing must be properly designed. However, design of permanent bracing is typically neglected entirely by architects, even though the governing design code (TPI 1) clearly requires that the "building designer" is responsible for bracing design.
In New Jersey, the Uniform Construction Code (UCC) requires a plan of permanent bracing for roof trusses. Yet, architects rarely include the "required" plan in their design plans and code officials almost never flag this glaring omission.
The position taken by many architects is that they believe they have the right to "delegate" bracing design to the truss manufacturer or the builder. However, such position is ludicrous since the architect knows full well that these parties have not agreed to, and most importantly, are not qualified to perform bracing design.
The main reason that architects neglect to provide bracing details on design plans is that they also do not know how to perform bracing design.
However, lack of bracing design is also due to the following reasons;
(1) Although standard details can be provided on plans, final design of permanent bracing can only be performed after roof truss design diagrams have been completed (by truss manufacturer). Building design plans must be at least partially completed before roof truss design diagrams can be prepared. Most often, the architect "completes" the plans before the truss manufacturer completes truss diagrams. To provide bracing design details, the architect would then have to perform "additional" design services, after the plans were supposedly "complete". Although bracing design could be specifically included in the agreement for architectural services, it almost always is not since the architect does not want to be responsible.
(2) Code officials will generally issue a building permit before roof
truss design diagrams have been completed. The issue of permanent bracing is then neglected.
Even if the architect specifies that the truss manufacturer (or builder) submit roof truss diagrams for "review", the architect generally does not provide bracing design details as part of any review comments.
As required by code, roof truss diagrams typically include small symbols to show the locations where permanent bracing is required. However, the governing design code (TPI 1) states clearly that design details for required bracing must be developed by the "building designer", not the truss designer.
The building designer should design bracing because bracing must be connected to some element of the building that is not designed by the truss manufacturer. This element must be designed to resist forces from the bracing.
If, as is almost always the case, building design plans do not include any bracing details for roof trusses, the builder and framing subcontractor tend to make up bracing details as they see fit, or simply omit any bracing. The predictable result is that bracing is all-too-often not installed or is installed incorrectly.
Often, a line of horizontal bracing is installed near the ends of web members, instead of near the mid-length as required. Such "bracing" is completely ineffective.
The writer has actually seen more than one case where a truss web member has been removed and used to "complete" a line of horizontal bracing.
Bracing Of Top Chords
Sloping top chord of roof trusses must resist compression force due to gravity (downward) load on the roof, such as snow load. Roof sheathing (plywood, OSB) provides effective lateral bracing when sheathing is nailed directly to top chords. This condition generally applies when trusses are spaced at 2 feet.
When truss spacing exceeds 2 feet, purlins are generally required to support roof sheathing. Purlins, which run across top chords of trusses, must then provide lateral bracing of the top chords.
Bracing For Web Members
Lateral bracing for truss web members is generally the key problem. Web members are either vertical or sloped, between top chord and bottom chord.
Long slender web members that resist compression force must very often be braced. Truss diagrams generally show the locations where bracing is required.
Some truss manufacturers will also note basic details of bracing, such as bracing size (1x4, 2x4) and even nailing to webs. However, providing such information does not release the building designer from providing complete bracing design, since additional important details remain necessary.
Bracing must be designed to resist force from the web member. Connection between brace and web must also resist this force.
The most frequent bracing method has been to use a horizontal line of bracing members (such as 1x4s or 2x4s) nailed across a series of web members.
Design force for lateral bracing has typically been taken as 2-percent of the design compression force in the web member. Recent studies have shown that this long-standing "rule" is usually adequate. However, in some cases, greater bracing force may occur.
Bracing force for an individual roof truss tends to be quite small. For example, using the 2-percent rule, bracing force for a web member that must resist 4,000 pounds compression is only 80 pounds.
However, for an assembly of roof trusses, buckling might occur in the same direction for every truss. Therefore, bracing force accumulates along the brace member. For a series of trusses with the same configuration, bracing force would simply be the number of trusses times the brace force for each individual truss. The number of trusses to be considered depends on the spacing between elements that will resist the accumulated bracing force.
Elements To Resist Bracing Force From Horizontal Braces
One basic building element that might be used to resist bracing force from horizontal braces is the gable endwall. However, the wall must be specifically designed to resist the accumulated bracing force.
A typical method for resisting brace force is to install sloped "collector" elements, at intervals, across the web members that must be braced. These sloped "collector" elements are installed on the opposite edge of web members from the horizontal braces.
Sloped collector elements must transfer bracing force to a building element that has capacity to resist accumulated bracing force. All-too-often, if the sloped collector is installed at all, it is only nailed to the truss webs. This type of installation should not be considered correct since it does not transfer force to any building element other than a truss web member that has not been designed to resist out-of-plane bracing force from other trusses.
To be completely effective, a sloped collector element must transfer force to the roof sheathing, which will act as a diaphragm, or to a floor or ceiling diaphragm below. This requires some type of blocking to connect the sloped collector element to the diaphragm. Providing proper design details for such blocking requires careful consideration of conditions with truss geometry.
Roof geometry for large houses is often complex, for appearance purposes. Numerous different truss configurations are then required. Such condition greatly complicates bracing design since web members of adjacent trusses are not aligned where truss configuration changes. For such cases, an alternate method of bracing should be used.
Bracing Individual Web Members
When lines of horizontal bracing are not practical, due to varying truss configurations, web members that require lateral bracing should be braced individually.
The most common method of providing individual bracing is installation of a "T-brace". Essentially, a T-brace is a board (typically 2x4) nailed flat across the narrow edge of the web, forming a T shape.
Another method could simply be reinforcing the web with another board of the same size, nailed to the side face of the web.
Bracing For Gable Endwall
When a prefabricated "truss" is used for the gable endwall, lateral bracing is often required for the high vertical members (which are essentially wall studs). Such bracing should not be tied to webs of adjacent interior trusses since those webs are not designed to resist out-of-plane force. See "Gable Endwall "Truss" - Design"
on this site for further information.