Engineering : What is Adequate Bracing?Home » Steel eNotes - Winter 2010 » Engineering : What is Adequate Bracing?
When it comes to trusses, bracing is a necessity, and cold-formed steel trusses are no exception. Members under compressive forces will buckle given the right (or wrong, depending your point of view) combination of force and member length. By way of example, take a wood yard stick by the ends and push them together. You will see what buckling is.
With trusses, the force in the truss member is the result of applied loads and truss geometry and generally speaking cannot be changed without making significant modifications such as reducing the truss spacing to reduce the load on each truss. Unfortunately this increases the number of trusses and increases $$$. A better solution: Provide bracing to reduce the member length.
When reviewing an Aegis Metal Framing truss drawing , bracing layout or Works submittal, the bracing specifications are clear. With Bracing Layouts and Works packages the material type, size, steel thickness and connections are clearly spelled out. Typical top chord bracing can be plywood, corrugated metal deck, composite sheathing or a purlin system. Typical web bracing will be hat channel, C-stud or other stiff element running laterally perpendicular to the truss member with periodic diagonal bracing.
Typical bottom chord bracing is directly attached wallboard or a system of lateral and diagonal braces.
Frequently, requests are made to supply alternative construction materials, and the question arises as to whether these are adequate substitutions for the specified bracing. Here is a review of some of the commonly applied materials in a roof system and their effectiveness for bracing.
Plywood or structural metal deck attached directly to the truss top chord:
Provides proper bracing of truss top chord when properly attached. Attachment is accomplished with screws, pins (and in some cases, welds), all with the appropriate fastener at the specified spacing: typically 6” to12” oc.
Standing Seam Roof:
Generally speaking, architectural standing seam roofing is not directly applied to the top chord of trusses. This material is not capable of supporting environment loads and does not provide adequate top chord bracing.
Purlin System for Standing Seam Roof:
When adequately fastened to the truss top chord typical Z purlins will provide bracing for the top chord. Diagonal bracing through the system is generally required unless the Standing Seam Roof can provide the lateral stability. Consult with the roofing system provider. When plywood or structural deck is attached to the purlin system, diagonal bracing is typically not required.
1 ½” Hat Channel:
A combination of lateral hat channels applied perpendicular to the truss web and diagonal hat channels that transfer forces to a resisting element provides for excellent web member bracing. The material thickness should be a minimum of 0.028”.
7/8” Hat Channel:
Just as with 1 ½” hat channel, the combination of lateral hat channel perpendicular to the truss wed and diagonal hat channel that transfer the force to a resisting element provides excellent member bracing. Limit the use of 7/8” channel to roof systems with truss spacing of 4’ oc or less.
C-shape and Track:
Same as 1 ½” hat. Apply required lateral and diagonal members. Minimum bracing member thickness of 0.028”.
Strapping is generally not considered to provide lateral bracing of web members
1 ½”, 7/8” Hat Channel, C-shape and Track:
As it is with top chord, sheathing attached to the purlin system on the top chord (assuming drywall is adequately attached to the purlins) provides bracing to the bottom chord. Diagonal bracing is typically not required in this scenario.
Drywall attached directly to the bottom chord:
When trusses are spaced such that drywall can span between the trusses (typically that is 24” or less) the drywall screw attached directly to the bottom chord is considered adequate for bracing the truss bottom chord. Typically a minimum thickness of ½” drywall is required. Maximum screw spacing of 12” oc is considered adequate for member bracing.
Resilient channel is a one-legged, hat-type channel formed from material approximately 0.019” in thickness. Resilient channel is specified in assemblies designed to obtain a fire or sound rating. For those assemblies, the resilient channel is directly attached to the truss bottom chord. Drywall or other ceiling material is then attached to the resilient channel. The assembly is very typical in commercial roof construction.
Due to the thin material composition and the overall design of the resilient channel, it does not exhibit the properties required to provide adequate bracing for the bottom chord. Even with drywall attached, adequate bracing resistance is not provided. Additional lateral and diagonal bracing in the form of hat channel or other products will be required to properly brace the truss bottom chord.
Hat channel used for fire rated assembly:
In some instances 7/8” hat channel is used in ceiling fire-rated assemblies. However, in order to satisfy the specific assembly requirements, 7/8” hat channel must be wire-tied to the truss bottom chord. This allows for some movement of the hat channel under high heat. Wire-tied hat channel will not provide lateral bracing. As with resilient channel, additional lateral and diagonal bracing in the form of hat channel or other products secured to the bottom chord will be required.
Mike Pellock, P.E.
Vice President - Technical