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  Systems Overview: Trusses

Where to Use Trusses
Cold-formed trusses are used primarily to create non-combustible roof systems that are sloped or otherwise complex in design.  Generally, this means buildings with a standing seam or shingled roof.
About Proprietary Truss Systems
Cold-formed trusses are typically created using one of the well-established proprietary systems designed specifically for this purpose, although c-stud trusses are also somewhat common in lighter load applications.

These systems differ from generic cold formed framing in the following ways:
  • Framing members will typically have considerably more bends than other shapes in order to deal with the higher axial loads of truss systems
  • The shape of the framing member is designed for compact shipping

Steel Construct Systems uses the Aegis Metal Framing truss system.
Including a Truss System in Your Project
The following sections will give you an idea of what a truss system typically looks like in practice.

Structural Sheathing
  • Shingles? Plywood is needed to provide a nail base. 5/8" FRT CDX plywood is typical.
  • Standing seam? Metal deck provides a suitable substrate and allows higher truss spacing. 22ga or 20ga B or F deck suits most situations.

Truss Spacing
The optimal truss spacing depends on the capabilities of the sheating that will go on top of them. While it is possible to add additional structural elements under plywood to increase the possible truss spacing, this tends to not be worth the trouble in practice.
  • Plywood sheathing typically requires support about every 2 feet
  • Metal deck can span 4-5 feet efficiently

Typically, the ceiling below trusses is either dropped acoustical ceiling or drywall on a flexible resilient channel or a floating furring channel system.  It is important to note that such a furring channel system cannot act as bottom chord bracing because it needs to allow the drywall to float independently of the trusses.  Should the connection be rigid, deflection of the trusses durring loading can damage the drywall, causing aesthetic and fire barrier issues.

Because the loads and details of each individual connection situation can vary greatly, the truss to bearing and truss to truss connections should be specified by the truss engineer.  In general, structural steel and cold formed steel are easy to connect to and concrete and bond beams are slightly less preferable but still practical; however, connections to wood and connections through built-up roofing should be avoided.

Permanent lateral bracing will be required to develop the full design strength of the trusses in all but the lightest and simplest of applications.  This bracing will almost always be present on the bottom chord, sometimes on the webs, and occasionally on the top chord if the sheathing is unable to provide adequate bracing for the top chord. The bracing requirements of the truss members should be clearly indicated in the truss engineering. Additionally, temporary bracing serves a critical role durring the installation--you can find out more about the industry's recommendations for temporary bracing at the CFSEI website.

Shear Resistance
The handling of horizontal forces within the roof system is closely intertwined with the truss system and we recommend that the truss system be used to transfer these forces wherever practical.  These force should include the horizontal forces placed on the trusses by wind and other loads and may also include diaphragm forces the trusses need to pass from the sheathing to shear resistance elements below.  These forces and their locations should be included in the contract documents so the truss engineer can account for them accordingly.

Catwalks and other open areas of moderate width are generally fairly easy to include within trusses.  However, one should be careful when locating bearings near a catwalk!  A bearing at each edge of a catwalk is good but an asymetric bearing near just one side of the catwalk or a bearing in the middle of the catwalk without bearings at the edges will create forces within the trusses that are very difficult to deal with.

Now that you have an idea about what your system will look like, you can get an idea about the cost using our budget tool:
Miscellaneous Design Considerations
Design wise, it is generally best if truss clear spans are kept to 60' or less, although greater spans are certainly possible. When designing the aspects of the building that determine the shapes of the trusses, we recommend considering these items to avoid unnecessary costs and complications:
  • Try to avoid situations where girder trusses have to carry long runs (20'+) of 16'+ trusses
  • To evaluate the practicality of a truss, calculate (truss span)^2 / (truss depth). a value <600' is good and a value >1200' is bad. This is especially important for girder trusses and situations with heavy loading.
  • Overhangs where the truss top chord extends 4'+ outside of the building start to affect cost considerably. This is amplified in high wind zones and at corners. This can be mitigated by creating a situation where the bottom chord also extends out to meet the top chord.
  • Carefully consider any minimum gauges you specify in the contract documents--this tends to have a large effect on cost and is unlikely to make the overall truss design any stronger because the designer will reduce the amount of webs and the material sizes for efficiency. Generally, this should only be done when needed to obtain a specific screw pull-out value, and then only on the relevant members.
  • Top chord live load is normally 16-30 lb/sq. ft.
  • Top chord dead load is normally 10-20 lb/sq. ft.
  • Bottom chord live load is normally 0-10 lb/sq. ft., except in the case of attics
  • Bottom chord dead load is normally 5-15 lb/sq. ft.
  • When specifying truss loading, the wording should indicate if the dead load does or does not include truss self weight.
  • Clearly define the location and magnitude of any shear values that need to be transfered.
  • We recommend delegating the engineering of connections and bracing to the truss manufacturer. (require stamped calculations)
  • Carefully consider how to handle wind loads on large gable ends--these loads can be deceptively hard to deal with at times.

If you have questions about any of these considerations, please feel free to contact us.
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