Engineering

  • Based on the recommendations of the IRC, IBC and ASCE 7, truss or rafter uplift connections should be designed for applied wind loads using MWFRS analysis.
  • Individual truss and rafter members should be designed using C&C generated wind loads.
  • The SBCA Load Guide includes information about uplift connections for structural building components.
  • Two engineers involved in the design of structural building components respond to a previous Technical Q&A on bearing area.
  • Both give their perspective on bearing area and ways that component manufacturers and truss designers can help engineers with this issue.
  • Each engineer discusses his preferred method for dealing with insufficient bearing area.
  • The Building Designer providing accurate wall or beam/header information in the Construction Documents is essential in order for the Truss Designer to arrive at the correct bearing width.
  • There are two key bearing related considerations—bearing capacity of the truss and bearing capacity of what the truss is sitting on; often, the bearing capacity of this material is an unknown and an assumption must be made.
  • The options for dealing with insufficient bearing need to be considered and specifically dealt with on a case-by-case basis.
  • Plywood and OSB design values are given; those that are doing repairs in your office should have a good feel for the similarities and differences.
  • Plywood and OSB generally have similar design properties with a key exception of fastener strength where plywood will require more fasteners to be used.
  • If a truss repair specifies only OSB, plywood should not be substituted without written permission from the registered design professional who prepared the truss repair design drawing.
  • The strength axis of a structural panel is the direction parallel to the grain of the wood fiber in the face and back surfaces of the panel.
  • The strength axis is usually the long dimension of the panel.
  • The IBC provides two tables with the allowable spans and loads (psf) for wood structural panel sheathing installed continuous over two or more spans with their strength axis perpendicular and parallel to the supports.
  • Standard metal connector plates are suitable for use in lumber with moisture content of 19 percent or less and in building applications with no exposure to corrosive substances.
  • Connector plates used in trusses exposed to high humidity or caustic chemicals may need to be treated to prevent corrosion.
  • A variety of protective coatings can be applied to truss places in situations where special protection is necessary.
  • There is often confusion in the marketplace about what criteria must be met to accept a new product and approve it for use in the intended application.
  • The IBC and IRC require products not approved within the code to be “at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.”
  • Manufacturers of products that do not have span tables or design values listed inside the IRC and/or IBC must provide design values for their products through independent testing.
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