Blocking For Lateral Displacement And Rotation
I am a building inspector and have some questions regarding how to apply IBC 2012 2308.8.5 (IRC 2012 R802.8) (similar IBC 2015 2308.4.6 & IRC 2015 R802.8) to trusses, especially those with high heels:
2308.8.5 Lateral support. Floor, attic and roof framing with a nominal depth-to-thickness ratio greater than or equal to 5:1 shall have one edge held in line for the entire span. Where the nominal depth-to-thickness ratio of the framing member exceeds 6:1, there shall be one line of bridging for each 8 feet (2438 mm) of span, unless both edges of the member are held in line. The bridging shall consist of not less than 1-inch by 3-inch (25 mm by 76 mm) lumber, double nailed at each end, of equivalent metal bracing of equal rigidity, full-depth solid blocking or other approved means. A line of bridging shall also be required at supports where equivalent lateral support is not otherwise provided.
Commentary: When the depth-to-thickness ratio (based on nominal dimensions) of joists and rafters exceeds 5:1, as would be the case in members larger than 2 inches by 10 inches (51 mm by 254 mm), the minimum lateral support required by Section 2308.8.2 is not sufficient to prevent lateral buckling between supports. Additional restraint is required. Sheathing, subflooring, decking and similar materials attached to each joist or rafter are considered to provide edge restraint. These requirements are cumulative. The lateral support required by Section 2308.8.2 applies to all joists. Additionally, members greater than 2 inches by 10 inches (51 mm by 254 mm) must have one edge held in line, and members greater than 2 inches by 12 inches (51 mm by 305 mm) must have one edge held in line as well as a line of bridging at each 8 feet (2438 mm) of span (which may be omitted if both edges are held in line). The section also prescribes acceptable bridging methods.
Is this requirement for framing true for all truss end bearing conditions regardless of truss heel height? What are the proper methods to achieve solid blocking? Is the combination of roof sheathing, gypsum wallboard on the ceiling and a hurricane clip on one side of a truss an acceptable substitution for solid blocking? How would one solid block a truss at intermediate bearing points? Do truss engineers account for rotation and lateral displacement in their truss design or is this the responsibility of the structural engineer or building designer?
Let’s tackle those questions one at a time, starting with the last one:
Do truss engineers account for rotation and lateral displacement in their truss design or is this the responsibility of the structural engineer or building designer?
The truss designer assumes (per appendix C, part 3 of the Truss Plate Institute's ANSI/TPI 1, explained in SBCA’s BCSI-B1 Summary Sheet) that the truss will be installed in-plumb and in-plane and will carry only in-plane loads. The building designer is responsible for designing the system to resist forces from rotation and lateral displacement.
Is this requirement true for all end bearing conditions regardless of truss heel height?
Yes, but the block doesn't need to go the full height of the heel to effectively block it and keep it from rotating.
What are the proper methods to achieve solid blocking?
Blocks, blocking panels, and structural panels (plywood, OSB) are all acceptable.
Is the combination of roof sheathing, gypsum wallboard on the ceiling and a hurricane clip on one side of a truss an acceptable substitution for solid blocking?
No, these are not a substitute for blocking.
How would one solid block a truss at intermediate bearing points?
It may not be necessary to install a full height block like a shear panel. One option is to use 4x2 blocks 22 1/2 in. long (for trusses spaced 24 in. O. C.) at the wall to stop lateral displacement and 2X4 blocks at the top chord nailed to the plywood diaphragm to stop rotation. Again, this is the building designer's decision.
See IRC 2012/2015 R802.8, IRC 2012 R802.10.3 and IRC 2012/2015 R602.10.8, which includes blocking details specific to braced wall lines and especially related to seismic concerns.
For additional guidance, see SBC Magazine, which has included a number of articles related to blocking: Truss Blocking Panels, Ventilation v. Eaves Blocking in Seismic Zones, Connections for Tall Heel Trusses and Technical Q&A on Blocking for Trusses at Braced Wall Panel Locations, Heel Blocking, Connecting Wood Trusses to Braced Wall Panels.