Truss Blocking Panels
Truss Blocking Panels
concept for the SBC Research Institute.
A new provision in the 2009 IRC, and carried through to more recent versions, is the use of blocking panels between roof trusses to connect the trusses to the braced wall panels below if the heel height is greater than 9¼". For trusses with heel heights less than 15¼", this blocking can be made of solid sawn dimensional lumber as shown in Figure 1A. However, if the heel height is greater than 15¼", a horizontal soffit panel or vertical blocking panel must be used as detailed in Figure 1B or 1C, or a blocking panel must be designed in accordance with accepted engineering practice.
Parallel chord roof trusses and energy heel trusses often exceed a 15¼" heel height and have need for this more elaborate method of blocking.
A set of plans and specifications will often provide the lateral load the blocking must resist and specify that the truss supplier design the blocking to resist that load. Ventilation requirements may be stated on the plans as well. Figure 2 shows an example of this type of blocking detail.
In the details shown in Figure 2, the blocking is to be designed for 300 plf of lateral load. The detail requires 2"x3" notches in the blocking panel for ventilation and requires the roof sheathing to be fastened to the blocking at 6" on center.
Component manufacturers are faced with two issues: (1) the design of a structural blocking component (aka, blocking panel) to resist the lateral load, and (2) the design of the connection of the structural blocking component to the trusses, roof sheathing and wall framing to resist that applied load while still maintaining adequate ventilation. These designs are difficult to analyze using engineering mechanics, due to the many different components that must interact to transfer the loads from the roof down to the wall. Componentizing this detail and getting paid for the technical work put into providing proper resistance of the loads is also essential and may be as complicated as the engineering mechanics.
To address these two issues, testing could be conducted to determine when blocking is necessary and to evaluate the capacity of heel blocks and their connections to the framing. Some of the factors that need to be investigated include the design of the blocking panel, effectiveness of partial height blocking, effect of different heel heights, and various blocking-to-framing fastening methods.
It is also important to think about this detail in the context of the overall building performance. Some of the questions that come to mind include, but are not limited to:
- Will a properly braced truss roof system actually rotate as suggested by the IRC provisions?
- What is the capacity of the roof and ceiling diaphragm as a system? Does the diaphragm performance change in the context of the interacting assemblies that constitute the building system?
- How is the lateral load distributed between the roof and the ceiling diaphragm and how much rotational (overturning) force will result from the eccentricity of the lateral load?
- What effect does gable end bracing have on the rotational restraint of the roof system?
The ideal test setup to evaluate these questions would contain a full roof assembly capable of simulating actual building construction.
Another benefit to testing this connection as part of a roof system would be the ability to define its capacity under loading from different directions such as uplift. The blocking panels will provide additional connections between the roof trusses and the top plate of the wall. These connections could allow a greater uplift force to be resisted.
A truss to top plate connection consisting of three 16d box (3½" long x 0.135" diameter) nails is allowed to resist up to 200 pounds of uplift force per IRC Section R802.11.1.
Roof assemblies shall have uplift resistance in accordance with Sections R802.11.1.2 and R802.11.1.3.
Where the basic wind speed does not exceed 90 mph, the wind exposure category is B, the roof pitch is 5:12 or greater, and the roof span is 32 feet (9754 mm) or less, rafters and trusses spaced not more than 24 inches (610 mm) on center shall be permitted to be attached to their supporting wall assemblies in accordance with Table R602.3(1).
In IRC Table R602.3(1) (at right), blocking between roof trusses is required to be fastened to the top plate with three 8d (2½" long x 0.113" diameter) toe nails.
The 200 pounds of uplift force allowed by the IRC does not take into account the nailing between the trusses, the blocking and the top plate, when blocking is provided. There is an unquantified load path resistance interaction present. If greater uplift resistance is present, the cost of providing a more complex blocking method may be partially offset by being more creative in applying uplift resistance connection systems.
Testing a full roof system would also allow the capacity of the truss to top plate connection to be evaluated under combined uplift and lateral loading. This loading condition is often evaluated using a unity equation, which takes the sum of the load divided by the capacity for each direction and sets it less than or equal to one, as shown in the following equation:*
The SBC Research Institute (SBCRI) is unaware of this unity equation being evaluated to verify applicability under combined loading conditions. SBCRI is also unaware of how the assumed wind loading condition applies to the roof structure in the real world and how the real-world loading condition gets resolved through the series of resistance connections that will exist. The applied loads and the resistance interactions seem more complicated than the simplifications that are provided by the code and unsupported by research analytics. Clearly, testing would allow for a better understanding of how combined loading affects the connection capacity and allow for more accurate designs.
SBCRI is working on refining the design for a roof assembly testing fixture that can be used to test structural elements and connections in lateral shear, uplift, and a combination of lateral shear and uplift forces (see Figure 3). This assembly will be ideal for testing different blocking methods. A clear goal will be to fully understand applied loads and resistances. Another goal may be to develop generic tested capacities for blocking elements that the component manufacturing industry could manufacture and sell to contractors to satisfy the IRC requirements. A final goal will be to create industry data defining the true performance characteristics of details like this. Once a better understanding is established, the creativity and innovation that exist in our industry certainly can come up with valuable resistance solutions. Ultimately, this type of information would provide roof trusses an added advantage over prescriptive approaches such as conventional stick framing.
*See strongtie.com/productuse/designer.html, strongtie.com/ftp/catalogs/c-hw12/C-HW12.pdf and strongtie.com/ftp/bulletins/t-01wfcm08.pdf