Why a Structural Boundary Member between a Truss/Rafter is known as aTruss Heel Blocking.
The use of blocking panels between roof trusses and braced wall panels below if the heel height is greater than 914" is a new provision carried through to more recent versions.As shown in Figure 1A, this blocking can be made of solid sawn-dimensional lumber.The horizontal and vertical blocking panels must be designed in accordance with accepted engineering practice if the heel height is greater than 1414".
There is a need for this more elaborate method of blocking for roof trusses that exceed a 1514" heel height.
A set of plans and specifications will give the load that the blocking must resist, and the supplier can design the block to resist that load.There may be requirements for air quality on the plans.This type of blocking detail is shown in Figure 2.
The blocking is to be designed for 300 plf of load.The detail requires the roof to be fastened to the blocking at the center with a notch in the panel for ventilation.
Component manufacturers are faced with two issues, one of which is the design of a structural blocking component to resist the load applied to it, and the other, the connection of the structure to the roof and wall framing.These designs are difficult to analyze using engineering mechanics because of the many different components that must interact to transfer the loads from the roof down to the wall.Getting paid for the technical work put into providing proper resistance of the loads is as complicated as the engineering mechanics.
Testing could be conducted to determine when blocking is necessary and to evaluate the capacity of heel blocks and their connections to the framing.The design of the blocking panel, effectiveness of partial height blocking, effect of different heel heights, and various blocking-to-framing fastening methods need to be investigated.
In the context of the overall building performance, it is important to think about this detail.Some of the questions that come to mind are not limited to.
A full roof assembly would be the ideal test setup to evaluate these questions.
The ability to define its capacity under loading from different directions would be a benefit to testing this connection as part of a roof system.There are additional connections between the roof trusses and the top plate of the wall with the blocking panels.The force could be resisted by these connections.
A truss to top plate connection consisting of three nails is allowed to resist up to 200 pounds of force.
The wind exposure category is B if the basic wind speed does not exceed 90 mph and the roof pitch is 5:12 or greater.
The blocking between roof trusses needs to be fastened to the top plate with three 8d toe nails.
The blocking and the top plate are not taken into account when the 200 pounds of uplift force is allowed.An unquantified load path resistance interaction is present.The cost of providing a more complex blocking method may be offset by being more creative in applying uplift resistance connection systems.
Testing a full roof system would allow the capacity of the truss to top plate connection to be evaluated.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 unity equation is being evaluated to verify applicability under combined loading conditions.The assumed wind loading condition affects the roof structure in the real world and how it gets resolved through the series of resistance connections that will exist.The applied loads and resistance interactions seem more complicated than the simplifications provided by the code.Testing would allow for a better understanding of how combined loading affects the connection capacity.
The design of a roof assembly testing fixture that can be used to test structural elements and connections in a combination of shear and uplift forces is being refined.Testing different blocking methods can be done with this assembly.The goal is to fully understand applied loads and resistances.A 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 requirements.The goal is 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 can come up with valuable resistance solutions.This type of information would give roof trusses an advantage over other approaches.
You can find strongtie.com/productuse/designer.html, strong Tie.Com/ftp/catalogs/c-hw12/C-HW12 and Strong Tie's website.