One of the questions I am asked most frequently is “Who is responsible for the truss-to-(fill in the blank) connection? One such example is the truss-to-wall connection. To answer this question, it helps to recognize there are two types of connections: a truss-to-truss connection and a “truss-to-everything-else-except-a-truss” connection. The Truss Designer is responsible for the former, and the Building Designer is responsible for the latter. Pretty simple, right? So why all the questions?
Some people incorrectly assume the Truss Designer is responsible for connecting the truss to everything the truss touches. Then, when the Truss Designer doesn’t specify a connection to something the truss touches (such as a wall), it prompts the question, “Hey, who is responsible for that connection? I thought the Truss Designer was!” In other cases, the person asking the question is actually challenging the answer, such as “Shouldn’t the Truss Designer be specifying the truss-to-wall connection? Why don’t they?” And finally, the question may be prompted at times when the project doesn’t have a Project Engineer (aka the Building Designer), so the question becomes, “Now who is going to specify that connection? It must be the Truss Designer, right?”
But the Truss Designer isn’t responsible for the truss-to-wall connection – and here’s why. Unless the scope of work has been expanded by contract, the Truss Designer is responsible for designing an individual component. The truss gets designed for a given set of specified loads, environmental conditions, serviceability criteria and support locations, all which are specified by the person responsible for the overall building: the Building Designer. Once designed, the truss will have a maximum download reaction and uplift reaction (if applicable) at each support location. Is that enough information to specify a truss-to-wall connection? No, it is not. First, the Truss Designer may not know what the truss is even sitting on; he or she may only know that the bearing is SPF material and 3 ½” wide. Is it a single top plate or double top plate? Is there a stud below the truss that can be connected to, or is the stud offset? Or, is the truss sitting on a header spanning across a wide window?
Second, even if the Truss Designer had all of the information regarding the bearing conditions, there is another problem. The Truss Designer has the reactions resulting from the loads applied to the truss. What about the reaction at the top of the wall (perpendicular to the wall) resulting from the lateral loads applied to that wall? And the shear loads acting parallel to the wall as a result of lateral loads applied to the end wall? These loads also need to be resisted by the truss-to-wall connection (hence, the F1 and F2 allowable loads that are published for hurricane ties), so the Truss Designer cannot select an adequate truss-to-wall connection based on the truss reactions alone.
Finally, there’s one more scenario to consider. Say a Building Department requires that truss-to-wall connections must be specified by the Truss Designer on projects that have no Engineer of Record. It wants to ensure trusses are adequately secured to the walls, and the Truss Designer may seem best equipped to determine those connections (this has actually happened in some places). The Truss Designer can find out what exactly the truss is sitting on, and can even calculate some approximate reactions for the top of the wall to conservatively take into account during the selection of the connection. Problem solved? Not entirely. That takes care of the top of the wall, but the load doesn’t stop there. So requiring the Truss Designer to specify the truss-to-wall connection only transfers the problem to the bottom of the wall. Who is going to address those connections?
While most people don’t think of the Truss Designer as being the person responsible for the connections at the bottom of the wall, many do think the Truss Designer should be responsible for the connections at the top of the wall. But because someone – namely, the Building Designer – still needs to ensure that a continuous load path has been satisfied by the connections in the building, does it really help to increase the scope of work of the Truss Designer to specify the truss-to-wall connection?
Let us know your thoughts in the comments below.