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Truss-to-Truss and Truss-to-Everything Else Connections

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?

Excerpt from ANSI/TPI 1-2007 Chapter 2 on Design Responsibilities

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.

Truss-to-Top Plate and Truss-to-Stud Connections

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.

Snow Loads vs. Top Chord Live Loads – A Historical Look at Snow Loading for Trusses

In my former life working as a consulting engineer, I reviewed many truss submittal packages. I remember during my reviews wondering how it was possible to get so much information on to an 8½ inch by 11 inch piece of paper. I also remember how a lot of what was being reported was difficult to understand without some help interpreting the information.

As many of you may know, Simpson Strong-Tie has ventured into the truss industry and we are now offering truss connector plates and software to component manufacturers around the country. So given my past experiences, I figure some of you might appreciate some insight into the engineering that goes on behind those truss submittal packages. So I have asked one of our great truss engineers, Kelly Sias, to put together some blog posts on the topic that we can share our knowledge with you. Kelly has worked in the truss industry for years and spent time as the Technical Director at the Truss Plate Institute. I am sure her blog posts are going to help all of us have a better appreciation for trusses.

Have you ever been involved in a discussion with someone on a project that ended with “but that’s the way we’ve always done it!”? I heard those words spoken by a contractor in my first engineering job when I tried explaining why his single stud would not work at a particular location. When he said something about his grandfather having always done it that way, I knew I could explain the calculations all day and it wouldn’t do much good.Continue Reading

Plated Wood Truss Hip End Styles

For many, the first day of summer means it is time to cinch up your favorite hip-hugging bathing suit and enjoy the warm weather. For the truss industry, it’s time to keep those hip-hugging bathing suits in the closet and take advantage of the favorable weather months by bidding and building as many jobs as possible. During the bid and build frenzy, there will be several hip end jobs leaving truss yards across the country, but what exactly is a hip end and what are the different styles?

Truss hip ends drawing — Roof with Multiple Hip Ends (blue), Plan View

The Structural Building Components Association website (SBCA) defines a hip roof as a “Roof system in which the slope of the roof at the end walls of the building is perpendicular to the slope of the roof along the sides of the building.” While framing terms differ by region, most trussed hip end systems will include hip trusses, jack trusses (end and side) and a rafter or corner girder truss. Hip end style and setback (distance from side or end walls to the hip girder truss) may also vary by building design and region.

In the western part of the country, a California Hip system is typically seen in many trussed structures. In this hip system, the hip truss flat top chord is dropped by the plumb cut of the jack top chord at the roof pitch. By doing this, the top chords of the end jack trusses can pass over and bear on the dropped flat top chords. As the height of the hip end roof plane increases, the height of the flat top chord also increases, though the interval at which the flat top chord height increases may vary by building design and region.

Truss Design: California Hip System — California Hip System, Plan View

Truss Design: California Hip Rendering — California Hip System, Rendering

East of the Rocky Mountains, the California Hip is rare and a Step-Down Hip system is more popular. Differing from the California Hip, a Step-Down Hip system is one where every truss under the hip end plane decreases in height, or “steps down” from the apex until it reaches the hip girder, which is placed at a pre-determined setback.

Less regional and more situational depending on the building design, are the Lay-In Gable, Dutch and Terminal Hip systems. The Lay-In Gable Hip system is one with many regional names and shares similarities with the California and Step Hip systems. Like the Step Hip, every truss steps down moving from the apex to the setback. Like the California, every truss flat top chord has a drop. However, the flat top chord is dropped by the plumb cut of a 1.5” member at the roof pitch, as the gable frame lays flat.

Roof System: Lay-In Gable Hip System, Plan View — Lay-In Gable Hip System, Plan View (Gable Frame shown in green for clarity)

In a Dutch Hip system, the hip end roof plane does not converge with the side planes to form an apex. Instead, the hip end plane pitches directly into the girder truss that is placed at a predetermined setback. Jack trusses then connect to the hip girder truss or to a ledger attached to the hip girder truss. This hip system is also referred to as a Dutch Gable.

Assuming like roof pitches and heel heights, a Terminal Hip system is one where the hip girder truss setback is half of the main truss span or building width. If pitches and heel heights vary, the girder truss is placed at the apex of the three converging roof planes, which could be more or less than half of the main truss span or building width.

While these are some common hip end styles in the truss industry, there are definitely others. Each style has its own advantages and disadvantages, and a discussion of those will be the topic of a future post.

What other types of hip end styles are you familiar with? Let us know in the comment section below.