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.

Fast forward several years to the present. The topic and audience are different, but the issue is still the same – it’s difficult to change the way something has always been done. Take snow loading on trusses as an example.  Historically, snow load has been lumped in as part of the top chord live loading on a truss.  A long-standing practice in many areas has been to take the ground snow load and simply enter it into the Top Chord Live Load (TCLL) box in the truss design software. Even the truss design standard, ANSI/TPI 1, and the IRC/IBC codes have included snow load as part of the top chord live load in the list of required design loads to be included on the truss design drawing:

List of required design loads to be included on the truss design drawing

List of required design loads to be included on the truss design drawing

The only problem is that snow load is not a live load, and no additional snow load considerations, such as unbalanced snow loads, are taken into account when it is applied as a live load in the design program.

This may in fact be acceptable at times, particularly when the full ground snow is used as the top chord live load. After all, this is in-line with the prescriptive approach taken in the IRC, as specified in section R301.6 Roof Load:

Roof Load

Roof Load

where Table R301.2.(1) is based on the local ground snow load. In many jurisdictions, the use of the full ground snow load for the balanced snow case is considered adequate to address any other snow-related effects including unbalanced snow loads.

The alternative approach is to treat snow loads as snow loads and live loads as live loads, and actually design the truss for the input snow loads and corresponding snow load design criteria. This puts all of the relevant snow loading parameters right onto the truss design drawing. However, because of the historical precedence to treat snow loads as live loads, this method has actually caused confusion in some Building Departments. Some departments see both a snow load and a live load and get confused by the live load. Some want to see snow load, but only the ground snow load. Others say they want to see a TCLL on the drawing and that’s it. Interestingly, the IBC-09 actually modified its provision regarding truss design drawings to remove snow load from the top chord live load provision and list it separately as part of the environmental loads:

Design Loads

Design Loads

Being from snow country (and actually being a fan of the white stuff every year), I might be a bit biased, but I think the IBC change is a change for the better.  Maybe it will help remind people that snow loads are not live loads. I’m not saying that ground snow shouldn’t ever be used as the roof design load; I’m just saying it should still be called (and reported as) a snow load. I think that’s an important first step to making sure everyone in the job is on the same page regarding what snow load considerations have (and have not) been included in the design.

What are your thoughts about snow loads being treated as live loads in the design of roof trusses? Let us know in the comments below.

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.

Step Down Hip System, Plan View

Step Down Hip System, Plan View

Step Down Hip System, Rendering

Step Down Hip System, Rendering

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)

 

Lay-In Gable Hip System, Rendering

Lay-In Gable Hip System, Rendering

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.

Dutch Hip System, Plan View

Dutch Hip System, Plan View

Dutch Hip System, Rendering

Dutch Hip System, Rendering

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.

Terminal Hip System, Plan View

Terminal Hip System, Plan View

Terminal Hip System, Plan Rendering

Terminal Hip System, Plan Rendering 

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.