By Seth Duncan
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Designating “Trusses by Others” on your building plans may sound trivial, but, all too often, the process for obtaining them is not. Those “others” who design and manufacture trusses from construction documents were asked what advice they would give to the building designers that create them. Everyone involved wants trusses with sufficient performance that are cost-effective, and they want a painless process for defining their requirements. Answering as many of their questions up-front as possible, along with ones they may not think to ask, is the best way to ensure you get truss designs that work without endangering your project’s timeline.
Dimensions
Most engineers will refer you to the architect’s drawings for the building’s dimensions, but some of them are so critical to truss performance that they are worth verifying before truss design begins: heel heights, overhangs, and bearing locations. The first two have to do with defining the roof envelope. Truss heel heights are generally determined by where the roof plane meets the bearing component (e.g. a wall or beam). These need to be clearly marked (Figure 1), and they also have to be tall enough to allow the chords of a truss to fit. Roomier heels often allow for more efficient truss designs too, which means savings on material costs. Overhangs, or any protrusion of the truss from the exterior of the structure, can experience significant uplift forces when they’re exposed to wind. These lengths, most commonly top chord overhangs, are easy for truss designers to miss, particularly when they vary within a project.
Truss bearings are typically going to be a wall or beam, and these need to have their locations and heights clearly conveyed (Figure 2)—again, especially when they vary within a project. Misplaced bearings have all sorts of ramifications for truss design, from causing unnecessary chord steps to heavy reinforcement in the wrong parts of the truss, and they are sure to confound the framers in the field.
Clash Prevention
Truss repairs can be costly both in terms of time and money, and one of the most preventable causes for truss repair is clashes in the field. This can come from HVAC requirements for openings for ductwork and ventilation (Figure 3) or plumbers needing space for supply pipes and drains. You don’t want to wait until the trusses are in place to find out they need to be shifted over or cut into, or to find out that new girders will be needed in order to accommodate other parts of your structure. This is especially important for HVAC running through a floor, as floor girders can be hard to design with large openings due to their restricted depth and webbing options (Figure 4). These are all good examples of where it can pay off to include the truss designer early in the planning stages.
Loading and Analysis
An overwhelming number of parameters affect the loading and analysis of trusses. Unless they receive explicit instructions, the truss designer is unlikely to deviate from the default settings in their software except when it helps them design the truss with cheaper materials. The defaults in their truss software may be overly-conservative, leading to costly overdesigning, while changing settings for which no parameters were provided to them could lead to trusses that meet all the specifications while underperforming on-site. Supplying all of the relevant constraints is the best way to avoid these problems.
Clear communication is essential when it comes to the project’s loading as a lot of factors go into developing the loads for a structure besides the basic live and dead loading for the chords. First, those dead loads may or may not account for the weight of the lumber itself, so a careful designer will assume that weight needs to be added in unless they are told that it has already been accounted for. Another common source of suboptimal loading is storage loads in residential attics. Most trusses need to be checked for whether a 42 inches high by 24 inches wide box can fit anywhere along its bottom chord, but IRC 2009 and newer editions allow storage loading to be omitted when the bottom chord is going to be covered by insulation and the application of the project is residential. There are also exceptions in some codes for structures without an attic access of 20 inches x 30 inches.
For wind loading, you may want to specify whether to use the Envelope or Directional procedure. The envelope procedure is a newer method that can be used for “low-rise” buildings with a mean roof height less than or equal to 60 feet and is typically the more accurate of the two methods for those structures.
Snow loading can be significantly affected by the conditions in which the structure exists. The terrain and exposure categories describe to what extent wind will help prevent snow buildup on the roof, the thermal factor takes into account whether or not the structure experiences sustained freezing temperatures below its roof in winter, and estimates of snow accumulation can be dramatically affected by the “slipperiness” of the roof as well as how well-ventilated it is.
Commercial structures in particular are more likely to have things like towers or other structures that require bracing to be attached to the roof (Figure 5). Experienced truss designers will notice items like this and either request clarification or make some assumptions.
Perhaps the most important inputs the building designer can provide into the truss analysis are the deflection limits. The building code has standards for this, of course, but you may find there are cases where the code allows for long-span trusses to deflect up to 2 inches, which can lead to problems like ceiling cracks. ANSI/TPI 1-2022 Table 7.6-1 has deflection limits that account for long-term deflection. Using more stringent deflection criteria and/or compensating for bottom chord deflection by specifying sufficient camber, or upward curvature, of the bottom chords may be necessary, especially for long, unsupported spans. The stakes are even higher for floor trusses, where certain flooring materials like stone, concrete, and tile are especially prone to cracking.
Material Cost and Design Time
There are many ways that insufficient planning up-front can lead to unnecessarily expensive trusses. One of the most common is the case of locating girders, or the trusses, that will support sets of trusses. Here are some rules of thumb: first, place a girder at a right angle to whatever it will be carrying whenever possible. That means to avoid placing girders on valley lines and at an angle relative to the wall. Angled truss-truss connections are significantly more expensive than perpendicular ones, so use the latter as much as possible. Another is to minimize the hip girder’s setback distance from the wall, ideally about 6 feet or less in residential construction. Again, the reason is to minimize the cost of the truss-to-truss connections, and, in many cases allowing for nailed connections in place of an expensive hanger.
On commercial projects, seeing if a girder is a viable option ahead of time may save money and frustration down the road. When loads are high, sometimes it is more cost effective to use a LVL or steel beam to support an area of the roof or floor as opposed to a massive girder. The last thing you want is to specify “Truss Girder by Others” only to find out when the trusses are being designed that a girder fails in that area. Having to change the architectural design of a building to accommodate an unexpected beam or column is something that can severely slow a project down.
Another way to reduce material costs is to keep the building as symmetrical and consistent as possible. The more times you can reuse a truss design, the less time designers have to spend creating truss designs and the more efficiently the trusses can be manufactured. The more you can avoid variations in wall height, roof pitches, etc., the more economical the trusses are going to be. It’s also helpful to consider allowing for trusses to have a consistent, even spacing. This convention comes from trussed roofs with plywood sheathing since plywood is sold in 8-foot sheets. Committing to a multiple of two for your truss spacing can significantly reduce framing time and material costs.
For any project, looking at the “worst case” truss design from the whole layout to find out if a design will work ahead of time is a good way to avoid costly redesigns down the road. The sooner you find what it takes to get that truss to work, the sooner you can update your cost estimates accordingly or begin the redesign process before too much work must be redone. Making the entire floor deeper or raising the heel height to improve truss performance is much easier at the beginning of the design process than at the end.
Connections and Bracing
Once truss design has been completed, there are still some truss-related items for which the structural engineer is responsible. The first is the plan for attaching trusses to their bearings. The finalized truss designs will include the reactions at each bearing, but a plan for attaching them to the building does not come with the trusses. In some cases, the earlier you consider this the better—you don’t want to wait until the trusses have been fully designed to realize there isn’t enough heel height to tie the trusses to the walls in a high-wind area.
The other remaining work is coming up with a bracing plan. Continuous lateral bracing is needed to prevent truss buckling under construction loads. Also, some individual truss designs are going to require permanent bracing, and it is the building designer’s responsibility to determine how to attach that bracing to the trusses as well as how to tie that bracing into the rest of the building.
Conclusion
The more complex the project, the more critical effective communication becomes, and the earlier that communication begins, the better. As Kirk Grundahl put it in a STRUCTURE magazine article in March 2020:
“One solution that works well is Building Designers and General Contractors (GCs) who commit to work with a specific CM [component manufacturer] early in the project life cycle. Communication and collaboration at the design development stage of any project solve many of the problems that typically present themselves during a deferred submission review and revision process.”
The increasing popularity of the Design-Build model means that building designers need finished truss designs much earlier in the submittal process, but since truss designers work for component manufacturers, getting truss designs typically means committing to use that truss manufacturer to produce the trusses up-front. The market is ripe for innovative solutions to this challenge. Whenever a truss designer does get involved, their needs are synonymous with the needs of an effective structure, and effectively anticipating those needs will lead to better outcomes for the quality and costs of all your truss projects. ■
About the Author
Seth Duncan is the Director of Operations for Truss Pal, a company that provides access to wood truss design and analysis information. When he’s not standing in for your buddy at the truss shop who helps answer your truss questions, Duncan likes spending time with his family outdoors. Connect with him on LinkedIn or visit trusspal.com.