structural DESIGN Deferred Design
Employing effective, early communication ensures you get accurate, on-time truss designs. By Seth Duncan
D
esignating “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 10 STRUCTURE magazine
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 Figure 2. Example of LVL beam specified without a clear their restricted depth height dimension on the structural drawings. 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
Figure 1. An example of truss overhang and heel height details is shown.