Powerful Tools Require Powerful Users – Designing with Today’s Tools

Technology and industrial advancements have simplified and reduced the intellectual and physical demands on jobs in the structural building components industry. Whether it’s the operator behind the saw, production staff setting up the table, or the designer in the office, computers have made the job faster and easier. Despite all of these advancements, it’s important for component manufacturers (CMs) to be mindful of some of the important lessons that can get lost in the shuffle of technological improvements.

Question

The issue recently came up at our plant of a “knowledge gap” between relatively new staff and those who have worked in the industry for decades. What are some examples of this gap that CMs could use to train staff?

Answer

For those of us who have been in the truss industry a while, we have seen amazing advancements in the tools used to design and manufacture components. Truss design engines have improved so much that, in many cases, they appear to do all of the work for Truss Designers. Therein lies the problem. In the plant, CMs have migrated to computerized saws and setup stations to speed up the fabrication process. This has been a great advancement in the plant and allowed CMs to get operators up to speed quicker and increase production. The down side to all this technology is that operators have lost some of the knowledge and “tricks of the trade” that were learned and passed down in years past.

The same problem holds true in the design realm but to an even greater extent. Design programs automatically perform many of the tasks that designers have done for years, from optimizing and aligning webs, checking inventory, matching splices, loading girders, selecting hangers, and applying wind and snow loads. Just like the saws in the plant, this has sped up the design process drastically; however, the user must beware. Even with the best technology, garbage in still equals garbage out. With program advancements and automatic loading from the layout, Truss Designers can fall into the trap of believing that all they need to do is clear up whatever truss design issue is causing the truss not to work and move to the next truss. This could be referred to as the “Visine Method” because all the designer tries to do is get the red out of the analysis screen. Seventy percent to 80 percent of the time, this method works, but let’s look an example where it doesn’t.

Example

A building measures 80' from outside of bearing to outside of bearing. The 80' span is framed with two 20' mono roof trusses at each end (see Figure 1) and a 40' common roof truss in the center (see Figure 2). The interior end of the mono trusses and the 40' common trusses will share a bearing located 20' in from each outside wall.  The 40' truss will have raised heels to match the depth of the monos at the common bearing. Unless special precautions are made, the design program may assume a 40' truss for the purposes of snow loading (see Figure 3) and not recognize that, when it is installed in the field, it is really an 80' truss over four bearings (see Figure 4). The 20' monos will be treated as a two mono trusses combined into a 40' span.

When these drawings/design parameters are sent into the Truss Design Engineer for preparation and sealing, he or she will look at the truss as an individual component and seal the design as it was input by the Truss Designer. The end result may be that the 40' center portion of the truss will be under loaded because the surcharge due to unbalanced snow load will be based on a 40' span and not the actual 80'.

Conversely, the 20' monos may have a heavier than required snow load applied to them. The program may assume that the peak of the mono is at the peak of the truss and will apply an unbalanced load to the peak of the truss, whereas, in the actual installed condition, this load doesn’t exist.

In order to design the trusses in this example correctly, the Truss Designer needs to have a good understanding of how the specific design software treats each special condition and the limitations on its ability to understand loading in the context of auto-loading features. The software provider can most likely offer a “work around” solution, if it is determined that a direct design approach is not possible.

The Truss Design Engineer reviews the design parameters of each truss as an individual component and, in turn, prepares and seals the design. The responsibilities of the Truss Design Engineer and the Truss Designer are clearly defined in TPI 1 Chapter 2 (see inset). It’s important to remember that the Truss Design Engineer relies upon the Truss Designer to take-off the proper loads from the building’s Construction Documents. The component must be defined and the design parameters input correctly in order for the truss to be designed correctly.

The snow and wind loading sections in design programs are very powerful tools, if used properly. These programs include many different input settings that are used to calculate loads in conformance with the requirements of the building code. While it is the Building Designer’s responsibility to determine and provide the correct information for each job, the Truss Designer needs to have a basic understanding of the loading conditions and the building design defined load path and how it relates to the job they are designing.

The technical department at each truss plant should have copies of the building codes for the areas in which the CM transacts business. They should also have a copy of ASCE-7 – Minimum Design Loads of Buildings and Other Structures. The SBCA Load Guide, sbcindustry.com/loads.php, is another excellent source of information. This spreadsheet is a loading code compliance reference tool that includes code-based equations that have been incorporated into load macros and calculators to check and verify the loads as defined by the Building Designer. Ideally, this tool is used by the Building Designer to provide the loading conditions for the project, which are then used to design the trusses. This information should be incorporated into training for all Truss Designers.

In addition to training, implementing a quality control (QC) back check process in the technical department is also crucial. This internal QC should not only review conformity to the Building Designer’s Construction Documents but also ensure that the applied loads are what would normally be expected for a project in the given location. Fully understanding the capabilities and limitations of our industry’s very powerful truss design software will help move jobs through the plant and avoid costly and time-consuming call backs or repairs.