You probably wouldn’t ignore a “warning sign” on the highway, as you can easily imagine the consequences of doing so. We can face similar situations when designing trusses, and the end result is that whatever we do in the software can end up in the real world somewhere on a jobsite.
Because of the nature of this job as a designer, it is common to lose touch with the reality of the rest of the process. A component designer can be jumping from one job to another during the day and be involved in several projects at the same time. We may fail to see or, worse, may ignore some warning signs along the way (either in an office or a remote location). Sometimes the result is that we simply are unaware of some warning messages.
One of these “warning signs” is the PLY-TO-PLY-NAILING issue. In MiTek, this message is oftentimes simply overlooked, as VersaTrus does not give you the Red “X” symbolizing a failed truss. Some designers think of it as just a warning and, in a rush, could overlook it.
Some facilities do not assemble plies in the plant and leave that responsibility up to the General Contractor or Framer. Some do assemble. This does not mean a designer can overlook it. Additionally, in recent versions of MiTek, ply-to-ply nailing is considered a failed truss and obligates the designer to correct it. This happens most often on roof girders which could require 3 or 4 plies.
Do we, as designers, truly understand what this “failure” means though? If our truss has a Green “check” next to it, then it works, right?
Actually, trusses work as a single entity of themselves as they apply to a truss system. These factors are totally contingent upon what settings are selected based on the reality of materials to be applied on the job site. However, when multi-ply trusses are combined, each single ply of a truss entity becomes joined together. Multi-ply trusses are commonly known as “girders.” A girder is a truss that carries another truss (or carries some type of load or framing on the project).
What this error message is telling you is that:
- The connection of the plies as they attach to each other is what is failing.
- The truss itself may work to carry the loading that attaches to it, but the fasteners of the ply attachment are inadequate.
- The total number of fasteners (nails/screws) is insufficient to be applied based on the selected chord and/or web size.
- The fasteners are shearing and must be upgraded.
When this failure occurs, we can have issues in the field such as:
- Shear transfer problems
- Unsafe installation
- Future failure and collapse
- Lives placed in danger
Maybe the main problem is the shear stress, because forces are acting parallel to a rectangular surface causing displacement and producing failure.
We have all been there…worked on a big job for several days, now are down to the last girder or two, and it just will not work. It passes, but has a ply nailing issue! We “play” with it a bit to try to get it to pass but nothing works. We are stumped. With 613 designs in the job, it will take a bunch of re-work (not to mention lost productivity) to get this job off our desk. What do we do?
There are 6 common ways to resolve this matter:
- Change chord size(s): The simplest and most efficient way is to bump the Top Chord or Bottom Chord sizes. Sometimes it is necessary to do both and, depending on the severity, you may even need to bump up web sizes.
- Verify that truss webs are being used to distribute load: Each manufacturer has its own Design Defaults. Some choose to transfer ply attachment only through the truss chords. It may be necessary, at times, to allow the webbing to be used also.
- Adjust connector type in engineering: The next alternative is to use the analysis to your advantage and adjust the “Nails and Screws” attaching the plies together. There, you can change your nail conditions or select screws. Selecting a stronger connector could solve your issue. Realize though, this method will require that you stock the specified fasteners, and that production staff may need to assemble the plies in the facility. On a truss like this, even if your salesman does not want them fastened in the plant, you probably should in this case. Leaving this up to the guys in the field may cause an additional problem that could be very costly.
- Verify layout: This problem commonly happens when a girder is carrying an important section of the roof, directly or indirectly (maybe is supporting another girder). So, check if there is a different way to configure truss placement. Determine if there is a way to avoid the girder or reduce the load. Also, you may consider LL reduction or spacing reduction to reflect actual girder spacing (rarely is a girder truly at 24” spacing).
- Review plans: Verify walls that you may not be considering as load bearing. If you have to select an interior bearing to be used, always check the support below so reactions are transferred to a member capable of supporting that load. If you are also providing the floor system (4X2 trusses or EWP), remember to add that extra loading and support to the system itself; if you aren’t, still make sure to provide all the info in your layout so your salesman and the builder are aware, and it can be reviewed by the Engineer of Record.
- Notify your peers and/or supervisors: If the problem is over your head, and you just are not sure what to do, then you can do something else: Ask for help! Chances are another staff member has encountered this before and can assist you efficiently to keep you moving forward.
Heed the Signs
Not knowing the answer — ask for help — is not a sign of weakness. It is a way to recognize the world around you and improve yourself and your perspective. If you have to use an unconventional solution, then notify the customer so they can discuss the issue with you and the other parties involved.
If you decide to overlook a warning, then it is up to you to take responsibility for consequences that may result. Maybe you are not the only one implicated in a decision. Remember there are a lot of people trusting in your judgment and even lives could be in danger for overlooking a detail. Of course, the software could be far from real failure, but it is designed to work on the safe side, so we as designers can implement a similar mindset.
Starting in the industry as a teenager, Christopher Gould has experience in all aspects of component operations, including one of his startup companies becoming a globally known entity. Currently, he is a Design Professional at KA Components in Otterbein, IN.