Recently, while conducting an ANSI/TPI audit, a splice caught my eye. When I investigated further, I realized that the bottom chord of the truss I was investigating had a very short, 2” chord segment. I immediately assumed the truss builders had cut a short block to make up for a miss-cut length of a chord segment. When I reviewed the shop drawings, I realized that the truss designer had designed a truss where the middle chord segment was 2” long. [For all images, See PDF or View in Full Issue.] See the excerpt of the cut list and note item B2: This is Not an optimal situation.
We could debate the merits of plating a 2 x 3 ½ block – will it hold together without splitting into pieces while it transfers bending forces, and will it save money – but I believe the better discussion would be about the software. Specifically, shouldn’t we understand the limitations of design software? When the truss builder on the line asks, “Is this right?”, the answer should be more than just, “Build it per the drawing.”
About 6 months ago, an attic design caught my eye. The design had a rather large attic room span. I wondered if the truss had interior bearing. I requested an engineered drawing showing the calculations, lumber, particulars. Wow! I was surprised. The truss was designed as a clear span. In my decades of truss design experience, I was amazed the truss worked. Upon closer inspection, I realized that the truss was run on 2-inch centers… That would be an expensive house. I guess you could save on sheathing cost? Unfortunately, the results were generated by a less than scrupulous designer. They could not get the truss design to work, so they kept reducing the spacing until the truss ran. I was told it was just to get some cutting and pricing. The cutting made it to the shop… not a good thing.
These two situations illustrate the need for having an engineering review by an experienced truss designer. Truss design software can be misused, so blindly using designs without a qualified evaluation can lead to unacceptable outcomes.
One of the best ways to accomplish this evaluation would be to have every design sealed. Many companies do this on every truss, and it’s a very practical way to reduce risk. If not for every design, however, at the very least designs that are outside the ordinary should be reviewed and sealed.
Let’s review. Are seals required?
- Some municipalities require sealed drawings on every truss, so it’s simply the cost of doing business.
- If you are not in the group above, are you off the hook? No. Keep reading.
- Even if seals are not required, seals are a very practical way to reduce risk.
- Sometimes designers make mistakes.
- Sometimes designers don’t have the experience they need.
- Sometimes designers do things that they know they should not.
- Truss design software is only a tool – it’s not an engineer in a box.
- It’s probably overkill to reseal that 24’ fink truss you build 100 times per year, but what is the real cost?
- At least seal the unusual designs.
- Consider sealing girders.
- Absolutely seal anything over 50’ or field-spliced or deep scissors.
- If the project ever gets to court, I can guarantee it’s going to need a licensed engineer’s review and seal. Why not cover your bets? Get your seals before you build it. Beat the Christmas rush!
Many, many years ago I heard this comment by a prospective plate customer directed at another person but intentionally conveying a message to me. As a connector plate consultant, I listened very carefully. According to this person, “If the software generates a design, then the connector company should stand behind the design.” I responded sarcastically: “Right, because people never make mistakes and calculators are always right.” It was a cynical response, but my point was that a computer, a calculator, or a pencil will depend on the input data.
Sealed drawings may not be required, but they will keep you from hanging out on a limb!
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.