Load Cases?

By Glenn Traylor

Years ago, truss design was focused on a simple calculation that determined the final forces that would be considered to be imparted on a truss component. These simple forces were used to determine the design. Things like snow, wind, and building materials weights were generally the only considerations made. Very quickly, however, the truss industry became more sophisticated and analyzed additional forces including seismic and long-term considerations. The main equation used is Newton’s Second Law:

F = Ma ; and we desire a = 0 ; so the sum of the forces on a truss are equal to zero or

∑F = (F1 + F2 + … + Fn) = 0 , where F is the forces on an object, M is the mass of the object, and a is the acceleration of the object. In truss design, we desire “a” acceleration to be zero except when dealing with seismic—but is that always the case?

Consider the active delivery of trusses from the roll-off trailer in this video posted on YouTube. The trusses even pick up speed as they slide downhill!

When trusses experience acceleration, a.k.a. movement, the forces acting upon these trusses can be tremendous. Potentially, the forces are a hundred times more than intended. More importantly, the truss designer would not calculate this sort of dynamic loading. But, how important is it to understand this situation? Is there any way to address this type of loading? How can we try to eliminate the “delivery load case”?

It’s a fact that truss delivery can be dramatic. Years ago, we used to deliver trusses with Barnes Trailers, which were pole trailers better designed for poles than trusses—talk about scary. But it’s also a fact that truss delivery doesn’t have to be traumatic. Today’s roll-off trailers are much improved from the olden days, and one of their advantages is being able to “sit” the trusses on the ground “gently” without the trauma.

Therefore, the question becomes: is it worth investing in more sophisticated equipment such as roll-off trailers? If you do, is additional training necessary? Example of a modern roll-off trailer

From the perspective of loads, both those loads acting on trusses and a load of trusses themselves, these are some items to consider.

1. When trusses experience acceleration, a.k.a movement, the forces acting on these trusses can be tremendous, potentially a hundred times more than an intended design.

2. Dynamic action can stress members and components past yield points, causing damage to your product.

3. Investing in decent delivery equipment will greatly increase your chances of eliminating dramatic deliveries.

4. Drivers should be trained to exercise extreme care when making deliveries.

5. Different types of lots, soil, and weather require different protocols.

6. Deliveries should be coordinated with the building contractor to designate a suitable drop area.

7. Customers should be informed of the need for a level area for the delivery.

8. When banding your trusses, keep the delivery in mind and use more banding than just what was necessary at the plant.

9. Consider nailing trusses together at tails with a 10d brite nail to help prevent sliding.

10. Use tools within your design software to upsize connectors to account for handling considerations or minimum tooth bite requirements for certain types of trusses.

11. Consider providing crane services to those clients with special situations.

12. Deliver only on very dark nights without moonlight—if you can’t see it, it didn’t happen.

13. Reread point #12 as an attempt at humor.

14. Create skid packages that bind trusses together so they are more likely to come off intact and together.

The most difficult part of a truss’s life is making it from the manufacturing plant to the roof. Let’s help the truss make the trip with as little drama as possible.

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.

Glenn Traylor Structural Building Components Industry Consultant http://www.linkedin.com/in/glenntraylor

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