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Component Manufacturing dverti$ dverti $ er
Don’t Forget! You Saw it in the
April 2023 #15285 Page #35
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Should We Consider “Delivery” Load Cases? By Glenn Traylor
ears 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 ;
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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 Continued next page
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