Truss Modelling Assignment
Discussion of results

Arch 324, Introduction to Structural Design, University of Virginia
Last updated Monday, April 15, 1996, at 9:14 AM Copyright © 1996, Kirk Martini
• Recall the following quote from the first lecture of the course:
Critical thinking is a rational response to questions that cannot be answered definitively and for which all the relevant information may not be available. It is defined here as an investigation whose purpose is to explore a situation, phenomenon, question, or problem to arrive at a hypothesis or conclusion about it that integrates all available information and that therefore can be convincingly justified.

--Joanne G. Kurfis
• That is a large part of this assignment: learning by looking and making hypotheses.

• Some examples:

 Regardless of whether it is right or wrong, this answer is well reasoned., concluding that one nail is pinned and two nails is fixed, implicitly assuming that a nail is a very strong and stiff connector. We'll test the validitiy of this hypothesis today. The discussion of supports is also well reasoned. This answer does a good job of arriving at a justifiable hypothesis that integrates the available information. This answer also identifies multiple possibilities and chooses the one that seems more justifiable. It would be good to include an explanation of why the support "can't" be fixed". This answer includes a fairly common error or applying boundary conditions to nodes that are not restrained. Boundary conditions represent the attachment of the modelled structure to something outside the model which prevents the model from moving. This answer also neglects to model the loads applied to the truss. Many people conclude that there are no applied loads since the the bridge carries only it's own weight. This is false. The model includes only a single truss (shown in blue below), not the entire bridge. Weight from the deck pulls down on the truss at each of the bottom chord joints where the deck is connected to the truss. In addition, this answer is unclear on the concept of support. The statement "there are no supports, and the thing justs rests there" doesn't quite make sense. Even if it justs rests there there is a support: a compression-only roller support. But, that's probably a better handling of the loads than this...

• An important question: How might we resolve the question of whether a two-nail connection is fixed or not?

• Last updated Monday, April 15, 1996, at 9:14 AM