Lab 4: Horizontal Load-Bearing System

The Objectives
In order to understand the role of intuition in structural design, a horizontal load-bearing structural system (bridge) will be built by groups of two. The objective is to design and construct a structural system with the greatest load to self-weight ratio. This exercise continues the investigation of materiality, the appropriateness of material choices and the comparison of modeling techniques. It will also demonstrate the importance of form and construction in the design of a system. Attention to craft is highly recommended. A chain is only as stong as the weakest link!

In order to facilitate the objective of the Lab, the structural system will be built both in the virtual and physical worlds. Multi-Frame should be used to help predict the behavior of the physical model.

The Rules
The rules for the design and construction of this structural system are as follows:

• it can only be made from cellulose materials (wood, paper, cardboard, etc.), non-metallic thread or twine, and any type of glue.
• it must stand without additional support
• it must freely span 30 inches
• a loading ring must be inlcuded below or within the structure so that a hook can apply the loading
• it must carry the minimum of a single 5# weight, applied, all at once, as a concentrated load to a ring which is built into the bridge somewhere along the plane of the midspan
• it must weigh no more than 6 oz. (about the weight of a chocholate bar)(use a postal scale)
• it must be elegantly and well built
• it cannot have any part of its structure below the plane of the top of the supports at any time
• it cannot be fixed to the supports in any way

The Documentation
This is a three part project. Thus, it is very important that all phases are well documented. The first lab report to be submitted must contain:

• freehand sketches of the bridge's structural concept
• free-body diagram of the bridge
• diagram of the assumed structural behavior (C or T)
• proposal for the materials to be used
• (gu)estimation of the bridge's final load bearing capacity
• a written description of the weaknesses of the design
• a prediction as to the potential mode of failure
• a print-out of Multi-Frame model of the bridge (2-dimensional or 3-D)
• a print-out of the axial forces and deflection diagram

After the completion of the final bridge "busting" takes place another lab report should be submitted. This should include the original report plus:

• drawings of the elevations and two principle sections of the final version of the bridge
• measured sizes and properties of the structural elements
• complete written verbal description of the design and construction process
• identification in color, on a photocopy, of the two sections of which elements are in tension and in compression
• a written verbal description of the mode of failure
• proposals as to how the load bearing capacity could be improved
• description of what was the greatest suprise during the entire bridge busting presentation

• the accuracy of the modeling techniques
• the lucid description of each of the models limitations
• the final load/weight ratio of the bridge
• the craft of construction of the physical model
• the understanding of the constructed structural system as demonstrated in the lab report