Lab 5: Bridge II : Exploration
The Problem
The problem is to build a virtual model of the bridge design and to use it to observe/predict the behavior of the bridge.
The Objectives
The objective is to learn the advantages and limitations of modelling mediums through direct experience.
An Interlude
There are essentially four approaches to analyze a structural system. Each method has distinct advantages and disadvantages and has been tried over time with varying degrees of success. These four methods are:
- a physical model of a specific scale and materials
- a graphical analysis (Graphic Statics)
- a numerical analysis (Newtonian)
- a computer analysis (Finite Element)
A full- or half-scale physical model yields information about the spatial and constructional characteristics, yet takes a long time to build. And, it is normally good for one test. A graphical model depends upon a simplification of both the structural system and material properties for easy analysis. Graphical and numerical techniques are traditional methods that have been used for the past century and can be repeated with ease. A computer analysis can offer the greatest flexibility for examining numerous geometrial or physical alterations to each system or element. It is usually rapid and can lead to very unexpected results. The difficulty of the last three techniques is the divorce of the analytical method from the physicality of the built structure. Each remains a simulation of the reality. However, the simulations are becomming increasingly exact. A computer simulation/analysis depends upon the user taking the time to not only input correct values, but also to check the answers to ensure that they fall within the bounds of reality.
The Process
Take the drawings of the model which you have prepared and build a two-dimensional and/or three-dimensional Multi-Frame model. The physical properties of the structural elements should be estimated for this initial analysis. As can be observed, the accuracy of the model's behavior depends upon the accuracy of the information that is given to the model.
The Documentation
The Bridge II lab report should contain:
- the material from the Bridge I Lab report
- free-body diagram(s) of the bridge
- diagram of the observed structural behavior in which the type (C or T) and magnitude of the internal forces are indicated
- a written description of the potential weaknesses of the design as predicted by/determined from the MultiFrame model
- a prediction as to the potential mode of failure
- a print-out of the Multi-Frame model (2-D or 3-D)
- a print-out of the axial forces and deflection diagrams
The Evaluation
The evaluation for this part of the exercise will be based upon the clarity and degree of completion of the lab report. It will also depend upon:
- the accuracy of the MF model
- the lucid description of each of the models' potentials and limitations
- the understanding of the constructed structural system as demonstrated in the lab report
Copyright © 1995, 1996, 1997 by Chris H. Luebkeman