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CIV102 Matboard bridge design project

The entire bridge design process is illustrated in the gallery below
The second CIV bridge project offered our team a great platform to perform everything we learnt in both CIV102 and Praxis I so far. This design opportunity allowed us, a group of three maturing engineering designers to combine both implementation of engineering design philosophy and practical application, both paper-based calculation and construction, so that  we could take them into actions. 
Overview

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This design project required engineering designers to pay a lot of attention to problem analysis process because of the large numbers of constraints and requirements listed in the instruction. Engineers do not want to waste time repeating problem-solving process just because they did not read the instructions carefully, neither do I. The highlighted instruction is place showing my subtle pre-design work.

Design Requirements

 

Objective

  • design a matboard bridge that can safely carry a model train to pass through

Constraints:

  • the train must pass through the bridge safely

  • the bridge must be at least 990m long

  • must be constructed by material given

  • two holes must be drilled at given position so load wires can pass through

Criteria:

  • two point load it can support (more is preferred, measured in Newton)

  • deflection of main span when two 250N loads applied (less is preferred, measured in mm)

  • high ratio between weight carried and self weight of the bridge (higher is preferred)

Preparation and iteration

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Before the design started, a lot of researches concerning the shape of the bridge were processed. Afterwards, several design alternatives were sketched in order to compare how shape of the bridge as a factor influences both materials used and assembly. We confirmed the final shape of our design by the following means, but not limited to:

 

  1. Comparing and referring to other successful design work(reference to the T- beam).

  2. Optimizing the effect caused by different dimensions by using software like Excel.

  3. Following the natural features of the bridge, which is illustrated by shear force and bending moment diagrams.

Design decision 

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As the result of iteration on selection of the shape of bridge, three different sections of the cross section were designed based on variation of the load on the bridge.

 

Each section changed its shape according to variation of the load distributed on the bridge. This design feature made the bridge more naturally reacted to uneven load distribution, and saved unnecessary materials from construction.

Test and
failure analysis

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As the result of load test, our designed bridge failed at the total load of 912N, 300N less than our predicted load, for which the glue failed between one of the webs and the flange in specific.

 

 As one of the two expected reasons of failure, glue failure was reasonable because for our design, the available glued area between webs and flange is limited. Another possible reason to failure was insufficiency of diaphragm near the support caused shear to occur.

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