Post Truss Bridge

744 Words 3 Pages
Bridge Design: Edmond Liu
My popsicle bridge is based on a truss design, which is a bridge design held up by a connected structure composed of many smaller pieces. In particular, my bridge is a variation of the Queen Post Truss Bridge, which has two central supporting posts to allow for a greater span allowing for larger contact points.
(http://pghbridges.com/basics.htm)

My design is modified to incorporate two extra posts towards the two contact points where the bridge rests on the desk, to increase the stability of the structure above the roadbed.
The roadbed itself is made from two 34cm long beams, which are each
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(Truss bridges are efficient at spreading forces on low bridge weight, low-distance applications)
Through testing, I found that small joints that are made from only two pieces of wood and glue are very weak, so I decided to focus the load on to only 4 main joints that would support most of the pressure placed on the bridge. This ability to spread the load over 4 joints is why I chose the Queen Post, instead of the kingpost which would be more suitable for this purpose most of the time.
Each of these joints are 5 popsicle sticks wide with glue between every layer, which also allows for horizontal struts to be attached at these points.
(http://www.historyofbridges.com/facts-about-bridges/truss-bridge/)
Physics:
The bridge was designed symmetrically, with the main load being a downwards force of (mass in kg) x (9.8m/s^2) N. Because of the symmetrical design, the four contact points share the normal force, which is a reactive force equal and opposite to the load force. In this case, it would be an upwards force of (force in N / 4) on each of the four contact
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Because there is such high stress along the middle of the top chord, I have made it an I-beam, where a beam running from the middle of one top chord to the other, helping to keep it stable to prevent any unbalanced load on either side of the top chord.
The tension forces on the bridge are found along the bottom chord (roadbed), where the force from the rod’s weight is distributed outwards towards the contact points, along with a deflection force pushing the middle of the bridge down. The vertical posts are also under tension, as the bottom chord is pulling down, while the top chord is pulling up because of the compression forces discussed

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