Wednesday, November 26, 2008

Popsicle Stick Bridge Testing

Popsicle stick bridge building competitions usually choose the winner based on the strength/weight ratio. Usually loads are applied at the mid span of the bridge by placing incremental weights. The load at failure (P) is then determined and divided by the bridge weight (W) to get the bridge performance.
In the DLSU CES Bridge Building Competition, a Universal Testing Machine (UTM) is used to test the strength and displacement of popsicle stick bridges. Since the load will be applied on the bridge deck, an innovative method was devised so that it can be applied to truss bridges. Two 14 mm steel bars spaced at specified distance are placed on the bridge deck. A hollow steel cage is then placed on the two steel bars. The UTM is then applied on the steel cage which then transfers the load to the two steel bars and then to the bridge deck. A displacement transducer is attached to the UTM to measure the displacement (D) in mm. The load is applied and the value of P (kg) is obtained. The value of P can be predefined or when the load dial of the UTM stops which indicates bridge failure. The bridge rating, S = 1000(P/D)/W is then determined. The S rating represents the bridge stiffness (kg/m) per unit weight (kg). For example, the bridge shown in the video below was tested and it’s P = 30 kg, D = 25 mm and W = 1.50 kg. Hence, it’s rating , S = 1000(30/25)/1.50 = 800 (kg/m)/kg. By using the S rating, strength, displacement and weight become parameters in ranking the performance of the popsicle stick bridges.

video

Video Clip: Popsicle Bridge Testing using a UTM at DLSU CE Lab

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