2017 Competition Guide
Deflection Measurement Equipment
There are many ways to measure vertical deflection. They range from simply using a yardstick to expensive digital devices. The rules do not specify a means for doing this. At a minimum the measurement system used should:
Click on the adjacent thumbnail to see some options used in the past. These options have included:
In at least one student conference where electronic means were used, the real time deflection is often projected on a screen for all to see. This is fine, however, it is not required. This information is not made public during the national competition--only the judges monitor the deflection at nationals. Team captains are required to verify the final deflection numbers but do not monitor deflection during loading.
It should be noted that added expense does not equate to accuracy and precision. As shown below, the device used at the 2011 ASCE Pacific Northwest Student Conference was simple to build, cheap (~$15.00 each, using materials laying around the shop) and as precise as any digital device. The repeatable multiple reader accuracy was 0.005" where as the electronic pots used at the national competition are only accurate to 0.007".
If you've come up with an innovative system, send in a write up of how it works along with images/drawings/video to firstname.lastname@example.org so we can share the ideas with others.
Dial gages are fairly common, however extensive experience indicates that these devices are extremely slow to set up correctly and tend to significantly slow down the competition. If dial gages are used, you should find quick means for mounting them. They are also problematic to read as reading them often requires competitors to get uncomfortably close to loaded bridges. Each of the two setups shown below use magnetically mounted dial gages which are much better than any other mounting system for dial gages. It is necessary to design something to mount the gages to as well. Also you need to make sure that the gages are set up in a way that they can measure the full rage of motion anticipated without having to be reset.
Electronic devices of various types have been employed more frequently in recent years. These devices can be expensive (particularly at the required strokes). They have many advantages in that the data can be obtained without having to get near the bridge. Some conferences have even hooked up the computers to projectors to show the deflections in real time to the audience (this does NOT happen at the national event!). You will notice that the LVDT setups shown have mechanical arms fabricated to both amplify the range of the LVDTs and to protect the devices in the event of a collapse of the bridge.
At the national competition, string pots are used and they are protected by mounting the devices in short sections of heavy steel tube sections as shown.
A Simple and Precise Measurement System
An inexpensive homemade measurement device was used at the 2011 ASCE Pacific Northwest Student Conference which was actually more precise than the digital devices used at the national competition. It also has the advantage of leaving a record of the deflection behind which can be independently checked. The Plexiglas guides were home made from Plexiglas available at the local builder's supply store and Plexiglas adhesive (ProWeld). The paper 3x5 cards (with lines) fit snugly in the guide and the sliding aluminum angle is cut to just fit within the guides. The cards are removed after the measurements are taken. The system proved fast and precise.
You can see the devices in action at towards the end of the video at: http://youtu.be/n4qcvL8eMCY