2025 International Bridge Building Contest Rules

In order to receive official wood and participate in this contest, a High School student must have placed first or second in a Regional Contest and be reported, by the Regional Coordinator, to Prof. Carlo Segre by e-mail: segre@iit.edu. Students may participate in person, by proxy or by mail entry.

This is a contest for individual high school students, not teams. The object of this contest is to see who can design, construct and test the most efficient bridge within the specifications. Model bridges are intended to be simplified versions of real-world bridges, which are designed to permit a load to travel across the entire bridge. In order to simplify the model bridge design process, the number of loading positions is reduced, and to allow the contest to proceed in a reasonable amount of time, only one loading position is actually tested. These simplifications do not negate the requirement that the bridge must be designed to accept a load at any of the positions. Bridges determined by the judges to not meet this requirement will be disqualified and tested as unofficial bridges.

1. The bridge must be constructed only from the official 3/32 inch square cross-section basswood included in the kit and any commonly available adhesive.
2. The offical basswood may be notched, cut, sanded or laminated in any manner but must still be identifiable as the original official basswood.
3. No other materials may be used. The bridge may not be stained, painted or coated in any fashion with any foreign substance.

1. The bridge mass shall be no greater than 25.00 grams.
2. The bridge (see Figure 1) must span a gap (S) of 300. mm, be no longer (L) than 400. mm, be no wider (W) than 80. mm, and be no taller (H) than 100. mm above the support surfaces. The bridge structure must extend below the support surfaces by no more (B) than 10. mm.
3. The bridge shall contain an "arch-type" structure which starts below the main support plane (see Figure 1) and spans between the supports. An arch uses curved members for its main load carrying members. For this contest, the arch shall be composed of either curved members or two or more straight segments arranged to approximate an A-frame or a multi-segmented frame. The arch element must make contact with the vertical faces of both support surfaces and must extend 10. mm or more above the support surfaces in the center of the span. Tie beams or tension members connecting the arch ends along the span are not allowed below the support surfaces.
4. The loading plane shall be horizontal and shall lie a distance (P) between 10. mm and 100. mm above the support surfaces.
5. The bridge must be constructed to provide for the loading plate (see section 3 below) at each of the two loading points 20. mm and 40. mm on either side of the center of the 300.  mm span along the longitudinal axis of the bridge.

1. The load will be applied downward, from below, by means of a 40. mm square plate (see Figure 2). The plate will have a thickness (t) of at least 6 mm but less than 13 mm and will have an up to 9.53 mm (3/8 inch) diameter threaded rod attached from below at its center with a standard hex nut. The plate will be horizontal with two sides parallel to the longitudinal axis of the bridge.
2. The load will be applied on the longitudinal axis of the bridge at one of two loading points: 20. mm and 40. mm on either side of the center of the 300.  mm span.
3. On the day of the competition, the judges will randomly decide which of the two loading positions will be used; it will be the same for all bridges.

1. On the day of the competition, contestants will place their bridge on the testing machine support surfaces, adjusting them by no more than ±2. mm to ensure contact with the downward projecting portions of the bridge and with threaded rod projecting from below at the designated loading position.
2. The loading plate will be placed from above on the threaded rod with two sides parallel to the longitudinal axis of the bridge and secured with a hex nut.
3. The load will be applied from below, with the contestant rotating the Vernier testing machine load wheel until bridge failure is sensed (see 4d). Competition loading will stop at 50. kg.
4. Bridge failure is defined as the inability of the bridge to carry additional load as sensed by the Vernier testing machine.
5. The bridge with the highest structural efficiency, E, will be declared the winner. Bridges failing above 50. kg will be considered to have held 50. kg for efficiency calculation.

1. All construction and material requirements will be checked prior to testing. Bridges failing to meet these requirements will be disqualified. If physically possible, disqualified bridges may be tested as exhibition bridges at the discretion of the builder and the contest directors.
2. If, during testing, a condition becomes apparent (i.e., use of ineligible materials, inability to support the loading plate, bridge optimized for a single loading point, etc.) which is a violation of the rules or prevents testing as described above in Section 4, that bridge shall be disqualified.
3. Decisions of the judges are final; these rules may be revised as experience shows the need. Please check our web site, http://bridgecontest.phys.iit.edu after February 12, 2025, to learn whether any changes have been made.