2018 International Bridge Building Contest Rules


These rules have been developed by the International Bridge Building Committee for the Forty-First International Bridge Building Contestto be held on Saturday, April 21 in Chicago, Illinois, USA. If you have a question that is not answered by our FAQ, please direct it to George Krupa at

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: Students may participate in person, by proxy or by mail entry.

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. Materials

  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 official 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.

2. Construction

  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 taller (H) than 150. mm above the support surfaces, and have a width (W) no greater than 80. mm.
  3. The bridge must be constructed to provide a horizontal support for the loading plate and at each of the two possible loading positions (see 3c). The bridge structure must allow the loading rod (see 3c) to be mounted from below.
  4. The bridge must be constructed to allow a 48 mm diameter, 300. mm long pipe (1.5 inch schedule 40 PVC pipe) to be passed horizontally across the bridge with the pipe’s lower surface on the loading plane (P) between 80. and 100. mm above the base of the bridge. This pipe must touch both loading locations simultaneously (see 3c).
  5. The bridge structure may not project below the support surfaces (see Figure 1).

3. Loading

  1. Competition loading will stop at 50. kg, loading will continue until bridge failure (see 4d).
  2. The load will be applied by means of a 40. mm square plate (see Figure 2) with a thickness (t) of at least 6 mm but less than 13 mm. A 9.53 mm (3/8 inch) diameter eyebolt is attached from below to the center of the plate. The plate will be horizontal and will be mounted with its edges parallel to the longitudinal axis of the bridge. Masses will be supported on a vertical loading rod suspended from the eyebolt. The minimum initial load will be 2. kg.
  3. The load will be applied with the center of the plate at one of two (2) possible locations on the longitudinal axis of the bridge: 30. mm to the left and 60. mm to the right of the center of the bridge span (see Figure 1). The two loading locations must lie in the same horizontal plane. This loading plane must lie a distance (Pbetween 80. mm and 100. mm above the base of the bridge.
  4. On the day of the competition, the judges will decide which one of the two loading locations will be used; ir will be the same for all bridges.

4. Testing

  1. On the day of the contest, contestants will center their bridge on the loading surfaces. They will have previously located the loading plate and 3/8 inch eye bolt to the selected loading position with the edges of the plate parallel to the longitudinal axis of the bridge.
  2. The load will be applied from below, as described in section 3 above. Competition loading will stop at 50. kg. However, loading will continue until bridge failure (see 4d).
  3. Bridge failure is defined as the inability of the bridge to carry additional load, or a load deflection of 25.4 mm under the loading location, whichever occurs first.
  4. 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.


E = Load supported in grams (50,000g maximum) / Mass of bridge in grams

5. Qualification

  1. All construction and material requirements will be checked prior to testing and may be checked after testing. Bridges failing to meet these requirements will be disqualified. Bridges disqualified prior to the start of the contest 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. If the disqualified bridge can accommodate loading, it may still be tested as an exhibition bridge as stated above.
  3. Decisions of the judges are final; these rules may be revised as experience shows the need. (Please check our web site, after February 5, 2018, to learn whether any changes have been made.)