General Information

The Bridge Strength Competition is an opportunity for middle and high school students to compete against each other to build the strongest bridge with the greatest efficiency. This competition is part of Anchorage Engineers Week, and will be held at the UAA Engineering & Industry Building on Saturday, February 18th, 10am-3pm. Participants will design and build a bridge at home or in class, and then Dr. Scott Hamel and his college students in the UAA chapter of the American Society of Civil Engineers will test the bridges' strength by crushing them! 

Participants must complete their bridge before the date of the competition. Materials are not provided to individuals.

Teachers who wish to assign the Bridge Strength Competition to their entire class can request materials while supplies last by contacting Dr. Hamel at sehamel@alaska.edu. Teachers may also request a classroom visit from a structural engineering expert to explain the principles and share advice for participants.    

Official Rules

Download the official rules to print at home or share in your classroom. 

Introduction

These rules are a modification of the rules for the International Bridge Building Contest.

Objective

Design and build a bridge which will support the greatest weight (maximum applied load) while meeting all the required specifications.

Specifications

1. Materials:
   1. You may use any commercially available Basswood that does not exceed 5mm x 5mm (3/16”x3/16”) in any cross-sectional dimension. There is no limit on the length of the pieces. Note: only 3/32x3/32 members are allowed for the International Bridge Building Contest, if you want to compete beyond Anchorage
   2. Glue is to be any commercially available wood glue or super glue. Hardened glue by itself may not be used as a structural member.  Non-wood fasteners, such as screws, may not be used. 
   3. The bridge may not be stained, painted or completely coated in any fashion. Decorative designs may be applied to the members provided they do not prevent judges from identifying the wood.
2. Construction:
   1. Mass: Bridges should be at or below 25 grams. Bridges more than 25 grams will be penalized by multiplying the max applied load by a reduction factor equal to: [(25 grams / actual mass)²].
   2. Length: The Bridge (Figure 1) must span (S) a 305 mm (12.0 inches) canyon opening. The bridge must sit on at least 25mm (1.0 inch) at each end, which means the overall length (L) must be at least 355mm (14.0 inches). The overall length (L) of the bridge cannot exceed 406 mm (16.0 inches). Bridges that are too short will not be tested.
   3. Width: The bridge must be no wider (W) than 70mm (2.75 inches). The width is measured at the loading surface.  There is no minimum width.  Bridges which do not meet these criteria will be penalized.
   4. Height: The height of the bridge above the support surface (H) may be no more than 177 mm (7.0 inches). There is no minimum height.  
   5. Load Point: The bridge must provide a horizontal loading plane (P) that is between 60 mm and 80 mm (3.15 inches) above the support surface. The support must accommodate one loading locations at the center of the bridge.  Any portion of the structure above the loading plane must provide clearance for the loading plate and for the cylindrical rod below the plate (Figure 2).
   6. Clearance: The bridge must have a minimum clearance (C1) of 60 mm (2.35 inches) in height above the support surfaces. This clearance extends a minimum length (C2) of 160 mm (6.30 inches) and be centered on the mid-span of the bridge. No part of the bridge structure may be built into this clearance area, and a 60 mm high by 160 mm wide block must pass cleanly under the bridge.
   7. Support: The bridge shall be supported by the bearing (sitting) on the horizontal support surfaces at each end. The vertical face of the canyon may not be used to provide support for the bridge, nor may supports sit in the water on the surface below the span (bottom of the canyon).  Bridges that touch the sidewalls or bottom of the canyon will be disqualified.  
3. Loading
   1. Loading Plate: Load will be applied by means of a 40 mm (1.60 in.) square plate (Figure 2).  The plate has a thickness (t) between 6 mm and 13 mm (1/4 inch and ½ inch).  The loading plate will be placed from above on a 3/8” threaded rod with two sides parallel to the longitudinal axis of the bridge and secured with a hex nut.  Force will be applied to the rod. 
   2. End of Loading: The largest supported load throughout the testing will be taken as the maximum applied load. Loading is stopped if the bridge breaks (i.e. an obvious peak is reached in the applied load measurement), or the bridge touches the sides of the load support or bottom of the load frame.

In summary:

• Maximum Mass: 25 grams
• Minimum Length (L): 355mm (14.0 inches)
• Maximum Length (L): 406mm (16.0 inches)
• Maximum Width (W): 70mm (2.75 inches)
• Maximum Height (H): 177mm (7.0 inches)
• Minimum Clearance (C1): 60mm (2.35 inches)

Tips and Suggestions

Triangles! The most efficient bridge designs use trusses, which have “holes” that are triangular. Common trusses are the Warren Truss, the Pratt Truss, and the Howe Truss.

To help you meet the weight limit, the approximate weights of some pieces are shown. Balsa wood is not used because it has a huge variation of density and strength.

High quality wood glue, such as Titebond II provides a strong and durable joint, but takes up to 24 hours to cure. Fast-dry (3 second) superglues do NOT form strong bonds, because they only bond to the surface. Slower (30 second) superglues can be as strong as wood glue. Polyurethane glue, such as Gorilla Glue, has similar strengths and expands to fill cracks, however it requires moisture to cure, so surfaces should be wetted.