Elevated Bridge Building New York State Coaches Clinic October th 2009 Fishkill, New York

What’s New Eliminated ¼” cross section for wood. Elevated Bridge combines Tower and Bridge New Base for bridge, it may be slippery. No width spec. Must pass a test block under bridge.

Truss design Uprights support load Diagonals prevent shear.

Bridge Loading When a load is placed on a bridge, it is divided among the members. Some members will be under tension some members under compression. Compression members should be as short as possible.

Truss Types Howe Truss Diagonals go down and out from center Pratt Truss Diagonals go down and in to center

Truss Types Continued Warren Truss No uprights SO Bridge showing test points and clearances. Loading Block Bridge Opening Test Base Standard Minimum Clearance

Rules The bridge must span 35.0 cm. for B division and 45 cm for C division supports must rest in Bearing zone that extends 5 cm beyond the clear span on a standard Test Base. The bridge may not extend below the top of the Test Base at any time, and may not be braced against any edge of the Test Base for lateral support. Maximum bridge height is 20 cm. The center portion of the bridge must be raised above the level of the Test Base, so that a Standard Minimum Clearance of 15 cm high x 15cm long for B Division and 30.0 cm long x 12.5 cm for C division high exists between the top level of the Test Base and the underside of the bridge. The bridge must support a Loading Block above the center of the opening in the standard Test Base. The loading point on the bridge must be constructed to permit placement of a chain or 6 mm (¼”) threaded eyebolt through the bridge and through the Loading Block, to support the load. There is no minimum or maximum width for the bridge. The Loading Block may be supported at any height above the Standard Minimum Clearance. The bridge must be a single structure, with no separate or detachable pieces.

Rules Continued The bridge is to be constructed of wood and bonded by glue. No other materials may be used. All construction materials are to be provided by the team. Particleboard, wood products, bamboo, paper, or commercially laminated wood may not be used. Individual pieces of wood used in the bridge can be of any size. Any type of commercially available bonding material (glue) may be used. Team members should be prepared to show or explain to the event supervisor what bonding material was used.

Design Work up a design and then build the first bridge. Use a large balsa such as ¼ X ¼. Check design insure bridge meets all rules. Build test jigs if needed to check clearances and heights. Inspect joints look for gaps or other poor joints. Mark any poor joint. Test Bridge, if bridge failed before 15 kg.

Check failure points, did bridge fail at marked poor joint. Improve building techniques and build again. If bridge failed not at joints design is flawed and needs to be changed. If the bridge holds all 15 kg build again with smaller wood.

Building Techniques Select good quality wood, insure grain runs the length. Mass major structural supports to insure balance. Use proper amount of glue. Use care in cutting do not tear wood. Joints must fit well. Use gussets, check grain.

Cross Members Cross members are used to hold structure straight under compression. Cross members are set at intervals so the structural members do not flex under given load. The density of the wood used will greatly effect the number of cross members needed.

Have patience First bridge constructed may not be a good design and may not have good construction techniques. Building is the best way to learn what type of designs work. Many variables will alter the outcome of a test. No two pieces of wood will perform the same. You must learn to check each piece.

Building Log Building log is important to help improve design. Record as much information as possible. Refer back to log to improve designs. Save old plans, compare efficiency.

Other tips Buy wood in bulk it will save money and time. Encourage team members to compete against each other. Always have other team members check bridge to the rules. Try different glues, test strength vs. weight.