Elevated Bridge

Overview

Elevated Bridge is a construction event that involves building and testing a bridge.

The main distinction of Elevated Bridge from other bridge events is that a portion of the bridge must be elevated or raised so that there is a "standard minimum clearance underneath the bridge, between the bridge and the testing base.

Elevated bridges are defined as follows. The bridge will be tested on a test base, which will contain a bearing zone. The bridge is only allowed to touch the test base in the bearing zone areas. The bridge must have a clear span, or distance between the bearing zone, as defined in the event specifications. A standard minimum clearance must exist between the bottom of the bridge and the test base underneath the center of the bridge. This can be easily visualized by imagining that the bridge must go over a block.

A hole will be located in the center of the test base, in the area underneath the standard minimum clearance. A loading block will be placed above the standard minimum clearance in this area, and a chain will hang from it and hold a bucket that will be loaded with sand.

The bridge may be constructed using only wood, of no greater cross section than 1/4", and bonded only by glue.

All appropriate descriptions of the aforementioned terms and size specifications can be found in your rules manual.

Designing

There are a countless number of designs for a bridge. Bridge designs are commonly referred to as trusses. There are 3 commonly known trusses: the Warren Truss, the Pratt Truss, and the Howe Truss.

Warren Truss

The Warren Truss is nice as it spreads the load evenly among its members. The top and bottom chords, however, carry more of the load.

Pratt Truss

The Pratt Truss contains right triangles in its design. The slants face the center of the bridge. The load is spread among the slants, which in the Pratt truss are tension members. However, the top chord and the vertical members also take quite a bit of the load and are compression members.

Howe Truss

The Howe Truss is similar to the Pratt Truss, except the slants face away from the center of the bridge. The difference with the Howe Truss is that the slants become compression members and the vertical members become tension members.

All three of these trusses have their pros and cons. It's up to you to decide which one you like best. Your choices are not limited to these designs. You can try designing your own truss with a bridge designer (link below). It is very helpful to design the truss structure in a truss analysis program. Many such programs exist. The simplest is from John Hopkins university, and can be found in the link below. Other, more featured programs, include MDSolids and Dr. Frame. The design of one face of the bridge should be constructed in the program. Then, the load can be applied to the center of the bridge, and the stress in each truss member can be calculated.

This information is extremely important when designing the bridge. Generally, it is better to use stronger wood in areas with greater stresses.

Once you have chosen your design, draw one side on a poster board. Make the picture life size. When you go to build the bridge, you can cut the pieces using your drawing.

Building

A completed Bridge

Prior to the competition, teams will construct their bridge. Bridges are not hard to build, but do take practice. The

more bridges you build and break, the more you will learn, and thus the more improvement you can make. It is important that you test as much as possible. Every broken bridge will help you improve with your next bridge.

The best way to build a bridge is as two 2-dimensional faces. A template can be drawn out on poster board. Attach wax paper over the template to ensure that the bridge is not glued to the template.

After both faces of the bridge are constructed, they must be put together. A jig that holds the sides parallel is useful in this situation.

Lateral bracing refers to bracing between the two sides of the bridge. Generally, lateral bracing is important on the top of the bridge, as that portion is in compression. When materials are in compression, they tend to bow and flex. The bottom of the bridge is in tension, which does not bow and bend, thus, lateral bracing is not required in these areas.

Construction materials

Bridges can be made out of any type of wood except those excluded in the rules, such as particleboard, plywood, bamboo, paper, etc. Balsa has the highest strength-to-weight ratio of any commonly available wood, and is readily available in many sizes at your local hobby shop.

When selecting balsa, it is important to ensure that all pieces used are of appropriate density. This can be found by weighing the wood with a pocket scale. One should also ensure there are no identifiable defects in the wood grain, as this may cause premature failure.You can buy balsa wood in sheets and cut out your own sticks or buy precut sticks. If you choose to buy in sheets, look out for the grain of the wood. There are 3 kinds of grain: A grain, B grain, and C grain. A grain is more flexible and is best for tension members. C grain is more stiff and good for compression. B grain is a combination of the two.

Basswood is also commonly used in bridges. It is stronger than balsa, but also much heavier. It can be very effective when used strategically.

Balsa or Bass?

Different bridges use different wood. It's completely up to you whether to use balsa wood or basswood when building your bridge. There are pros and cons to both:

Balsa pros: Lightweight, cheaper, easier to cut/sand, changes mass in different humidity.

Balsa cons: Stiffer than bass, weaker, inconsistent density.

Bass pros: Stronger, holds together better at joints, doesn't crush, same strength throughout stick, bends more.

Bass cons: Heavier, more expensive.

The kind of wood you use is completely up to you. You can build strong bridges with either kind of wood. Depending on your design, you might want one wood over the other. If you have many pieces in your design, you might consider balsa, as it is lightweight. But if you have few pieces but still want a strong bridge, bass is the way to go. It's best if you experiment with both kinds of wood and see what you prefer.

Glue

The wood may be bonded by any type of glue. This includes wood glues, cements such as Duco or Ambroid, and CA. CA works very well for bonding bridges. It is available in various viscosities. Thin CA will seep into the wood at the joints, thereby strengthening the overall joint. It also cures in seconds, facilitating the construction of the bridge. Wood glue is also strong in bonding any type of wood, although it is heavy. It also takes much longer to dry. When using any type of glue, it is important to use the appropriate amount. A little glue can go a long way, and using too much glue is a very easy way to drastically increase the mass of a bridge.

Testing

Before testing, the bridge must be weighed.

During testing, the bridge will be placed on the test base, so that it touches only within the bearing zone. The competitors will then assemble and place the loading block on or within the bridge, so long as it is above the standard minimum clearance. An s-hook will be used to suspend a chain from the loading block, and the chain will support a bucket that is approximately one inch off the floor.

Contestants will then load sand into the bridge until it fails or holds 15.000 kg. At that time, the bucket of sand will be weighed, and the structural efficiency determined.

Structural efficiency is found by dividing the mass held by the bridge by the mass of the bridge itself. Generally, these measurements will be in grams, although any units of mass may be used, provided they are the same.

Links

New York Coaches Conference

Garrett's Bridges 5 steps

The Diary of a Bridge Builder

Bridge Designer

Bridge Diagrams