1. Engineering Terms
Bridge Unit

2. SPAN
The distance between two intermediate supports for a structure, e.g. a beam or a bridge. A span can be closed by a solid beam or by a rope.

3. Dead Load
The weight of the structure itself. In the case of your project, it will be the weight of your balsa wood truss bridge.
Live Load
Any weight that is added to a structure such as traffic, snow, wind, earthquakes, etc.

4. Compression
The force that squeezes an object together. Whatever is under compression typically shortens as a result.

5. Tension
A force that tends to pull something apart, typically making it longer. An example of this would be stretching a rubber band.

6. Neutral Axis
The neutral axis is an axis in the cross section of a beam (a member resisting bending) or shaft along which there are no longitudinal stresses or strains.

7. Member
A single part of a bridge, such as a beam, truss, girder, etc. used in conjunction with other members to create an entire bridge design.

8. Gusset
A metal plate used to unite multiple structural members of a truss.

9. Depth
The overall height of a bridge from the bottom of the substructure to the top of the superstructure.

10. Shear
A strain in the structure of a substance produced by pressure, when its layers are laterally shifted in relation to each other.

11. Sub-Structure
The substructure consists of all parts that support the superstructure. The main components are:  Abutments or end-bents  Piers or interior bents  Foundation  Footings  Pilings

12. Super-Structure
The component of a bridge which supports the deck or riding surface of the bridge. The superstructure consists of the components that actually span the obstacle the bridge is intended to cross. It includes:  Bridge deck,  Structural members  Parapets, handrails, sidewalk, lighting and drainage features

13. Why are Triangles the Strongest Shape?
The triangle is the strongest shape due to the rigidity of its sides, which allows them to transfer force more evenly through their sides than other shapes. Triangles are used extensively in the construction of buildings and bridges. When a load is applied to the top of a triangle, the sides are under compression, while the bottom is under tension.

14. Forces That Destroy Bridges
Wind
Snow
Rain
Earthquakes
Live Load

15. What Needs To Be Done To Keep Bridges Safe Over Time?
Regular Maintenance including painting
Annual inspections

16. Where Does Compression Occur on a Truss Bridge?

17. Beam Deflecting Under Compression