1. Wooden Bridge Construction

2. Strength & Joinery Wood is very strong parallel to the grain ONLY
Wood glues very well on the face grain and edge grain (sides) but poorly on end grain
Bond is often stronger than wood itself
Wood has high strength in tension and compression but only moderate in shear

3. Gluing 2 pieces side by side will double its strength
Gluing 2 pieces o top of each other will quadruple its strength
Pieces that are too short can be over lapped

4. Triangles & Trusses A triangle is the strongest geometric shape
Which bridge is stronger?

5. Types of Trusses
Various types of trusses are used in bridge design/construction
Designed & tested by bridge & mechanical engineers

6. 8 Common Types 1. King Post/Queen Post Simplest type of truss
Contains the fewest members
Used for simple roofs or short span bridges
2 angled supports leaning into a common vertical support
A bridge would require 2 king post trusses with a road surface or bridge way between them

7. 8 Common Types Warren Truss One of the simplest yet strong
Consists of longitudinal members joined by angled cross members
Forms alternately inverting triangles
Ensures all members are not subject to torsion – only compression and tension
Equilateral triangles

8. 8 Common Types Members alternate between tension and compression
Members in the centre are subject to compression and tension (compensate for live loads)
Red = tension Blue = compression Green = no load
Centre Load
Spread Load

9. 8 Common Types Pratt Truss Includes vertical members and diagonals
Opposite to Howe Truss
Shorter, vertical members are in COMPRESSION
Often used in railway bridges
Good use of iron
Typically uses more iron
Iron beams for diagonal members
Many variations
Centre Load
Spread Load

10. 8 Common Types Howe Truss Includes vertical members and diagonals
Opposite to Pratt Truss
Longer, angled members are in COMPRESSION
Suitable for longer spans
Typically uses more wood
Wooden beams for diagonal members
Centre Load
Spread Load

11. 8 Common Types
K –Truss
Shorter compression members than other truss designs
Shortened vertical members (compression)
Shorter compression members = more resistance to buckling
Most complicated and hardest bridges to build
Centre Load
Spread Load

12. 8 Common Types Bollman Truss Type of suspension truss
Inefficient due to each panel functioning on is own rather than aiding each other like the previous types of trusses
Extremely stable
Stiff deck – good for railway bridges
Easy to assemble

13. 8 Common Types Cont. Fink Truss Most common roof design truss
V-shaped members
Overlapping king post trusses connected by smaller member in BLANK
Capable of spanning longer distances
Single = 33ft, double = 54 ft
Quick, cheap, capable of carrying large loads, relatively long life span

14. Making Trusses When making a truss:
Make a full size drawing on grid paper.
Cut the pieces to fit the drawing.
Tape the drawing to a table.
Glue the pieces in place to the drawing gluing the joints. Use glue sparingly.
Glue joints. Use glue sparingly.
When dry, pull the paper off and remove carefully.