Wooden Bridge Construction. Strength & Joinery Wood is very strong parallel to the grain ONLY Wood glues very well on the face grain and edge grain (sides)

Wooden Bridge Construction

Strength & JoineryWood is very strong parallel to the grain

ONLYWood 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

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

Triangles & TrussesA triangle is the strongest

geometric shapeWhich bridge is stronger?

Types of TrussesVarious types of trusses are used

in bridge design/construction◦Designed & tested by bridge &

mechanical engineers

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

2. 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 Common Types

• Members alternate between tension and compression

• Members in the centre are subject to compression and tension (compensate for live loads)

8 Common Types

Centre Load Spread LoadRed = tension Blue = compression Green = no load

3. 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

8 Common Types

Centre Load Spread Load

4. 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

8 Common Types

Centre Load Spread Load

5. 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

8 Common Types

Centre Load Spread Load

6. 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

8 Common Types

8. 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

8 Common Types Cont.

Making TrussesWhen making a truss:1. Make a full size drawing on grid paper.2. Cut the pieces to fit the drawing.3. Tape the drawing to a table.4. Glue the pieces in place to the

drawing gluing the joints. Use glue sparingly.

5. Glue joints. Use glue sparingly.6. When dry, pull the paper off and

remove carefully.