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 grainBond is often stronger than wood itselfWood 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 strengthPieces 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 TrussesVarious types of trusses are used in bridge design/constructionDesigned & tested by bridge & mechanical engineers
6 8 Common Types 1. King Post/Queen Post Simplest type of truss Contains the fewest membersUsed for simple roofs or short span bridges2 angled supports leaning into a common vertical supportA 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 membersForms alternately inverting trianglesEnsures all members are not subject to torsion – only compression and tensionEquilateral 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 loadCentre LoadSpread Load
9 8 Common Types Pratt Truss Includes vertical members and diagonals Opposite to Howe TrussShorter, vertical members are in COMPRESSIONOften used in railway bridgesGood use of ironTypically uses more ironIron beams for diagonal membersMany variationsCentre LoadSpread Load
10 8 Common Types Howe Truss Includes vertical members and diagonals Opposite to Pratt TrussLonger, angled members are in COMPRESSIONSuitable for longer spansTypically uses more woodWooden beams for diagonal membersCentre LoadSpread Load
11 8 Common TypesK –TrussShorter compression members than other truss designsShortened vertical members (compression)Shorter compression members = more resistance to bucklingMost complicated and hardest bridges to buildCentre LoadSpread 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 trussesExtremely stableStiff deck – good for railway bridgesEasy to assemble
13 8 Common Types Cont. Fink Truss Most common roof design truss V-shaped membersOverlapping king post trusses connected by smaller member in BLANKCapable of spanning longer distancesSingle = 33ft, double = 54 ftQuick, 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.