POPSICLE BRIDGES: How To Engineer Bridges and Structures Institute Of Electrical And Electronic Engineers, Phoenix Section Teacher In Service Program / Engineers In The Classroom (TISP/EIC) Arizona Science Lab www.azsciencelab.org “Helping Students Transfer What Is Learned In The Classroom To The World Beyond” Material Sources This presentation includes material copied from these web sites: HowStuffWorks — “How Bridges Work,” http://science.howstuffworks.com/engineering/civil/bridge.htm PBS “Building Big — The Labs,” http://www.pbs.org/wgbh/buildingbig/lab/index.html PBS “Building Big — Bridge Basics,” http://www.pbs.org/wgbh/buildingbig/bridge/basics.html Oracle Education Foundation, ThinkQuest, http://library.thinkquest.org/J002223/types/types.html YouTube — “Tacoma Bridge,” http://www.youtube.com/watch?v=3mclp9QmCGs Bridge Types — Beam Structure Bridges, http://www.pennridge.org/works/brbeam.html National Grid for Learning — The Bridges Project: Bridge Types, http://www.bardaglea.org.uk/bridges/bridge-types/bridge-types-intro.html Bridge Basics — A Spotter's Guide to Bridge Design, http://www.pghbridges.com/basics.htm MAY 2014 ARIZONA SCIENCE LAB 2 What Is A Bridge? • A bridge is a structure built to span a valley, road, body of water, or other physical obstacle. • There are more than 500,000 bridges in the United States. • Do you know how they work? • Why are some bridges curved while some are straight? • Engineers must consider many factors before determining the size, shape, and overall look of a bridge. MAY 2014 ARIZONA SCIENCE LAB 3 What Is The Problem In Building A Bridge? • Most bridges are supported only at each end where it meets the terrain. – support in the middle of a bridge requires a vertical pillar, that may not be possible. • The weight of the people, cars, trains, on the middle of the bridge has to be supported by the two ends. • Somehow the weight in the middle of the bridge has to be transferred to the two ends. MAY 2014 ARIZONA SCIENCE LAB 4 An Experiment! • Place a 2” x 10” x 8’ piece of lumber between two work benches, with the flat side horizontal • Have one, then two, then three students stand on it • What happens? • Now turn the lumber so it is on edge and clamped at each end in the work benches • Have one, then two, then three students stand on it • What happens this time? • How do you explain the difference in results? • Understanding this is key to understanding how bridges work! MAY 2014 ARIZONA SCIENCE LAB 5 What Is A Force? In physics, a force is any external agent that causes a change in the motion of a free body, or that causes stress in a fixed body Or In Simpler Terms . MAY 2014 ARIZONA SCIENCE LAB 6 What Is A Force? It can also be described as a push or pull that can cause an object with mass to change its speed or direction ( to accelerate ) or which can cause a flexible object to deform. In this workshop we will deal with forces acting on flexible objects! MAY 2014 ARIZONA SCIENCE LAB 7 Compression, Tension & Shear Forces • Compression = squeezing • Tension = stretching • Shear = sliding • Torsion = twisting • All materials are stronger in compression, tension and shear than in twisting (torsion) or bending • Bridges are designed to maximize compression, tension and shear forces while minimizing torsion and bending forces MAY 2014 ARIZONA SCIENCE LAB 8 Beam Bridge • The beam bridge consists of a horizontal beam supported at each end by piers in the banks. – A log bridge thrown across a stream or river is the oldest and simplest beam bridge. • The weight of the beam pushes straight down on the piers / banks. • The farther apart its piers / banks, the weaker the beam becomes. • Beam bridges rarely span more than 76 meters / 250 feet. MAY 2014 ARIZONA SCIENCE LAB 9 An Experiment! • Put a large rubber ball on the table and push down on it – What happens? What does the ball do? What do you feel? – That is the force of compression • Take a bungee cord and have two of you pull it between you – What happens? What does the bungee cord do? What do you feel? – That is the force of tension • Take a swim noodle and have two of you twist the two ends in opposite directions – What happens? What does the swim noodle do? What do you feel? – That is the force of torsion MAY 2014 ARIZONA SCIENCE LAB 10 An Experiment! • Shear force is more difficult to demonstrate • Imagine a square sheet of thick cardboard • Suppose you apply a force to the top of the square while the bottom is held fixed in in a work bench • The force will try to change the shape of the square into a parallelogram by making the elements of cardboard slide relative to each other • We say the card board is experiencing a shear force MAY 2014 ARIZONA SCIENCE LAB 11 Beam Bridge: Forces • When something pushes down on the beam, the beam bends • Its top edge is pushed together (compression), and its bottom edge is pulled apart (tension) Any weight sitting on the center of the beam will be transferred to the two ends of the beam sitting on the river banks (for example) W • If the supported weight becomes very large a point will be reached when the beam bends and breaks LOG RIVER BED W/2 W/2 MAY 2014 ARIZONA SCIENCE LAB 12 Why Are Support Beams Always Oriented With The Depth Greater Than The Thickness? The location of the compression/tension forces! MAY 2014 ARIZONA SCIENCE LAB 13 Making The Beam Stronger • A single beam spanning any distance experiences compression and tension • The top of the beam experiences the most compression, and the bottom experiences the most tension • The middle of the beam experiences very little compression or tension • If the beam were designed with more material on top and bottom, and less in the middle, it would better handle the compression and tension forces better – For this reason, I-beams are more rigid than simple rectangular beams MAY 2014 ARIZONA SCIENCE LAB 14 Wooden I-beams (“Engineered Wood Beams”) Are Now Used In House Construction • Engineered wood I-beam is a structural component of top and bottom flanges, connected with a plywood flange of various depths • Engineered wood I-beams are primarily used for floor systems but also found in some roof applications MAY 2014 ARIZONA SCIENCE LAB 15 The Shape Of A Structure Affects How Strong It Is MAY 2014 ARIZONA SCIENCE LAB 16 Triangulation • The triangle is a very strong structural form. • The triangle is used in structural designs to reinforce and support weight. • All structures on this page rely on the strength of the triangle. MAY 2014 ARIZONA SCIENCE LAB 17 Triangles Can Be Assembled Into A Beam Structure • This wooden beam has been made from lengths of 2x4 studs joined together in triangular shapes. • Because of the triangles, the beam is very strong. • This beam extends the concept of the I-beam. • The center of the beam is made up of the diagonal members of the truss, while the top and bottom of the truss represent the top and bottom of the beam. • The top and bottom of the beam contain more material than its center. MAY 2014 ARIZONA SCIENCE LAB 18 These Structures Are Known As “Trusses” • The truss is made up from triangular designs and used to support more weight and span more distance. • A truss structure is much lighter than a solid beam of the same strength. • For this reason they are used both in building construction and bridge construction. • A truss is always under compression and tension. MAY 2014 ARIZONA SCIENCE LAB 19 House Construction Uses The Strength Of Triangles To Make Strong, Light Roof Supports Over Wide Spans From 2x4 Wooden Studs MAY 2014 ARIZONA SCIENCE LAB 20 Wooden Roof Trusses For Houses Come In A Variety Of Shapes MAY 2014 ARIZONA SCIENCE LAB 21 Truss Bridge • The truss bridge is an assembly of triangles. • A truss bridge is basically a fancy beam bridge. • The triangular supports span across the top and sides of the bridge. • There are also trusses across the bridge at top and bottom to give it side-to-side torsional (twisting) strength! MAY 2014 ARIZONA SCIENCE LAB 22 There Are A Large Number Of Truss Designs Used For Bridges MAY 2014 ARIZONA SCIENCE LAB 23 Let’s Build A Truss Bridge! MAY 2014 ARIZONA SCIENCE LAB 24 Let’s Build A Truss Bridge! • You will work in teams of two. • You will be given: – 200 Popsicle sticks – A hot glue gun • Your challenge is to design and build a truss structure bridge that will: – Span a gap of 61 cms / 24 inches between two work tables – Support a weight of 23 kg / 50 pounds at the center point of the bridge – Use no more than 200 popsicle sticks • The load weight will be placed on the upper surface of your bridge so do not worry about building road surfaces through the bridge! • Don’t forget to include side-to-side torsional (twisting) strength! MAY 2014 ARIZONA SCIENCE LAB 25 OTHER TYPES OF BRIDGES • There are other ways to design a bridge. • They all involve the same compression, tension and shear forces as the truss bridge. MAY 2014 ARIZONA SCIENCE LAB 26 Arch Bridge • The arch bridge has great natural strength • Thousands of years ago, Romans built arches out of stone • Today, most arch bridges are made of steel or concrete, and they can span up to 800 feet • Arches can also be set above the deck as on the Sydney harbor bridge in Australia MAY 2014 ARIZONA SCIENCE LAB 27 Arch Bridge: Forces • The design of the arch, the semicircle, naturally diverts the weight from the bridge deck to the abutments • Arch bridges are always under compression • The shape of the arch itself is all that is needed to effectively dissipate the weight from the center of the deck to the abutments MAY 2014 ARIZONA SCIENCE LAB 28 Starrucca Viaduct • A stone arch bridge that spans Starrucca Creek near Lanesboro, Pennsylvania • It was the largest stone rail viaduct in the mid-19th century.