Engineering Structures

Tacoma Narrows Bridge Collapse “Gallopin Gertie” I-35 Bridge Collapse I-40 Bridge Collapse

3 Types of Bridges Beam Arch Suspension (cable-stay)

Key Terms Span – the distance a bridge must go over without support Abutment – the structure that supports the end of the bridge

Typical Beam (Truss) Bridges Howe Truss Pratt Truss Warren Truss

The Featherweight Challenge Working as a member of an engineering design team (2 students per team), design a paper truss using the least amount of paper that can support the weight of a one liter bottle of water. Design Requirements: Your paper truss must be at least 5” tall and 11” long and must use all 5 of the dowel rods on the testing station to support the weight. You may only use the following materials: 1 – 8.5” X 11Sheet of Paper 5 – Reinforcement Tabs Additionally, students may use a pair of scissors, hole-punch, ruler, and pencil.

Paper Bridge Design Challenge

Intro To Truss Bridges A truss bridge uses a triangle based lattice work design to add rigidity to the supporting beams, greatly increasing their ability to dissipate compression and tension. Once the beam begins to compress, the force is dissipated through the truss.

How Truss Bridges Work In a beam bridge, the top of the beam has the most compression, the bottom the most tension, and the middle very little of either. If you look at the truss as a single beam, the majority of your material is at the top and bottom, creating more rigidity where it takes on the most force.

How Truss Bridges Work Additionally, the use of triangles within the truss allows it to dissipate the load throughout the truss work.

Popular Designs WARREN TRUSS The Warren truss is perhaps the most common truss for both simple and continuous trusses. For smaller spans, no vertical members are used lending the structure a simple look (illustration #1.) For longer spans vertical members are added providing extra strength (illustration #2.) 1 2 Matsuo Bridge Company - http://www.matsuo-bridge.co.jp/english/bridges/basics/truss.shtm

Popular Designs PRATT TRUSS The Pratt truss is identified by its diagonal members which, except for the very end ones, all slant down and in toward the center of the span. Except for those diagonal members near the center, all the diagonal members are subject to tension forces only while the shorter vertical members handle the compressive forces. This allows for thinner diagonal members resulting in a more economic design. Matsuo Bridge Company - http://www.matsuo-bridge.co.jp/english/bridges/basics/truss.shtm

Popular Designs HOWE TRUSS The Howe truss Is the opposite of the Pratt truss. The diagonal members face in the opposite direction and handle compressive forces. Matsuo Bridge Company - http://www.matsuo-bridge.co.jp/english/bridges/basics/truss.shtm

Physical Forces Compression - A pressing force that squeezes a material so that it becomes more compact. Example - The weight of the building on its lower columns caused a great deal of compression, causing buckling.

Physical Forces Tension - A stretching force that pulls on a material. Example - The vertical cables of suspension bridges must remain in tension at all times because of the continuous weight of the roadway and cars. Snapping will occur if the tension becomes to great.

Truss Strength A single beam spanning any distance experiences compression and tension The top has more compression The bottom has more tension For any bridge to be strong all parts must be tied together allowing all the parts to work together to share the load

Load Load is the force applied to a structure Live Load – weight of objects placed on the structure (cars, trucks, trains, etc.) Dead Load – weight of the materials that make up the bridge (beams, road bed, etc.)

Racking Racking occurs when a force is applied to a rectangular geometric shape.

The Solution to Racking You can overcome the racking problem by including diagonals. A triangle is the strongest geometric shape when vertical forces are applied

Joinery Joinery is where 2 or more pieces of wood meet When designing and building a bridge you must consider the forces that are acting upon each of the bridges components

Shear Shear is stress that is applied parallel or tangent to the face of material.

Lateral Bracing Lateral bracing is also critical to a bridge’s design and ability to carry a load. Lateral bracing serves to break the top chord into smaller sections, giving it more strength. Think of it as the truss on top of your bridge.

Bridge Resources http://www.garrettsbridges.com/design/trussdesign http://168.170.14.135/webs/teachers/bstreeter/Engineering/Notes.pdf http://www.garrettsbridges.com/design/lateral-bracing

Competition Resources Bridges – a very comprehensive site (http://www.teachers.ash.org.au/jmresources/structures/bridges.html) Building Big - Bridges (http://www.pbs.org/wgbh/buildingbig/bridge/index.html ) Westpoint Bridge Designer Software (http://bridgecontest.usma.edu/ ) Johns Hopkins Bridge Designer (http://www.jhu.edu/virtlab/bridge/truss.htm)

Structural Engineering Challenge OVERVIEW Participants will model a through bridge of a recognized truss style for destructive testing. The bridge will be destructively tested to determine design efficiency. CHALLENGE Working individually, with material constraints, participants have an opportunity to construct a bridge that reflects knowledge of engineering design and construction concepts.

Structural Engineering Challenge REGULATIONS Dimensions The structure must be exactly 12” in length, have a minimum internal width of 2”, and a minimum internal height of 3” Materials The following may be used as structural pieces 12' of 1/8” X 1/8” balsa wood 8’ of 1/8” X ¼” balsa wood to be used as horizontal members (this 8’ of material may be made of 1/8” laminated balsa wood) Lamination of the material is prohibited (with the exception of the 8’ of 1/8” X ¼” Joints are to be glued together with Elmer’s School Glue or blue, green or pink structures glue (no gorilla or super glue) An adhesive spread past ¼” of the joint is prohibited. Coating of structural members with adhesive is prohibited. The structure must rest on the top of the abutments. The center of the beam must provide clear passage for the one half inch (½") test rod.

Structural Engineering Challenge EVALUATION The structure is weighed before testing and the weight is recorded on the evaluation form. An increasing load is applied to the structure via the test block until the structure fails. The test block is three quarters of an inch (¾") thick, two inches (2") in width and six inches (6") in length with a ½” hole to accommodate the testing device The failure weight is recorded on the evaluation form. The efficiency is determined by the failure weight x 4.54, divided by the weight of the structure in grams. The efficiency is rounded off to three (3) decimal places and recorded on the evaluation form. The highest numeric efficiency is the winner. In case of an efficiency tie, the greatest weight held by the tied entries will be declared the winner. Awards will be given for first, second, and third places from each level. Structures that violate guidelines will receive a deduction of 20% of the greatest weight held for the first violation. Structures are not to be tested if: there are two (2) or more rule violations. the structure cannot be placed on the tester. the testing device cannot be placed in the center of the structure. straight pins are left in the structure.