Mr. Shepherd’s Bridge Project OBJECTIVES Research actual bridge designs Design a model bridge based on research Submit construction drawings for approval Build a bridge with toothpicks that can be tested for structural strength ALLOWABLE MATERIALS Regular round or flat wooden toothpicks Elmer's white glue Paper for the roadbed (optional) String for suspension designs only; 3 meters max (nylon offers high resistance to breakage under tension) RESEARCH AND SUBMITTAL OF APPROVAL DRAWINGS Each student will research bridge designs from literature or actual structures noting bracing points and reinforcements and how each design takes into account gravitational and load forces and the materials used. Each student will then create a bridge design and submit a drawing for approval (drawings should include dimensions (m) and 3 aspects of the bridge... a side view, a top view, and end view). Approved drawings will be returned and used to construct the bridge. Drawings can be actual size or scaled down; if you scale down you must include a drawing scale. Take your time on your drawings, it is not a race!!! DESIGN AND DRAWING CHANGES As you begin to construct your bridge, you will undoubtedly want to make improvements (changes) to your design. Design changes are allowed. However, you must revise your drawings showing the changes you intend to make and submit them for approval prior to proceeding with construction. Once change approval has been secured from Mr. Shepherd, construction can continue (be aware that I may make minor adjustments/improvements to your requested changes). SPECIFICATIONS Your target maximum weight of the finished bridge is 80 gm. (a total weight more than 80 gms. will be allowed but carries a grade reduction penalty...the greater the overage, the greater the penalty). Any design can be used as long as the roadbed is flat and unobstructed to allow a matchbox car to travel its length. The roadbed does not actually have to support the car; the car is used only to check clearance (i.e. the roadbed can be made of paper to save on weight). The bridge must be free standing and allow for a 2 cm. high x 30 cm. wide board to pass under the bridge while the bridge rests on a flat surface. The bridge will also need to span across my plastic recycling tubs which are placed 12” away from each other on top of my desk. Only the materials listed may be used to build the bridge. Excessive amounts of glue may not be used as part of the structure, i.e. the bridge or bridge joints may not be completely covered with glue. LOAD TESTING PROCEDURE Each bridge will be placed on two support structures (my two recycling tubs 12” apart edge to edge). An 8" long, 1/2" diameter wooden dowel will be set across the center of the bridge on top of the road surface (set perpendicular to and on top of the road). Heavy-duty wire is looped over each end of the dowel which supports hanging weights below the bridge. 1 kg weights are added until all the weights are used up (total of 11 kg. / 25 lbs.) or the bridge structurally fails (breaks). IMPORTANT DATES Check your individual class calendars for due dates; NOTE: there are two due dates for this project... design drawings are due first, then actual bridges). #1) Design drawings are due by the end of class on Friday 12/11 #2) Bridges are to be completed on a date that will be chosen on Wednesday of next week. GRADING RUBRIC Students will fill out and turn in their grading rubric for their bridge on their test day. All students must turn in a grading rubric form! Toothpick Bridge Project RUBRIC (turn in after bridge test) Date ___________ Student__________________________________ Class_____________ Weight applied ________________ gms. Bridge weight __________________gms. RATIO: ( weight held in gms / bridge weight in gms ) = __________ POSSIBLE RAW POINTS: Raw Points Earned Calculated Ratio Bridge Weight Bridge Weight Bridge Weight Bridge Weight up to 80 g 81-100 g 101-120 g 120+ g 100+ 100 97 94 90 90-99 99 96 93 89 80-89 98 95 92 88 70-79 97 94 91 87 60-69 96 93 90 86 50-59 95 92 89 85 40-49 94 91 88 84 30-39 93 90 87 83 20-29 92 89 86 82 10-19 91 88 85 81 1-9 90 87 84 80 DEDUCTIONS (APPLIES FOR ALL BRIDGES): 1. Clearance infractions: (underside height, pier width, car clearance, -1 pt. each): CIRCLE ONE: No deductions -1 -2 -3 2. Bridge deviates from approved drawings (-1 to -10 pts): CIRCLE ONE: No deductions -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 Raw Points Earned Less deductions - NET PROJECT GRADE Bridge Building Websites to visit for detailed information: http://www.garrettsbridges.com/category/design http://www.mhhe.com/engcs/engmech/beerjohnston/vm/ppreview/CHAP06_files/frame.htm http://www.pghbridges.com/basics.htm http://nisee.berkeley.edu/godden/ http://bridgepros.com/ http://www.pisymphony.com/toothpick/toothpick1.htm Bridge Building Basics Because of the wide range of structural possibilities, this Spotter's Guide shows only the most common fixed (non-movable) bridge types. Other types are listed in the Bridge Terminology page. The drawings are not to scale. Additional related info is found on the other Terminology pages which are linked to the left. The four main factors are used in describing a bridge. By combining these terms one may give a general description of most bridge types. span (simple, continuous, cantilever), material (stone, concrete, metal, etc.), placement of the travel surface in relation to the structure (deck, pony, through), form (beam, arch, truss, etc.). The three basic types of spans are shown below. Any of these spans may be constructed using beams, girders or trusses. Arch bridges are either simple or continuous (hinged). A cantilever bridge may also include a suspended span. Examples of the three common travel surface configurations are shown in the Truss type drawings below. In a Deck configuration, traffic travels on top of the main structure; in a Pony configuration, traffic travels between parallel superstructures which are not cross- braced at the top; in a Through configuration, traffic travels through the superstructure (usually a truss) which is cross-braced above and below the traffic. Beam and Girder types Simple deck beam bridges are usually metal or reinforced concrete. Other beam and girder types are constructed of metal. The end section of the two deck configuration shows the cross-bracing commonly used between beams. The pony end section shows knee braces which prevent deflection where the girders and deck meet. One method of increasing a girder's load capacity while minimizing its web depth is to add haunches at the supported ends. Usually the center section is a standard shape with parallel flanges; curved or angled flanged ends are riveted or bolted using splice plates. Because of the restrictions incurred in transporting large beams to the construction site, shorter, more manageable lengths are often joined on-site using splice plates. Many modern bridges use new designs developed using computer stress analysis. The rigid frame type has superstructure and substructure which are integrated. Commonly, the legs or the intersection of the leg and deck are a single piece which is riveted to other sections. Orthotropic beams are modular shapes which resist stress in multiple directions at once. They vary in cross-section and may be open or closed shapes. Arch types There are several ways to classify arch bridges. The placement of the deck in relation to the superstructure provides the descriptive terms used in all bridges: deck, pony, and through. Also the type of connections used at the supports and the midpoint of the arch may be used - - counting the number of hinges which allow the structure to respond to varying stresses and loads. A through arch is shown, but this applies to all type of arch bridges. Another method of classification is found in the configuration of the arch. Examples of solid-ribbed, brace-ribbed (trussed arch) and spandrel-braced arches are shown. A solid-ribbed arch is commonly constructed using curved girder sections. A brace-ribbed arch has a curved through truss rising above the deck. A spandrel-braced arch or open spandrel deck arch carries the deck on top of the arch. Some metal bridges which appear to be open spandrel deck arch are, in fact, cantilever; these rely on diagonal bracing. A true arch bridge relies on vertical members to transmit the load which is carried by the arch. The tied arch (bowstring) type is commonly used for suspension bridges; the arch may be trussed or solid. The trusses which comprise the arch will vary in configuration, but commonly use Pratt or Warren webbing. While a typical arch bridge passes its load to bearings at its abutment; a tied arch resists spreading (drift) at its bearings by using the deck as a tie piece. Masonry bridges, constructed in stone and concrete, may have open or closed spandrels A closed spandrel is usually filled with rubble and faced with dressed stone or concrete. Occasionally, reinforced concrete is used in building pony arch types. Truss - simple types A truss is a structure made of many smaller parts. Once constructed of wooden timbers, and later including iron tension members, most truss bridges are built of metal. Types of truss bridges are also identified by the terms deck, pony and through which describe the placement of the travel surface in relation to the superstructure (see drawings above). The king post truss is the simplest type; the queen post truss adds a horizontal top chord to achieve a longer span, but the center panel tends to be less rigid due to its lack of diagonal bracing.