Model Bridge Construction: Balsa Wood Design and Engineering

[00:00:00][Music] we live in a mobile society travel from city to city and across state lines is a daily occurrence for many people most of us don't give a second thought to the highways tunnels and bridges we encounter on our regular road trips however in order to enjoy travel mobility somebody had to design and then build the structures that enable us to span waterways valleys and mountain passes in this video we will guide you in the process of designing and then building a balsa wood bridge the DVD is organized by chapter so you can navigate to the sections most appropriate we will start with some basic beam and joint strength activities and then advance to bridge design and construction when you're ready to begin working on your bridge the first thing to accomplish is to create some design sketches typically an engineer will generate several sketches to help them develop different design ideas we recommend that you refer to the pits Co bridge book for some trust ideas and design samples there's also space in the book for you to sketch your ideas it's a good idea to complete some sketches with super structures and some with both super structures and sub structures [Music] civil engineering is a branch of engineering related to the building of municipal systems such as roads bridges dams communication towers and other facilities at this point you should have several design sketches and be ready to pick one and begin the next step in the process is to create a three view sketch that will show the top side and end views of the bridge again referring back to the pits go Bridge book for examples use the grid paper to draw your views to full scale if you are using balsa wood then each member will be 1/8 inch wide if you are working with basswood the strips will be three thirty-seconds of an inch wide if your bridge is for a competition other than a classroom contest you will need to follow the rules for that event your instructor will provide you with the exact span of the bridge before you start your three view drawing finally the roadway surface inside the bridge must be open to allow a block of wood that is one inch thick and two inches wide to pass through its entire length the block will be used during the testing of the bridge at this time locate the graph paper and your list of bridge specifications before moving on [Music] racking is a kind of stress that distorts a square or rectangle causing it to become a parallelogram to strengthen the shape a diagonal brace is added converting the rectangle into two triangles this makes the figure stronger and minimizes the racking effect we're back and moving ahead with the three view sketches on the large grid sheet sketch the side view of the bridge to full-scale the side view is the one you would see if you were in a boat on the river and the bridge was in front of you again referred to the pits go bridge book for an example welcome back you should have the side view of your bridge completed next draw the top view to full-scale and align it directly above the side view the top view is what you would see if you were in an airplane looking down at the top of the bridge [Music] [Applause] we're almost finished with the three views at this time draw the end view this is the view you would see if you are in a car approaching the bridge again you may want to refer to the pits go bridge book for an example [Music] the efficiency of a model bridge can be determined by the load of the bridge can hold the formula for efficiency is failure mass divided by the mass of the structure at this point you should have all three views of your bridge completed we are now ready to begin the construction phase gather the following materials [Music] [Music] [Music] [Music] [Music] when you have all the necessary supplies we can begin first use masking tape to attach your full-scale 3-view sketch or drawing to a piece of foam board cover the drawing with waxed paper and tape it in place if a Pitts co-construction kaddy2 is available slide the foam board into the slotted ends of the Cadi [Music] before we move on with the construction of the bridge we have a couple of recommendations for you to think about when constructing a wooden bridge the first thing to understand is that the joints are often the most important aspect of the project once a joint fails the integrity or strength of the entire structure is weakened this weakness can cause a chain reaction that leads to the total failure of the structure when building your bridge remember that glue does not work well on the end grain of wood you will construct a stronger bridge if the joints are made with the side grain of the wood we've got a few more recommendations for you to consider when constructing your bridge here's the next one when possible set a diagonal brace between two parallel pieces this technique helps prevent racking you lay the diagonal piece across the Timbers and mark your cut with a knife next you'll want to mark on both ends where the piece lies across the parallel pieces and cut the marks then cut off the tips of the pieces so they fit into the corners and glue the piece in place [Music] [Applause] here are some more general construction tips if you ever have trouble fitting a piece in a tight spot try picking it up with a pair of tweezers instead of your fingers when laying the wood on your board use as many pins as needed to hold the pieces in place you can cross two pins over a strip of wood to hold it secure when cutting make sure you are working on a protected surface and make the cuts away from yourself you can also use the pits go timber cutter to cut the wood pieces [Music] [Music] here's our final suggestion for constructing your bridge before gluing the bridge joints use a hobby knife to scrape the wood where the joints will be glued the natural oils from your hands touching the wood prevents the best possible bond to the glue scraping the joint surfaces helps remove dust and oil [Music] models are used by all kinds of engineers to provide a three-dimensional example of how a final building bridge or structure will look models also enable the engineer to attempt different solutions to problems prior to building the structure now that you've heard all the tips and recommendations it's time to begin building the bridge first cut strips of wood to fit the outline of the bridge use the hobby knife or pits Co timber cutter to cut the wood to ensure a snug fit cut the pieces a little long on the first cut and then trim the end to fit if a pits go little termite sander is available use it to sand the ends to fit pin each piece into place after you have pinned the outline of the bridge glue the pieces in place surfaces where strips connect should be coated with a thin film of glue use pins to hold the pieces flat [Music] [Applause] we're back and moving ahead with the bridge construction after your outline has been glued into place fill in the wood strips that make up the other members of the first side of the bridge make sure the joints are completely dry before moving the pins [Music] after the first side has dried remove the pins and carefully lift the side off the wax paper place the finish side on a flat surface repeat the same procedure to build the second side [Music] at this point you should have both sides of the bridge cut and glued next you will need to cut and construct the cross braces that support the roadway of the bridge build each of them exactly the same length now you are ready to work on the top of the bridge cut the cross braces that support the top and make each of them exactly the same length here's an important note in these instructions we're showing the top braces being attached first but depending on your design it might be best to attach the Rodeway cross braces first before you begin the step review your design and start with whichever set of braces will best stabilize your structure [Music] [Applause] we're moving on with the bridge construction pin one side of the bridge so it's perpendicular to the foam board the top should be facedown blew the top braces to the side use a square to ensure the side is perpendicular to the cross brace members [Music] for the next step add glue to the other end of the braces and pin up the other side parallel to the first side make sure all the pieces fit together squarely and are not loose allow the glue to dry [Music] after the glue is dry pull out the pins and carefully remove the bridge from the work surface turn it over and if necessary pin the bottom of the bridge to hold it in place apply glue to the sides where the roadway cross braces will be attached put the cross braces in place and if needed position the braces in place with paper clamps while they dry at this point you should check all joints to ensure they are well glued and that all members are securely in place if you used clamps remove them as well if everything is solid and secure then you're finished [Music] [Applause] congratulations on finishing the bridge when you are ready to test the bridge you can navigate to the testing chapter on the DVD to check out other activity kits visit our website at wwlp.com [Music] you [Music]