NudeLobster Posted February 17, 2011 Report Share Posted February 17, 2011 Ok, so here's the project. Each of us build our own wooden tower to be tested in a structural stress analyzer to see who's design can hold the most weight. The requirements are: Base must be at least 3" wide, no more than 5" widetop must be at least 1" wide, no more than 5" widemust be 12"-15" tall and, we have 15 rods of basswood that measure 1/8"x1/8"x24"....so in other words, 30 feet total of material, or 360" This is the design I had built, but when I annotated it, it came out to near 475" in material It is a 5" square base, 3" square top, 12 inches tall. 12" long vertical beam down the middle of each side. Horizantal cross braces every vertical inch, with diagonal braces going from the center out in each quadrant. here is a picture: http://i213.photobucket.com/albums/cc92/pocketrocket1331/Mobile%20Uploads/1297915363.jpg now, what is the best way for me to cut over all material while keeping as strong as possible? same design, but scaled down? or less horizontal beams and thus utilizing longer diagonal beams? input would be great -Justin Link to comment Share on other sites More sharing options...
Mix_67 Posted February 17, 2011 Report Share Posted February 17, 2011 (edited) 3 dimensional tower or 2d truss? EDIT: sorry missed the last line. it is 3d. Look at and experiment with bending some of the members. pre stressing members can really add to the strength. Joint construction is the weak link in projects like this, so minimize joints. Just a lowly ME's opinion. Edited February 17, 2011 by Mix_67 Link to comment Share on other sites More sharing options...
NudeLobster Posted February 17, 2011 Author Report Share Posted February 17, 2011 It would be a 3d free standing tower. We only have to design one side because the front and back will be the same with the perimeter framing, and the two sides being really just cross bracing to connect the front and back, because laminating two perimeter frames the teacher said. If the front and back bracing is different than the sides, than I need to sketch both sides, but I figured the best design would be uniform bracing. -Justin Link to comment Share on other sites More sharing options...
Chad Posted February 17, 2011 Report Share Posted February 17, 2011 It it subjected to lateral loads or just vertical? Why not use a triangualr shape (3 corners as viewed from top)? that frees up material for more supports, and a triangle is a very stable shape. How about a conical shape? 5" base and ~3" top. Link to comment Share on other sites More sharing options...
NudeLobster Posted February 17, 2011 Author Report Share Posted February 17, 2011 (edited) It is subjected to just vertical force. There is a rod that goes through the middle of the tower, which pulls a plate down on top of the structure until it snaps. Now triangular...I never thought of that. I am going to start playing with some Idea's there. I just measured out a 5 inch sided equilateral triangle and I don't think that will cut it, because it won't fit around the base of the machine. the machine has a raised base that is about 2.5 inches squared, which would not fit inside the 5 inch sided equilateral triangle for a conical shape, I don't know how I would easily build a conical shape out of the straight wooden rods we have as material...A cylinder would be best I would think, but there is no easy way to build anything circular out of straight pieces. It would take a very long time to accurately build -Justin Edited February 17, 2011 by NudeLobster Link to comment Share on other sites More sharing options...
Chad Posted February 17, 2011 Report Share Posted February 17, 2011 It is subjected to just vertical force. There is a rod that goes through the middle of the tower, which pulls a plate down on top of the structure until it snaps. Now triangular...I never thought of that. I am going to start playing with some Idea's there. I just measured out a 5 inch sided equilateral triangle and I don't think that will cut it, because it won't fit around the base of the machine. the machine has a raised base that is about 2.5 inches squared, which would not fit inside the 5 inch sided equilateral triangle for a conical shape, I don't know how I would easily build a conical shape out of the straight wooden rods we have as material...A cylinder would be best I would think, but there is no easy way to build anything circular out of straight pieces. It would take a very long time to accurately build -Justin If it's not subject to side forces, it doesn't need a wide base. A moslty straight up and down narrow structure would probalby hold the same weight, but require less bracing leaing more material for the verticals. An equalateral triangle with a 2" base and 1-1/2" top would fit with ease and leave a lot more material. Most structures I've seen in the real world use bracing at 45* for loads like you are subject to. On strong lateral loads or high twist stresses, they tend to be more acute, oriented in the direction of the undesired forces at about 30*. Link to comment Share on other sites More sharing options...
Fanta Posted February 17, 2011 Report Share Posted February 17, 2011 (edited) http://www.fallingpixel.com/products/14941/mains/000-3d-model-RadioTower_thumbnail01.JPG For every "X" they have a lateral support. Should cut down half of your lateral. Does it HAVE to be square based? Most radio towers are actually triangular based as well. Essentially just do a truss type designhttp://www.euro-lift.com/images/TR-BD-Truss.jpg Edited February 17, 2011 by Fanta Link to comment Share on other sites More sharing options...
NudeLobster Posted February 17, 2011 Author Report Share Posted February 17, 2011 Like I said, it doesn't have to be a square, but it does have to be 3" wide across the whole base, so a triangle sadly won't work. for a triangle to work, it would have to be able to inscribe a 3x3 square inside of it, which a 5" wide equilateral triangle cannot do The X idea is good though, I'll try that and see what material annotation I get then -Justin Link to comment Share on other sites More sharing options...
Chad Posted February 17, 2011 Report Share Posted February 17, 2011 Tringles are really good for lateral loads and twist, which is why they are used for tall narrow towers. Just stright up and down, they aren't stronger, but may yield better material efficiency in the end. Link to comment Share on other sites More sharing options...
NudeLobster Posted February 17, 2011 Author Report Share Posted February 17, 2011 for the X brace Idea, would it be better to have the cross braces splitting the X like in the above picture, or having a cross brace at the top and bottom of the X's? in the picture above it seems like the diagonal braces spanning from inside out, would buckle easily without a cross beam supporting it. Am I right? and Chad, if triangles aren't as efficient for vertical forces, than what would be the best bracing system to use? -Justin Link to comment Share on other sites More sharing options...
Chad Posted February 17, 2011 Report Share Posted February 17, 2011 the triangular braces are there to prevent twist and lateral loads, in your scenerio you will have very little. it's the horizontal braces that keep the verticals from bowign under load, that is where your focus needs to be. How about a hexagional or pentagonal shape? are you allowed to use internal bracing (inside the structure)? or just 2D walls that get bonded together. With 5 or more sides you can do internal bracing witout getting in the way of the load rod up the middle. Link to comment Share on other sites More sharing options...
NudeLobster Posted February 17, 2011 Author Report Share Posted February 17, 2011 (edited) the triangular braces are there to prevent twist and lateral loads, in your scenerio you will have very little. it's the horizontal braces that keep the verticals from bowign under load, that is where your focus needs to be. How about a hexagional or pentagonal shape? are you allowed to use internal bracing (inside the structure)? or just 2D walls that get bonded together. With 5 or more sides you can do internal bracing witout getting in the way of the load rod up the middle. we can use internal bracing as long as it can fit around the 3x3 base of the test machine, though material use makes that idea look to be impossible. I used fanta's idea with the x beams, and I got 362.75" total... 2.75" too much :mad: :mad: I will talk to my teacher tomorrow though, because that is with the bracing measuring all the way to intersection. with the 1/8"x1/8" square material, it will actually shorten each diagonal brace approximatively 3/16", with 96 total braces shortened by that length, it would be 18 inches less than I annotated, so I should be able to use this design: http://i213.photobucket.com/albums/cc92/pocketrocket1331/Mobile%20Uploads/1297920810.jpg This design, annotated literally, would be 362.75" total material, but when annotated to take into account the material width, it would come out to somewhere around 344.75" -Justin Edited February 17, 2011 by NudeLobster Link to comment Share on other sites More sharing options...
garychoffmann Posted February 17, 2011 Report Share Posted February 17, 2011 Basswood is easily bent, what about making a coil spring? Link to comment Share on other sites More sharing options...
NudeLobster Posted February 17, 2011 Author Report Share Posted February 17, 2011 Basswood is easily bent, what about making a coil spring? we can use Basswood and only basswood...it used to be balsa wood but he said the basswood is denser? -Justin Link to comment Share on other sites More sharing options...
Chad Posted February 17, 2011 Report Share Posted February 17, 2011 Try a panel similar to the first one, but with out the center vertical. also make it with fewer horizontals but placing the diagonals at 45*, not the ~25* as shown. Imagine a square with a diagonal, and a bunch of those stacked on top of each other. Now do this design in a hexagon (as viewed from above). You'll end up with the same number of verticals, but less bracing material per panel, which can then be used internaly. Looking down on the tower, you'll see the advantage of internal bracing. If you just make a square, each vertical only has 2 axies of support, with internal bracing, you can easily double that or more. Perhaps an otigon, the first design would support that if you folded it down the middle. Link to comment Share on other sites More sharing options...
Shelby Posted February 17, 2011 Report Share Posted February 17, 2011 if the load is only excert'd straight down , then the more vertical rods the better , large area X's will cause force to be direct'd out ward i'd have to go with Chad a 5 side'd tower would suport more weight , if it was evenly apply'd straight down Link to comment Share on other sites More sharing options...
MikeMeyerhoff Posted February 17, 2011 Report Share Posted February 17, 2011 Like everyone is saying your verticle members are what will carry the load so the more the merrier. You want to keep them as short as possible for two reasons, the longer they are the more easily they will bend and the more material you use. You want to keep the verticle members as close to verticle as possible for two reasons, the more angle they are the longer they get and the more angle they are the more horizontial force is created within the structure. So maximize the number of full length verticle members you can get and use the rest of the material for bracing. You don't need much diagonal or x bracing in the verticle plane, maybe two or three pieces. The role of this bracing is to keep the whole structure from toppling sideways. Most of your bracing should be in a horizontal plane between verticle members. By bracing the verticle members horizontally you keep them from bending. The most effecient place to brace is always at the middle point of the member. But in your case having two sets of braces at the 1/3rd points would probably be best to give you a freestanding structure even if the joints aren't perfect. mm Link to comment Share on other sites More sharing options...
Skullzaflare Posted February 17, 2011 Report Share Posted February 17, 2011 3 dimensional tower or 2d truss? EDIT: sorry missed the last line. it is 3d. Look at and experiment with bending some of the members. pre stressing members can really add to the strength. Joint construction is the weak link in projects like this, so minimize joints. Just a lowly ME's opinion.if made out of balsa wood like it most likely is, the fiber of the wood is the weak point, not the joint, elmers wood glue tears the wood long before the joint breaks i remember making those balsa bridges in tech ed long ago, build it, then he puts a bar in the center (scale equal of a small box truck) and slowly starts adding weight to the box below hanging on the cable to the bar Link to comment Share on other sites More sharing options...
Elkidmino Posted February 17, 2011 Report Share Posted February 17, 2011 Triangles and diamonds are the way to go. Take a look at what fanta posted and don't look at them as X's but diamonds that terminate at a cross brace. It's all about distributing the load. Look at the basswood and take note of how the grain lies. You the pieces with a parallel grain for the uprights and longer cross braces, and anything with perpendicular grain should be thrown out, it's not to be used since it's weak. Use an exacto knife to cut it. Build a basic outline shape and then start making your cross braces and diamond shapes by placing the pieces of wood up to your structure and marking your cuts. Make sure you cut on the outside of the line b/c the exacto eats up material. Good luck man! Your structure will fail faster if it twists. I know it's loaded vertically, but things twist when they can't compress. Design it to resist that and it'll do well. Link to comment Share on other sites More sharing options...
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