Building a Bulsa Bridge

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Through-shot bridges Building A Bulsa Bridge
for Competition
© 2002
Over-shot bridges

Each year there are contests stretching from local to nationwide on the building of bridges by using matchsticks, popsicle sticks or bulsa wood. Because the rules change each year, many previous constructs may or may not be allowable in the current year. Thus this page is meant to offer some general insights into the construction of bridges.

The first thing you should do is learn from what others have learned. Go to the experts. Take a drive around and sketch each of the different kinds of bridges that you see. Be sure you label their main construction materials. That means short ones as well as long ones. That means culverts as well as world-class bridges that span great rivers or straits. That means the common overpasses on highways and railways as well as bridge-tunnel combinations.

The next thing you should do is classify them. The greatest dicotomy is dividing them between those your car went OVER versus those your car went through (overshot versus through-shot bridges, respectively).

The results of this should show you that overshot bridges are by far the most common as they range from culverts to the common cloverleaf overpasses. And a lot of concrete was used to build them (or stones, if you are looking at the bridges that the ancient Romans built - and they are still standing!). In your category of through-shot bridges including suspension and "box" bridges, you will see that steel is the preferred material. Of course, you may see a few very old wooden bridges such as the "Covered Bridges of Madison County," and there are odd wooden trestles scattered about in coal yards.

Now you should ask yourself some fundamental engineering questions: why aren't box bridges made of concrete? And why are overshot bridges generally made of concrete? What are the characteristics of concrete and of steel. In a very few words: cheap concrete can stand compression, and steel can withstand tension.

Ask yourself: which is the more important force in both classes of bridges - resistance to compression or resistance to tension?

But you are asked to build a bridge out of wood. What are the characteristics of wood? Look at nature. Tall trees weigh many tons, and all of it must be supported by a small, imaginary disk of wood down at the base. Wood very strongly resists compression. A short length of standard American fir "2x2" (1.75 x 1.75 inches actually) can withstand a compression of more than 25,000 pounds when pressed end to end, but only about 2,000 pounds pressed on it sideways over a 1.75 inch length will crush it to paper. Thus wood should be used in building compression resistant structures - those normally seen built of concrete. Meanwhile bulsa wood is a far lighter wood than is fir. Even so, its resistance to crushing from end to end is surprising. A square inch piece can easily support more than 600 pounds - providing that the block is standing so that its grain is vertical. Thus whenever wood is used in construction it should be used for its compressive strength, and not much for its tensile (pulling) strength. Thus, Strategem #1: IF AT ALL POSSIBLE, build an overshot bulsa bridge.

Some contests include an interesting wrinkle: "efficiency." What is the load the bridge can carry per unit weight of the bridge? Often the trick here is to make the lightest bridge possible. As a joke, one wag suggested laying a stick of bulsa over the gap. This would be like laying a board across a small creek. Sure, it wouldn't support much, but then that stick didn't weigh much either, and the efficiency might be astonishing! In any event, the strategy might be wise to build a "minimalist" bridge.

There is a big bit of engineering falacy in contest bridges: they are measured only in the centers. In real bridges loads move from one end to the other, and must be supported all the way across. Thus your minimalist bridge need only offer significant support in the middle.

And one final bit of engineering advice seen in most concrete bridges: there are no diagonals - all the force is downwards: all the force is supported by the "legs" or columns of the trestle.

Pillar bridgeThus in the best of all competition worlds, here is what your bridge should look like: you have a central, vertical pillar, perhaps with a hole down its center to accomodate the contest's measuring device. The lower end rests in the hypothetical riverbed. Then going from shore to shore is a single, very thin strip of wood - the hypothetical roadway. Upon the central pillar you have a small platform that conforms to the specifications of the load. Then stretching "shore to shore" and lying upon the platform is a thin ribbon of wood to symbolize the roadway. Now imagine: the pillar is a slice of bulsa log, or a composite built up of many strips of bulsa wood. You could place tons on top of that pillar. You win! Even the measuring device is broken!

And that is exactly the reason that trestles are so common!

Canyon bridgeOh, oh! Your rules may state that central pillars are not allowed. Now what to do? Again, thinking "overshot bridge" because those are compression resistant bridges, you must divert the force from being straight down onto a pillar to one that goes off to the sides, say in the walls of a canyon that the bridge is spanning.

Minimal canyon bridgeExcept that the judges aren't going to supply you with a canyon, and you need to build one! Fortunately, wood does have considerable tensile strength that can be harnessed - IF you do it right. Alas, the tensile strengths needed in box bridges require joints that also have good tensile strength - and glue is not good for that. THUS, stay away from box bridges! So, consider a bridge such as shown at the right. A bridge like this is very light, and all the stress is angled off on the diagonals. To prevent the diagonals for separating (a la canyon walls), there is a tie-piece across the bottom. At the ends of the tie-piece are glued "stops". Gluing in this fashion is very strong. The hazy circles indicate weak glue joints, but they are only for aesthetic purposes to simulate an upper roadway. A bridge like this was built 16 inches long, and had an efficiency of at least 750, and upheld 12 kilos without breaking. It's breaking point is unknown.

More Fun With Bulsa

Wooden Arches: Another property of wood is its relative stiffness. But this does not mean that wood cannot be bent into circles. Embroidery hoops are an example, just as are the ribs of wooden canoes. There are, of course, tricks to making permanent bends in wood. Most of those tricks employ boiling or steaming the wood and then slowly deforming it into the desired shape, and allowing it to dry either at room temperature or in an oven. In a way you probably have done this when you have chewed on your long-depleted popsicle stick. Soon the soggy wood could be bent. Bending the wood allows you to keep the grain always at its compressive peak all along its curved length. It would be, for example, an excellent form over which to lay bricks until the mortar hardened. (Concrete and bricks are also very strong compressively, and don't rot like wood does.) But wooden arches are good for real bridges, but not the best for bridges where only the center strength is important.

"A-Frames:" You have taken a trip around your area to look at the various types of bridges. Now turn your attention to your own home where the roof or ceilings are types of bridges - AND usually made of wood! Go up into your attic, and go look at the trusses used in buildings. Roofs in temperate climates must be strong considering that tons of snow can be added to them in winters. Also a considerable weight of roofing - shingles - is added. These real roofs have roofing and snow buildup on their slanted sides - on their upper sides - overshot bridges! Thus you see that they have diagonals to keep the sides from sagging. But suppose the weight is only added at the peak of the roof. Then all the side-support would mostly not be needed. Nevertheless, one part of the normal roof truss is still needed: the long horizontal piece that keeps the two ends of the A-frame from separating. This is a tensile problem. In homes with short spans, that horizontal "tie" is still usually another wooden component, but in larger spans over churches, for example, they are usually made of steel rods or cables (although often covered with wood for aesthetics). Architects of old cathedrals solved that problem by adding "flying buttresses." Again: this is like the minimalist, canyon-spanning bridge, shown further above.

Box bridges are not commonly built anylonger probably because they restrict the heights of trucks' needing to pass through them, and because they must make use of lot of upper components from which the roadbed must hang (tensile). That is not appropriate for either wood or concrete. Use of steel is called for and suspension bridges use much less steel than do box bridges. (Perhaps the best example of a huge box bridge is one of the oldest of that type - the rail bridge over the River Forth near Edinburgh, Scotland.)

Fancy Bridges: So you would rather win the beauty prize rather than the contest! You will want to build some sort of box bridge - like most of those done by your competitors. You will not win! You will only have a magnificent collapse - UNLESS: You do careful craftsmanship of making interlocking joints that don't depend so much on the strength of glue.

Suggested bridgeSUGGESTED BRIDGE (for efficiency): This is a modified "A-Frame" having a small flat area on top for the load. With all two-sided bridges, it is imperative that both sides be "congruent" - absolutely equal in size and shape, so that the whole will sit level and not have any wobble, which would strain unevenly under pressure. It is particularly important the the top, load-bearing section of the bridge be absolutely level so that the applied load will press on both sides of the bridge evenly - again, not producing uneven stresses that would lead to torque (twisting) and aggrevating early collapse. Note how the two different circled joints are made. In the upper ones, the top horizontal piece sits like a keystone in an arch. Pressure only makes it fit tighter, rather than sliding between the diagonal A-frames. The lower circled connection is joined so that the A-frame diagonals dig into the lower horizontal strip, and cannot slip outward as they are stopped from doing so by the "shoes" glued at the ends of the horizontal strip. Various diagonals are pictured because they may help prevent the last little bit of torque that preceeds collapse.


Choosing the woodAll bits of wood are not alike. Carefully look at the grain in each piece. Make sure that ALL the grain runs from end to end in your pieces. Any cross-graining is begging for early collapse.


Assembling your beamsIt is important for straight beams to remain straight and not bend, bow and finally snap due to their distortion under stress. On the otherhand, if you are making arches or otherwise bending the wood to form laminated arches, you want the wood grain in a specific manner, as shown in the diagram to the right.

Building beams


Constructing pillarsPillars, if allowed, should be massive. Some rules don't allow the gluing of components lengthwise. In that case, you "box" them with small bits of wood as shown in the lefthand figure. Because many of the measuring devices use to test and demolish the bridges use a weight that hangs down through the middle of the bridge, your pillar must have a hole down through its center. Shown is the end-view of a 4x4 boxed pillar. Of course, you would probably want a pillar that was at least 8x8 with a 2x2 hole down its center. It would thus have 60 vertical pieces to bear the load!


Making a symmetrical bridgeWhen you look at the side of your bridge, you will probably want it to be symmetrical - one end's being the mirror image of the other end. This is easy: sketch the left end of the bridge on the left half of a long piece of paper, and then fold the paper at the halfway point so that the first image is on the OUTSIDE. Trace through the paper onto the blank piece. When you open the paper, the whole symmetrical bridge will be there.


Making jigsAs stated, one of the very important things you must do is make your bridge's side congruent. One way to do this is to make one side, then trace it on paper, and finally glue your other half together as it lies on the tracing. Some builders might want to pre-stress some of their parts in ways that counteract the eventual applied weight. To do this a jig is needed - especially for making arches and other curves. In this latter event, you should be prepared to make a half-dozen or so pieces and then choose the two that most closely match.


Types of glue jointsIn general there are three types of glue joints you might make. The "butt-end" glued joint is very weak, because glue does not set well on the end-grain of wood. For any joints that must take separation stress, this is a strong no-no! It would be better to overlap the side of the butt-end with the neighboring piece and glue it there. Glue bonds strongly to the sides of wood. In fact, glue is usually stronger than the wood itself, and the more side-grain you can stick together, the better, as in the third part of the diagram.

Strengthened T-JointIf you must have a T-joint, you should try to gain permission to use a strapping as shown here. It is important that the square edges of the longer piece (the horizontal one shown here) be rounded so that the strapping can be molded around it without the wood filaments' breaking. Many glues can be applied to hot, wet wood. Use clamps until the wood is dry.


Even the builder of the Statue of Liberty built several duplicates (smaller ones, of course) so that they could be tested for various factors. You should do the same! You probably have been told what the winning bridge last year was able to withstand before collapsing. You want yours to be stronger (or more efficient). Build a model and test it. If it can uphold lasts year's final catastrohic weight, you have a good chance of winning this year. If you bridge is able to do that - and you are going for the efficiency award, then consider what superfluous pieces are on your bridge and remove them. The lighter you make your model before collapse, the more efficient it will be. Once you have done your testing, then set about making your final bridge. You may want to make two of them. Test them for being able to withstand just a little less weight than you used to devaste your model. If they can take it, take the one you think was built best and submit that - WITH CONFIDENCE!

And a couple of final words:

G O O D   L U C K !

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