WHICH MODEL BRIDGE DESIGN
SUPPORTS THE MOST WEIGHT?
The purpose of this project was to find out which model bridge design would support the most weight. Engineers and bridge-builders could put the results from this project to good use. It was believed that that the arch bridges would support the most weight. The materials used were craft sticks, wood glue, commercial weights, gram scale, and ruler. The required supports were constructed especially for this project. All amounts of weight were measured in grams. The bridges were constructed. Then the supports were constructed. Commercial weights were placed in the middle of each bridge to test it. More weights were added until each bridge collapsed. The final amount of weight was recorded in grams. The arch bridges held an average of 1485 grams. Pier bridges held an average of 1051 grams. Beam bridges held an average of 1248 grams. Truss bridges held an average of 1017 grams. Cantilevers held an average of 1220 grams. Suspension bridges held an average of 1226 grams. Cable-stayed bridges held an average of 1161 grams. All bridges generally held the same amount of weight. The arch bridges held a little more than the others did. The arch bridges were in the 1400-1500 gram range. The others were in the 1000-1200 gram range.
Information indicates that because of its properties, an arch bridge will support the most weight. Almost everything that is built to take great pressure is curved, such as the hull of a submarine.1 Arches support heavy objects such as the solid stone walls of the Coliseum in Rome.1 Therefore, an arch bridge should support the most weight.
Historically, the arch bridge, an improvement over the simple beam bridge was invented and perfected by the ancient Romans.2 Early arch bridges consisted of large stone blocks wedged together to form an arch.3 Sophistication of construction of stone masonry arch bridges reached its peak during nineteenth century.2 When first used, they were built with masonry blocks.3 Nowadays arch bridges with short spans are made of concrete or wood and those with long spans are built of concrete or steel.4 The forces on an arch bridge extend outward and downward (a condition called compression).3 An arch is higher in the center than the ends, acting almost entirely on compression.3 In an arch bridge the main structural system supporting the deck is a curve member (or members), higher vertically at its center than its ends, acting almost entirely in compression with its compressive load being maintained by thrust against immovable abutments.3 Therefore arch bridges must be built with stone, brick, timber, cast iron, steel, and reinforced concrete because they take compressive loads well.3 Arch bridges constructed with steel or pre-
Arches alone have great natural strength, when combined with supports called abutments (see next paragraph), they posses even more strength.2 The few bridges that are supported by more than one arch make use of a pier between the arches.2 This suggests that a bridge supported by more than one arch is strong enough to support whatever it is supporting only when theres a pier added. That changes an arch bridge to a combination arch/pier bridge.2 The main load carrying system in a modern arch bridge consists of two ribs supporting a deck by the use of spandrel columns (see next paragraph) or by suspenders.5 The deck of an arch bridge can either be on top of or be under the curve of the arch.2 The second design of arch bridge is preferred in situations where the road level is relatively low, and there is an active waterway close below the bridge.6 The famous Sidney Harbor Bridge is of this type.6 In addition to these two types of arch bridges, there is a third type.5 This type does not make use of abutments or spandrel columns.5 Instead, the ribs do the work of supporting the arch.5 One end of each rib is tied to the other end, and the thrust of the ribs is supported by structural tension.5 The tie is about deck-level and the deck hangs below the curve of the arch.5 It is fact that all these types of bridges can carry traffic on top deck uninterrupted while not hampering traffic below them.2 The choice of deck placement depends on the engineer and the surrounding area of the bridge.2 Arch bridges are easy to construct.2 A single, strong arch supports most arch bridges.2 When something pushes down on an arch, its force is evenly distributed throughout the arch.7 Arches have ribs, which normally end at inclined faces, termed skewbacks.5 While arch ribs are essentially compression members, they are also subjected to bending due to partial live loading, contraction due to shrinkage in the case of concrete ribs and the transfer of the deck load to the rib at discrete locations.5
Engineers must design arch bridges so that the sides of the arch do not spread apart and collapse the bridge.2 Thats where spandrel columns and abutments come in.2 Spandrel columns transfer the load of the roadway to the arch, which bears the weight of the bridge.2 Abutments are crucial to arch bridges.2 Abutments, as previously mentioned, are supports at both sides of an arch bridge at the base of the arch. When something pushes down on an arch bridge, its weight is carried outward along the curve of the arch to the abutments.5 As said before, arch bridges act on compression. The compressive load is maintained by the thrust against the abutments.5 Without the abutments, the ends of the arch would spread apart under any weight.3 Abutments also help support the arch itself.2 It is important to have sufficient abutments for an arch bridge.2 If they are too small, the bridge will not support much weight.2 If they are too big, they can cause unnecessary construction difficulties.2 Abutments are solid structures, if they were hollow, they would be too light to effectively support a bridge.2 Abutments or other types of supports are needed on all types of bridges, but they are especially important for arch bridges.2 To sum it up, there cant be one cubic meter abutments for a five hundred meter span.2
A bridge supported by a single arch cannot have a very long span.2 There must be a gradual and significant curve throughout the entire arch for it to posses the most strength.2 The best curve is usually found in a short to medium span bridge.2 That is because for a long span arch bridge to gradually and significantly curving, it must be a very tall.2 A tall arch is much harder to construct than a shorter arch.2 If a tall arch is not wanted, two or more arches must be constructed.2 That causes untold difficulties in constructing spans over deep water and gorges.2 The longest single arch bridge in the world, the New River Gorge bridge in Virginia, is five hundred eighteen meters in length, suggesting that arch bridges lose their ability to effectively support large amounts of weight when they exceed that span length.6 Some long-spanning arch bridges are supported by a combination of piers and arches.2 Therefore to have a single arch supporting a bridge effectively, it must be a short to medium span.2 On the other hand, spans of less than about of fifty meters generally are not economical because the cost of construction of the curved arch ribs far exceeds any advantage of an arch over other types of bridges for such a short span.2
Devices called hinges are sometimes used in arch bridges.2 Without hinges, arch ribs extend continuously from skewback to skewback and are fixed at skewbacks so no rotation can occur and thus the bridge is called a fixed arch.2 There can be two or three hinges in an arch.4 The three-hinged arch has hinges at the center and at the skewbacks, and the two-hinged arch has hinges only at the skewbacks.4 These hinges permit the transfer of the main compressive loads from one section of arch rib to another or to the abutments without transfer of bending movements.4 Hinges at the skewbacks simplify only the abutment design, but hinges at the crown and the skewbacks simplify the design of the arch rib as well as the abutments, eliminate stresses due to temperature changes, and allow the arch to tolerate minor movements of its abutments.4 In scaled-down models, hinges are not easily shown.
The weight carrying capabilities of the other types of bridges may seem similar to the arch bridge, but in reality, they are quite different.6 The beam bridge, although easy to construct, cannot be very long in length, and certainly cannot support very much weight.2 If piers are added, the beam bridge becomes a pier bridge and can span a very long distance.6 The longest bridge in the world, the twenty-four mile long Lake Ponchartrain Causeway in Louisiana is a pier bridge.2 The major drawback is that it is very difficult to build in deep water or in a gorge.2 The cantilever bridge, a refinement of the pier bridge, features cantilever arms8 made up of scaffolding-like trusses over on the sides and occasionally under the bridge.8 This bridge is also held-up by piers.8 Although this bridge can support a little more weight than the pier or the beam bridge, the arch bridge can support much more.8 Another drawback is that the cantilever bridge cannot span a very long distance without extra piers.8 As said before, building extra piers is difficult in deep water.8 Suspension bridges, undeniably beautiful and graceful, are used for long spans.9 A suspension bridge like the Golden Gate Bridge, with only two towers can span fifteen hundred meters or more.9 The deck of a suspension bridge hangs from two thick, usually steel, cables.9 The cables, in turn, run through the towers and are fastened to anchorages on either shore.9 The major drawback of suspension bridges is that they can collapse in strong winds because they are light and flexible.9 This danger was highlighted by the collapse of the Tacoma Narrows Bridge in 1940.9 Even though they can span very long distances, suspension bridges cannot support very much weight because of their flexibility and low weight.9 Cable-stayed bridges, such as the Sunshine Skyway Bridge a modification of the cantilever bridge, are relatively new in the United States.9 Although they resemble suspension bridges, cable-stayed bridges support their deck very differently than suspension bridges.10 Unlike suspension bridges, the cables on a cable-stayed bridge are connected directly to the towers.10 Usually, two towers are placed in a bridge with the ends of their cables touching.10 This type of assembly provides the bridge with a long center span.10 Cable-stayed bridges are sturdier, and can support more weight than suspension bridges, but not as much as arch bridges.10
If properly constructed and adequately supported, an arch bridge will support more weight than each of the following types of bridges: beam, pier, truss, cantilever, suspension, and cable stayed. Therefore, an arch bridge will support the most weight.
1. Construct a testing area in a flat place with two supports, thirty-five cm apart
and fifteen cm from the ground.
2. Construct fifty-six bridges (eight of the following types: arch, beam, pier, truss, cantilever, suspension, and cable-stayed) from Popsicle sticks and adhesive, making sure the sticks and adhesive are of the same kind for all the bridges. Each bridge must span a distance of forty cm and have a height of fifteen cm.
3. At the testing area, place one bridge on the supports.
4. Put the one hundred-gram weight in the middle of the bridge.
5. Increase the amount of weight by one hundred grams eight times, then fifty
grams seven times, then ten grams ten times and finally, one gram as many times as it takes by placing the weights on top of the first weight till bridge collapses. Make sure no one walks by testing area during test.
6. Record the final amount of weight.
7. Repeat steps 3-6 for each bridge.
All bridges held generally the same amount of weight. The arch bridges held a little more than the other bridges. They were in the 1400-1500 gram range. The other bridges were in the 1000-1200 gram range.
The bridges would not stand up on their own, so a support at each end had to be constructed. Balancing the weights on the bridges required patience. Clamps were used to hold the bridges during gluing.
The bridges supported different amounts of weights because each type has different construction. The arch bridges supported the most weight because of the great natural strength of the arch. The pier bridges supported the least weight because the supporting piers broke during construction.
QUESTIONS FOR FURTHER STUDY
1. Bridges made of what material will support the most weight?
2. What adhesive best supports a laden bridge?
3. What type of supports best support a laden bridge?
4. What type of weight best destroys a bridge?
5. Does the air temperature determine how much weight a bridge can support?
1. Growing Up With Science.Vol. 2. Webster Unified, Tarrytown, 1990. P 97-98
2. Salvadori, Mario. Building: The Fight Against Gravity. American Books, Saddle Brook. 1979. P 86-96.
3. The World Book Encyclopedia. Vol. 2. World Book Incorporated, Chicago. 1997. P 600-603.
4. The New Encyclopedia Britannica. Vol. 2. Encyclopedia Britannica I Incorporated, Chicago. 1995. P 510-512.
5. McGraw-Hill Encyclopedia of Science and Technology. Vol. 3. McGraw-Hill Incorporated, USA. 1992. P. 54-64.
6. Nova firstname.lastname@example.org.
7. Cuevas, Mapi. Physical Science. Holt, Rinehart, and Winston, Austin. 1994.
8. Growing Up With Science.Vol. 3. Webster Unified, Tarrytown, 1990.
9. Llewellyn, Claire. How Things Work. Scholastic Incorporated, London, 1995. P 22-23.
10. The Handy Science Answer Book. The Carnegie Library of Pittsburgh, Pittsburgh. 1994. P. 379-383.