Bridge Building Introduction:
This lab was designed to explain the concepts of tension and compression in structures such as bridges. The main goal is to understand the role of tension and compression forces in building bridges. The one specific type of bridge used in this this lab is a Wa Warren rren Truss Truss bridge. The compression compression and tension forces act in different different ways ways in different types of bridges. This experiment will analyze the forces in a Warren Warren Truss bridge, and see how the correct combination of compression and tension make the bridge possible.
Background:
There are many types of bridges. For example, beam bridges, truss bridges, arch bridges and suspension bridges. All All of these look different, and are made using using separate techniques. techniques. uspension uspension bridges such as as the !olden !ate "ridge in an Francisco use cables. "eam bridges use horizontal beams to support weight. Truss Truss bridges, the type analyzed in this lab experiment, make use of triangular structures, structures, in order to spread the weight out. There are many types of truss bridges, but the one built in this lab is a Warren Tr Truss uss bridge, named after #ames Warren. A lot of truss bridges are built by adding $ertical supports between the triangles of a Warren truss bridge, to add stability. stability. %owe$er, %owe$er, Warren Warren truss bridges are are still $ery stable stable and capable of handling load. &ue to the web configuration of these bridges, the compression and tension forces are split between different beams. 'nder a uniform load, different beams are sub(ect to either tensile or compressi$e stress. As a load crosses the bridge, the beams are sub(ect to stress re$ersal, distributing the load, and making it easier for the bridge to handle it. Figure ) shows the calculations of tension *in red+ and compression *in blue+ for a simple Warren Warren truss bridge with a weight going across across it. The The equilateral triangles triangles make it it possible for the the diagonal forces forces to be equal. Figure shows a diagram of the distribution of forces in a simple Warren truss bridge.
Figure 1: The distribution of a load in a warren truss bridge. Figure 2: Compression and Tension forces acting in a Warren truss bridge
These bridges are great for small distances, but the triangular structure becomes a problem for longer spans. -onger spans mean the truss depth has to be greater, increasing the size of the triangles. This makes the panel points become more spaced apart, making the framing of the bridge less efficient. The diagonal beams length also increases, and this (ust means that the bridge wont be able to resist buckling as much. That is wh y these days, most truss bridges include $ertical beams as well. There arent many Warren truss bridges left anymore, but a few small ones can be spotted in countries like pain, !reece or /exico. Figure 0 shows a truss bridge. 1t has $ertical supports, but all warren truss bridges do now, since this is more stable.
Figure 3: 127th treet !ridge" Coo# Count$" %llinois
Figure &: 'n older Warren truss bridge at (umford Falls" )aine Procedure: 1. 2. 3.
2arefully and strategically build a bridge using the kit pro$ided. 3nce all the beams are in place, take the long blue strip and fit it between the bridge, like a road. ince there are only 4 load cells a$ailable, this experiment must be done in se$eral trials. "efore e$ery trial,
build the load cells into the desired positions along the bridge and take note of the placement. 4. The first trial must be used to pro$e symmetry of the bridge. o, place the load cells on the same positions 5. 6.
along the bridge, on opposite sides of the bridge, as shown in figures ) and . 3nce the load cells are fit in, connect them to the computer using the equipment pro$ided. 3pen &ata tudio and start a new experiment and collect data as you roll down a car across the road on the
bridge. 7. /ake a table out of the collected data of the tension and compression forces, and export it into excel.
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8. 9.
'se 5xcel or /AT-A" to graph this data on a time $s. force graph. Take the load cells out of the current placement, and put them in the next positions to be tested. 6epeat steps 4 to 7 until you ha$e collected data on e$ery beam of the bridge. ince you pro$ed the symmetry of the bridge, you only need to perform the experiment on one side of the bridge. 1t is assumed that the results will be the same for the other side.
Figure -: oad cells 1.1 and 1.2" used to test s$mmetr$.
Figure *: +,ample of a wa$ to setup the rest of the trials"
Results:
Figure 78 9lacement and labeling of the load cells on the bridge *Trial ), testing for symmetry+
Figure :8 9lacement and labeling of the load cells on the bridge *Trial ;<+
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/raph 1: 'cti0it$ for load cell 1.1 Tension
/raph 3: 'cti0it$ for load cell 1.3 Compression
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/raph 2: 'cti0it$ for load cell 1.2 Tension
/raph &: 'cti0it$ for load cell 1.& Compression
/raph -: 'cti0it$ for load /raph *: 'cti0it$ for cell load2.1 cell 2.2 Compression Tension
/raph 7: 'cti0it$ for load cell 2.3 Tension
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/raph : 'cti0it$ for load cell 2.& Compression
/raph 5: 'cti0it$ for load cell 3.1 Compression
/raph 14: 'cti0it$ for load cell 3.2 Tension
/raph 11: 'cti0it$ for load cell 3.3 Tension
/raph 12: 'cti0it$ for load cell 3.& Compression
/raph 13: 'cti0it$ for load cell &.1 Compression
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/raph 1&: 'cti0it$ for load cell &.2 Tension
/raph 1-: 'cti0it$ for load cell &.3 Tension
/raph 1*: 'cti0it$ for load cell &.& Compression
/raph 1: 'cti0it$ for load cell &.* Compression
/raph 17: 'cti0it$ for load cell &.- Compression
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/raph 15: 'cti0it$ for load cell -.1 Compression '6 Tension
/raph 24: 'cti0it$ for load cell -.2 Tension
/raph 21: 'cti0it$ for load cell -.3 Compression
/raph 22: 'cti0it$ for load cell -.& Tension
/raph 23: 'cti0it$ for load cell -.- Compression Discussion:
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All bridges must show symmetry, i.e., the forces on one side of the bridge must be the same as the forces on the other side. 3therwise, the bridge will be unstable, and will fail. Trial ) of this lab shows this. -oad cell ).) and ). were places at the same positions along the bridge, but on the opposite sides of the bridge. Their graphs were $ery similar and showed tension at the same point of time. The same goes for load cells ).0 and ).4. We were able to thoroughly pro$e symmetry on the bridge.
As shown in figure , a truss bridge, forces on the beams at the bottom and the diagonal beams in the center are tension forces. 2ompression exists on the beams at the top and the diagonal beams on the outside. "ased on figure , the load cells that should ha$e shown compression are .) and <.<. The slope is positi$e a ma(ority of the time in the graphs of these load cells, showing compression. The beams at the bottom *., 0., 4., <.0 and <.4+ all $ery clearly show a negati$e slope in the graph, indicating that there is tension. The graphs for the beams at the top *.4, 0.4 and 4.4+ ha$e a positi$e slope for a ma(ority of the time, so there is compression. Another type of beam tested was a beam that connected the sides of the bridge, o$er the top. 1t showed a positi$e slope followed by a negati$e slope, showing compression followed by tension, due to its placement.
The results of the graphs of the rest of the beams *the diagonals+ werent all the same, like the pre$ious ones. There was some tension and some compression. This is because due to the triangular structure and the distribution of forces, some of the diagonals are under compression while others are under tension. 3$erall, the results from the graphs are as expected. 1 expected all of the diagonals to show tension more, but realized that they showed both tension and compression.
u!!ar":
This lab showed the distribution of tension and compression in a Warren truss bridge. 1t showed how the triangular structure helps the bridge carry loads easier by balancing out the tensile and compressi$e strength. The beams at the top of the bridge show compression *positi$e slope+ and the ones at the bottom show tension *negati$e slope+. As for the diagonal beams, some show compression, while other show tension because the triangular structure distributes the force.
Works 2ited
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=-ecture ) ; 9hysic of /aterial.= Lecture 21. >.p., n.d. Web. < ept. ?). @http8www.ic.sunysb.edu2lassphy)4)mddoku.phpBidCphy)4)8lectures8)D.
=/odel "ridge &esign.= Garrett's Bridges. >.p., n.d. Web. E ept. ?). @http8www.garrettsbridges.comdesignwarren;trussD.
=Warren Truss.= Bridgehunter.com: Historic Bridges of the United States. >.p., n.d. Web. E ept. ?). @http8bridgehunter.comcategorytagwarren;trusspageD.
=&a$id !uise The 5$olution of the Warren, or Triangular, Truss The #ournal of The ociety For 1ndustrial Archeology, 0. The %istory 2ooperati$e.= David Guise | The vo!ution of the "arren# or Triangu!ar# Truss | The $ourna! of The Societ% &or ndustria! (rcheo!og%# )2.2 | The Histor% *oo+erative . >.p., n.d. Web. 7 ept. ?). @http8www.historycooperati$e.org(ournalssia0.guise.htmlD.
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