If models of three different types of bridges (truss bridge, arch bridge, and beam bridge) made of popsicle sticks are tested for their weight bearing capacity, then truss bridge will be able to withstand the most weight, followed by arch bridge, and beam bridge will withstand the least amount of weight. This is because triangle is a strong and rigid shape which is used in truss bridges, arch is another strong shape as it allows load to spread out evenly, which is used in arch bridge. The only supports in beam bridge are vertical piers, which makes them weaker compared to truss and arch bridge. 

KEY TERMS- 

Civil Engineering- A STEM field that focuses on designing and maintaining physical environments in which human live, like buildings and bridges.  

Truss bridge- A truss bridge is a type of bridge where the load bearing structure is composed of trusses.  

Arch bridge- An arch bridge is a type of bridge where the main supporting elements are arches.   

Beam bridge- A beam bridge is a type of bridge where a horizontal beam is being supported at each end.   

Load- Forces that a structure is designed to oppose.  

Dead load- Loads that include the weight of the bridge itself or any other permanent object on the bridge.  

Live load- Temporary load on a bridge (cars, trucks, trains, bicycles, people, etc.).  

Enviornmental load- Temporary loads that act on a bridge due to environmental instances and the weather conditions (snow, wind, earthquakes, hurricanes, tornadoes, rain, etc.).   

FORCES THAT ACT ON BRIDGES- 

Compression- A pushing force (compression force)  

Tension- A pulling force   

Torsion- A twisting force (wringing a cloth)  

Shear- When there are two opposite forcings acting on the same point. Shear occurs when two parts of a structure are forced in opposite directions (ripping a paper).  

The main bridge types are suspension bridge, truss bridge, beam bridge, and arch bridge. 

Warren Truss  

This uses diagonal beams that form equilateral triangles (triangles with three sides of same length). When a force (load) is applied to one of the corners of a triangle, it is distributed down to each side. Two sides of the triangle are squeezed, which is compression, and the third side of triangle is pulled, which is tension. The equal distribution of force gives triangle remarkable stability. This is known as principle of distributive forces. Because of this principle, no matter how much force you apply on a triangle, it won't change shape unless its sides are compromised. Triangles are rigid shapes, once you set the lengths of the sides, the angles are fixed. The triangles shape cannot be changes without altering the lengths of its sides, which makes triangles a rigid structure. It also means it can withstand forces from any direction without changing shape. Stability and rigidity are the two characteristics which makes triangles a very strong structure for bridges.  

 

Arch Bridge  

The curvature of an arch bridge gives the structure more strength than some other structures. If something heavy were to travel on an arch bridge, weight will change the bridge with a downward, sagging force. The support columns on the bridge allow the weight to travel across the entire structure with consistency. Arch bridges get stronger over time. This is because the compression placed on each side will flatten the arch, creating a U that is less rounded. This distributes the weight of the deck better and provides more stability. However, to make a strong arch bridge, engineers must create the perfect arch. Arch bridges are always under compression. Tension in the arch is negligible. Natural curve of arch and its ability to dissipate the forces outward reduces effects of tension from the underside of the arch. Arch allows load to spread out instead of pushing straight down, which makes it a strong shape for bridges. Parabolic arch is considered the best arch shape. Arch bridge resists shear through their shape as arch spreads load evenly across the bridge. 

  

Beam Bridge  

This is the simplest form of bridge in which the deck is supported by vertical structures. Load causes horizontal compression on the top of the bridge. At the same time, horizontal tension occurs at the bottom. The supports carry the loads from the beam by compression vertically to the foundations. Beam bridges are weakest and used for only short distances. Their only supports are provided by vertical piers. The further apart its supports, the weaker a beam bridge gets. In continuous beam bridge, load is distributed across several piers, and it can be used for longer distances as compared to simple beam bridge. Beam bridges are also known as girder bridges. 

Independent variable  : Shape of the bridge (Truss, Arch , Beam)  

Dependent Variable : Weight bearing capacity of the bridge  

Controlled factors are length and width of the bridge, type of material used to make models.  

Uncontrolled factors are height and weight of the bridge. 

TO BUILD TRUSS BRIDGE-  

First, glue three layers of popsicle sticks together and  end to end  

 to make the base that run the length of the bridge.   

Second, put the two base sticks parallel to each other and glue sticks on top to create the deck.  

Third, glue triangular trusses on both sides of the base.  

Next, create two more of the bases for the top deck. Glue popsicle sticks on  

 top of the base to create another deck.  

Lastly, glue the deck on the insides of the triangular trusses.  

Repeat these steps to create two more truss bridges.  

TO CREATE ARCH BRIDGE-

First, glue three layers of popsicle sticks together end to end to make the base that run the length of the bridge.   

Second, put the two base sticks parallel to each other and glue sticks on top to create the deck.  

Third, grab some popsicle sticks and lay them down in the shape of an arch. Glue them together and cut excess popsicle   

sticks to create two arches.  

Next, Glue arches onto the bases of the bridge.  

After that, glue three support columns on the arches vertically.  

Lastly, glue popsicle sticks on top of the bridge to join the two arches together.  

Repeat these steps to create two more arch bridges. 

TO CREATE BEAM BRIDGE-

First, glue three layers of popsicle sticks together end to end to make the base that run the length of the bridge.   

Second, put the two base sticks parallel to each other and glue sticks on top to create the deck.  

Third, lay and glue popsicle sticks down on the deck to create supports for the piers.   

Next, glue two popsicle sticks together and cut the curved parts off and make sure length is same for all piers..   

Create eight of these piers.  

After that, glue  piers on each support in upward direction. Piers are at same distance.  

Then, create another deck, but with the supports on the bottom.   

Lastly, glue the second deck on top of the beams. The beams must be  

 aligned with the supports.  

Repeat these steps to create two more beam bridges.  

Prepare a loading block by glue three popsicle sticks together.  

Weigh all nine bridges and note reading in lab book. 

TO TEST OUT BRIDGES- 

First, take two identical shoe racks and place them parallel to each other.  

Second, tape some cardboard on the show racks to make them even and support the bridge.  

Third, place the bridge on top of the cardboard and in between the two shoe racks.  

Fourth, attach rope to loading block and to a bucket on the other end. Weigh this and keep on adding flour to the bucket it until it weighs two kilograms.  

Next, put the loading block and top of the bridge and leave the bucket hanging.  

Start slowing adding 1.5 kg weight each time to the bucket until the bridge starts to bend. Once it starts bending, start adding 600 grams and then 100 grams packs of flour into the bucket until the bridge collapses.   

Repeat this procedure to test all nine bridges and note the readings in the logbook. 

TRIAL 1-

I have started the first trial of my experiment in my basement. The date is February 25th, 2024 and the time is 9:05 p.m.   

Truss bridge- Started to bend at 6kg  

Arch bridge- Started to bend at 2.4 kg  

Beam bridge- Started to bend at 5 kg 

Bridge Design	Weight in kg
Truss Bridge (T1)	8.9
Arch Bridge (A1)	6.9
Beam Bridge (B1)	8.6

TRIAL 2-

I have started the second trial of my experiment in my basement. The date is February 25th, 2024 and the time is 9:30 p.m.   

Bridge Design	Weight in kg
Truss Bridge (T2)	8.2
Arch Bridge (A2)	6.7
Beam Bridge (B2)	8.7

TRIAL 3-

I have started the third trial of my experiment in my basement. The date is February 25th, 2024 and the time is 10;00 p.m. 

Bridge Design	Weight in Kg
Truss Bridge (T3)	8.1
Arch Bridge (A3)	7.6
Beam Bridge (B3)	7

 This science fair experiment was conducted to check which one of three bridge structures, truss bridge, arch bridge and beam bridge, would be the strongest. To check the strength of each bridge model a rope was tied to loading block placed on top of each bridge and tied to a hanging bucket to check the load at which bridge would collapse. Truss bridge carried most load (average of 8.4 kg ) before collapsing followed by the beam bridge (average of 8.1 kg) and arch bridge carried an average of 7.07 kg before collapsing. For the arch bridge, the shape of arch needs to be perfect, shape of arch bridge for trial 3 was more symmetrical and able to carry most load. If we consider live load and dead load both, truss bridge is still strongest with average weight bearing capacity of 8.65 kg, followed by beam bridge with average weight bearing capacity of 8.35 kg and arch bridge carried least weight bearing capacity of 7.27 kg. An interesting observation was that when the arch bridge collapsed, it did not split while collapsing. This is because the shape of the arch can resist shear force. Strength to weight ratio is the mass of load that broke bridge divided by mass of bridge.​

•  Truss bridge- 8400g/250g = 33.6grams​

•  Arch bridge-7070g/200g=33.35grams ​

• Beam bridge-8100g/300g=27grams ​

• Truss bridge has the highest strength to weight ratio, which shows that triangles are light weight structures efficient of carrying more load.

The purpose of this experiment was to investigate the effect of bridge design (Truss , Arch and Beam) on the weight bearing capacity of the bridge. To conduct this experiment, I built models of three different types of bridges using triangles for Truss bridge , Arch for arch bridge and vertical piers for Beam bridge as explained in the procedure to build bridge models. Model bridges were tested using flour and metal weights for their weight bearing capacity with the help of a hanging basket from the roof of the bridges using loading block and rope. I had started with 2 Kg hanging weight for each model and weights were added with increment of 1.5 Kg , 600 grams and 100 grams until the bridge collapsed and reading was noted . It was hypothesized that as result of this experiment truss bridge would be able to withstand most weight followed by arch bridge ,and beam bridge would be able to withstand least weight. This proved to be partially correct because truss bridge was able to withstand most weight(8.4Kg) but beam bridge was the second most strong for weight bearing and was able to withstand 8.1 Kg followed by Arch bridge which was able to withstand 7.07 Kg. Truss bridge was able to withstand most weight due to stability and rigidity of the triangles used in bridge design. Beam bridge was able to withstand weight because of continuous supports at short spams rather than having only two supports at the end of the bridge. Supports carried the load from the beam by compression vertically to the foundation. For the arch bridge, the shape of arch needs to be perfect , shape of arch bridge for trial 3 was more symmetrical and able to carry most load. If we consider live load and dead load both truss bridge is still strongest with average weight bearing capacity of 8.65 Kg, followed by beam bridge with average weight bearing capacity of 8.35 and arch bridge carried least weight of 7.27 Kg. An interesting observation was that when the arch bridge collapsed, it did not split while collapsing. This is because shape of arch can resist shear force. Conclusion is supported by background research and websites in bibliography. 

Bridge engineers use similar principals as used in this experiment to design real bridges. Understanding load bearing capacity helps them create safe and efficient bridges. Popsicle stick bridges help demonstrate how wood or popsicle sticks can withstand weight. Engineers also choose materials based on strength, durability, and cost. So, understanding weight-bearing capacity helps with material selection. Engineers assess existing bridges for safety and maintenance regularly. Load tests help figure out weak points and other factors that might affect the weight bearing capacity of a bridge and how to improve them. Strength to Weight ratio helps them reduce costs of materials and increase life span of the bridge.

There could be reading errors in kitchen and bathroom scale. There could be construction errors, especially when you are using hot glue gun. Hot glue dries very quickly and can lead to weaker joints sometimes. While constructing beam bridges, glue would dry out very quickly while adding vertical piers to top deck due to large number of piers. Paper clips can be used on joints and to add strength and bind popsicle sticks together. There could be design errors, especially in arch bridge as it's hard to build perfect arches. Cutting popsicle to  smaller pieces and joining them together can create perfect arch designs. I could have added more supports to the arches to help give strength and support to the arch bridge. 

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