DESIGN THE BEAMS for a cheap bridge

In summary, the bridge will be 20 feet wide and constructed of four by eight ft plywood sheet which is placed on top of two composite beams that are 4 ft apart. The beams need to take all the load which is 500 lb/ft distributed over the length of each composite beam. The plywood are not attached. The design and build the beams using any of the following materials: 3 plywood, 4 feet by 8 feet by 1/2 in thick, properties unknown; 16 pine wood planks, 1 inch by 10 inch ACTUAL cross section, 25 feet length; 16 " " " " , 2 inch by 4 inch, actual cross section,
  • #1
wildleaf
25
0
Need to build a bridge 20 foot wide gully. The bridge is 4 ft wide and will be constructed of four by eight ft plywood sheet which is placed on top of 2 composite beams that are 4 ft apart (see link for picture).The 2 composite beams need to take all the load which is 500 lb/ft distributed over the length of each composite beam, (see link for the picture). The plywood are not attached. DESIGN AND BUILD THE BEAMS using any of the following materials:

- 3 Plywood, 4 feet by 8 feet by 1/2 in thick, properties unknown.

- 16 pine wood planks, 1 inch by 10 inch ACTUAL cross section, 25 feet length
- 16 " " " , 2 inch by 4 inch, actual cross section, 25 feet length
Pin has Young's Modulus of 2 MSI, and a desgin tens/compression strength of 4000 psi and a desgin shear strength of 500 psi ( don't use safety factors)
-2000 steel nails, 0.125 in diameter, 4 inch long
Steel has Young's modulus of elastic of 30 MSI and shear strength of 20,000 psi
-Liquid adhesive/glue, allowable shear stress 50 psi
- crosscut-wood-saw
-Paper, pencil and calculator for the the design

PICTURE: http://i51.tinypic.com/io3td4.jpg

I have NO CLUE where to start or what I should do. If someone would explain it so that I will be able to do it.
 
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  • #2
You don't think you could design a simply supported beam 20 feet long which can take the specified load?
 
  • #3
I don't know how to start it.
 
  • #4
The self weight of a bridge is (usually) the largest load it carries so you start by making an assumption (guess) as to the self weight. Add the imposed loads and use structural theory to design to carry the total.
This will give a working model that can be refined until the strength matches the loads.

go well
 
  • #5
Start your design like this:

1. Determine the maximum bending moment for a simply supported beam with L = 20 feet and with the specified load of 500 lbs/ft.

2. Draw the shear force and moment diagrams.
 
  • #7
Studiot: I don't think you have to make an assumption for the weight bridge.

SteamKing: I have found max moment of 75,000 and the max shear of 5000. Do you use the max shear force and use in "q = VQ/I " ?? In order to use that I will have to get the I, right?
You would find out what q and using q and the shear strength for the nail we can find the shear stress? Or do we use T (tau) = q/t ??

turbo-1: Sadly that link does not help me.
 
  • #8
Use the max. bending moment and the strength properties of pine to determine the section modulus necessary to support the load so that the beam is not overstressed in bending.
 

Related to DESIGN THE BEAMS for a cheap bridge

1. How do you determine the best beam design for a cheap bridge?

The best beam design for a cheap bridge is determined by considering factors such as the bridge span, load capacity, and material cost. A thorough analysis is done to determine the most cost-effective design that can still meet safety and structural requirements.

2. What materials are commonly used for cheap bridge beams?

Materials commonly used for cheap bridge beams include wood, steel, and concrete. These materials are relatively inexpensive and have been proven to be strong and durable enough for bridge construction.

3. How do you calculate the load capacity of a beam for a cheap bridge?

The load capacity of a beam for a cheap bridge is calculated by using the beam's dimensions, material properties, and the expected load that it will need to support. This calculation is an essential part of ensuring the safety and stability of the bridge.

4. What factors should be considered when designing beams for a cheap bridge?

Several factors should be considered when designing beams for a cheap bridge, including the bridge span, load capacity, material cost, and construction techniques. Other factors such as the location and environmental conditions should also be taken into account to ensure the bridge's longevity and safety.

5. How can I ensure the beams for my cheap bridge are strong and durable?

To ensure the strength and durability of the beams for a cheap bridge, it is crucial to follow proper design and construction techniques. Beams should be designed to withstand the expected load and be made from quality materials. Regular maintenance and inspections should also be conducted to ensure the beams remain strong and safe for use.

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