Compression, tension, and shear stresses combine in ...



Task : Compression, tension, and shear stresses combine in anything we try to build. By carefully choosing and arranging parts, a mechanical or civil engineer can design a structure that will utilize the inherent strength of the building materials and compensate for their inherent weaknesses.

Your task is to use a limited supply of materials with low inherent strength to build a bridge that will most efficiently support the greatest possible weight. Your design must allow for a bus to travel down the length of the bridge as well as the testing block to fit across the center of the bridge. You may build a bridge alone or as a team (of up to 3 total people) - you may work with anyone taking regular physics. AP students must work with other AP students.

Specifications: You are required to build a truss-style bridge that meets the following specifications:

• Overall length across roadway exactly 60.00 cm

• Overall length across top 20.00 – 60.00 cm

• Minimum height of bridge (from below roadway to top of bridge) 15.00 cm

• Maximum height below roadway 7.00 cm

• Overall width of bridge(maximum outside to outside width) 10.00 cm at all points

• Maximum mass of finished bridge * 80.00 grams * this includes glue!

• Minimum load 12.5 pounds

• Maximum load 122.5 pounds

Digitally Drafted Diagrams –must include above listed dimensions AND the dimensions for the block and bus openings (see below).

BUS ( YOU MUST PROVIDE A CLEAR PATHWAY DOWN THE SPAN OF THE BRIDGE SO THAT THE TESTING BUS CAN PASS ACROSS THE BRIDGE. THE OPENING MUST BE 5.75-CM WIDE BY 7.25-CM TALL.

BLOCK ( You must provide a clear opeining in the SIDE OF THE bridge so the testing block fits into the side! The Roadway must be level with the table top when testing!

• Length of testing block = 9.8cm

• Height of testing block = 3.1cm

• Width of testing block = 6.3cm

You do not need to build a physical road surface on your bridge, an open framework is fine (but the block must have a place to rest). Also, the testing block MUST be placed on your roadway (see photos on webpage). It WILL NOT BE PLACED ON THE TOP OF YOUR BRIDGE. (See disqualifications)

Precise measurement and construction is VERY important so that the bridge will fit into the testing frame. If it does not meet specifications you may have to remove some of your bridge members, which will weaken the structure. (See disqualifications)

Materials and Building:

You will have to supply the materials for this project. You are only permitted to use the following materials:

• Balsa wood (1/4 inch thick or thinner) Balsa wood is available at any craft, art or hobby store.

• Any one-part adhesive Epoxies and Resin-based adhesives are NOT permitted!

Buy your materials soon – area stores could be out of stock if you wait until the week before the due date to purchase materials!

Grading * the 35 pts will be converted into 100pts for the final grade

Preliminary Diagrams and Photos - DUE January 5th (Tuesday) = 20 Homework points

The Diagrams – You will create bridge plans using some digital drafting/drawing program or NEATLY hand drafted (using straight edges and appropriate scaling). If making digital plans, it could be something fancy like C.A.D.D. or Google SketchUp or it could be something simple like the drawing tool(s) in a Word document.

All drawings must:

* be NEAT and to (some indicated) scale.

* include dimensions (overall length across roadway, overall length across top, height of (total) bridge, height below

roadway, overall width, and other crucial dimensions)

* show 3 views: side elevation, top elevation and bottom elevation

Revised (if necessary) Diagrams and Photos - DUE January 19th (Tuesday) = 10points

The Diagrams – You will create bridge plans using some digital drafting/drawing program or NEATLY hand drafted (using straight edges and appropriate scaling). If making digital plans, it could be something fancy like C.A.D.D. or Google SketchUp or it could be something simple like the drawing tool(s) in a Word document.

All drawings must:

* be NEAT and to (some indicated) scale.

* include dimensions (overall length across roadway, overall length across top, height of (total) bridge, height below

roadway, overall width, and other crucial dimensions)

* show 3 views: side elevation, top elevation and bottom elevation

* the mass of the finished/constructed side of the bridge

You must also submit a (physical) photograph proving that at least one entire side of the bridge has been constructed (not just cut and laid out, but put together and glued). It is not acceptable to bring a camera to the teacher and just show them the picture - you have to submit a physical photograph (although it does not have to be on photo paper).

Bridge - DUE February 3rd (Wednesday) = 25 points

The Bridge – You will RESEARCH tension, compression, shear and (existing) bridge styles and use this information to construct a balsa wood bridge. This bridge will be scored on the following criteria: meeting the specifications (40%); holding a minimum weight of 12.5 pounds (20%); and the efficiency ratio (40%). Your score on the efficiency ratio which has been established using data from the past results.

NOTE – We will have 120 pounds available to test your first bridge. If your bridge does not break under this load, then your efficiency ratio will be based on 120 pounds maximum.

|Plans and Preliminary Photos |Refer to the details outlined above for the requirements. |

|(20 pts) |The following deductions will be made for missing/incorrect elements: |

|*homework side of grade |-2 sloppy (not done digitally or sloppy) |

|You cannot earn negative |- 1 = for each missing elevation (on the diagrams) |

|points |- 1 = missing required dimensions (on the diagrams) |

| |- 1 = did not include a scale (for the diagrams) |

| |-4 = does not show all 3 views |

|Plans and Preliminary Photos |Refer to the details outlined above for the requirements. |

|(10 pts) |The following deductions will be made for missing/incorrect elements: |

|You cannot earn negative |-5 = no photo submitted |

|points. |- 2 = photo submitted but it does not clearly indicate that one full side is complete |

| |-2 = no mass included (for the constructed side of the bridge) |

| |-2 sloppy (not done digitally and sloppy) |

| |- 1 = for each missing elevation (on the diagrams) |

| |- 1 = missing required dimensions (on the diagrams) |

| |- 1 = did not include a scale (for the diagrams) |

| |-4 = does not show all 3 views |

|Bridge Score | |

|(25 pts) | |

| | |No* = AUTOMATIC - 4 |Major Rule Violations |

|Specifications met? |Yes = 10 / 10|maximum points available 6/10 |No and NOT testable. |

|(10 pts) | |(but could be lower – depending on violations) | |

| | | |Zero! |

| | |*still “testable” | |

| | | |

|Minimum weight held |Yes = 5 / 5 |NO = 0 /5 |

|(5 pts) | | |

| | | |

| | |

|Efficiency |See Ratio Scale |

|(10 pts) |Maximum points available are 12/10 |

| |*if your bridge did not meet specification you cannot earn ANY extra credit! |

| | |

Ratio Scale

500 + = 12

475 - 499 = 11.5

450 - 474 = 11

425 - 449 = 10.5

400 - 424 = 10

375 - 399 = 9.5

350 - 374 = 9

325 - 249 = 8.5

300 - 324 = 8

275 - 299 = 7.5

250 - 274 = 7

200 - 224 = 6

175 - 199 = 5.5

150 - 174 = 5

125 - 149 = 4.5

100 - 124 = 4

75-99 = 3

50-74 =2

below 50 = 1

Disqualification:

If bridge does not meet ALL of the specifications, it MAY be tested but there will be a point deduction and it will NOT be eligible for the class competition.

Your bridge may be TOTALLY disqualified (earning a zero) for the following reasons:

• Laminating of balsa wood – this is NOT permitted – will result in a zero.

• Stacking pieces with adhesive will be regarded as lamination and the bridge will earn a zero.

• Building a non-truss bridge – box trusses are NOT acceptable! This will result in a zero.

• Using materials other than those listed. Make sure that you purchase BALSA wood, NOT BASS wood. Also pay attention to the type of glue that you apply. This will result in a zero.

• Excessive weight – You will lose points for going over the weight limit. You must get your bridge to 80.0-g before it is tested. If you need to remove 5.0-g or more you will not be permitted to test your structure (and will earn a zero)!

• Illegal I-beams – Use of I-Beams is NOT permitted (bridge will earn a zero)

• Illegal joints – acceptable joints are: single overlap; double overlap, butt joint, gusseted plate joint, and dovetails. Overlap on any one joint may not exceed 1.0-in2.

• Not leaving an open roadway on the structure or sufficient space for the testing block or bus.

• Painted structures are NOT permitted. No stickers or tape.

• Soaking/coating members in resin or adhesives is NOT permitted!

• Non structural members - You may not have members on your bridge to simply meet qualifications.

NOTE – If you have to question if something is legal, assume that it IS NOT. Anything questionable could get your bridge disqualified!

Your teacher reserves the right to refuse to test any bridge that fails to meet the specifications. Bridges that are not done in the spirit of the competition will not be tested. A non-tested bridge will receive a non-droppable zero.

Important Dates:

▪ The Preliminary Building Plans are due in class on January 5th (Tuesday)

▪ The Building Plans and Photo of ½ built bridge with mass are due in class on January 19th (Tuesday)

▪ READ the rules above and refer to the RUBRIC for grading details.

If you are absent from school your plans and bridge still need to be in school on the due date!

▪ Your Bridge is due in class on February 3rd (Wednesday)

▪ To impound you will be asked to put a post-it note with your name(s) on the bridge. Your teacher will then measure and mass it and enter this data into a spreadsheet.

If you are absent from school your bridge still needs to be here on this date.

No class time will be devoted to design or construction of the bridges. If you decide to work with a partner be sure that you will be able to set aside A LOT of time with this person to work on the project.

-----------------------

Testing

Block

Length of block – this goes ACROSS the width of your bridge

Height of block

Width of block

Efficiency Ratio = weight held (pounds) x 456 (grams/ pound)

Mass of structure (grams)

Deductions

length/span too long or short ( -2 per cm over/under

width too narrow ( -2 per cm under

width too wide ( -1 per cm over

height too short ( -1 per cm under

height under deck too much ( -1 per cm over

mass too heavy ( -1 per gram over

You must have it under 85 grams or it won’t be tested (score of zero)

Illegal joints ( -1 for every infraction

block doesn’t fit ( you have to rip your bridge apart to get it in there (

bus doesn’t fit ( see rules for “block doesn’t fit”

Use of I-beams or H-beams ( public ridicule and a score of zero!

Use of basswood ( here’s a box of tissues…cry to someone else …you will get a zero.

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