Mrs. Nowlan's Grade 7 Science

Unit 4: Structures and Forces

Leaning Tower of Pisa

Tacoma Bridge collapse

 

Dates to Remember

December 18 - 20: Internet activity on Forces and and Loads. Due January 10

January 7: Review of Forces, Loads and Stresses

January 8: Examining Forces activity page 310.

 

Definitions

External forces: forces or stresses that act on a structure from outside of it e.g. planes on the twin towers.

Internal forces: forces or stresses that act on a structure from inside. Internal forces are produced by external forces.

Deformation: the change of shape that occurs after an external or internal force has been applied to a structure. Deformation can lead to permanent damage or failure of a structure.

Dead load (static): is a permanent force acting on a structure e.g. gravity

Live load (dynamic): is a changing or non-permanent force that acts on a structure e.g. mass of snow on a roof, wind that blows down trees.

Tension: the force produced by pulling a material.

Tensile strength: is the largest measure of tension force that a material can stand before it breaks.

Shear force: a force that bends or tears a material by pushing parts of it in different directions.

Shear strength: measures the largest shear force that a material can stand before ripping apart.

Compression forces: crush a material be squeezing it together.

Compressive strength: measures the largest compression force that a material can stand before it loses shape or breaks.

Torsion forces twist a material by turning the ends in opposite directions.

Torsion strength: a measure of the largest torsion force that a material can stand and still spring back into its original shape.

Bending forces: a combination of tension and compression forces.

 

 

 

Useful sites

Internet Activity:

Purpose: this activity is designed for your group to present information, visuals and examples of forces, loads and stresses to an audience that includes your teacher, classmates and parents.

Task 1:

In your group, plan how you are going to present the information. Look through the web sites, definitions and text book to see what information need to be covered. Divide up the work between the group members.

Task 2:

http://www.pbs.org/wgbh/buildingbig/lab/forces.html

Using the above site and the definitions to the left, make a scrap book, poster or computer generated notes (Word or PowerPoint) that includes all the 5 types of forces, a description and picture of what they are and an example of what they do.

Where possible, have some examples or visual aids to show how the different forces affect structures.

Task 3:

http://www.pbs.org/wgbh/buildingbig/lab/loads.html

The Stoney Trail bridge in Calgary has to withstand many different types of loads that could possibly lead to its collapse. Using the above site, show what these loads are and how they affect the bridge. Your group must determine how best to demonstrate how different stresses (such as mass, wind, earthquake, temperature) might affect this bridge. This might be a good opportunity to get someone to take you skating at Bowness Park over the holiday- then you can examine the bridge for yourself. If you are there on December 27th - you will probably bump into your teacher - the world's worst skater!!

	Photographs courtesy of "Calgary Bridges: Teacher Resource and Field Trip Guide", 1999, Calgary Board of Education and Calgary Science Network
Bridge description: Stoney Trail is a beam bridge. It is a segmentally constructed, incrementally launched, post-tensioned concrete structure. Under the bridge there is a cable-stay pedestrian structure. The pedestrian structure is a steel truss girder with a concrete deck. The incremental launch design was chosen for a couple of reasons. It was a less expensive alternate design submitted by the low bidder and it provided a method of constructing the bridge with minimal impact on the south bank of the river. The south bank, along this section of the river, contains one of the few stands of Douglas Fir trees in this area. Prominent Feature: The magnitude of the bridge is the prominent feature. It has spans of 102 meters, 40 meters up from the river valley and a 4.5 meter deep girder. Walking through the bridge on the inside of the girder is comparable to walking through an LRT tunnel. Material: Stoney Trail Bridge is constructed of cast-in-place concrete abutments, piers and superstructures. The box girder and deck (superstructure) was cast in segments then post-tensioned with steel reinforcing cables and pushed from the north abutment to the south. Strength: The Stoney Trail Bridge gets most of its strength from the miles of post-tensioning cables located inside the girder. Post-tensioned: means that the concrete was reinforced with flexible sheet metal tubes along the line of tension. After the concrete hardens, steel cables are strung through these tubes and then tensioned (pulled tight) against the concrete beams. Bridge Structure: Information and diagram courtesy of "Calgary Bridges: Teacher Resource and Field Trip Guide", 1999, Calgary Board of Education and Calgary Science Network, p.84.

Evaluation:

1. Written and visual definitions of five forces. (10 marks). Bonus of 2 marks for visual examples.

2. Application of information about loads to Stoney Trail bridge (10 marks).

3. Clarity of information (5 marks)

4. Creativity and effectiveness of presentation (5 marks).