pg3

                                             Exteranl and Internal Forces Pg. 7

Review of External Forces:
External forces affect structures from the outside. they are divided into two groups, live loads and dead loads.
Live loads include:
a) strong winds; b) impact forces; c) objects on or within the structure.
Dead Loads include: the wright of the parts that holds the structure.

Review of Internal Forces:
Internal Forces affect structures from the inside. They are dividen into four forces:
a) Tension is the force that streches structures
b) Compression is the force that squashes structures
c) Torsion is the force that twists or rotates structures
d) Shear is the force that pulls apart structures.

The four drawings were posted in classroom using the overhead projector. Students must have copied the correct answers.

3.13: Choosing Structures (page 162-163) Pg. 10

1. Define the three types of structures. Give 2 examples for each type.
a) Shell Structures: It has a solid surface and a hollow interior; e.g., eggs, shoeboxes, aircraft wings
b) Solid Structures:Has only one part and contains no hollow spaces; e.g., walls, sams, phone poles, etc.
c) Frame Structures: Are formed from a combinaiton of parts; e.g., goal posts, hydro towers, griders that make up a bridge

2. Solid structures are also called mass structures because their strength comes form their mass.

3. The main use of Solid Structures is as supports.

4. Because of their flexibility, frame structures are better at handling torsion and tension forces than solid structures although they do not resist compression as well.

5. An important advantage of frame structures is that they use less material than solid structures.

6. Why are some structures a combination of more than one structural type?
shell, solid, and frame structures have different advantages and disadvantages with respect to loads. Therefore, combining two or more types in a single structure combines each type's strength and compensates for each one's weaknesses.

7. List the advantages and disadvantages of using a frame structure as a helmet.
Advantages:       lightweight, allows air circulation, reduces the amount of material required for construction
Disadvantages: holes in the frame may permit entry of pointed objects, more complicated to fabricate compared with a shell structure.

8. Open Quiz: Structure Shapes:

Fill in the blanks: Fill in the blanks using the word box below, and then press "Check" to check your answers.
dam                forces                 frame               helmet                  hollow         hydro tower           mass        shape
shell               skeleton             solid                triangular             weak             weight                   shape

When designing a structure, you need to consider the forces that will eventually be applied to the structure (eg. weight, wind, traffic, etc.). You can then determine the best structure shape to handle the load.

A shell structure is a solid, domed surface, with a hollow interior. Force applied to the shell is transmitted through the whole structure. Because the interior is hollow, the structure can be of low mass. Strong shells can be made out of weak material, because it is the shape of the shell that gives the structure its strength. A helmet is an example of a shell structure.

A solid (or mass) structure is one part, with no hollow spaces. It relies on its own weight to resist applied forces. A dam is an example of a mass structure.

A frame structure is formed from a combination of parts usually joined at the end, to form a skeleton which resists the forces applied. It often makes use of the triangular shape to increase strength. A hydro tower is an example.

3.14: Finding Stability in Symmetry (pg 164-165) & 3.17: Strengthening Structures (pg 170-171) Pg. 10b

Read pages 164-165. Define Symmetry.

Symmetry: a body or structure is symmetrical if it can be divided in half with each half becoming a minor image of the other.

Strengthening Methods: Read pages 170 & 171. Answer the following questions:

1. A truss is a frame that takes advantage fo the strength of th tiangle by linkeng many of them togerher. Each triangle in a truss is made up of 3 members or parts. Trusses can be used to support other parts of the structure or on their own as an althernative to solid structures such as beams

2. A gusset is a piece of solid material used to reinforce a seam or a joint. Gussets cab be used to resist tension, compression, or torsion forces.

3. A tie is a support that does its work by resisting tension forces.

4. A strut is a support that resists compression forces.