Describing Structures Topic 3

Designing Structures Designers need to consider 4 main parts for a successful structure: Function Aesthetics Safety Cost Materials &Environmental Impact

Function What does a structure do? Contain, transport, shelter, support, lift, fasten, separate, communicate, break, hold Loads are forces (weight) a structure resists Live loads change Cars on a bridge, snow on a roof Dead loads stay the same the weight of the structure

Function How does the function of each of these structures differ?

FUNction – Rube Goldberg This Too Shall Pass List as many functions as you can see in this video!

Aesthetics Aesthetics is the study of beauty in art and nature – does it look good? ‘Beauty is in the eye of the beholder’

Safety Safety very important to designers Better safe than sorry is the basis of a margin of safety. Extra strength in case of failure - Many roofs in Canada have a margin of safety for snow load! Cost is sometimes challenging to balance with safety

Safety Not all safety concerns can be foreseen! Tacoma Narrows Bridge The designers did not take into account the effect the wind in the area would have on the design

Safety Forth Rail Bridge Safety vs. Cost

Safety - Earthquakes

Cost – Choosing Materials The properties of a material must serve the purpose of a structure. Would you set sail in a concrete canoe?

Materials – Composites Composites combine multiple materials They bring together strengths of materials and balance weaknesses Rebar tensile strength Concrete Compression Strength Concrete and steel are able to resist opposite types of strength. What are some other composite materials?

Materials – Layered Layers of a material are called laminations Often, layers will be put together in a particular way to balance weaknesses Think of napkins! Juice boxes Coolers, fridges, walls Windshields What are some other layered structures?

Materials – Woven & Knit Woven or knit materials are made up of linked threads which distribute forces through the whole structure They offer flexibility and can be layered Cotton clothing, rugs, netting, screens

Materials – Flexibility Pressed or melted substances can also offer flexibility Plastic wrap, aluminum foil, felt, paper Flexible materials are great for structures that need to be folded, rolled or molded If they are lightweight it can make transport, storing, and use more convenient Tents, parachutes, clothing

Think about it! What allows these hard structures to be bendy?

Material Choice Consider: Cost Appearance Environmental Impact Energy efficiency How would these be considered in choosing materials to build a house?

Materials

Design – Joints Materials often need to be connected, joints are typically weak points Mobile joints – allow movement Door hinges, elbows, bike chains Rigid joints – are locked in place Dovetail joints, rivets, nails, welds Can you think of other examples?

Design – Joints Fasteners Interlocking shapes Ties Adhesives Nails, staples, bolts, screws, rivets, dowels Interlocking shapes Folded seams, dovetails, puzzles, Lego, zippers Ties Thread, string, rope: shoes, drawstrings, Adhesives Thermosetting (hot glue guns), solvent based (crazy glue), epoxies - stronger bond = stronger chemicals Melted bonds Welds, soldering

Forces, Loads & Stresses Topic 4

Forces External forces are stresses that act on a structure from outside Weight of a car, wind blowing, snow; live loads They cause internal forces – stresses within the materials of the structure Internal stresses can change the shape or size of a structure this is called deformation. Deformations can be repairable or cause a structure to fail

External Forces – Loads A dead load is a permanent force, acting on a structure. This includes the weight of the structure itself. A live load is a changing, or non-permanent force acting on a structure. This includes the force of the wind ,the weight of objects on a structure Impact forces (collisions with the structure) are another type of live load.

Internal Forces – Tension Tension forces stretch a material by pulling its ends apart Tensile strength measures the largest tension force the material can resist before failing.

Internal Forces – Compression Compression forces crush a material by squeezing it together. Compressive strength measures the largest compression force the material can resist before it loses its shape or fails.

Internal Forces – Shear Shear forces bend or tear a material by pressing different parts in opposite directions at the same time. Shear strength measures the largest shear force the material can resist before it rips or fails

Internal Forces – Torsion Torsion forces twist a material by turning the ends in opposite directions. Torsion strength measures the largest torsion force the material can withstand and still spring back into its original shape.

Internal Forces – Bending A bending force is a combination of tension and compression

Internal Forces

Forces on Structures What forces are present in a swing set? How can the design of a swing set resist these forces? Page 324

Resisting Stresses The strength of a material is often determined at the level of particles Steel has high tensile strength – it has strong forces pulling the particles together Graphite has low sheer strength – the particles are arranged in layers with weak bonds between layers – used for writing Rubber has high torsion strength – each particle is attracted in all directions and can return to its place easily See page 314

FORCES LAB You will be in small groups and will rotate through stations to test the different types of forces. Lab write ups must be completed in your booklet and handed in!