1. Structures and Designs
By Mr. R. Gallagher

2. Vocabulary Thrust lines Equilibrium Elasticity
Imaginary lines of force caused by loads
They are transmitted through all parts of the structure to the ground
Equilibrium
When a force is balanced by another force
Elasticity
The ability of a material to return to its original shape and size when a load is removed
Some materials are so brittle that instead of changing shape they break without warning

3. History of Towers
People have been constructing buildings since ancient times
4 story houses were made 4000 years ago and the Romans made 10 story buildings
Towers are mostly vertical structures that support a load.

4. Constructing Towers
Until modern times, structures were designed and built using the method of trial and error
Each builder would build a structure higher than the last until the structure failed
Modern engineers use mathematics, physics and computers to design structures
High strength reinforced concrete has replaced stone, wood and brick
Engineers use; computers to perform structural analysis, scale models and wind tunnels to better understand a structure’s strength

5. Types of forces on structures
Dead and live loads
load – forces applied to a structure
Dead loads
Permanent loads that do NOT change or move
The weight of the structure itself is a dead load
Live loads
Those that move and change
Ex. Wind, melting ice, truck moving over a bridge

6. Types of forces on structures
Static and dynamic loads
Static load
Loads that do not change or change slowly
loads that build up gradually over time, or with little dynamic effects
Ex. Ice melting on a bridge
Dynamic load
Loads that change quickly
Ex. A gust of wind

7. Stress and Strain Stress Strain
Is the amount of force placed on an object
Is measured in Newton's
Strain
Is the distance an object deforms under stress

8. Types of Stress Compression The tendency to push or squash a material
A material under compression is always shorter

9. Types of Stress Tension The tendency of a material to be pulled apart
A material under tension is always longer

10. Types of Stress
Shear
Occurs when a material has two parallel but opposing compression forces
One part slides past the other
Ex scissors

11. Types of Stress Torsion The tendency of a material to be twisted
Ex a wrench tightening a nut

12. Types of Stress Bending Placing a load in the center of a span
The top part compresses while the bottom tenses

13. Recall Types of Stress

14. Recall Types of Stress

15. Strong Shapes Triangles compression compression tension
Are a strong shape
Used in most structures
When under a load one side of the triangle is always in tension which gives the shape strength
Have greatest support when the force is applied to a corner not in the middle
compression
compression
tension

16. Strong Shapes Squares/rectangles
May work well for columns but are not as strong as cylinders
When placed under a load its shape will distort
Making two triangles from the square will increase its strength

17. Strong Shapes Cylinders
Is a strong shape when force is applied from the ends
It is strong because there aren’t any corners and the force is distributed evenly around the cylinder
Strong against compression

18. Strong Shapes
Arches
Arches support loads anywhere along their curves unlike a triangle
A disadvantage is that arcs push out at the base
Therefore abutments at the sides must be used to prevent the arc from flattening
abutment

19. Strong Shapes Domes Use the same principles as arcs
Are really many arcs placed together
Used in stadiums so that columns do not get in the way of the playing field

20. Trusses
A. Howe Truss

21. B. Pratt Truss

22. C. Warren Truss

23. Beams I Beam Shape of an “I” Very strong and economical
Used in houses, buildings and bridges
Can be steel or wood

24. Beams Box Girder Are “U” shaped and reinforced
Used mainly in bridges of lighter pay load

25. Beams
T beam
                                                                                                                              
Double “T” Beam

26. Bridges Beam Bridges Bending stress
Supported by the ends on the embankment
When the span is too great columns are added for support
The shapes of columns and girders make a difference in the strength of the bridge

27. Bridges Truss Bridges In 1890’s the train became popular
Bridge builders needed to strengthen their beam bridges
It had to withstand massive weight and vibrations
They solved this by using trusses

28. Bridges Old Arc Bridges
For 1000’s of years arc bridges were made from wedge shaped stones called voussoirs
They were placed over a wooden framework
The very middle and last stone placed in the arc was called the keystone
The framework would be removed and was held in place by the pressure on the keystone
As always arches need abutments

30. Bridges Modern Arc Bridges
Today arches are constructed out of concrete and steel
Concrete arches are still made in the same way as stone arches
Steel arches are made by connecting curved steel beams

31. Bridges Suspension Bridges
The best to build over large bodies of water
A tower is built at either end, usually in the water
A cable is strung from one side of the water to the other over the two towers
On either side of the water on the embankment the cables are attached to huge concrete blocks called anchorages
A roadway is hung from the cables
The roadway is made strong by adding trusses along the side

32. load

33. Cofferdams A giant box that holds back the water
Provides a safe work area for excavation

34. Caissons A huge open ended box is set on the riverbed
The box is called a caisson
When air is pumped in, water is forced out
This provides a water tight space where people can dig up mud an stones
When they reach the bedrock the caisson is filled with concrete and the tower is built on top of it

35. Bridges Cable Stayed Its towers connect directly to the deck
They do not need the large anchorages that suspension bridges do

36. Bridges Cantilever Bridge 2 beams project from opposite piers
Each beam is support at one end like a diving board
The two beams meet in the middle of the span

37. Substructures Pile foundation All that is below the main floor
The building sits on it
Pile foundation
Large columns are dug or pounded into the ground

38. Substructures
Raft foundation
Extends past the edge of the building

39. Substructures
Box foundation
Like your basement at home

40. Superstructures All that is above ground Steel Skeleton construction
The frame of the building is constructed from steel columns and beams
Often have cross braces

41. Superstructures Curtain Wall
Does not carry any of the load of the building
It is attached to the steel frame work