Structures and Designs By Mr. R. Gallagher

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

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.

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

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

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

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

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

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

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

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

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

Recall Types of Stress

Recall Types of Stress

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

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

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

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

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

Trusses A. Howe Truss

B. Pratt Truss

C. Warren Truss

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

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

Beams T beam                                                                                                                                Double “T” Beam

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

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

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

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

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

load

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

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

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

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

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

Substructures Raft foundation Extends past the edge of the building

Substructures Box foundation Like your basement at home

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

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