# Structures and Designs

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25 Νοε 2013 (πριν από 4 χρόνια και 7 μήνες)

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Structures and Designs

By Mr. R. Gallagher

I.
Vocabulary

A.
Thrust lines

Imaginary lines of force caused by loads

They are transmitted through all parts of
the structure to the ground

B.
Equilibrium

When a force is balanced by another force

C.
Elasticity

original shape and size when a load is
removed

Some materials are so brittle that instead
of changing shape they break without
warning

II.
History of Towers

People have been constructing
buildings since ancient times

4 story houses were made 4000
years ago and the Romans

Towers are mostly vertical

III.
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

IV.
Types of forces on
structures

A.

forces applied to a
structure

i.

change or move

The weight of the structure itself is

ii.

Those that move and change

Ex. Wind, melting ice, truck moving
over a bridge

Types of forces on
structures

B.

i.

Loads that do not change or change
slowly

time, or with
little

dynamic effects

Ex. Ice melting on a bridge

ii.

Ex. A gust of wind

V.
Stress and Strain

A.
Stress

Is the amount of force placed on
an object

Is measured in Newton's

B.
Strain

Is the distance an object deforms
under stress

VI.
Types of Stress

A.
Compression

The tendency to push or squash
a material

A material under compression is
always shorter

Types of Stress

B.
Tension

The tendency of a material to be
pulled apart

A material under tension is
always longer

Types of Stress

C.
Shear

Occurs when a material has two
parallel but opposing
compression forces

One part slides past the other

Ex scissors

Types of Stress

D.
Torsion

The tendency of a material to be
twisted

Ex a wrench tightening a nut

Types of Stress

E.
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

VII.
Strong Shapes

A.
Triangles

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

tension

compression

compression

Strong Shapes

B.
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

C.
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

D.
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

E.
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

VIII.
Trusses

A. Howe Truss

B. Pratt Truss

C. Warren Truss

IX.
Beams

A.
I Beam

Shape of an “I”

Very strong
and
economical

Used in
houses,
buildings and
bridges

Can be steel or
wood

Beams

B.
Box Girder

Are “U” shaped
and reinforced

Used mainly in
bridges of

Beams

C.
T beam

Double “T” Beam

X.
Bridges

A.
Beam Bridges

Bending stress

Supported by the ends
on the embankment

When the span is too
great columns are

The shapes of columns
and girders make a
difference in the
strength of the bridge

Bridges

B.
Truss Bridges

In 1890’s the train
became popular

Bridge builders
needed to
strengthen their
beam bridges

massive weight and
vibrations

They solved this by
using trusses

Bridges

C.
Old Arc Bridges

For 1000’s of years arc
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

D.
Modern Arc Bridges

Today arches are
constructed out of
concrete and steel

Concrete arches are
same way as stone
arches

Steel arches are
curved steel beams

Bridges

E.
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

the cables

along the side

i.
Cofferdams

A giant box that
holds back the
water

Provides a safe
work area for
excavation

ii.
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

F.
Cable Stayed

Its towers connect
directly to the deck

They do not need
the large
anchorages that
suspension bridges
do

Bridges

G.
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

XI.
Substructures

All that is
below the main
floor

The building
sits on it

A.
Pile foundation

Large columns
are dug or
pounded into
the ground

Substructures

B.
Raft foundation

Extends past
the edge of the
building

Substructures

A.
Box foundation

Like your
basement at
home

XII.
Superstructures

All that is
above ground

A.
Steel Skeleton
construction

The frame of
the building is
constructed
from steel
columns and
beams

Often have
cross braces

Superstructures

B.
Curtain Wall

Does not carry any