Structures and Designs

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25 Νοε 2013 (πριν από 3 χρόνια και 11 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


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


II.
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.


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.
Dead and live loads


load


forces applied to a
structure

i.
Dead loads


Permanent loads that do NOT
change or move


The weight of the structure itself is
a dead load

ii.
Live loads


Those that move and change


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

Types of forces on
structures

B.
Static and dynamic loads

i.
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

ii.
Dynamic load


Loads that change quickly


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
lighter pay load

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
added for support


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


It had to withstand
massive weight and
vibrations


They solved this by
using trusses

Bridges

C.
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

D.
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

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


A roadway is hung from
the cables


The roadway is made
strong by adding trusses
along the side



load

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
of the load of the
building


It is attached to
the steel frame
work