Transcript Aspect Ratio

Bridges
bridge, span
a structure that allows
people or vehicles to cross an
obstacle such as a river or
canal or railway etc.
 physics,
the physical influence that
produces a change in a
physical quantity;
(Force=mass x acceleration)
Disaster,
Something that may occur
when the laws of physics are
ignored.

Your Mission
Over the next week you will have a
chance to experiment with bridge
designs in an attempt to span a gap and
overcome the force of gravity while
carrying a load.
 But first…let’s make some
considerations.

Static vs. Dynamic:

Static: Not moving.
 Static Load: Forces
on a nonmoving
structure.
 Dynamic: In motion
or changing.
 Dynamic Load:
Forces changing due
to movement of
structure.
force
A little quiz:

Identify the static
and dynamic system.
Types of Forces continued

Tension : A stretching
force. Pulls molecules
apart.

Two people pulling put
the rope in tension.
Types of Forces continued

Compression : a pushing
force. Compacts
molecules.

Ex: The weight lifter’s
body is compressed by the
barbell.
A little quiz:

Tension or
compression?
Types of Forces continued

Bending : A
structure subjected
to bending is being
stretched and
compressed at the
same time.

Where are the
tension and
compression forces
on this member?
Tension
Compression
Types of Forces continued

Shear: A force
resulting from two
forces acting in
opposite directions.
 A shear force is
created where two
opposite forces try to
cut, tear or rip
something.
Types of Forces continued

Scissors, Another
Example: The two
handles put force in
different directions on
the pin. The force
applied to the pin is a
shear force.
Types of Forces continued

Torsion : A turning
or twisting force.

EX: As ends of the
plastic ruler are
turned opposite
directions, the ruler
is said to be in a
state of torsion.
Frame Structures

A bridge is a type of
frame structure.
 Frame structures are
made from many
small parts, joined
together. Bridges
cranes and parts of
this oil rig are just a
few examples.
Frame Sructures

The different parts of
a frame structure are
called members.
Each type of
member has a
different job to do in
supporting the
structure.
Structural Forces & Members

Beam: A Beam is a
piece of material
supported at both
ends.

When a beam is
loaded the top is
compressed and
the bottom is in
tension.
Compression
Tension
Members

Beams used in
larger structures take
many different forms,
some are simply
solid, some are
hollow, and others
have special crosssections to provide
strength and rigidity
Members

Columns: The
vertical pieces
supporting a beam
are called columns.
Members

Cantilever : A
cantilever is a beam
which is supported at
one end only.

Cantilevers are
used where it is not
possible to have a
support at both
ends (a diving
board for instance).

When a cantilever is
loaded, the top
surface is in tension
and the bottom is in
compression.
Tension
Compression
Structural Forces & Members

Using a computer desk as an
example different forces can be seen
working
PART A: Is in tension because the
weight of the computer is stretching it.
PART B: Is under compression
because the weight above it is
pushing downwards and compressing
it.
PART C and D: This is the same
member but on the inside
compression is taking place and on
the outside it is being stretched
(under tension).
Structural Forces & Members

Tie: A member
mostly under tension
forces is called a tie.
Structural Forces & Members

Strut: A member
mostly experiencing
compression is
called a strut.
Structural Forces & Members

The vertical column
experiences both
tension and
compression.
More examples of struts and ties

(WALL)
 The beam is held in
position by a steel
rod. The weight of
the beam is
stretching the rod
(tensile force).
More examples of struts and ties

(ROOF)
 The roof beams are
under pressure from
the weight of the tiles
on the roof
(compressive force).
The floor beam is
being stretched
(tensile force).
More examples of struts and ties

(FLAGPOLE)
 The wires on either
side of the flagpole
are being stretched
(tensile force).
Why is the pole
under a compressive
force ?
Making Structures Rigid

When forces are
applied to a simple
four-sided structure it
can be forced out of
shape quite easily. A
structure which
behaves in this way
is said to be nonrigid.
Making Structures Rigid


By adding an extra
member the corners are
kept from moving apart.
The structure cannot be
forced out of shape, and
is said to be rigid.
Notice that the additional
member has formed two
triangles in the structure.
Making Structures Rigid

Most frameworks are
built using a
combination of struts
and ties to make
triangles. Triangles
make very strong
and rigid structures.
This is called
triangulation
Making Structures Rigid

An alternative to
triangulation is to use a
gusset plate. A gusset is
a piece of material used
to brace and join the
members in a structure.
A triangular gusset plate
has been used here but
they can be made in a
variety of shapes.
A Bridge Example

The bridge below is
a common type
called a Box Girder
Bridge.
A Bridge Example

Triangulation
distributes the weight
of any vehicle or
pedestrian crossing
the bridge. The
weight is distributed
through all the
members which
increases the load
the bridge can hold.
A Bridge Example

This type of bridge is
favored by the Army.
The Army Engineers
have transportable
bridges like the one that
can be dismantled and
transported anywhere in
the world and
reassembled. They are
bolted together and are
semi-permanent
structures. They are
useful for short spans.
Aspect Ratios
Aspect Ratios are used to describe
everything from television sets, to
photographs, to aircraft wings, to movie
projection formats, to tires, to basically
anything that can be described as having
a length and a width.
 Let’s take a look.

Aspect Ratio

Graphics and photos are described in terms of aspect
ratios. If a graphic has an aspect ratio of 2:1, it means
that the length is twice as large as the height. The
term is also used to describe the dimensions of a
display resolution. For example, a resolution of
800x600 has an aspect ratio of 4:3.
Aspect Ratios

Basically,
aspect ratio is
a measure of
how square
something is.
 It is calculated
by dividing
length by
width.
2 cm
10
cm
AR = 10:2
= 5:1 or just “5”
How can Aspect Ratio be applied to
Bridges?
Let’s do a demo shall we?
 What we should have found out.

Short and Wide is better than…
 Long and Narrow (at least for bridges)
 A piece of spaghetti is stronger if it is pulled
than if it is pushed.
 In fact, an aspect ratio of 20:1 is ideal for
bridge supporting members.

Pulling vs. Pushing

Ok. So…describe the pulling and
pushing forces on this bridge.
Forces Involved in Bridge Building
http://www.pbs.org/wgbh/buildingbig/bridge/index.html
Some bridge types that you might
want to think about.

Trestle and Arch
 Trestle
The Beam Bridge
The Forces on the Beam Bridge

Beam Bridge: Forces
When something pushes
down on the beam, the
beam bends. Its top
edge is pushed together,
and its bottom edge is
pulled apart.
Types of Beam Bridges

Continuous span

Swing Bridge
The Truss Bridge

Consists of an assembly of triangles.
The Forces on the Truss Bridge

Every bar in this
cantilever bridge
experiences either a
pushing or pulling
force. The bars
rarely bend. This is
why cantilever
bridges can span
farther than beam
bridges.
Suspension Bridges

Can span 2,000 to 7,000 feet -- way
farther than any other type of bridge!
The Forces on the Suspension
Bridge

In all suspension bridges, the
roadway hangs from massive
steel cables, which are
draped over two towers and
secured into solid concrete
blocks, called anchorages, on
both ends of the bridge. The
cars push down on the
roadway, but because the
roadway is suspended, the
cables transfer the load into
compression in the two
towers. The two towers
support most of the bridge's
weight.
Cable-stayed Suspension Bridge

Cable-Stayed Bridge
The cable-stayed bridge,
like the suspension
bridge, supports the
roadway with massive
steel cables, but in a
different way. The cables
run directly from the
roadway up to a tower,
forming a unique "A"
shape.
The Arch Bridge
The Forces on the Arch Bridge

The arch is squeezed
together, and this
squeezing force is
carried outward along
the curve to the supports
at each end. The
supports, called
abutments, push back
on the arch and prevent
the ends of the arch
from spreading apart.
(Back to Flight- ) Wing Aspect Ratio:
The aspect ratio is a measure of how square the wing is. It is calculated by
dividing the wing length from tip to tip by the wing width. Generally, higher
 Describe the
aspect ratios are more efficient during low speed flying.
diagram using the
forces drag, lift,
and weight.
 Why does a high
aspect ratio work
well for this low
speed glider?