1. Mr. Ilko

2.  Tacoma Narrows Bridge – 2 mins
 http://www.youtube.com/watch?v=P0Fi1Vc
bpAI
 Building Collapse
 http://www.youtube.com/watch?v=uKeENd
yIluI&feature=related

3.  Anexternal force is a force that is applied
on a structure from something else
• Such as walking into the wind or swimming
against the waves
 Theforce we feel everyday is GRAVITY,
gravity is the external force acting on
everything on the planet

5.  Thedownward force acting on us due to
the Earth’s downward pull
 MASS is the amount of matter that the
Earth is pulling down
 When we leave Earth the gravity is
removed and we are able to float because
our mass is not being pulled down

7.  Gravity acts on a structure everywhere, but
there is an imaginary point where balance
is maintained
• THAT IS THE CENTER OF GRAVITY
 Thelocation of the center of gravity
determines the stability of a structure

8.  Inorder for us to increase a structures
stability we use two main methods;
• 1. Increase the Base compared to the height
• 2. Place the majority of the mass close to the
ground

9.  Stand up and balance on one foot….
• 1.
• 2.
• 3.
 CanI balance a meter stick on one finger
focusing only on its center of gravity?

10.  Take a ruler and balance it on your finger
 How did you get it to do that?
 What do you notice about the ruler?
 Moveyour finger one centimeter to the left.
Now another, and another what
happens??

11.  Symmetry is a balanced arrangement of
mass that occurs on opposite sides of an
object
 Fora structure to be stable, its mass must
be distributed equally around the center of
the structures base

17.  When structures are built, the creators
consider the load that structure will have to
resist
• A book shelf resists its weight and the books
• A bridge resists gravity, maybe a car, and the
materials its made from
 Load is split into two different types;
• Static
• Dynamic

18.  Is
the weight of the structure itself and the
non-moving load it supports
 Nails, wood, steel, cables are a few examples
 Even though these parts make up the load,
they create a external force because
gravity is pulling on these items
 Static Load is also known as DEAD LOAD

20.  Is the opposite of static load
 It
is an external force that moves or changes
with time
 These can change very quickly such as a
sudden gust of wind or an earthquake
 An example of a dynamic load is cars driving
over a bridge, or students walking up stairs
• Dynamic is also known as LIVE LOAD

22.  Bridges
are prime examples of structures
designed to withstand loads
 Different
conditions call for different
bridges because all bring their own pros
and cons

23.  Is the most common type of bridge used
 Is flat and is supported at its two ends
 Longer beam may be supported by
addition piers

25.  Isa light weight but strong bridge, made of
trusses (triangle-shaped frames) along its
sides

26.  Hangs between two ends that hold it up
 Modern ones have very tall towers on
either end that support its main cables that
hold up the bridge
 The main cables are anchored into
concrete, and the smaller cables support
the roadway

28.  Designed to withstand very heavy weights
 The dynamic load of people or cars causes
each piece of wood or stone to push against
the next one
 This
push eventually leads to the end
supports embedded into the ground
 The ground pushes back strengthening the
bridge