Engineers design all structures with enough
strength to withstand the force and load that
will be placed upon them. Generally loads are
masses resting on a structure, but may also
be forces such as wind, impacts, vibrations,
bending or internal twisting distortions
transferred through the structure. There are
two types of loads:
Dead loads are
static. They do not
change or move.
Engineers also refer
to these as static
The weight of the bridge is a
Live loads are those
that move and change.
Take the example of a
bridge, the weight of the
vehicles, wind, snow/ice
build up, thermal
contracting are all
examples of live loads.
Live loads that change
in value quickly (i.e.
wind gusts)are called
In the picture above both the
weight and momentum of the train
represent a live load.
Dynamic loads are
the most difficult to
predict and prepare
for. Here is a video
image of the
Bridge (1940) that
wasn’t designed well
enough to support
Click on the above picture to
learn more about the
collapse of this bridge.
Stress and Strain
Loads stress a structure by pulling or
pushing which can result in bending or
is the measure of the
amount of force placed on an object.
As you witnessed in the previous video,
the stress from the load was too much
for the bridge to handle and therefore
Types of Stress
Forces are always pushing or pulling on
structures. Five types of stress on structures
to push or squash a
to make a material
shorter or more
i.e. Garbage Truck
Compression is pushing
Tension is a pulling
force. It tends to
make a material
i.e. A sling shot.
The weight of the water jug is
causing tension on the rope
supporting it. It is a pulling
Shear occurs when
a material is
divided by two
These forces are
One part of the
material slides past
the other part.
Shear strength is the ability of a material to resist
being fractured by opposing forces acting of a
straight line but not in the same plane, or the
ability of a metal to resist being fractured by
opposing forces not acting in a straight line.
Think of scissors, which is easier to cut, Paper?
Cardboard? Metal? Which of those would have
more shear strength? The thicker the material
the more shear strength it has.
Bending occurs when a load is placed on or near
the center of a horizontal beam. When bending
occurs, the top of the beam is in compression and
the bottom in tension.
Torsion is a twisting force. Torsion is
usually caused by dynamic forces such
as exhibited in the Tacoma Narrows
Now that we have learned about the
forces that cause stress and strain on
structures, we have to apply design and
construction techniques to combat
those forces. To do that we need to
study shapes and the effect that they
have in supporting structure.
Rectangles and Squares
Columns and braces are joined at 90
Easily deform under pressure
Need diagonal bracing to support loads
and to withstand stress
Make for strong towers
The more triangles, the
stronger the tower.
towers by keeping them
The diagonal bracing
prevents buckling or
deformation of the
Arches support loads at
any point along their
curve. Triangles only
support at the edges.
Arches are more likely
to push out at their base
Trusses are beams (top and bottom chord)
that are made of two horizontal pieces joined
in the middle by a series of triangles. The
shape and design of the triangles can vary.
Trusses are lighter, stronger, and more cost
effective than a solid beam of the same
After the infamous 1906 earthquake in
San Francisco, California, it was
realized that wood and steel framed
construction was superior to that of
buildings made of brick and mortar
An earthquake produces a series of
waves that move horizontally across the
ground causing buildings to sway from
side to side. Therefore a rigid structure
can only withstand minimal shaking
Therefore some flexibility in a building is
necessary. Steel and wood
construction offer this flexibility.
Earthquake Resistant Structures
Buildings between 5 and 20 stories are
at the most risk to earthquake damage.
Taller buildings tend to be more flexible
because they are designed to withstand
high winds. However, non
material may fall and cause damage to
Earthquake Resistant Structures
The base on which a structure is built
also plays an integral role in it’s safety.
Dry sandy soil tends to settle causing
the building to settle. Wet soil tends to
liquefy. In both instances buildings may
stay intact but may tilt or even fall over.
To prevent this from happening,
buildings are built onto “piles”
Steel enforced concrete piles are built
deep into the earth’s surface to help
transmit the load of the building into
more solid dry ground. Even in
Saskatoon, due to frost heaving the soil,
buildings and decks are built onto piles.
Base Isolation Technique
The base isolation technique supports
the entire building on bearings made
from rubber and steel which act as
springs. So, as the earth is moving
below the structure, the springs isolate
the structure from the movements thus
reducing earthquake damage.
That concludes our study on
engineering terms. Please consider
what you have learned when designing
and constructing your tower. The
earthquake simulator is designed to
shake, twist, and distort your building.
What can you do to design and build a
structure that can withstand the vigor of