Some of the common structures we encounter
every day are buildings, bridges and monuments.
There are also other structures.
For example, space station is a structure.
Body of an airplane is a structure.
Hull of a modern cruise liner is a structure.
Chassis of a car is also a structure.
Today we want to talk about a special kind
of a structure called, Trusses.
Truss structure is widely used in the design
of bridges and roofs.
The function of a truss is to transfer the
loads from one location to another location,
typically to the foundation and to ground.
Next time you go to Costco, or home depot,
or to a large warehouse, look up, and you
will see many steel members arranged in a
way to support the roof.
What you are looking at is a roof truss.
A roof truss takes the weight of the roof,
and the snow on the roof if you get snow where
you live, and transfers that load to the walls
and to the foundation.
A bridge truss takes the weight of the cars
and trucks and transfers it to the support
pillars.
What makes Truss special compared to other
structures?
Well, trusses are lightweight compared the
large load they can support.
They do this by carrying the load very efficiently.
Truss structures use triangle element in the
design, which makes the structure rigid.
Let us talk about his.
I am going to make a structure with four members
connected by pin joints.
I am going to do this with Lego pieces.
Here I have four members.
One, two three, four, and they are connected
by pin joints.
And this structure, when I apply a load is
not stable.
It collapses.
Not stable.
Now take a look at this structure made of
three members, and this forms a triangle.
This structure is stable and rigid.
The triangle geometry is inherently rigid
and will stay together unless you break it.
So the basic building block of a truss is
a triangle.
Large trusses are constructed by attaching
several triangles together.
A new triangle can be added to the truss by
adding two members to this triangle.
A truss constructed in this fashion is known
as a simple truss.
Here I have a model bridge and this bridge
is made of simple truss on each side.
With this basic introduction, we care now
ready to analyze trusses.
Before we start, I want to make sure we keep
in mind the following four assumptions made
in the analysis of truss structures:
Truss members are connected at their ends
only.
You can see this in this bridge.
They are all connected at their end points.
Truss members are connected by pins joints
only.
A truss is loaded only at the joints.
For example, here, here, here.
This means the load can applied only at these
joints in the bridge, and not in between the
members.
The weight of the truss member is neglected
These assumptions lead to another interesting
fact.
Since the weight of the member is ignored
and the member is loaded only at the ends,
each member is subjected to axial loads only.
Let me remove a member from this model bridge
now and show this.
This member takes only axial load.
Therefore, each member can only be in tension
or compression.
Such members are called as two force members.
So each member of this model bridge is a two
force member.
So we can define a truss as a structure entirely
made of two force members connected by pin
joints.
This understanding of trusses make truss analysis
very easy.
Let’s now consider a simple truss and perform
statics analysis in the next video.
