In our discussion about structures, we dealt
with trusses so far.
Trusses are special type of structures.
They are composed entirely of two-force members.
As you can imagine, not all structures can
be made of just two-force members.
Frames are structures that contain at least
one non-two-force member.
I would prefer to call a non-two force member
as a multi-force member.
Frames are rigid structures.
They are designed to be stationery.
Examples of frames are many, and they are
everywhere.
You can see the frame structure of a house,
when it is being built.
An ironing table is a frame.
A simple folding chair is a frame.
Let’s take a closer look at the folding
chair.
Once you unfold the chair and set it on the
floor, it is designed to stay as a rigid structure.
You don’t want a chair moving, or shaking
when you sit on it.
So, chair is a frame.
To confirm our understanding, let’s take
a closer look at this folding chair again.
There are three members that make this chair.
One of them is a leg that is extending all
the way from the floor to the back rest.
The second leg goes from the floor to the
seat, which is the flat surface where you
sit.
The seat connects the two legs.
If you look at the first leg.
It is resting on the floor.
We can assume that as a roller support so
there is going to be a support reaction.
This leg is also connected to the second leg
with a pin joint, so we know that there are
two support reactions there as well.
This leg is also connected to the seat with
a pin joint so we know there are two more
support reactions there.
Finally, when someone leans on the backrest,
this member is going to experience an applied
force on the backrest.
So, this member has forces applied at four
locations.
This cannot be a two-force member.
This is a multi-force member.
You can see the second leg of this chair is
also not a two-force member as it has reaction
from the floor, reaction at the pin with the
first leg, and at the point with the seat.
Clearly this folding chair is NOT a truss.
It is a Frame.
So, frame is a structure containing at least
one multi-force member and designed to remain
stationery.
Our goal in the analysis of frames is to determine
forces acting on the members.
To accomplish, we will follow a simple process.
