Truss is a structure made of two force members.
In order to support the large loads applied
to the structure, trusses must be rigid.
As we have seen the simplest structure, that
is rigid, is a triangle.
Please keep in mind, when I say rigid, do
not assume that a truss will not deform at
all, when loads are applied.
Trusses and all other structures undergo very
small deformations under the action of forces,
while retaining their original shape and structural
integrity.
You will learn about these deformations and
related calculations in the strength of materials
course.
When the deformations are excessive, the structure
will fail, and you will lean about failure
analysis in another course.
That’s why the basic building block of all
trusses is a triangle.
One way to construct a truss is to start with
a basic triangle element such as triangle
ABC and keep adding additional triangular
elements using two new members.
We can continue this process, and a truss
that can be constructed in this fashion is
called a simple truss.
Although it seems all trusses are made this
way, this is not the case.
Such trusses are called compound trusses.
Take a look at this example.
This is a compound truss.
The advantage of simple truss is that it makes
it easy to check the structural integrity
and solvability of a truss using simple relationship.
Assuming “m” is the number of members
in a truss and “j” is the number of joints,
then that relationship is m = 2j -3.
Let’s take a look at this example.
This truss is known as a warren truss.
Let’s count the number of members.
I see there are 19 members in this truss.
There are 19 members, 11 joints and 3 support
reactions.
Substituting these numbers in our equation,
we find 19 = 19
This basically means the number of equations
to be solved is the same as number of unknowns.
In other words, this is a statically determinate
truss.
If this relationship is NOT satisfied, then
the structures may be unstable or statically
indeterminate.
Let’s take a look at some examples now.
