Zero Force Members – Lesson 15

Take a look this simple truss. Let us find determine member forces of this
truss using method of joints. You know how to solve this problem based on
our earlier discussions. Typically, I will draw the FBD of the entire
structure. Calculate the support reactions. We will then draw the free body diagram of
each joint and calculate the member forces. Some of you may even start with a joint instead
of drawing the free body diagram of the entire structure. For example, joint C in this case, where there
are only two unknown forces. I encourage you to pause this video now and
solve this problem yourself for all member forces using method of joints. It should not take too long to solve this
problem. Ok. I assume you have solved the problem. You can now continue with the video to verify
your answers. Your analysis should show that members CD
and BC do not carry any load at all. These members are not in tension or compression. Such members are called zero force members. You may think why we need such members if
they don’t carry any load. In reality, you often need zero force members
for many reasons: One, often, they are there for maintaining
structural integrity Two, increase stability of the truss Three, provide support if there is any change
in the loading condition And finally, sometimes zero force members
are added to make a truss structure look nice. I will discuss this when we solve more problems
later. As you can see, if there is a way to identify
these members without drawing free body diagrams, you can speed up your calculations. This also helps to develop a better understanding
of trusses. It turns out, we can actually identify zero
force members by inspection, just by looking at it using a few rules. So let us go over these simple rules. These rules are easy to remember and apply. You will get good at it if you practice this
a few times. In the worst case scenario, you can still
draw the free body diagram of each joint and solve the problem. So don’t be worry too much. Ok. Take a look Joint C in this case. Joint C connects two members CD and BC. Notice, these two members are not collinear. They form an angle. Also notice, there is no pin or roller support
at this joint. This means Joint C has no support reaction. There is also no applied or external load
at this joint. So there are three conditions. 1) Two members are non-collinear No external load No support reaction I can now put these facts into a single rule. When two non-collinear truss members form
a joint, and there is no external load or support reaction applied to the joint, then
the two members must be zero-force members. This rule confirms what we know from our calculations. I hope you can now apply this rule to any
truss problem, and identify the zero force members. We will look at some examples a bit later. Now let us take a look at another scenario. I took the same problem and changed the loading
condition. Now the applied load is at Joint C and there
is no load at joint D. Please pause the video now. Go ahead and do the calculation for the member
forces one more time using free body diagrams and come back. Ok, I hope you completed the calculations. Now that you have member forces, check which
members are zero. You will find that members BC and CD are no
longer zero-force members. They carry load. But member BD is a zero force member. You will also find members DE and CD carry
equal loads. If you look at joint D, notice there are four
obvious conditions here. 1) Two members are collinear Third member makes an angle with the other
two There is no external load And there is no support reaction. Now I can put these facts into a single rule. When three members form a truss joint, for
which two members are collinear and the third member makes an angle with the other two,
then the non-collinear member is a zero-force member, if there is no external load or support
reaction is applied to that joint. The two collinear members carry equal loads. Knowing these two rules will make your life
easy in solving truss problems. Let us now take a look are some example problems
and see if we can identify the zero force members quickly without doing calculations.

10 thoughts on “Zero Force Members – Lesson 15

  1. awesome sir..i have not found this much of clear cut explanation yet..awesome sir…iam really big fan of u..please continue your services at affordable cost of every one even poor sections

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