# 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.

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

is there a way to identify a zero member in a joint with 4 members? with 5 members?

Thanks so much

Thanks sir ,very nice can u explain more topics

Sir, your really outstanding professor, thank you sir

Very nice sir … please make more videos..

great and easy to understand

Thankfull to you sir.

Thank you sir

Appreciate you sir!