# Zero Force Members



## Calixico (Aug 7, 2010)

I understand that if a vertical member is between two colliner members and no force are applied at that joint, the force in the member is zero. If a force is applied at the joint, the internal force in that member is equal to the applied load. Am I correct in my solution to this problem---- that the answer if 10k and 10k, answer choice 'C" ? If the two 10k load was removed from joint "B" and "D", then the two vertical members would become zero force members. Could someone please confirm this for me.


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## ALBin517 (Aug 9, 2010)

Calixico said:


> I understand that if a vertical member is between two colliner members and no force are applied at that joint, the force in the member is zero. If a force is applied at the joint, the internal force in that member is equal to the applied load. Am I correct in my solution to this problem---- that the answer if 10k and 10k, answer choice 'C" ? If the two 10k load was removed from joint "B" and "D", then the two vertical members would become zero force members. Could someone please confirm this for me.



C looks right to me.

And the problem seems to fit the normal NCEES format.


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## civilized_naah (Aug 9, 2010)

Calixico said:


> I understand that if a vertical member is between two colliner members and no force are applied at that joint, the force in the member is zero. If a force is applied at the joint, the internal force in that member is equal to the applied load. Am I correct in my solution to this problem---- that the answer if 10k and 10k, answer choice 'C" ? If the two 10k load was removed from joint "B" and "D", then the two vertical members would become zero force members. Could someone please confirm this for me.


In fact, the verticality (is that a word?) of the member is immaterial. The generic rule is: If all the forces (members forces + externally applied forces) at a pinned joint can be grouped along TWO LINES OF ACTION (not necessarily perpendicular), then (i) any force that acts alone must be a zero force member and (ii) any forces that appear in a pair (no other forces along that line) must be equal and opposite.


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## Badger (Aug 10, 2010)

Calixico said:


> I understand that if a vertical member is between two colliner members and no force are applied at that joint, the force in the member is zero. If a force is applied at the joint, the internal force in that member is equal to the applied load. Am I correct in my solution to this problem---- that the answer if 10k and 10k, answer choice 'C" ? If the two 10k load was removed from joint "B" and "D", then the two vertical members would become zero force members. Could someone please confirm this for me.


I believe answer C) 0, 10 is correct.

If you use the method of joints at pt J, the force in member JB = 0, since members KJ and JI are colinear (in dirct line with one another). Or Sum in y = KJy - JIy; in x direction KJx = JIx.

Using the method of joints at pt E, the force in member GE = 10k.

The force in member HD is zero also.

If you remove the 10k loads at B &amp; D, the force in vertical member JB is still zero, and the force in member HD is still zero.

This looks like a good prctice question for the morning breath.

I think I am right, but my statics is pretty rusty, so maybe somebody can confirm.


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## MA_PE (Aug 10, 2010)

badger? we don't need no stinking badger!

Sorry, I could not resist.


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