Determining a zero force member visually

Hello,

When given a truss structure, how does one tell visually just by looking at the structure which members are zero force members?

 

[PhanthomJay]

From Wiki:

• "If only two members meet in an unloaded joint, both are zero-force members.
  
  
  

• If three members meet in an unloaded joint of which two are in a direct line with one another, then the third member is a zero-force member.
  
  

• If two members meet in a loaded joint and the line of action of the load coincides with one of the members, the other member is a zero-force member."
  

An unloaded joint is a joint where no external forces are applied , or a joint where there is no external reaction force.

A loaded joint is a joint where external forces are applied , or a joint where there is an external reaction force.

 

[nvn]

Woopydalan: See also post 2345373.

 

So that means if you have 4 members at a joint, then none of the members can be zero force members? I am looking at a homework problem and this is the case where 4 members meet and yet one or more of the members is a zero force member. I thought it was only 3 members meeting

 

[AlephZero]

So that means if you have 4 members at a joint, then none of the members can be zero force members?

No, it just means you can't decide if they are zero force members by looking at that joint. But the other end of each member is connected to a different joint, and that joint might tell you it is a zero force member.

Of course it is also possible for a member in a complicated structure to have zero force just by happenstance, but that's not very interesting.