Identify the zero force member D600 N 4 m 4 m 6 m Tm D 700 N

Identify the zero force member D600 N 4 m 4 m 6 m Tm D 700 N 2 m 500 N

Solution

It is very easy to identify zero force members, look for three membered joints, two of which are in a straight line, i.e. their line of action is same, and the third one has different line of action. In such case the third member is a zero force member, provided there is no other force acting on the joint.

1) EC is a zero force member, because other two members meeting at joint C are having same line of action, and there is no other force acting at the joint.

2) This one is interesting. Initially one can tell that BG is a zero force member. But as BG is a zero force member, so if we erase member BG from the sight, now we look at joint G and we observe that GC becomes a zero force member as well, because the remaining two members GA and GF have same line of action. Similarly if we look at joint C, as GC is zero force member, it makes FC as zero force member as well. Now at joint F, as FC is zero force member, this makes FD as zero force member too. So one has to look the structure again after erasing a zero force member and see if it has resulted in the formation of another zero force member.

So zero force members are : BG, GC, FC and FD