Statics- Truss method of joints- null members?

Question

Im having trouble identifying null / zero force members for the method of joints, i know i few rules: a member that is adjacent to two colinear members is null if the group are not in contact with a force, or somthing like that, but still, its a bit frustrating, is there any easy way of identifying them?? also should the bottom horizontal truss members = to the total quantity of the forces acting on the truss?

Answer 1

Typically, its a zero member when you have two non-collinear members connected to a joint that has no external loads or reactions applied to it or when three members with two collinear are connected that has no external loads or reaction applied to it.  Unless you are taking a test, do the method of part or section to find the forces in the member and they should come out as zero when analysis is done.  The bottom chord to a truss member is not equal to the total quantity of the forces acting on the truss.  If you have diagonal members that make of the web, the external forces are being distributed to all those members in the web and in the bottom and top chord.  The full external load when applied to members are distributed so you may find the forces or slight less or higher in member externally but if you take each member, and split it into vertical and horizontal forces and split all the external forces into horizontal and vertical forces.   The sum should be zero.  IF they are not, you did something wrong and need to check to make sure you got your external reaction correct and begin the analysis at the support using the method of joints.

Answer 2

The next question is: Can the null members be deleted?  After all they carry no load, so what good are they?  Can't they be deleted and save some money and make a more elegant design?

No!  They are needed to prevent buckling.