Truss question!!

For a pin jointed truss that is statically determinate both externally as well as internally (i.e. no. of members=(2xno. Of joints-3), suppose if the axial force in one of the members is zero, what is the contribution of such member?If I remove that member the truss will become unstable(i.e. no. of members<(2xno. Of joints-3)) at the same time that member is not in a state of stress. Is it that the member contributes just for stability?Can anyone clearify?What should be the minimum cross sectional area of that member?

Help please

 

[wiktor]

The zero member is just for stability of the truss (but you can remove the node and you will not need this member anymore).
Usually these are being used to reduce the length of the compression members.
The cross section of this type of member it's not so important - the moment of inertia (both directions) counts.
The general rule is that the member used to brace compression member should be able to develop approx. 10% of the force in the member being braced.

 

[JAE]

If your truss is significantly long, you might want to consider an unbalanced load, in which case your zero force member will probably receive some load.

Unbalanced loads are sometimes required by code (wind loads on pitched roofs, snow loads) while some are developed at the discretion of the engineer (unbalanced live loads).

For a floor truss, there will always be times when live loads will not be uniformly placed over your tributary area. Therefore, your zero-member may be required.

 

[Trussdoc]

In a symmetrical truss that has partially or fully parallel chords, the web at the center will have zero axial force under uniform load. However, this doesn't mean you can arbitrarily remove that member. The shear at the ends of the panel will not be zero and will induce added shears and moments in the chords. Also, under an unbalanced snow load, or office loading, the center web force is no longer zero.

Removing the web may not make the truss unstable, but will force you into an indeterminate analysis.

 

[cv]

For the truss theoretical problem, you can remove the member since you keep the structure in triangles.
For any truss in pratice, you have always nonzero axial force members, 'cause truss is a much more complex structure.


CV
carlosvalinhas@netcabo.pt