russ axial forces ` section size

huangyhg

truss axial forces ~ section size
manually when i used to analyse a truss (whether by joint, section or graphical method), i never used to consider the section sizes, as far as i remember. for long, now i use staad or sap. by modifying the section sizes, i see slight change in results (axial forces). i am not changing any geometry and ignoring self weight. what caused the results to be changed?
check out our whitepaper library.
self weight?
i am asking program to ignore self weight, still values change
the differences probably have to do with axial shortening and shear deformations.
are you modeling with pin joints, or fixed joints?  if with pin joints, there should be no differences.  with fixed joints, the member size will affect the amount of "secondary" stresses caused by bending of the truss   
i sedcond spats' idea.  it is probably having to do with the axial shortening/elongation of   
the relative stiffness of the members is a factor in determining forces in indeterminate structures. unless there are second order effects being considered, i would expect the results for a truss to be the same when all members have the same size and modulus regardless of the size you choose.
are you loading the truss only at the joints? are all   
relative stiffness doesn't matter when you analyze a pinned joint truss. re  
i am using pin joints. size of top chord and bottom chord and diagonals are different. now if applied loading is same, and i ignore self weight, should not truss axial force remain same irrespective i change the section sizes of top or bottom chord or diagonal.
spats is getting my point, but do we all agree on it? 100%
are you sure that the program is treating all joints as truly pinned?  
in order to get a viable solution and prevent the truss from rotating in space, you have to fix at least one end of one