BA says.... “The end pieces are not fixed. You should model the end supports as pin and roller because this is the way they are deemed to act.” There are a number of different definitions of fixity, the worst being what the software assumes when you tell it fixed/fixed. The program assumes no rotation or translation in any direction, not a likely real world condition. He says this for several reasons:
(1.) That is how we learned basic simple beam theory, one end pinned and the other on a roller. Do they still make any effort to teach that in school these days, or isn’t that theory needed any longer since the software takes care of all that stuff, in its own befuddling way. And, leaves us wondering why we get funny stresses (forces) in some of the truss members. Or worse yet, causes us to allow a significant error to go unchallenged, or corrected. But, you are questioning it, so, good on you.
(2.) It’s been done this way for years, that assumption simplifies the analysis significantly, and has not caused stl. jst. failures. It allows us to ignore the secondary axial affects, for a very good approximation of the real world conditions.
(3.) To design a single simple open web stl.. jst., we can’t afford to spend more than six months worrying about secondary affects which we have learned over time do not lead to problems with the final design. However, there have no doubt been Master’s and Ph.D. theses done on these very subjects for the SJI. If you have two angles for t&b chord members, and sq. or round bars for the diags.; the members are all pretty stiff w.r.t. axial loads, the primary force system; but the diags. are considerably less stiff (strong) w.r.t. bending than the t&b chords are, and it is tough to get our heads around exactly how the welding btwn. the two would transmit a moment anyway. Suffice-it-to-say that the welds or diags. might yield a little, over small areas, to accommodate these secondary moments. This same thinking is not true (or proper) for larger structural trusses when we are talking about W14 t&b chords and W14 diags. In these cases those secondary effects must be accounted for, in detailing, welding and analysis.
Olivei34 says.... that the welded ends don’t sound much like pinned and roller conditions. But, this assumption is required for our typical simplified analysis approach. Olivei34, answer these questions, answer your own question/problem, and post your thinking and we can talk some more. Assuming you control the stl. jst. defections to reasonable limits, what is the difference in the straight length (chord length) of the top chord, and the curved length when deflected, 16ths or .01's of an inch, if you can measure it. Are there things (details) within the roof system which can accommodate this length change without causing bldg. or jst. failure, or things which will resist it, and make it less critical? Then again, if you tell the software the ends are fixed, what does this length change mean in the way of axial strain (thus, large stresses or forces) in the top chord? End fixity will also induce bending moments which the top chord can’t really accommodate in the immediate area of the jst. seat, so the members or the numerous welds will yield a bit to tolerate some moment, so we call this condition pinned. Again, various degrees of fixity, or exact definition. There is a moment on the end of the jst. to the extent that the first diag. and the reaction point are eccentric by a few inches, and this must be accounted for in the jst. design, but it is not a fixed end moment. Compare your first run/model with one which is pinned and roller at the reactions, the t&b chords are continuous members, but the diags. are pinned/simple supports for the t&b chords at each panel point. In either case, the moments and forces should balance at each of the panel points (nodes), and you should be able to see how your fixed/fixed assumption radically effects the member forces/stresses.
