RISA And OWSJ
RISA And OWSJ
(OP)
I am modelling an OWSJ on RISA with just a typical uniform load of 900 lbs/ft.
The joist is 12'-0 long all with 24" panels.
I have made the end pieces fixed and every other node on the program unrestrained.
After analyzing the and checking the chords I notice that the axial force in the top chord is almost half the force that is in the bottom chord.
This is obviously wrong. I am expecting a similar force in both the top and bottom chord.
Has anybody run in this type of problem. I know I am not modelling this correctly.
The joist is 12'-0 long all with 24" panels.
I have made the end pieces fixed and every other node on the program unrestrained.
After analyzing the and checking the chords I notice that the axial force in the top chord is almost half the force that is in the bottom chord.
This is obviously wrong. I am expecting a similar force in both the top and bottom chord.
Has anybody run in this type of problem. I know I am not modelling this correctly.






RE: RISA And OWSJ
Supports: pin - roller?
Are the top chord and bottom chord segmented (separate members) between panel points?
EIT
www.HowToEngineer.com
RE: RISA And OWSJ
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: RISA And OWSJ
So both bottom and top chords are parallel and have been modelled as continous members along the joist. Panels are at 610 mm apart.
In the end if I model with both ends as pinned restraints or as a Pin-Roller system I get different results.
What is the appropriate way to go about this situation?
Again these are just typical joist, not a arched or scissor type joist.
RE: RISA And OWSJ
I don't have RISA but in STAAD I typically design trusses or owsj with all members specified as TRUSS type (i.e. only take axial loads). I'm sure RISA has a similar command. This way you don't have to worry about changing all of the end conditions
RE: RISA And OWSJ
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.
BA
RE: RISA And OWSJ
RE: RISA And OWSJ
Examine your own RISA output and tell us the magnitude of the horizontal reaction at each end of the joist. And then tell us whether the supports can supply that reaction without deflecting.
Welding a shallow joist shoe to its support does transfer some moment, but it is usual to ignore it as the shoe is not capable of carrying much moment and the weld is not designed to carry moment.
Joists may be made continuous by welding both top and bottom chords to the supports, but that is not standard practice.
BA
RE: RISA And OWSJ
(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.
RE: RISA And OWSJ
The Pin-Pin or Pin-Roller question is one that comes up frequently here at RISA. I generally recommend a Pin-Roller assumption for trusses. But, I also tell folks that they can run some easy tests to see if that assumption is realistic.
Run the analysis with pin-pin boundary conditions. Make note of the (probably very large) horiztonal reaction at each of the two supports. This is the force that the end-support of the truss is required to resist in order for that boundary condition assumption to be valid.
Then run the analsyis with the pin-roller boundary conditions. Make note of the horizontal deflection that you get at the roller end of the truss. Generally speaking this will be a relatively small deflection. In order for the boundary condition assumption to be valid, this is the type of deflection that the supports must allow.
At that point it is usually clear that the real supports will be much closer to the pin-roller assumption than the pin-pin. If it's not entirely clear then I tell them to using the pin-pin assumption but with the horizontal restraint being given a spring constant commensurate with whatever structure is supporting the truss. At that point you will get a result in between pin-roller and pin-pin, but you will probably notice that the results will be much closer to pin-roller.
RE: RISA And OWSJ
RE: RISA And OWSJ
RE: RISA And OWSJ
But for a typical joist the modeling is pin pin everywhere. The joist fabricator than makes sure that the webs have enough contact length for the welds to resist the forces properly. In general the weld all around versus specifying weld lengths. They don't want the guys in the shop having to think to much. IF more weld is needed than you add a gusset plate.
John Southard, M.S., P.E.
http://www.pdhlibrary.com/
RE: RISA And OWSJ
BA