Truss Joints: Pinned or Rigid?
Truss Joints: Pinned or Rigid?
(OP)
Do you assign the joints of a truss-type structure as pinned or rigid?
I always use the joints as rigid but the static matrix is bigger and the analysis procedure is slower in order to calculate bending moments, shear and axial stresses.
Does anyone noticed any problems in modelling trusses with all joints assigned as pinned?
I know it's faster but only axial stress are given by the analysis soft, and I'm concerned about possible eccentricity of loads on truss joints in the assembly stage.
Thanks,
MP
I always use the joints as rigid but the static matrix is bigger and the analysis procedure is slower in order to calculate bending moments, shear and axial stresses.
Does anyone noticed any problems in modelling trusses with all joints assigned as pinned?
I know it's faster but only axial stress are given by the analysis soft, and I'm concerned about possible eccentricity of loads on truss joints in the assembly stage.
Thanks,
MP






RE: Truss Joints: Pinned or Rigid?
You can evaluate the effects of eccentricities by using the axial load calculated with pinned connections multiplied by the maximum eccentricity. Anyway a good design should avoid excessive freedom for adding eccentricities at erection.
prex
motori@xcalcs.com
http://www.xcalcs.com
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RE: Truss Joints: Pinned or Rigid?
RE: Truss Joints: Pinned or Rigid?
RE: Truss Joints: Pinned or Rigid?
But... actually we design "rigid joints" when detailing a real trusses. We make joints details with a steel pieces a lot more rigid than the bars they join (with a lot of bolts and go on...).
Then, rigid joins are not so far than reality.
In other hand (on my own experience), if the bars have not a big sections, the diference in the analysis results are minimal.
Regards
Hernán
RE: Truss Joints: Pinned or Rigid?
It is in contrast a much more complicated activity where the behaviour of the structure is compared against suitable behavioural models in order to realistically decide if it will fail or not.
This approach is required because not all the stresses are equally important to determine the failure: an example is in the local stresses generated in the connections of a truss-type structure as a consequence of the absence of a real pinned joint. Those stresses are secondary (deformation controlled quantities) and will normally cause failure only as a consequence of fatigue, not by yielding.
This is the main reason that allows for calculating truss-type structures with unpinned (e.g. welded) connections as if they were pinned: this procedure is not only simpler and faster, but also gives results closer to reality in predicting the failure of the structure.
prex
motori@xcalcs.com
http://www.xcalcs.com
Online tools for structural design
RE: Truss Joints: Pinned or Rigid?
RE: Truss Joints: Pinned or Rigid?
Your question had been subject of research by early founders of structural theory, as Timoshenko. If you read his book in 'Moment Distributin Method', you will find a chapter in 'Secondary Forces on Trusses' in which the behavior of pined-joint trusses is compared with the trusses with fixed joints. Results show that bending moments are negligible and the process of taking fixed joints will affect the overall structural weight only by a small percentage. As a rule, if the axis of members are coinciding at joints, taking the joint fixed, even if the members are tubular and welded connection, will not show considerable bending moment. You can prove this by once analysing a truss with fxed joints and then re-analyse it by the joints released for rotation. If you have offset in the joints i.e. brace axis do not coincide on the same point on the chord, or if you are dealing with fatigue problem as in long span bridges, you should respect the effect of fixity on the joints.
RE: Truss Joints: Pinned or Rigid?
At this moment I implore for your pardon (what a boring cascade of words!!!).
(I apologize too for my bad english)
Herny
RE: Truss Joints: Pinned or Rigid?
RE: Truss Joints: Pinned or Rigid?
I'm not saying that the calculation with pinned joints is safer, because indeed is the contrary: if you calculate the bending stresses due to joint fixities you'll get higher total stresses and then you'll estimate a smaller safety factor for the structure.
Nor am I saying that the error in taking pinned joints in place of fixed ones gives a good or bad approximation of real stresses: of course the difference is known to be quite small in most cases, but there may rare occasions where this difference is not so small (particularly when the trusses are not very slender).
What I am really saying is that the increase of stress due to joint fixities has no effect on structure's failure (at least up to a quite large limit), so that if the engineer's goal is to evaluate as best as possible the factor of safety with respect to failure (and indeed I believe this is often the case, especially for important structures) those stresses should not be included in the analysis.
prex
motori@xcalcs.com
http://www.xcalcs.com
Online tools for structural design