Truss axial forces ~ Section size
Truss axial forces ~ Section size
(OP)
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?






RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
Castigliano
RE: Truss axial forces ~ Section size
If you set up a stiffness matrix for a truss you can see the direct impact taht EA will have on the forces.
RE: Truss axial forces ~ Section size
Are you loading the truss only at the joints? Are all members pin ended?
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
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 member at each and every joint. How you do that could affect the result, giving you the difference in results.
Mike McCann
MMC Engineering
RE: Truss axial forces ~ Section size
In a statically determinate truss, should not make the least bit of difference.
The only think that comes to mind, albeit a very unlikely scenario, is that somehow, when you change your sections, the program is adjusting the precise location of the center of the member. This could happen if, for example, you define the location of your top chord members as "Top of Member". When you change these sizes, the distance between your bottom chord members and the neutral axis of the top member is modified slightly. This changes the geometry and could lead to different results.
Just a guess.
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
You're getting a bit upset.
The way you modeled your indeterminate trusses is not valid, as the distribution of force is based on RELATIVE stiffnesses. Modeling ALL the members one size or another would not affect the results of an indeterminate truss.
In a determinate truss, in theory, you should be ably to place ANY member of any size anywhere in the truss and get the same results (assuming you are not checking for failure)
RE: Truss axial forces ~ Section size
You repeating exactly what I have said.
RE: Truss axial forces ~ Section size
you got my point
RE: Truss axial forces ~ Section size
Run your model using different sections within one model. Do not chnage load but change sections only. Make it determinate truss. See if reults are changing. IF not , teach me what you did. If results are changing than why?
some points above are very logical as chnage in c/c member spacing, shear and axial deformations
RE: Truss axial forces ~ Section size
Are the pins truely pins or top and bottom chords continuous with pins on the chord members only. I found that modelling pins (true pins) is sometimes a pain in these analysis prgrams. Also are you doing a primary analysis or secondary analysis?
gues my thinking is in line with mssquare48!
RE: Truss axial forces ~ Section size
how would the diplacements affect the results in a statically determinate truss?
RE: Truss axial forces ~ Section size
I am doing primary analysis
(From STAAD using Perform static check)
I ran both ways
top and bottom chords continuous with pins on the chord members only and T & B chord discontinuous.
RE: Truss axial forces ~ Section size
Just saw your ante penultimate post. Sorry about the redundancy.
RE: Truss axial forces ~ Section size
First, figure out if this is actually a determinate truss. If so, then you are modeling something wrong or you need to get new software. If indeterminate, then I would try to get the results to be the same with all 2 force members first, no self weight, no loads applied between joints. If none of the above works, please consult someone in your office.
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
Cheers !
RE: Truss axial forces ~ Section size
My money is that you have pins at each end of your truss creating an indeterminate system.
you need a pin and a roller to get discount the effect of stiffness, thus making it determinate.
RE: Truss axial forces ~ Section size
I think this is a rational approach.
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
Cheers!
RE: Truss axial forces ~ Section size
Yes, I believe so.
minorchord2000
Your analysis/design procedure sounds fine, it doesn't address the OP's dilemma though.
RE: Truss axial forces ~ Section size
correct?
RE: Truss axial forces ~ Section size
I believe.
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
But then why does the model give moments on t and b chord if he has pins on all joints?
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
i had this problem in my undergrad steel class project.
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
Galambos
Even if he has pinned-pinned instead of pinned-roller supports the forces in the pinned-pinned models should still be the same although not correct.
RE: Truss axial forces ~ Section size
In a space frame relative stiffness will matter and will change the load paths within the truss members as there is no way to specify all members as "Truss" members in a space frame without runnign into instability problems. In a space frame truss design the larger the capacity of the section, the more load it will "suck" from surrounding members. Truss design using STAAD is can be an iterative process.
RE: Truss axial forces ~ Section size
Pin support one end, roller on the other, and specify every single damn memeber as a "truss".
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
r = number of external supports for the truss
j = number of nodes in the truss
a) m + r = 2j is statically determinate
b) m + r < 2j is statically unstable
c) m + r > 2j is statically indeterminate
Case c applies for me
72+3 > 2 x35
soeven with roller support and all pin joints, truss can be indeterminate
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
The software doesn't distinguish structures from statically determinate or indeterminate. It just calulates member stiffness and assembles the stiffness matrix of the structure. Thats why it asks for member sizes. It then inverts the stiffness matrix to solve for the diplacements and then solves for actions.
Maybe when it inverts the matrix it just comes up with slightly different values with different member stiffnesses.
Its just an error due to the nuber crunching.
Maybe!
Cheers!!
RE: Truss axial forces ~ Section size
let's just see a screenshot...that should solve it.
RE: Truss axial forces ~ Section size
If only some members are changing, then the axial force would be different with a pin-pin connection; if all members are changing to something else, then yes, the axial force should be the same.
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
a pin-roller truss on the other hand is a very different story.
RE: Truss axial forces ~ Section size
RE: Truss axial forces ~ Section size
Exactly my line of thinking. For a critical form structures for example determinant is never zero, but very small. Matrix structural analysis is always an approximate method(albeit with high accuracy!) Thats why with different member sections it comes up with slightly different solutions. However I would expect these to be insignificantly small, with the hand solution which doesn't depend on section properties being the exact solution.
Again an example of computers going about solving simple problems the hard way and getting close to the correct answers. Fortunately we humans know a quicker and in this case exact way of solving the problem.
What this proves is that no computer program is perfect.
RE: Truss axial forces ~ Section size
This was for a determinate, pin jointed, node loaded truss.