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Truss axial forces ~ Section size

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

Self weight?

RE: Truss axial forces ~ Section size

(OP)
I am asking program to ignore self weight, still values change

RE: Truss axial forces ~ Section size

The differences probably have to do with axial shortening and shear deformations.

RE: Truss axial forces ~ Section size

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 member.  This in turn has a small effect on the axial load.

Castigliano

RE: Truss axial forces ~ Section size

I sedcond spats' idea.  It is probably having to do with the axial shortening/elongation of members.
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

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 members pin ended?

RE: Truss axial forces ~ Section size

Relative stiffness doesn't matter when you analyze a pinned joint truss. Remember he's comparing to simple statics and graphical methods.

RE: Truss axial forces ~ Section size

(OP)
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.

RE: Truss axial forces ~ Section size

(OP)
Spats is getting my point, but do we all agree on it? 100%

RE: Truss axial forces ~ Section size

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 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

Agree with spats-

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

If the truss is indeterminate then it does matter!  

RE: Truss axial forces ~ Section size

I just checked an indeterminate truss with all the members as W4X13 and another case with all W44X335 and got the exact same axial forces. If this is actually a determinate truss, then all the above talk about stiffness and deformation is pointless and your model is flawed.  

RE: Truss axial forces ~ Section size

hayenwp..

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

frv,

You repeating exactly what I have said.   

RE: Truss axial forces ~ Section size

(OP)
frv
you got my point

RE: Truss axial forces ~ Section size

(OP)
haynewp
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

dgkhan

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

Spats

how would the diplacements affect the results in a statically determinate truss?

RE: Truss axial forces ~ Section size

(OP)
civeng80
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

heyenwp-

Just saw your ante penultimate post. Sorry about the redundancy.

RE: Truss axial forces ~ Section size

You can have slight differences if you are mixing flexural members with 2 force members. I asked before "Are all members pin ended?" because of this. If you use all one section for an indet. truss that has a ratio of bending/axial stiffness=1 and then switch all to another section where the bending/axial stiffness=1.1, then the force distribution will be different.

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

It's a long shot; do you have a sliding support at one end?

RE: Truss axial forces ~ Section size

Hope Im not missing anything here, but if the top chord is continuous then its not statically determinate truss.  Put pins on the top chord (sometimes not easy to do)  if you can and run the analysis again because I think you have secondary stresses.  Also roller support as has been said before.

Cheers !

RE: Truss axial forces ~ Section size

apsix beat me to it and is dead on.  

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 use STAAD Pro and am designing a 130' long pedestrian bridge using tubes throughout.  I model all the members using fixed nodes, and then release the end moments in all the web members.  This results in moments and axial forces in the top and bottom chords, while the web members only resist axial force.

I think this is a rational approach.   

RE: Truss axial forces ~ Section size

Galambos, it can still be internally indeterminate.  

RE: Truss axial forces ~ Section size

Moment in top and bottom members = difference in axial forces.

RE: Truss axial forces ~ Section size

Correct me if Im wrong anyone but if top chords are continuous then its statically indeterminate even with the roller support right?

Cheers!

RE: Truss axial forces ~ Section size

civeng80
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

So then it answers the Op's delema?

correct?

RE: Truss axial forces ~ Section size

for the benefit of analyis comparison between hand/model the top and bottom chords would need to be jointed where they meet diagonal members, otherwise the anlysis would not be valid. When sizing members the top and bottom chords may need to be considered as a compression/tension member over the full length if not braced at each connection.

I believe.   

RE: Truss axial forces ~ Section size

OP ran analysis with chords discontinuous which I take as meaning pinned at each web member connection.

RE: Truss axial forces ~ Section size

if the model is calculating moments in t and b chord then this will account for differences in results? Simple statics won't due this as all forces are axial.  

RE: Truss axial forces ~ Section size

Exactly herewegothen.

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

if hes applying a udl rather than loads at the node points then there will be some moment in the chords albeit so minimal as not not to effect the design?  

RE: Truss axial forces ~ Section size

plus the model will determine moments based around deflections of nodes. Hence as there will be deflection, there will be moments. As the deflection becomes less and less so does the moment in the chords I would think.  

RE: Truss axial forces ~ Section size

again, pin and roller supports....not pin and pin.

i had this problem in my undergrad steel class project.

RE: Truss axial forces ~ Section size

doesn't eliminate the moment in the chords from a UDL or as the model calculates, from deflection of nodes.  

RE: Truss axial forces ~ Section size

Are you loading the truss at node points only and is every member pinned at both ends including the top and bottom chord members. If so then the forces (there will only be axial forces) shouldn't change. The deflection is the only thing that will change.

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

If you are using STAAD, are you analyzing a "plane" frame or a "Space" frame? This is of PARAMOUNT importance in specifying member releases.
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

STAAD will absolutely allow you to analyze a plane frame truss and specify ALL members as "truss" members. No "pin" connections need to be specified so long as you have a plane model. Can't get any simpler.
Pin support one end, roller on the other, and specify every single damn memeber as a "truss".   

RE: Truss axial forces ~ Section size

Member sizes should not matter in a theoretical analysis that ignores selfweight.   

RE: Truss axial forces ~ Section size

(OP)
1. Let: m = number of members in the truss
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

Just a thought here.

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

patswfc, i disagree completely.

let's just see a screenshot...that should solve it.

RE: Truss axial forces ~ Section size

I second galambos, with a caveat..  

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

deadhorse

RE: Truss axial forces ~ Section size

I second galambos, no caveat.  

RE: Truss axial forces ~ Section size

Civeng80 has the answer. For a pure detrminate truss(axial forces, pinned joints, loads at joints) hand solutions by method of sections or joints is exact. FE is ALWAYS an aproximate solution. It does not distnguish between determinate or indeterminate. Changes in A change the stiffness matrix and change the residual errror. That is why there ar MINOR differences betwwwen member sizes. When you do an FE problem, ever notice the static check is not quite zero? Same concept. Know the limitation of your tools.

RE: Truss axial forces ~ Section size

I'm not sure you will be able to solve a pin-pin truss by using hand solutions methods of sections or joints.

a pin-roller truss on the other hand is a very different story.

RE: Truss axial forces ~ Section size

Did anyone ask what percentage the results were changing by?  It may be small enough to ignore, given the inherent inaccuracy of computers and the algorithms used to solve matrix math.  Also, if the truss is nowhere near 0,0,0 there may be other round-off errors in simply the coordinate system.  It also looks like some of the members are coming together at fairly acute angles, presenting ill-conditioned equations to be solved - depending on the algorithms used there may be errors there.  Did the OP ever run the same analysis twice to see if there were small differences between them?

RE: Truss axial forces ~ Section size

DRC1

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

I don't use FEA but using my classical matrix analysis software resulted in identical results to 6 decimal places for a range of relative section sizes.
This was for a determinate, pin jointed, node loaded truss.

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