Truss axial forces ~ Section size 1

[dgkhan]

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?

 

[csd72]

Self weight?

 

[dgkhan]

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

 

[spats]

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

 

[Castigliano]

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

 

[Lion06]

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.

 

[haynewp]

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?

 

[spats]

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

 

[dgkhan]

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.

 

[dgkhan]

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

 

[msquared48]

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

 

[frv]

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.

 

[haynewp]

If the truss is indeterminate then it does matter!

 

[haynewp]

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.

 

[frv]

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)

 

[haynewp]

frv,

You repeating exactly what I have said.

 

[dgkhan]

frv
you got my point

 

[dgkhan]

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

 

[civeng80]

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!

 

[civeng80]

Spats

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