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?

 

[dgkhan]

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.

 

[frv]

heyenwp-

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

 

[haynewp]

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.

 

[apsix]

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

 

[civeng80]

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 !

 

[Galambos]

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.

 

[minorchord2000]

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.

 

[haynewp]

Galambos, it can still be internally indeterminate.

 

[Tomfh]

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

 

[civeng80]

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

Cheers!

 

[apsix]

civeng80
Yes, I believe so.

minorchord2000
Your analysis/design procedure sounds fine, it doesn't address the OP's dilemma though.

 

[civeng80]

So then it answers the Op's delema?

correct?

 

[oneintheeye]

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.

 

[apsix]

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

 

[oneintheeye]

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.

 

[civeng80]

Exactly herewegothen.

But then why does the model give moments on t and b chord if he has pins on all joints?

 

[oneintheeye]

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?

 

[oneintheeye]

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.

 

[Galambos]

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

i had this problem in my undergrad steel class project.

 

[oneintheeye]

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