Hi,
I am trying to model a composite beam (steel beam + concrete slab).
For this purpose I modeled my composite beam as a steel beam and plate elements.
I applied then an offset distance 0.95 m to the steel beams. Now I see that the steel beam gets a very large axial force and the Mz moment at mid span is very small !!!
If I apply MEMEBER RELEASE (Fx) to the steel beams, the Mz moment would be correct, but what happents if I apply temperature load to the plate elements?
The axial force in the beams will be zero so this approach won't work properly.
I am really confused the way STAAD calculates section forces.
Do you have any clarification on this?
babmou
Mr.Babmou
you have released Mx at both ends which in my view should not be done for any member unless there is some compelling reason for doing so, as the beam would become free to rotate on its axis. That is why you are getting warning messge of instability in your output file.But this is not the reason that you are getting a large amount of axial force in your beam after analysis.You have applied member offset in Y direction to your beam. This is inducing secondary forces in the beam on analysis and is one of the reasons that your are getting axial force in the beam due to gravity loads as well in addition to temperature load. Temperature load is bound to cause axial force if Fx of the member is not restrained. Secondly your concern is that you have released Fixed But Support for Fx but you are still getting Fx in the member after analysis .The reason is that member has got offset in Y direction but not the support. You are therefore bound to get axial force due to induction of secondary forces unless you relese the ends of beam on the right hand side for Fx or you remove offset from the member.The analysis of beam will be carried out by Staad as per conditions prevailing at the new location, effected after offset.
I shall be glad to know if any other member of the Forum has a different view in the matter.
Sureshprsharma
Hi Sureshprsharma,
Thanks for your reply.
I have used the EXPLICIT definition method in STAAD to create a composite beam.
STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 12-Oct-10
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 20 0 0;
MEMBER INCIDENCES
1 1 2;
DEFINE MATERIAL START
ISOTROPIC CONCRETE
E 2.17185e+007
POISSON 0.17
DENSITY 23.5616
ALPHA 1e-005
DAMP 0.05
ISOTROPIC STEEL
E 2.05e+008
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-005
DAMP 0.03
END DEFINE MATERIAL
MEMBER PROPERTY EUROPEAN
1 TABLE CM HE1000B CT 0.25 FC 55000 CW 8 CD 25
CONSTANTS
MATERIAL STEEL ALL
*
SUPPORTS
1 PINNED
2 FIXED BUT FX MY MZ
LOAD 1 LOADTYPE Dead TITLE LOAD 1
MEMBER LOAD
1 UNI GY -100
LOAD 2 LOADTYPE Live TITLE SHRINKAGE
TEMPERATURE LOAD
1 TEMP -30
PERFORM ANALYSIS PRINT STATICS CHECK
FINISH
The steel beam and concrete slab act monolithically and as expected for load case 1, the max Mz=5000 kNm, but it doesn’t work for the temperature load (Shrinkage).
I modified my model (Composite Beam.std) in such a manner you described, however I still get axial force in the beam.
Using OFFSET make the structure look graphically great but not statically.
Do you know a way to get around this?
PLEASE SEE THE POST PROCESSING RESULT OBTAINED FROM THE DATA FURNISHED BY YOU. I AM NOT GETTING Fx. YOU MAY HAVE SOME OTHER IPROBLEM WHICH PERHAPS I AM NOT ABLE TO COMPREHEND.
Hi Sureshprsharma.
Thanks for your follow up on this issue. Really appreciate it.
As you see when I use the in-built Composite Beam option in STAAD, the maximum mid span moment will be Mz=5000 kNm and no axial force is induced either as you have verified in your excel sheet, but the problem is this model doesn’t work when temperature load is applied.
My intention is to create a model comprising of beams and plate elements like “Composite Beam.std”.
The question is how I can create a model to get same Mz and no axial force for pure bending due to gravity loads?
I tried different ways such Master-Slave and skipping the OFFSET command, however I still get axial force.
I don’t have any other software to verify if I get the same section forces.
I know many users have the same difficulty in understanding the way OFFSET works.
Have you any suggestions?
I will be away for two days, but please keep on this discussion as it is very important for many other users too.
Regards
Dear Babmau,
Now what I get from your deliberations that you wish to have the same result for the beam modelled with steel section and plate as that for the composite beam out of sections stored in the software. It is not possible because Staad not only uses the material properties but the sectional properties inbuilt in the section table for the analysis so as to behave monolithically.
If any one carries a different opinion, it would be welcomed