Sum of reactions vs. sum of values in seismic analysis

[Anonymous]

Hello, we have performed an Seismic analysis and a subsequent response spectrum analysis to determine the Base Shear .

A review of the results it appears that the sum of reac. and the sum of val. are not in compliance as they do when loads are applied to the model by the user.

A  search on this forum showed that there was a difference in the way the two results determined  ( http://forums.autodesk.com/t5/Robot-Structural-Analysis/Reaction-Table-query/m-p/3949469#M14317  ).

It is obvious that the total of reactions in sum are the total base shear of the building but how can we show the individual forces in the supports? 

Reactions in Diagrams for buildings show the sum of Val. but we want to display the sum of reac. 

How can i know the force in different supports of the building that in sum wil be the base shear? Can this be viewd in a tabel?  

Does the differenc in sum of val. and sum of reac. become bigger as a result of torsonal modes? 

I see this as a fundemental question for how to interpret the results form the seismic analysis i RSA2014

We use RSA2014 Version 27.0.5.4679 (x64)

The spectrum are calculated using NS-EN 1998-1 +NA.......

[Anonymous]

Does anaybody have a reply to the topic? 

[Anonymous]

No one? 

[t.sautierr]

Each shear force in each wall : reduced results for pannels

Precision :

I found a quite small difference : 2670 + 0.3 * 313 = 2763.9 vs 2765 kN .... -> matter of precision I think.

[Anonymous]

Please explain?

[t.sautierr]

following the load table you gave if you make load case X + 0.3 load case Y you find what is in the comb X+0.3Y

[Anonymous]

Yes, but that is not the issue. As you se ther is difference between sum of val. and sum of rec. The displayed forces are sum of val. but the "real" forces that the structure has do withstand are the sum of reac. ( Base shear) 

[t.sautierr]

Oh ok ... are you sure ou have selected all the panels when you did the screenshots?

[Anonymous]

Yes. I would like a coment form Autodesk regarding there selection on how too calculate the piles forces.

I would like the forces that d not come from sum of val. because the real forces are the sum of reac. (they are calculated according to "real"  theory).

The sum of val. change by selectoin of the mode in the program and that give the sign of the nodal force. If the bulding have torsional effects, the sum of val. can be non conservative. And sense all the displayed results are the sum of val. and not the sum of reac. how may you get the real force in ie. piles,  ore walls???

[t.sautierr]

Attach your model please because I'm not sure there is a real problem. (see attached pics)

[Anonymous]

Its because of the CQC- and the selection of mode signs as you say!

Can you tell me how i can get the reaction force that in sumation wil give me the sum of reac. and not the sum of val.?

You see, that is the correct seismic load that the foundation have to withstand.

The way i do it now is to select a mode that may not bee the main mode in the X, or Y- dir. But wil give me a sum of val. that is larger than the sum of reac. and in that way get safe results. Any thogths?  

[t.sautierr]

Forget about the sum of value ..... does not have a meaning  in seismic analysis.

For each wall, focuss on reduced results and proper comb (CQC or SRSS).

To check the overall precision, check if for each load case (load case and not comb), the sum of reaction is the sum of effort.

That's All.

[Anonymous]

Tanks but when i check the reaction table for each of my walls suports the value that is displayed i the sum of val. Not the sum of reac.  

I see that the redsed results for each story will give me the correct force that is obtaind by (CQC/SRSS) but i can't obtain the support reaction from this. 

Do you se my problem? How may i obtain the correct value i the supports?  

[t.sautierr]

Reduced results for pannels and not results for from stories.

Can you attach your model please.

[jenyzosin]

hello

Try to establish your main mods (ex: X=1 and Y=2) and then put this in Combination sign tab from Analysis Parameters.