EC8 - reduction problem of design spectrum by behaviour factor q

[Anonymous]

Hi,

if we use Eurocode 8 window in seismic nalysis we have a problem becasue Robot doesn't reduce spectrum with behavior factor q. It is also visible in calculation notes after calculation if we take any factor.

Only posibility is to use spectral option and define exact spectrum who is already reduced by q for exact type of soil, acceleration and behaviour factor q. This we can do handly in Excel and with .spe file insert in Robot.

But in that case we don't have opportunity to take eccentricities according to eurocode and that may be a problem.

Why Robot doesn't reduce linearly vertical ordinate of design spectrum by behaviour factor q ?

By Eurocode for ultimate limit state we need to use spectrum for 475 year period reduced by q (design spectrum), and for service limit state 95 y return period (elastic spectrum). So by my opinion only is correct to use 2 spectral cases. Sometimes, depends on the type of soil and values of q, spectrum have the same shape. 

But if we allready have EC8 window I don't understant why Robot doesn't calculate according to Eurocode requirements.

Regards

rm

[Artur.Kosakowski]

Try to define two EC8 seismic cases with the same parameters but different behaviour factors. Run analysis and check values of displacements and forces for each of them. You will see that qs were used to reduce forces automatically. 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi Artur,

I provide short test with concrete frame structure, where I had two behaviour factors.

A. q=3.0

B. q=1.0

In attachment I sending you my brifely analysis in Word with conclusions and model in Robot.

Actually, Robot gives good forces who are reduced with q, but displacements are allways for q=1. According that we need take care about displacement analysis according to EC8. This is not a problem, only problem is if we don't know for this fact in our analysis.

Calculation note for different q gives good shape of spectrum.

So finally, my apologies .....  Yes, Robot counting behaviour factor for forces but not for displacements. Hope you have some answer or explanation for this.

Regards

[Artur.Kosakowski]

Please check: 

SEISMIC ANALYSIS PARAMETERS (EC 8 - GENERAL)

"Note: The behavior factor q reduces the design spectrum; this way it also decreases seismic loads and the resulting internal forces. However, as specified in point 3.2.2.5, the displacement based on the design response spectrum is multiplied by the behavior factor q. Thus, usually such displacement is not reduced in relation to the elastic response spectrum."

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Yes, that is correct.

but for ULS in seismic case we are desling with 475 year return period spectrum. That spectrum we reduce with q. 

And it works OK, forces are OK.

For service limit state (damage limitation or interstorey drift limitation) we have to analyze displacements for 95 year return period.

So, the formula in EC8 say:  If you calculate displacements for return period spectrum 475 year you need to convert that displacement values for 475 to 95 year.

In that case when we have results of displacements for 475year we multiply it to behaviour factor to get him back to elastic spectrum and then we multyply again with factor ''ni'' to return it 95 years acceleration and spectrum. Factor ''ni'' depends from important factor, and usually for ordinary buildings is 0.5.

And in Robot window there is no possibility to define important factor who define displacements for serviceability seismic action.

Regards

[Artur.Kosakowski]

Robot considers these points:

4.3.4 Displacement calculation

(1)P If linear analysis is performed the displacements induced by the design seismic

action shall be calculated on the basis of the elastic deformations of the structural

system by means of the following simplified expression:

ds = qd de (4.23)

where

ds is the displacement of a point of the structural system induced by the design

seismic action;

qd is the displacement behaviour factor, assumed equal to q unless otherwise

specified;

de is the displacement of the same point of the structural system, as determined by

a linear analysis based on the design response spectrum in accordance with

3.2.2.5.

The value of ds does not need to be larger than the value derived from the elastic

spectrum.

NOTE In general qd is larger than q if the fundamental period of the structure is less than TC (see

Figure B.2 ).

[Anonymous]

Yes, I agree with you. EC8, 4.3.4.  explain that in detail.

Also 4.4.3.2. define Limitations for interstorey drift. EC8 do not give restrictions regarding total displacements, only give interstory drift limits.

There is reduction factor ''ni'' which takes into account the lower return oeriod of the seismic action associated with the damage limitation requirement.

And for example for imoprtnace class III reduction factor ''ni'' is 0.5.

As I said before calculation is OK, only in interstorey drift analysis we have to manually muliply Robot displacement values with reduction factor ''no'' according EC8, 4.4.3.2.

Regards

[Anonymous · Accepted solution]

Dear Artur,

In the meantime I found the post:

http://forums.autodesk.com/t5/robot-structural-analysis/ec8-reduction-factors-amp-displacements/td-p...

where you gave the answer about reduction factor ''ni''.

Robot doesn't consider that factor. 

That is not a problem. Everyrhing other works fine.

Thank you.

Best Regards

rm