Eurocode-8 : Seismic RSA analysis 1

[dynamic jiala]

Hi All

In ACI code, the RSA analysis scale factor is Ig/R.
How about in Euro code what is the scale factor? some of my friends said no need to define scale factor in Euro code. some said use g=9800 and some said just use the default value

Anyone has any better answer on this? and how it affect the base shear compaare to lateral load analysis method

 

[HTURKAK]

That depends on your choice and purpose of analysis. The EC 8 notations totally different. The gravitational acc . g=9.81 m/sec2 is used with SI units.

You are expected to construct spectrum applicable for your specific case and the common practice,and the unit of spectral acc .Sa is m/sec2. Always remember, The units shall be consistent ..

Regarding your second question, I understand Equivalent lateral force procedure which assumes the fundamental period T1 will be dominant.
If you provide more details , you may get better responses.

Below , you will see EC8 design response spectra (Type 1, soil type C) copied and pasted from the book(Seismic Design of Buildings to Eurocode 8 Edited by Y. Elghazouli )

 

[Eagleee]

what does this scale factor do in aci? is it accounting for energy dissipation mechanisms? if so, then the eurocode factor which does this is 'q' and called a behavior factor and its choice depends on several things. although you asking above if it could be 9800 leads me to think you are referring to something totally different as this 'q' factor is usually around 2.

 

[dynamic jiala]

Hi all

Thanks for your response, i am asking the the attached imagine value in Etabs as coulded

Thanks

 

[dynamic jiala]

Thanks both for your great assistance and answers.

Sorry i was talking about Etabs.
I think the EC code just use 9800 default value is correct which is g. because the q value has been specified in both static and RSA analysis. so no need to scale again. not like ACI code

 

[HTURKAK]

g=9800 mm/sec2 . What are the units that you have preferred ? If you define the units mm,kg,sec and q =1 ( response modif. factor) , and importance factor gamma=1.0, the scale factor will be 9800.

But the spectrum acceleration shall be unitless.. that is, spectrum acceleration shall be percentage of g.

How did you construct the spectrum ? what are the units ? what is the applicable q and importance factor?

Never forget to work with consistent units and if you provide more info, you will get better responses..

 

[insanias16]

@ dynamic jiala

Did you manage to figure this out? I'm also confused on the scale factor input in ETABS.

 

[Blackstar123]

I'm also confused on the scale factor input in ETABS

Regarding scale factor, two important things you should know about ASCE response spectrum is:
1. Its an "Elastic" spectrum
2. Technically, it has a "silent" g in the denominator i.e., Sa/g.

Earthquake force on any structure is basically Mass times Acceleration. Thus, to extract the value of spectral acceleration Sa, we need to multiple the acceleration values form spectrum with g. We simply divide by R to convert the spectrum from elastic to inelastic. I is the importance factor.

 

[hardbutmild]

The scaling is not constant through the whole frequency domain, You should modify the response spectra to get the design value as HTURKAK mentioned.

Unrelated to the question, you should use 5% eccentricity for diaphragm eccentricity (if this option is what I think it is).

 

[insanias16]

For UBC/ACI its very clear on how to modify spectrum by adjusting scale factor in ETABS, but for Eurocode 8 I am not certain.

Do I need to modify the scale factor (default 9800m/sec2) if I have already defined the behavior factor (q) when creating the response spectrum?

 

[hardbutmild]

Do I need to modify the scale factor (default 9800m/sec2) if I have already defined the behavior factor (q) when creating the response spectrum?

Try it. If changing the behaviour factor changes internal forces then you don't need to additionally modify the scale factor, if it doesn't then you have to do it manually (it probably does it automatically).

 

[HTURKAK]

I remember some 50 yrs ago.. data was provided with punched cards to compiler so to the computers.Nowadays the speed increased enormously and softwares changed but the computers still never does quite what we want ,but only what we tell them..

The scale factor (default 9800m/sec2) is O.K for,
- The spectrum acc. values percentage of gravity ,
- The units are, N,mm, sec ,
- The importance factor I=1.0 and
- R or (the behavior factor (q) in EC) =1.0

In this case , the calculated forces and displacements will be elastic.

If you want the design forces , with say q=4.0 , the importance factor I=1.5 and the units N,M, sec, g=9.8 m/sec2 and the scale factor will be = 9.81*1.5/4.0=3.678

Remember,the units shall be always consistent..

If this respond does not answer to your question, provide more details ..