Spectral Acceleration

[teckert]

If the period in the first mode of my six story split level frame is 0.72 seconds. It was analyzed using El Centro Earthquake North-South.

How do i read the spectral acceleration graphs and determine where the locations on the spectral acc. impact the frame the most?

Thanks for your help.

 

[ishvaaag]

Not sure if this answers even a bit what you want but the normalized spectral acceleration charts are of similar structure. In them, if you have a stiff structure, with then small or limited displacement, and a short fundamental period (so high frequency of vibration), you are seeing the biggest responses in terms of forces on the building. If, contrarily, your structure is softer, you will see less solicitation in terms of force and have more displacement. So the response of your structure at a given mode as measured by some response spectrum chart is given by the ordinate in the chart. Since there are many modes, there is a superposition of expected responses, each having a contribution. Quoting what the RISA 3D manual says...

"In the response spectra analysis procedure, each's of the model's modes is considered to be an independent SDOF (single degree of freedom) system. The maximum responses are calculated independently. These modal responses are then combined to obtain the model's overall response to the applied spectra"

The combination is made upon a "mass participation" for the mode. You can see an example of such evaluation at page 5-11 of

Design of Concrete Buildings for Earthquake & wind forces
S.K. Ghosh
August W. Dormel Jr.
PCA 1992, Skokie, Illinois

Simpified methods, as in Spain's code NCSR-02 and others, also ensure a mass participation factor.

Then, a statistical combination on such participations ensues, and a code of signage must be given upon what you get some variability on the seismic forces accounted for, i.e., you don't get for a given structure a single set of response spectra solicitations but if you restrict yourself to a single signage and combination rules.

That the values so obtained may need be upscaled to some minimum value otherwise calculated is just a precautory measure allocated by the code, as are as well

-making the calculation for a minimum eccentricity of the masses, or adding some eccentricity to the resultant shear forces per floor

-applying the seismic forces along a rosette of directions

I particularly distaste the complication of the added eccentricity because introduces uncertainties of application blackboxed by every particular code in the programs used. It would be better in my view either consider a range of vertical loads or/and a bigger safety factor for EQ in the combinations, reducing such uncertainties, leading to a more consistent statement of the earthquake forces which long term can be more useful upon experience to good tuning of the codes.

 

[GregLocock]

Your question isn't too clear, are you asking where the biggest response was? Or where to hit the structure to get the biggest response? Do you mean the word "impact'?

It might be useful to post some of the data you don't understand.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.