Equivalent seismic horizontal loads

[JokaSteel]

Hello everyone,
I am a beginner in the seismic analysis and I am stuck in two points. To illustrate my first question, I will take as example a 2 rigid floor building. My doubt is after the response spectrum analysis (suppose horizontal load) is performed; If I want to perform an static analysis considering the equivalent horizontal forces generated in the response spectrum step, how do I do that, I mean, what horizontal forces do I put on the floors? I read some threads about the base shear. Do I need to put the base shear in the static model, in the contrary direction?

Another lacking of sure: Is there a way to calculate the percentage of the response spectrum results with respect to the weight of the building?
Since now, I appreciate any help.
Joka.

 

[msquared48]

The base shear is distributed to any particular floor in accordance with the moment generated at that particular floor level in proportion to the total overturning moment.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[asixth]

@JokaSteel

What analysis package are you using. A response spectrum analysis should give you the "real" storey shear for a particular building stiffness and mass distribution whereas the equavilent static will give you a generic one (usually with larger storey shears). If the analysis package cannot give you the force distribution between the lateral elements than you will need to extract the shears of the lateral elements between floor and sum them to get the storey shears.

Storey shears from response spectrum depends on the spectrum curve and building stiffness and lateral acceleration. The rules I go by for my typical locality is between 2.5 to 5% of the building mass. 2.5% for relatively flexible structures and 5% as the stiffness increases.

When I perform a response spectrum analysis I like to apply the accelerations at 0deg, 30deg, 45deg, 60deg and 90deg. There is no need to give apply them in the contrary direction because the base shear extracted will be equal to the corresponding angle in the first quadrant.

 

[JokaSteel]

Thanks Mike and asixth for your answers. Maybe I am a step before that.

The reason I want an equivalent static stress is that I want to sum up this load (from response spectrum) with wind load (this one I get from an static analysis). How can I combine this two loads, as I combine wind load with dead load, for example? How I get the base shear to distribute it over the floors?
many thanks.
Joka.

 

[asixth]

Without a computer package very difficult. The storey shears from an response spectrum analysis is an extraction, not an input

 

[JokaSteel]

Thanks again, asixth.

I have the nx-nastran package. I will see in the documentation if there is a easy way to do that. Do you known if it is possible with nx-nastran?
Joka.

 

[frv]

Why combine wind and seismic?

 

[JokaSteel]

Hi frv,

The wind load is an example only. I would like to learn how to do such kind of combination. As asixth said, a computer package is necessary. I am searching if my package do that (femap/nastran) and also for another package that do such sum "internally". Do you have any suggestion?

Thanks.
Joka.