Modal Analysis According to EC 8 1

[jmramos]

I have a doubt about the interpretation of clause 4.3.3.3 of EC 8, when it establishes that the sum of the effective modal mass for the modes to be considered in the modal analysis, must represent at least 90% of the total mass of the structure. I suppose that the mass of the structure referred to in the clause, refers only to the part of the mass of the structure above the ground floor. In the case of structures with underground floors, including perimeter earth retaining walls, the masses corresponding to the floors below the ground level should not be considered, because they vibrate according to the oscillatory movement of the soil and, therefore, do not introduce additional efforts above-ground structure. Am I right? Thanks in advance.

 

[Agent666]

It kind of depends, the mass/levels considered really depends on at what level the loads from ground are being resisted or transferred into the structure.

Just because a basement is more rigid than a superstructure doesn't mean it should be ignored, the basement still contributes to the dynamic response of the structure after all and often the lateral loads are still reliant on the foundations under the basement to transfer the base shear into the ground.

 

[HTURKAK]

If you specify the item that you have doubt , we can discuss in detail..


This is true for a certain point of view.. For example , in order to check the minimum EQ requirements, the mass of flexible upper portion should govern.


This is not true.. moreover, the rigid basement floors ( having short periods ) will attract more seismic forces including the EQ active soil loading...

The buildings having box type rigid basement floors may be analyzed in two ways..

1- Analyze the flexible upper portion seperately and combine the effects with rigid portion. In this case, the seismic force for the basement floors will be Fbeq= ag* W / q ag= PGA anf q =behavior factor for basement =1.5 + seismic forces from flexible upper floors+ seismic active soil loading.


2- Analyze the whole system with q=1 .. and you will notice that the rigid basement floors will be active at higher modes.. In this case, you may consider the soil springs for horizontal and vertical direction.. Then , apply different q values for upper and lower portion.

If this is a real question, pls provide more info. to get better responds.

 

[jmramos]

Thank you HTURKAK for your valuable contribution. i understood the explanation YOU gave me, but I wanted to put another one. Performing the structure modeling for example, in Sap2000, how is it possible to introduce two behavior coefficients in the same analysis, for the lower and upper portion?

 

[HTURKAK]

I am retired eng. for some years and don't know the new versions of SAP 2000. But i want to give opinion free from software.

You have two options for analyzing a bldg with rigid basements .

1- Dynamic analysis say RSA of the whole structure flexible upper portion together with rigid basement storeys . In this case, you should consider the passive thrust of the soil assuming horizontal springs with half spring constant and foundation soil with vertical springs. The behavior factor q for upper portion say qf =6 and for rigid portion qr=1.5. You are expected to run the model for two separate q values. The effects of first run with qf =6 will be valid for flexible upper portion and second run effects with q=1.5 will be valid for basement floors.

2- You may perform dynamic analysis for the upper portion only assuming the foundation level is top of the basement and design the flexible portion elements for the effects q=6 . Then find the reactions . Use these reactions for the design of rigid basement storeys but multiply the seismic effects with qf/qr ( in this case 3.0 ) and separate analysis of the basement storeys for user defined seismic forces and for upper portion reactions.


If the rigid basement is with several storeys, be careful for the detailing of walls, passive soil loading, and transfer slab may develop acc. to position of core walls.

 

[jmramos]

Once again, my sincere thanks. I understood your explanations very well. Sorry for the audacity (or annoyance), but do you know of any valuable books that you can recommend, dedicated to mathematical expressions for calculating the spring coefficients? Cordial greetings

 

[HTURKAK]

Dynamic soil modulus ,spring coefficients should normally provided by geotechnical group...

However, in order to get the concept, I will suggest you to look;

- PRACTICAL SOIL DYNAMICS by MILUTIN SRBULOV
- ASCE GEOTECHNICAL EARTHQUAKE ENGINEERING AND SOIL DYNAMICS PROCEEDINGS OF THE CONFERENCE
- FEMA Provisions for Foundation and Soil structure interaction,

and the following docs. at links :

https://www.nehrp-consultants.org/publications/download/nistgcr12-917-22.pdf

https://www.nehrp-consultants.org/publications/download/nistgcr12-917-21REV1.pdf

P.S. I found the following document ( Eurocode 8: Seismic Design of Buildings
Worked examples ) at the following link :

https://eurocodes.jrc.ec.europa.eu/...ismic_Design_of_Buildings-Worked_examples.pdf

 

[jmramos]

once again, my thanks for the helpful collaboration. Best regards