Dynamic or Static Analysis

[knandwana]

Hi :

While performing a seismic analysis. how should one determine which method to use a) Dynamic Analysis or b) Static Coefficient Analysis ?

Thanks,

Kapil

 

[JAE]

Which code? Might help in getting answers from folks.

Usually static analyses are used only when the structure in question is fairly uniform in shape, stiffness, etc. and located in low to moderate seismic areas.

The IBC (i.e. ASCE 7) specifically tells you when you can and cannot use a static procedure - read through the text and you will see where they list the requirements.

 

[bazflexure]

If you are using UBC 97 chapter 16 explicity defines the requirment. You need to check your builind for against different irregularities(soft story, weight irregularity etc) and soil profile type (F needs dynamic)

 

[RRDBE]

what will be difference in base shear in % for say 70 m height structure, if we calculate by the two methods Static and Dynamic method...

 

[JAE]

I think the codes require the designer, when using the Dynamic procedure, to also use at least use 80% (someone check me) of the Static force.

 

[CivE0]

RRDBE,

I don't know if there is a rule of thumb like that. It would first depend on what type of static analysis you're comparing to and the type of building you have. The ASCE7 for example over or underestimates the base shear and drift depending on the type of lateral system you have. The difference is in uncertainty and will not necessarily give you favorable results.

Another factor that's perhaps just as important is the Sds value from ASCE7 and how it corresponds to the time histories in the area. In high seismic regions in the state of California for example, the Sds values are not always conservative.

Each case is different and requires some preliminary study to come up with an estimated difference in demand on the building.

 

[structurallyyours]

as per indian codes if response spectrum badse shear are less than seismic coe. base shear then seismic coefficient base shear shall be considered

 

[structurallyyours]

Dear Kapil,
The choice of method is a judgement of structural engineer. If you ask me I will prefer seismic coefficient. If the structure is complex then only I will go for response spectrum method.

The preference for seismic coefficient method is due to it's simplicity to use. Also combining seismic coefficient results is also easy. Reviewing results after analysis is also easy. If the engineer is doing seismic analysis for the first time then he should first try with seismicc coefficient method and the go for responce spectrum.

For responcespectrum method there are many complexities. The main part in analysis is interpretation of results and for which the structural engineer should have sound knowledge of dynamics of structures.

Another problem with responce spectrum is that after analysis the results will not have any sign because of modal combination. Then doing column design you need totake care because if you will use unsigned forces then column will get designed for minimum axial force and maximum design moment. This will lead to very high % of steel in RCC structures. For doing footing design if you require raft or combine footing, design can be done only after attaching signs to the responce spectrum results.

I hope the info above will help you in taking decision.