31 posts in this topic

On 1/12/2017 at 2:47 PM, Abid Qasmi said:

interesting discussion!!! but how can we check T i.e the fundamental time period in ETABS?



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Dear Waseem,

Your first two points from NEHRP are logical as they are directed towards the implementation of an upper limit on time period to be used in seismic forces evaluation to eliminate the possibility of excessively flexible structure.These provisions are present in all building codes and seem to be logical and valid.

For usage of Ta when Tactual < Ta,

The RS curve you have shown indicates Ta & Tc both on constant acceleration zone , so whether you use Ta or Tc you will get the maximum ground acceleration or in ELF terms whether you use any of them equation of max base shear will govern.So both are equal mathematically.

However, ASCE endorses what you are saying. But as long as usage of Ta (approximate period) when Tactual < Ta is concerned it still seems illogical to me and will be associated with potential inconsistencies, some of them could be as follows,

  • Drifts evaluated from Ta will be lesser than actual, therefore building separation widths (most importantly) will be under estimated.
  • Code defined equation of max base shear will be of no value for a certain height of structures, while following this.
  • As the actual seismic forces will be greater than that used in design, therefore structure will require high energy dissipation demand than anticipated, as members will be yielded and extended to inelastic range of stresses earlier (due to under estimation of seismic forces) than anticipated stage (associated with R used and detailing performed).

Many other issues will also arise due to under estimation of seismic forces that you can also consider.

Therefore, although this document is published from a credible source but they didn't justify the adequacy of this provision and didn't address the probable consequences.So it seems a non-engineered dictation to me.

Practically this situation is very rare but I will go for UBC97 in such situation rather than following this.






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Dear Umar,

First-off, many thanks for the explanation and your interest in the topic. Highly appreciated. Perhaps, the RSA image added to the confusion than doing anything good, i guess.

We are in the same boat as far as T within the range of maximum spectral acceleration is concerned, however, kernel of the discussion revolves around the "scaling of dynamic base shear with respect to ELF base shear". As such, analysis time period (used in RSA graph), even if lies within the maximum acceleration range might give dynamic base shear less than ELF V.

For this, and as required by certain codes (American e.g.), for scaling up the dynamic base shear, we need to establish a "Time Period" for static procedures and calculate static base shear.

So I wanna twist our discussion in another way. Let me ask, if T(dynamic) and T(static) are same, do we get same base shear from both methods?

Static procedures V depend on R; a compound factor for ductility, damping, inelastic/elastic response etc. On the other hand, spectral acceleration from RSA is a function of Ca, Cv and damping but not specifically R.

Dynamic base shear is the product of mass matrix, acceleration and participation factor of each mode then we add up all the modes by some type of combination, right. Now, even if time periods were same, and even if the structure behaved in fundamental mode (SDOF concept of RSA) with very rigid torsion response, base shears from both methods would still be little different due to partial active mass in dynamic analysis (maybe 90%?).

Hence (and correct me), saying that T corresponding to Vmax of static procedures if used in RSA would result in same base shear would not be appropriate, i guess.

Moreover, in my opinion, RSA analysis (although based on actual earthquake records), is a tool where you need to put up some boundaries; upper and lower. Whereas the empirical time period limitations make more sense as they were calculated based on actual buildings and earthquakes through statistical analysis. That might be the reason FEMA recommends using Ta.

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On ‎1‎/‎5‎/‎2017 at 9:28 AM, Rana said:



I reached at the similar conclusion on this ambiguity.




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On 7/28/2016 at 8:49 AM, ILYAS said:

ETABS 2015 have  option in design that whether you consider minimum eccentricity or not


Etabs version 9.7.5 also have the option whether to condider minimum eccentricity or not


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