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Non Linear Dynamic Analysis and the number of records

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For those of you who are NDP experts, or have had experience in this field,
here is a question:

In non linear time-history analysis, we face, amongst others, the following

1.- The selection of the records
2.- The number of records to use, or
3.- The allowed  one-sigma confidence bandwidth in the mean response of the

SEAOC Blue book and FEMA 273 suggested the use between 3 to 7 records;
using the maximum structural response if you use 3 as the sample size, or
the mean response for 7 or more records.

As everyone knows, the results are very scattered, even if two records have
almost the same Response Spectrum they would yield very different
structural responses.

The dispersion for the intensity ,Sa[fo] (Response Spectrum Acceleration
at the fundamental structural period ), is on the 50%-90% range, depending
on [fo]. If we need a 95% confidence level, and a +o- 10% bandwidth,  means
use a very large number of records, if my figures are accurate, between 100
to 324, being a function of dispersion.

The use of SEAOC Blue book or FEMA 273 Guidelines (3 to 7 records), implies
a one-sigma confidence band of 30% or more.

So, this  question arises: How many records to use? Or, in other words,
which bandwidth should be used in the mean structural response and at which
confidence level?

Also, if any, please feel free to comment on the normalization procedure or
the scaling parameter to be used on the record's sample.

TIA for your answers.

Adolfo Galvez, MSc.
Lima - PERU

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