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Re: ASCE 7-02 Time period in Metric

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Anantha:

My guess would be due the equation being imperical in nature and a likely
some round-off effects when they generated the values to be used in the
code.  You can bet that the real values are not 0.055 and .75 (and maybe
even the 0.02 for ft).  They likely took the meters to ft conversion
(about 3.28084 using the value from my 1st edition LRFD manual) and raised
it to the .75 factor (although they may have used a "more precise" value
than 0.75) and then multiple that value by 0.02 to get the "equivalent"
meters (i.e. 0.055) value.  The problem is that more than likely the
actual values from research for the ft empirical equation is something
other than exactly 0.02 and 0.75, thus the resulting value is likely
something not exactly 0.055.  In otherwords, they did not want to use some
long number with a lot of sig figs in the code and thus rounded stuff off
to make it look "nicer".

For example, if the value was something like actually 0.0226 (which rounds
to 0.02) and 0.754325 (which rounds to 0.75), you would take 3.28084 to
the 0.754325 power and get 2.450307.  Now, multiply that by the 0.0226 and
you get 0.055376, which rounds to 0.055.  So, they likely thought that it
would look "cleaner" to use 0.02, 0.055, and 0.75 rather than 0.0226,
0.055376, and 0.754325.  PLEASE NOTE THAT MY VALUES ARE A SPECULATIVE
EXAMPLE TO ILLUSTRATE THE POINT...I DON'T KNOW WHAT THE ACTUAL VALUES ARE.

In addition, if your conversion factor did not match EXACTLY the
conversion factor that they used, then more "error" would be introduced.
Based upon the 14.6 m and 47.88 ft that you stated, you conversion factor
would be roughly 3.27945, which is different from the factor that I used
from the LRFD steel manual.  Using the factor that I used, it would be
47.9 ft for 14.6 m.

So, my quess is that is a round off error issue largely.

Regards,

Scott
Adrian, MI


On Wed, 27 Sep 2006, Anantha Narayan C.K. wrote:

> Greetings,
>
>   This might be the dumbest question on the forum, but could anyone please let me know what I am doing incorrectly.
>
> Reference : ASCE 7-02 Eq 9.5.5.3.2-1
>
>   Ta = Cu * hn ^ x
>
>   where for "All other structural systems" Cu = 0.02 (0.055 in metric) and x = 0.75 [TABLE 9.5.5.3.2]
>
>   For a building that is 14.6 m high (= 47.88' )
>
>   Using English Units, Ta = 0.02 (47.88)^0.75 = 0.36 s
>
>   Using Metric Units, Ta = 0.055 (14.6)^0.75 = 0.41 s
>
>   Could anyone please tell me as to why there is a discrepancy between the results?
>
>   Thanks
>   Anantha
>
>
>
>
>         VTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVTVT
>
>   ANANTHA NARAYAN, E.I.
>   Structural Engineer
>   Bliss and Nyitray Inc.
>   Miami, FL - 33134
>
>
>
>
>
>
> ---------------------------------
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