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<p class=MsoNormal>Art:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I’m sorry for delaying my response, my time isn’t
always mine. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Your evaluation is essentially correct, my previous
evaluation was an overly simplified evaluation (equation 16.56) and
didn’t properly reflect the topic. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>It was never a question of which code to pick as a baseline.
I don’t hold the view that the 99 SBC has any particular advantage over
the IBC (except that it’s easier to use from a designer’s standpoint).
The lower base shear found in the 03 IBC with Memphis amendments reflects the level
of hazard for 1:500 return period shown on the 1996 maps your prepared for
NEHRP. The 1:500 return period is a traditionally accepted level of hazard
found to be appropriate for building with a 50 year life. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Recurrence periods for performance levels of natural hazards
except for earthquake are 500 years or less (ASCE – 7.02):<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Local Flooding
100-year<o:p></o:p></p>
<p class=MsoNormal>Regional Flooding
500-year<o:p></o:p></p>
<p class=MsoNormal>Snow
Loading
50-year<o:p></o:p></p>
<p class=MsoNormal>Wind Loads
500-year (previously 100-years)<o:p></o:p></p>
<p class=MsoNormal>Rain
Loads
10-year<o:p></o:p></p>
<p class=MsoNormal>Ice
Loads
50-year<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Thus we found the use of a recurrence interval 50 times greater
than the statistical life of the building to be meaningless – in effect becoming
a level of hazard matching the maximum probable event. This is a level of
hazard seldom used for the design of any structure. As an example, the
level of hazard used for the Hernando Desoto Bridge seismic retrofit was substantially
less; in spite of its importance to the region. The Tennessee’s Department
of Transportation’s structural engineer told me that the reason for not
meeting the 1:2500 year criterion was the cost. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The difference between 10%PE maps in the 99 SBC and the 10%PE
maps found in 03 IBC reflects the improved state knowledge of your maps over
the SBC maps. The 10%PE 03 maps reflect the earthquake hazard perceived by the
public in this region. The Memphis amendments encourage the building owner to
build to a level that best reflects his sensitivity to casualty and property
loss.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>However, in the event that the 99 SBC was used as a baseline
code, you’d find that the differences between 99 SBC and the 03 IBC are
quite different depending on location in the county and the height of the
building. Using only 100 foot tall buildings skews the conclusions. The
predominance of the buildings in the county are less than 5 stories. And those
in the Northwest portions of the county are almost exclusively 1 and 2 story buildings.
Outside of downtown Memphis any building in excess of 5 stories is unusual. Therefore
we added weight to the impact to the shorter building more than to the taller
buildings.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Looking at a cross section of buildings across the county we
found the following in Shelby County:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Table 1<o:p></o:p></p>
<p class=MsoNormal>03IBC(Mem.)/99SBC<o:p></o:p></p>
<p class=MsoNormal>Shelby County 2-story 5-story 10-story<o:p></o:p></p>
<p class=MsoNormal>Southeast 0.90 0.77 0.61<o:p></o:p></p>
<p class=MsoNormal>Middle 0.94 0.70 0.56<o:p></o:p></p>
<p class=MsoNormal>Northwest 0.94 0.75 0.60<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Since we are using the 99 SBC as a baseline code it would
only be fare to compare the 03IBC (without amendments) versus the 99 SBC. See
the evaluation below.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Table 2<o:p></o:p></p>
<p class=MsoNormal>03IBC/99SBC<o:p></o:p></p>
<p class=MsoNormal>Shelby County 2-story 5-story 10-story<o:p></o:p></p>
<p class=MsoNormal>Southeast 1.73 2.47 1.96<o:p></o:p></p>
<p class=MsoNormal>Middle 1.87 2.94 2.33<o:p></o:p></p>
<p class=MsoNormal>Northwest 2.91 4.77 3.79<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>These tables, like your evaluation, assumed the Response factor
did not contribute to the evaluation, and we used a D Site Class (the dominant Site
Class in the county.) Even still, in order properly evaluate the differences in
the code the resultant base shears should include redundancy and overload
factors prescribed differently in the IBC than in the SBC. The overstrength
factor, usually 2 or 3 applies all cases equally, while in most cases the redundancy
factor would not apply to table 1. A conservative multiplier for all the
entries in both tables would be 1.5. In so doing we see that the
differences between 03 IBC (MEM) versus 99 SBC become relatively small. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Because of these differences illustrate in Table 2, Memphis
elected to utilize all three codes, the 03 IBC with amendments to the map, the
99 SBC, and the 03 IBC without the amendments. This best represented the
perceived hazard, and allowed the building owner to select the code that best
represented his threshold of risk. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Considering the above whys is USGS opposed to individual
communities to selecting their own seismic risk? <span style='font-family:"Arial","sans-serif";
color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>Joseph
Tomasello, PE</span></b><span style='font-size:12.0pt'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>5880
Ridge Bend Rd.</span></b><span style='font-size:12.0pt'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>Memphis,
TN 38120</span></b><span style='font-size:12.0pt'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:12.0pt'> <o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>Phone:</span></b><span
style='font-size:12.0pt'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>(901)
761-2016 office</span></b><span style='font-size:12.0pt'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>(901)
821-4968 direct</span></b><span style='font-size:12.0pt'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>(901)
412-8217 mobile</span></b><o:p></o:p></p>
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