[CEUS-earthquake-hazards] The balance

[Joe Tomasello]

Art:

 

I'm sorry for delaying my response, my time isn't always mine. 

 

Your evaluation is essentially correct, my previous evaluation was an overly
simplified evaluation (equation 16.56) and didn't properly reflect the
topic. 

 

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.  

 

Recurrence periods for performance levels of natural hazards except for
earthquake are 500 years or less (ASCE - 7.02):

 

Local Flooding                          100-year

Regional Flooding                    500-year

Snow Loading                            50-year

Wind Loads                             500-year (previously 100-years)

Rain Loads                                10-year

Ice Loads                                   50-year

 

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. 

 

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.

 

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.

 

Looking at a cross section of buildings across the county we found the
following in Shelby County:

 

Table 1

03IBC(Mem.)/99SBC

Shelby County             2-story             5-story             10-story

Southeast                     0.90                 0.77                 0.61

Middle                         0.94                 0.70
0.56

Northwest                    0.94                 0.75                 0.60

 

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.

 

Table 2

03IBC/99SBC

Shelby County             2-story             5-story             10-story

Southeast                     1.73                 2.47                 1.96

Middle                         1.87                 2.94
2.33

Northwest                    2.91                 4.77                 3.79

 

 

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. 

 

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. 

 

Considering the above whys is USGS opposed to individual communities to
selecting their own seismic risk? 

 

 

Joseph Tomasello, PE

5880 Ridge Bend Rd.

Memphis, TN 38120

 

Phone:

(901) 761-2016 office

(901) 821-4968 direct

(901) 412-8217 mobile

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