[CEUS-earthquake-hazards] The balance

[Wang, Zhenming]

Hi Joe,

The return period defined in the national seismic hazard mapping is not comparable to the mean recurrence intervals of flood, wind, and other natural hazards. This may be one of the reasons that the users have difficulty to select design ground motions. Attached are comments on the national seismic hazard maps (2007 update).

Thanks.

Zhenming

________________________________
From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov [mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf Of Joe Tomasello
Sent: Saturday, January 19, 2008 3:17 PM
To: afrankel at usgs.gov; ceus-earthquake-hazards at geohazards.cr.usgs.gov; 'Seth Stein'
Subject: [CEUS-earthquake-hazards] The balance

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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