[CEUS-earthquake-hazards] no "right" answer

[Joe Tomasello]

Klaus et al:

This would not be the case in Memphis, for that matter, West Tennessee or
East Arkansas. There are two basic reasons for this:

1) the seismic hazard is generally mapped as being much higher than in New
York with wind loads are smaller.
2) Most sites in the New Madrid Seismic Zone are significantly less than
Class B. Ranging from Site Class C to F; the most prevalent site class is
Class D.

Parts of Kentucky, Missouri, and Southern Illinois may be a little different
having less cover over load bearing rock. My experience has been that the
overburden ranges from 10 to 40 feet in parts near the Mississippi River.  

I almost sure you couldn't have your peculiar situation - I'll look into it
sometime this week. It's my recollection that if earthquake controls the
building along one axes then it is to be analyzed with seismic forces
applied "separately and independently in each of two orthogonal directions"
(Seismic Design Category B structure was assumed).  


Joseph Tomasello, PE
5880 Ridge Bend Rd.
Memphis, TN 38120
 
Phone:
(901) 761-2016 office
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-----Original Message-----
From: jacob at ldeo.columbia.edu [mailto:jacob at ldeo.columbia.edu] 
Sent: Saturday, February 16, 2008 9:55 AM
To: Joe Tomasello
Cc: 'Wang, Zhenming'; ceus-earthquake-hazards at geohazards.usgs.gov
Subject: Re: [CEUS-earthquake-hazards] no "right" answer

Joe, Zhenming, all:

Re wind vs seismic:

When NYC discussed seismic codes in the 90s, several studies were published
by
then NCEER (later MCEER) that addressed this comparative issue. For regular
buildings the cross-over from seismic to wind load dominance occurred (for
NYC
code winds = 100mph) generally for heights between 10 and 20 stories, where
below 10 stories generally seismic loads controlled the design, above 20
stories wind generally controlled, and between 10 and 20 stories it depends
on
the details of shape and materials (i.e. mass) of the buildings.

For highly irregular buildings, because of torsional loads, seismic
dominates
over wind more prominently.

There can exist peculiar situations: if you have a tall UN-HQ-curtain-like
building, tall, and narrow in one direction, and long in the other: wind
loads
control design along the "short axis" of the building, seismic along the
"long
axis", since the wind loads depend on the surface area availble, while
seismic
depends on the inertial forces, and hence the mass of the building as seen
along a given axis.

Klaus Jacob
=====================
Quoting Joe Tomasello <JT at reavesfirm.com>:

> The demarcation line of where wind load controls the design is dependent
on
> the seismic load and the mass (weight) of the building. Wood buildings
> without masonry veneers that fall into design categories A,B, and
sometimes
> C (see tables 9.4.2.1a and b ASCE 7) Most all other types of buildings,
> e.g., concrete, steel frame etc wind will normally control only in Design
> Category A. As a rule of thumb, I start to look for seismic to control the
> building design Ss approaches 0.20g or as Sd approaches 0.10g.  The
> residential code may be a bit different; however our experience has shown
> that the R-IBC generally follows the regular IBC code.
>
>
>
> Bear in mind that there are prescriptive requirements for buildings in all
> design categories A-F.  However in design categories A and B these are
> usually minimal and are systemic in any good structural design.
>
>
>
> As an example of how far reaching this code is, normally you would think
of
> Nashville as being "seismically quiet."  However we did an 8 story
concrete
> shearwall hotel built in 06-07 designed under the IBC 2003. It fell in
> Design Category B, Sds = 0.218, Sd1=0.098. Base shear for seismic was more
> than twice that of wind. The net effect was to add considerable cost to
the
> building above that would have been required by the Standard Building Code
> even though it is likely that seismic would have controlled - the forces
> would have approached those of wind and the wind loads would have been
> lower.
>
>
>
> Joseph Tomasello, PE




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