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Re: Design ground accelerations

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Lynn,

It seems that, as others have pointed out, that the geotech report is
implying (or recommending) that you design this project according to a
performance base code.  As others have pointed out, it seems in line with
some of the FEMA stuff.

You can, however, likely still compare what was done using the UBC code
with these performance requirements (to a certain degree).  Keep in mind,
that while not truly a performance based code (this means that a desired
performance level is picked then the building designed according to the
desired performance level), the UBC _DOES_ have an intented performance
level.  The difference is that the code dictates required level of
performance, while a performance based code would allow the designer to
choose (with some restrictions).

Now the first step would be to equate the terminology.  The geotech report
seems to be using one set of terminology (i.e. "without suffering
significant damage" and "without collapsing") while FEMA documents use
terminology like "immediate occupancy", "operational", "life safety", or
"near collapse".  If you can get to the FEMA terminology, then from there
you should be able to equate that to the performance level that the UBC
uses (which is "life safety" if I recall correctly...the memory tend to
blur this late at night).  I would likely consider "without suffering
significant damage" either as "immediate occupancy" or "operational",
while "without collapsing" as "life safety".  Thus, it would seem to me
that the 0.8g recommendation would be the comparable to what you have done
with the UBC requirements.

If so, then you need to determine what design acceleration the UBC
provisions have "determined" for you building.  This can be done in a
couple of ways in theory.

The simplest for comparison purposes would be to determine a design
response spectra per the UBC code (see section 1631.2 and Figure 16-3).
>From this design response spectra, you can determine that seismic
acceleration that the building would be expected to see by using the
building's period.  This could be compared with the 0.8g value.  The only
slight problem with this is that you likely did not design the structure
using a design response spectra (i.e. did not do a dynamic analysis) but
rather the using the Static Force Procedure.  This could result is some
minor practical differences, but should still get you to the same basic
design level except eliminating some of the over conservative levels of
the Static Force Procedure (it is designed to be overly conservative when
compared to more "exact" analysis that take into account more actual
information about the site and building).

This leads to the second option, which I will likely get a little fuzzy on
since I am a little rusty with my seismic theory (too much living the good
life here in "non-seismic" land [and for those purests, yes, there is not
such thing as a non-seismic area...just low seismic risk]).  All sesimc
design is about taking the seismic acceleration (some value times g) times
the mass of the building to determine the lateral forces that the building
will see.  This includes the Static Force Procedure.  When you come up
with V=some number*W, this is essentially acceleration times mass (do a
little algebraic manipulation...W is mass time g, so it becomes some
value times g times the mass of the building).  Where I get a little
rusty is the effect of R in the equation.  You will need to be careful
that you compare the right "some value" times g with 0.8g (i.e. include
the effect of R or not).  As you are likely aware, the R accounts for the
level of expected inelastic response from you selected structural system.
Without including R, you would be designing the structural system for a
higher load such that the building would remain completely elastic up to
that higher load value (which means some REALLY big members and lots of
$$$ for your client).  Since we know that all structures will have some
ductility (inelastic response), we use the R value to reduce that design
force because we know that even though the earthquake load will exceed the
elastic capacity of the system, the system will still have some expected
inelastic capacity.  Thus, your problem will be which do you compare with
0.8g...the value with the effect of R or without the effect of R.  This is
were I am definitely rusty...I want to say that it would be without the
effect of R (after all, the geotech guy and the earthquake is looking at a
black box when considering the building...that is, they don't know how it
will respond to the building), but I could be wrong.  Hopefully, someone
less rusty will jump in.

This still leaves you with the problem of proving that the building will
suffer "no significant damage" for the 0.6g earthquake.  To be kind of
blunt, that is not really the code official's or the geotech's concern
(it is the owner's concern if they included it in design scope of work),
but I am not really advocating saying that to him or her.  The code's (and
code official's) role is to assure that life safety is not threatened, not
to assure minimal economic (or even personal bodily to a certain degree)
damage.  The UBC code even states this...take a look at section
1626.1...it states:

"The purpose of the earthquake provisions herein is to primarily to
safeguard against major structural failures and loss of life, not to limit
damage or maintain function."

Thus, the UBC code provisions (which are the code official's "bible") are
designed to prevent loss of life, which does not mean that some people may
unfortunately be injured (seriously or not).  It is not aimed to prevent
building owners from have serious economic loss due to damage to the
building and building contents.  This added level of assurance can be
designed for, but it is above and beyond what is required by code.

HTH,

Scott
Ypsilanti, MI


On Wed, 11 Sep 2002, Lynn wrote:

> We have received plan check corrections on a Fire
> Station we designed.  The governing Code is the 1997
> UBC.
>
> The plan checker has sited a section in the
> Geotechnical report called "seismic shaking", that says
> the building should be designed for .6g without
> suffering significant damage, and .8g without
> structural collapse.
>
> The plan checker then goes on to site the California
> Building Standards Administrative Code as requiring
> that the building be designed for the ground
> accelerations given in a Geologic Hazards Evaluation.
>
> The plan checker is asking for additional structural
> calculations showing that the building is able to
> withstand peak ground accelerations of .6g's without
> significant damage, and .8g's without structural
> collapse.
>
> What we have done is just used the 1997 UBC, taken into
> account the near source factors, and factored again for
> an essential facility.  Beyond that, I am not sure we
> are required to do anything.
>
> The language used by the plan checker seems to relate
> more to NERHP provisions and the IBC design
> methodology.
>
> Has anyone else come up against this, and if so, what
> have you done to satisfy the plan checker.
>
> Thanks
>
> Lynn
>
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