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RE: Question about seismic lateral deflection

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Cliff,
The issue is intended to prevent the building from "pounding" into the
adjacent structure during cyclic motion caused by an earthquake. With
that said, both structures need to be reviewed as each will move
independent. Each building will move independent and the drift from each
must be considered. The problems are not great where setbacks are
provided, but where one building is constructed on the lot line, it
becomes your responsibility to account for the drift from the structure
your are designing.

Personally, I believe that drift from static deflection caused by wind
loading could create a problem as well. Wind will cause a lightweight
building to deflect and gusts should create some cyclic response
increasing the possibility of pounding.

The BOCA code (which I am not familiar with) is the final word.
Regardless of what the building official will allow, you are responsible
to comply with no less than the minimum standard defined in the code.
IMO, the current UBC makes drift calculations difficult, but they
generally calculate close to the historically used deflection limits of
0.005H per story.

Dennis S. Wish, PE
California Professional Engineer
The Structuralist.Net Information Infrastructure

Website:
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-----Original Message-----
From: Cliff Schwinger [mailto:cliffws(--nospam--at)home.com]
Sent: Monday, January 21, 2002 7:25 PM
To: seaint(--nospam--at)seaint.org
Subject: Question about seismic lateral deflection

Hello. I am a structural engineer in Philadelphia, PA. and I've been
"lurking" on this list for about a year. I have been involved in an
ongoing debate with some of my peers regarding an issue related to
seismic design and I thought I would ask my question on this list to see
if any of you seismic "experts" might be able to enlighten me. (Please
pardon me if my questions are elementary as I am still on the "seismic"
learning curve.)

The BOCA Code is the governing building code for most of our projects.
The debate centers around two sentences in Section 1610.0 (Earthquake
Loads) as they pertain to a building we are designing in our office that
is in Seismic Performance Category "C".  They are:

"THE TOTAL DEFLECTION OF A BUILDING DUE TO SEISMIC DESIGN FORCES SHALL
NOT ENCROACH ON AN INTERIOR LOT LINE."

and

"ALL PORTIONS OF THE BUILDING SHALL BE DESIGNED AND CONSTRUCTED TO ACT
AS AN INTEGRAL UNIT IN RESISTING SEISMIC FORCES UNLESS SEPARATED
STRUCTURALLY BY A DISTANCE SUFFICIENT TO AVOID CONTACT CAUSING DAMAGE TO
THE STRUCTURAL SYSTEM OF THE BUILDING UNDER TOTAL DEFLECTION AS
DETERMINED BY SECTION 1610.4.5.1"

The first sentence seems to indicate that when designing a structure in
a congested urban area, we have to tell the owner that their building
has to be set back from the property line a sufficient distance so that
when the "big one" hits Philadelphia (that's another question for
another time) the building will not "lean over" the property line onto,
and perhaps into, an adjoining building.  The dilemma is this: "What
obligations do we have to set our new building back EVEN FURTHER than
our computed deflection in order to account for the adjacent 60-year old
building that was constructed right up against the property line?!  In
other words, if we estimate that our building will drift 1 foot and the
adjacent building will drift 1 foot then are we obligated to tell the
owner to design his new building 2 feet back from the property line?!
And how to we even estimate how much someone else's 60-year old building
will drift under a seismic event?

The second sentence seems to indicate that when we place an expansion
joint in new building, the joint has to be sufficiently wide so that two
independent sections of the building do not "bang" into one another
during an earthquake. This would result in a huge joint cover! (The
adjacent building segments have different periods.)  What, if any,
solutions are there to this "problem" of a big expansion joint cover?
What do you California engineers do? We are considering eliminating the
joint and just "locking" the building segments together.  It also
appears that when seismic forces are involved and you are looking at
expansion joints in buildings - sliding joints are out of the question.
With the movements involved during an earthquake, double columns appear
to be the only way to go.

Thanks for input anyone can provide.

Cliff Schwinger, P.E.

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