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Re: Question about seismic lateral deflection

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Cliff,

Without delving into the probabilities of the "big one" hitting PA, this is
a common issue in EQ country.  Property lines are property lines, this is a
legal issue more than anything. Buildings must not project over a property
line without a legal easement.  The UBC does not have the specific language
you refer too but the intent is similar.

The way that this is enforced for seismic separation is typically as
follows:  There is an identified problem of existing structures on common
lot lines being too close together.  As existing structure is replaced with
a new structure, the new structure is required to be set back from the
property line by the required seismic deflection of the new structure only.
It is not necessary for you to also accommodate the adjacent existing
buildings deflection as well.  If this was required the city would be
granting an easement to the adjacent building.  The concept is one of phased
compliance, eventually as all the buildings are replaced the required
separations will be in place.  Until then, the structures will probably hit
in a seismic event, but the building code does not have the power to require
either mandatory retrofit of the existing structures away from the property
line or too penalize the owner who upgrades first by requiring the granting
of easements.

The issue of seismic separation is intended for portions of the structure
that due to geometry or lateral force resisting system differences will not
respond and act together.  Picture a one story shearwall building with a two
story moment frame entry Porte cochere.  The two structures will not respond
in phase with one another therefore the interface between the two portions
must have a seismic separation.   There are several commercially available
rated joint systems that can accommodate floor, wall, and roof separations.
There is no such thing as a "no damage" structure under typical economic
concerns.  However there is controlled damage designed to be easily repaired
in specific areas.  The important issue for seismic separation is that the
LFRS be allowed to perform as intended, and the non-LFRS structural members
accommodate the required movement without danger of failure or threat to
life safety.  Most of the Architects are familiar with the problem and have
preferred joints to address water issues and other architectural concerns.

Based on what you indicate (different periods for the two portions of the
structure) each would have an independent lateral system and a suitable
joint separation.


Paul Feather

----- Original Message -----
From: "Cliff Schwinger" <cliffws(--nospam--at)home.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, January 21, 2002 7:24 PM
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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