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Re: Question about seismic lateral deflection

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Cliff,

	I have a couple of points for your consideration.

  1.)	If your neighbor's building is "to the property line" when a
setback is required you appear to have an encroachment problem which may
require a legal agreement.  In Calgary this would be called a "third
party agreement" and would involve the two neighbors and the local
building authority.  It may well be that the neighbor has to bear some
of the cost of your excessive setback.

  2.)	Regarding the expansion joint, two or three ideas come to mind. It
may be suitable to provide a hinge rather than a separating joint.  You
might consider "bumpers" and allow the two sides to actually come into
contact (I don't think much of this idea; but you might not want to
dismiss it out of hand).  Perhaps, with appropriate architectural and
structural detailing, you may be able to find a way to eliminate the
need for an expansion joint altogether.  This last idea would be my
personal preference.

	I hope these thoughts are useful to you.

				Regards,

				H. Daryl Richardson

Dennis Wish wrote:
>
> 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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>
> ."The truly educated never graduate"
> -----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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