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A seismic question with no (easy) answer?!

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How do engineers working on projects in earthquake prone areas
(California) handle expansion joints in existing buildings when
additions or other modifications are made to the structures and the
building code requires the existing structure to conform to current
seismic requirements for new structures?

Here is a fictitious example (but one that will be very common in the
years ahead under the IBC Code in the northeastern U.S.):  

-	A 20 year old eight story structural steel building was designed
for future vertical expansion for 4 additional floors.  

-	The building has a 1 ½? expansion joint down the middle and the
two halves are separated by double columns.

-	The 4 additional floors are now being added ? but only on the
north side of the expansion joint.  No work is being done to the
existing structure on the south side of the expansion joint.

-	This fictitious building is located in an area of low
seismicity.  When the original structure was designed the building code
required no consideration of seismic forces.  The present code is IBC
2000 and there are sizable seismic loads that must be considered because
the building is an ?essential facility? and is founded on poor soil.

-	The buildings will ?bang? during the design earthquake.  The
?banging? was not an issue prior adding the new floors, but now that the
new floors are being added, IBC 2000 requires the lateral load resisting
system in the half of the building with the new floors to be upgraded to
meet IBC 2000 requirements.  The building code does not require the
structurally independent framing on the other side of the expansion
joint to be upgraded because no work is being done to the existing
structure on that side of the expansion joint.  Even if we did upgrade
the lateral load resisting system in this half of the building, the
existing expansion joint is so small that the halves would still bang
during the design earthquake. (All floor slabs align at the expansion
joint, so there is no potential for floor slabs to ?knife through?
columns.)

The questions I have concern banging of the building halves at the
expansion joint:

1.	What do we do about the ?banging building? situation? Even if we
were to cut the slab back and put in a big expansion joint, the double
columns are close together and they?ll hit each other. In fact maybe
?banging (steel) columns? are not so bad due to the ductility of steel.
2.	The code doesn?t prohibit buildings from ?banging? ? it just
says that there can?t be any ?damaging contact?. If building segments
bang together there may be lots of small damage, but I would think that
if both buildings are of the same construction type, if all of the
floors align on either side of the expansion joint, and if the
structural framing is ductile, then banging action would not cause
collapse.  What is the definition of ?damaging contact??  If damaging
contact means damage that causes a threat to public safety, then I would
say that slab edges hitting each other are not a concern.  On the other
hand if facade components ?bang? at the expansion joint (same 1 ½? gap
at expansion joint in facade), then that would be a threat to public
safety and that condition would have to be corrected.
3.	What is done in California when renovations or additions are
made to old buildings with expansion joints and those joints are too
small to prevent contact during a big earthquake?

I heard that there is software that can actually model the ?banging
building? phenomenon.  Is this type of analysis common in California? 

Can anyone shed some light regarding practical solutions to this
problem?  As I said at the beginning of this post, the specific details
that I outlined in this post all relate to a fictitious building, but
this problem will have to be addressed in many renovation projects here
in the northeast U.S. where I live when IBC 2000 kicks in.

Thanks.

Cliff Schwinger


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