# Seismic Joint Size

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: Seismic Joint Size
• From: Peter Griem <griem(--nospam--at)slamcoll.com>
• Date: Thu, 28 Mar 2002 12:44:14 -0500
```I interpret the building codes to read that the seismic joint between two
buildings must be sized by:
1. taking the elastic deflection calculated for each building (say by RAM,
2. multiplying that value for each building by the appropriate Cd factor of
the lateral system (to account for additional plastic deflection)
3. adding those amplified deflection values for each building together (so
that if the buildings move toward each other in an earthquake they don't
crash into each other)

First question - do you agree that that is the way the code READS?  Second
question - do you agree that is the INTENT of the code?  Third question - Do
you size seismic joints this way?

Here's the problem - take a building categorized in a lower Seismic Hazard
Exposure Group, e.g. SHEG I in the BOCA or IBC.  The allowable story drift
(inc. Cd) = .020hsx.  For a four story building at 12' per story, the
allowable deflection at the 3rd floor would be (3)x12'x12"/'x.020=8.6in.
Say the design shows that two adjacent MF bldg are limited to 75% of that
value - you need a seismic joint between them of .75x8.6x2= 13 in.  A 13" jt
at the third story of a building.  Try to explain to the architect -
especially in a low/moderate seismic zone.

How do you or others you know treat this issue.  I would especially like to
know how this is treated on the west coast.  What is the largest
manufactured seismic joint that can be found to bridge a corridor?

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