Chabot College Seismic Safety Report
Grand Court Arcade Structure
Facilities Committee Presentation
April 23, 2015
Presenters:
Russell Berkowitz – Forell Elsesser structural engineers
Ernie Yamane – Steinberg Architects
The grand court colonnade is a major feature of the Chabot campus
Aesthetics
Shade / Shelter
Connector of inner buildings
Options moving forward include:
1. Do nothing
2. Remove structure
3. Renovate/modify existing structure
4. Replace structure
The strengthening of the arcade is not currently mandated by DSA, i.e. there are no
triggers. DSA triggers include:
Cost of remodeling structure exceeds half of the replacement value
Weakening of the existing structure through modifications
Adding load to the existing structure
It should be noted that none of the seismic strengthening done at Chabot have been
mandated. It has all been done on a voluntary basis as the “right thing” to do to protect
our campus community in the event of an earthquake.
Seismic evaluations are done by using two seismic event standards:
BSE-R expected to occur once every 225 years, or a 20% chance of occurring every 50
years. Magnitude M6.2 to M7.0
BSE-C expected to occur once every 975 years, or a 5% chance of occurring every 50
years. Magnitude M6.8 to M7.3.
In the standard arcade design the weakest points are at the base of the columns and at
the connection of the columns to the beams. These areas will not flex enough in an
earthquake.
In a BSE-R event these connections would be overstressed by a Demand-Capacity
Ratio (DCR) of 1.1 to 1.8 or 80% overstressed. In this event it is expected that the
arcade would suffer major damage and be non-repairable.
In a BSE-C event the column connections would be overstressed by a DCR of 1.5 to 3.4
or by over triple. In this earthquake event there would be high likelihood of a loss of
stability and potential failure of the structure. Said another way, if a Loma Prieta sized
earthquake happened along the Hayward Fault the grand court colonnade has a high
likelihood of collapse.
Other sections of the grand court canopy system were examined.
The B1300 PAC entrance canopy is a structurally detached and independent system.
The “clamshell” arched entrance cover is anchored by two huge concrete walls and is
OK as is. 6-8 columns on either side of the arched canopy would need to be wrapped
in a high tensile fabric adding roughly ½” to the column size.
The Fire Truck Gateway between buildings 800 and 900 is believed built in early 2000
and is probably adequate if built after 1976 when the building code changed.
The two-story columns supporting the roofs of two-story buildings with connections to
the canopy would not behave well in an earthquake. The DCR would be 2.5 to 3.5.
The solution will probably be to sever the arcade from the columns.
The exterior ramps to B100 are required for exiting from the second floor library. The
ramps are structurally separate from the arcade and can probably be adequately
strengthened with a tensile fiber wrap.
The walkway connections from B300 and B2300 to the B100 second floor are heavy
because of the walkway component and may require strengthening.
It was noted that tensile fiber wrap would not be a workable solution for most of the
arcade because of its unrestrained nature.
If weight was removed from the colonnade (i.e. remove the precast concrete double
Tees and roof) there would probably no longer be a stability issue. Possibly this could
be replaced with a lighter weight cover. Any removal of the arcade roof would need to
address lighting replacement.
Input from the committee included:
Before and after theater performances, the covered walkway is used by large crowds
The covered walkway helps keep our building entries dryer
Replacing the roof with a translucent material could help light darker areas like B200
We would not like to lose the cover
The next step will be to research solutions and develop associated costs.