4.5 Alterations in
Evaluation and mitigation of existing GSA buildings shall
meet the requirements of ICSSC RP 6 (NISTIR 6762),
Existing Buildings and
Standards of Seismic Safety for Existing Federally Owned
and Leased Buildings with the following modifications:
Historic Structures
Evaluation of existing buildings shall be in accordance
with the provision of the ASCE Standard, Seismic
Evaluation of Existing Buildings, ASCE/SEI 31). The
Alteration requires ingenuity and imagination. It is
primary objective of the Standard is to reduce the life-
inherently unsuited to rigid sets of rules, since each
safety risk to occupants of federal buildings and to the
case is unique. It is recognized that total compliance
general public. Life-Safety is the minimum performance
with standards may not be possible in every case. Where
objective appropriate for federal buildings.
serious difficulties arise, creative solutions that achieve
Seismic rehabilitation of existing buildings shall be in
the intent of the standard are encouraged.
accordance with the provisions of Prestandard and
Where a historic structure is to be altered, special
Commentary for the Seismic Rehabilitation of Buildings
documents will be provided by GSA to help guide the
(FEMA 356). Life-Safety is the minimum acceptable
design of the alterations. The most important of these is
performance level for existing Federal buildings. FEMA
the Building Preservation Plan (BPP) which identifies
356 further provides for an extended level of performance,
zones of architectural importance, specific character-
Immediate Occupancy, where required to meet the
defining elements that should be preserved, and standards
agency's mission. ASCE/SEI 31, ASCE Standard, Seismic
to be employed. For some buildings a detailed Historic
Evaluation of Existing Buildings, and FEMA 356,
Structures Report is also available. See Chapter 1: General
Prestandard and Commentary for the Seismic
Requirements.
Rehabilitation of Buildings, provide the basis for defining
these performance objectives, evaluation criteria and if
General Design Considerations for Structural Upgrading
necessary, mitigation, are identified.
If shown by ASCE/SEI 31 evaluation that the desired
Seismic Performance. The performance objective of a
performance level is not satisfied, the rehabilitation of the
seismic upgrade is Life Safety, defined as the safeguarding
building to attain the desired performance level shall
against partial or total building collapse, obstruction of
substantially satisfy the Basis Safety Objective criteria of
entrance or egress routes and the prevention of falling
FEMA 356, including the use of both the BSE-1 and BSE-2
hazards in a design basis earthquake.
earthquake criteria.
Not all seismic deficiencies warrant remedial action.
It should be noted that the hazard level (ground motion)
Seismic upgrading is an expensive and often disruptive
used in ASCE/SEI 31 to evaluate buildings is based on
process, and it may be more cost effective to accept a
earthquakes with a 2% probability of exceedance in 50
marginally deficient building than to enforce full
compliance with current code requirements.
115
STRUCTURAL
ENGINEERING
4.5
Alterations in Existing Buildings and Historic Structures
Revised March 2005 PBS-P100
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