4.5 Alterations in Existing Buildings and Historic Structures
Alteration requires ingenuity and imagination. It is inherently unsuited to rigid sets of rules, since each case is unique. It is recognized that total compliance with standards may not be possible in every case. Where serious difficulties arise, creative solutions that achieve the intent of the standard are encouraged.
Where a historic structure is to be altered, special documents will be provided by GSA to help guide the design of the alterations. The most important of these is the Building Preservation Plan (BPP) which identifies zones of architectural importance, specific character-defining elements that should be preserved, and standards to be employed. For some buildings a detailed Historic Structures Report is also available. See Chapter 1: General Requirements.
General Design Considerations for Structural Upgrading
Seismic Performance. The performance objective of a seismic upgrade is life safety, defined as the safeguarding against partial or total building collapse, obstruction of entrance or egress routes and the prevention of falling hazards in a design basis earthquake.
Not all seismic deficiencies warrant remedial action. Seismic upgrading is an expensive and often disruptive process, and it may be more cost effective to accept a marginally deficient building than to enforce full compliance with current code requirements.
Evaluation and mitigation of existing GSA buildings shall meet the requirements of ICSSC RP 6 (NISTIR 6762), Standards of Seismic Safety for Existing Federally Owned or Leased Buildings. with the following modifications:
Evaluation of existing buildings shall be in accordance with the provision of the Handbook for the Seismic Evaluation of Buildings–A Prestandard (FEMA 310). The primary objective of the Prestandard is to reduce the life-safety risk to occupants of federal buildings and to the general public. Life-Safety is the minimum performance objective appropriate for federal buildings.
Seismic rehabilitation of existing buildings shall be in accordance with the provisions of Prestandard and Commentary for the Seismic Rehabilitation of Buildings (FEMA 356). Life-Safety is the minimum acceptable performance level for existing Federal buildings. FEMA 356 further provides for an extended level of performance, Immediate Occupancy, where required to meet the agency’s mission. FEMA 310, Handbook for the Seismic Evaluation of Buildings–A Prestandard, and FEMA 356, Prestandard and Commentary for the Seismic Rehabilitation of Buildings, provide the basis for defining these performance objectives, evaluation criteria and if necessary, mitigation, are identified.
If shown by FEMA 310 evaluation that the desired performance level is not satisfied, the rehabilitation of the building to attain the desired performance level shall substantially satisfy the Basis Safety Objective criteria of FEMA 356, including the use of both the BSE-1 and BSE-2 earthquake criteria.
It should be noted that the hazard level (ground motion) used in FEMA 310 to evaluate buildings is based on earthquakes with a 2% probability of exceedance in 50 years (2%/50 years). On the other hand, the hazard level used for a rehabilitation design in FEMA 356 is based on compliance with the Basic Safety Objective (BSO). The BSO requires compliance with both the BSE-2 earthquake (2%/50 years earthquake accelerations) at the Collapse Prevention Performance Level and with the BSE-1 earthquake (the lesser of the accelerations from the 10%/50 years earthquake or 2/3 of the 2%/50 years earthquake) at the Life-Safety Performance Level. The earthquake accelerations associated with the 2/3 of the 2%/50 years earthquake will result in significantly higher seismic design values than those resulting from a 10%/50 years earthquake in some areas of the country.
Upgrade Priorities. It may not be practical to upgrade an entire structure to current requirements at any one time. Whenever upgrading is only partially done, the first priority should be given to items that represent the greatest life safety risk, such as the lateral force-resisting system, unreinforced masonry bearing walls or both.
Seismic Upgrades for Historic Buildings. Historic buildings should meet the same life safety objective as other buildings. Decisions made to preserve essential historic features should not result in a lesser seismic performance than that required by ICSSC RP 6. See Chapter 1.
Seismic Strengthening Criteria for Nonstructural Elements. Where deficiencies in the attachment of elements of structures, nonstructural components and equipment pose a life safety risk, they should be prioritized and those elements with the greatest life safety risk strengthened first to meet current code requirements.