SEISMIC ANALYSIS OF EXISTING BUILDINGS

PSM has been providing seismic upgrade of existing buildings for over forty years. More recently, we have upgraded Essential Facilities with Enhanced Performance Objectives.

Several developments have greatly affected seismic upgrade design since we began in the 1970’s. Building codes and standards, driven by research and observation of damage from earthquakes, have specifically addressed existing building systems and historic construction materials, allowing structural engineers to analyze these components more rationally. This was accompanied by an increase in sophistication and usability of analysis software, greatly increasing the ability to review behavior of existing systems and upgrade options.

The other major change has been advances in understanding of regional seismicity throughout the country. When seismic design criteria was first introduced into Building Codes, the entire country had only 4 (later 5) Zones of seismic force level. Seismic analysis now relies on mircozonation, where seismic ground motions are determined by Zip Code.

SEISMIC UPGRADE OF EXISTING BUILDINGS

Seismic upgrade of existing facilities starts with information gathering, including client needs, available drawings and other information, material testing and analysis, and site reconnaissance. A formal seismic study is then performed, using ASCE 41. Seismic deficiencies are identified, and a path for further analysis is determined. Significant computer modeling of the existing structure is usually performed as part of this study.

Once these stages are complete, the selection and analysis of upgrade elements begins. In addition to iterative computer modeling of the selected schemes, coordination with other building elements, especially architectural and mechanical systems, is conducted. Finally, Construction Documents are completed.

Construction of a seismic upgrade project will require extensive structural engineering support, to address unforeseen field conditions, and alternates proposed by the contractor. An occupied building will present major challenges unless tenants are relocated during construction. PSM has extensive experience in seismic upgrade for occupied facilities. KIRO Television Broadcast House is an operating studio, which restricts noise and work hours. Magnuson Park Building 11, partially occupied during construction, included sensitive youth populations in therapy, and ongoing operations of recreation facilities.

HIGHER SEISMIC STANDARDS FOR ESSENTIAL FACILITIES

Building codes have long addressed the need for Essential Facilities to be designed to a higher seismic level than normal buildings, as they need to function after a major earthquake. More recently, ASCE 41 has given owners and their consultants specific tools to select Performance Levels above Life Safety. An Enhanced Performance Level for Essential Facilities will help to maintain the functioning of this type of facility after an earthquake.

Both Structural and Non-Structural Performance needs to be addressed for an Essential Facility, because of the need to remain operational. Non-structural seismic issues, including mechanical systems, ceilings, access floors, and other items, needs to be considered for an Essential Facility.