Pond Ingenuity Winter 2021

A recently completed retrofit of Hangar 3052 at Joint Base Lewis-McChord demonstrates how to utilize the Seismic Evaluation and Retrofit of Existing Buildings Standard to make facilities better able to withstand earthquake forces. As the federal government continues to make a push for existing buildings to be more resilient and able to withstand increased seismic and wind loads, enhancements to structures are becoming required as part of renovations. Knowing when these evaluations and enhanced retrofits are required is important to identify early on in a project’s scope of work. A recent evaluation and retrofit of Hangar 3052 at Joint-Base Lewis-McChord, Wash., illustrates that need to determine scope early. By following ASCE Standard 41-17, Seismic Evaluation and Retrofit of Existing Buildings, and applying results from evaluations, the design team was able to provide cost- effective retrofits that worked with the existing structural elements and brought the facility into compliance with the required seismic codes. Washington State is a high-risk location for seismic activity, with the second highest risk in the United States of large and damaging earthquakes. A total of three large fault lines run through a state that is home to the Cascadia subduction zone, which has the potential to produce some of the largest and most destructive earthquakes in the world. While seismic retrofitting can be crucial anywhere, it is especially important at Joint Base Lewis-McChord. DETERMINING NEEDS Existing federally owned buildings must follow design guidelines within UFC 3-301-01, which requires the use of Recommended Practice 8 for determining appropriate seismic evaluation and retrofit efforts. An ASCE 41 evaluation is required if anticipated renovation costs exceed 50 percent of the plant replacement value for Seismic Design Category C—or if anticipated renovation costs exceed 30 percent of the plant replacement value for Seismic Design Categories D, E, and F. Exemptions to these requirements are defined in Recommended Practice 8 and most commonly apply when the building is in a low seismic area and immediate occupancy following an earthquake event is not needed. The building risk category must be determined based on the function of the building and the seismic design category is based on geographical location and soils classification. A more critical building in a high seismic area is likely to require an ASCE evaluation than a less critical building in a low seismic area, as the requirement for risk mitigation is greater. By extending the life of the building through renovations, it becomes critical to reduce the risk of damage to the building structure from seismic events. For all existing buildings requiring an ASCE 41 evaluation, the basic performance objectives are the minimum required goals. More stringent performance objectives may be required or elected to be used for some buildings. Performance objectives, also referred to as targeted building performance levels, are intended to determine the structural damage expected when the building experiences an earthquake of a specified severity. The least damage will occur when the Operational Performance Level is selected, which requires the building to remain fully operational immediately after an earthquake event. The most severe damage will occur when the Collapse Prevention Performance Level is selected, which requires the building be able to be safely evacuated after an earthquake event but cannot be re-occupied until extensive repairs are performed. The cost and effort associated with the least amount of damage allowed is greater than that required when more severe damage is allowed. CONDUCTING THE EVALUATION There are two parts to the ASCE 41 process for an existing building: evaluation and retrofit. Evaluation determines deficiencies in the existing structure that prevent it from achieving selected performance objectives. Retrofit alters or strengthens existing systems or elements to bring it into compliance with performance objectives. The evaluation process for an existing building is organized into three tiers. Tier 1 is required for all buildings requiring seismic evaluation. Tier 2 applies when deficiencies are identified in the Tier 1 evaluation and the building is classified as a common building type. And Tier 3 is used when deficiencies are identified in the Tier 1 evaluation and the building is not classified as a common building type. “Common building types” are defined in ASCE 41. The building must also conform to the height limitations based on level of seismicity to qualify for a Tier 1 or 2 evaluation. The reasoning is that buildings in one of these common types have similar behavioral characteristics of the lateral-force resisting system in a design seismic event. At Hangar 3052, the primary lateral force-resisting Undertaking Seismic Evaluation & Retrofit Published in The Military Engineer Magazine. 12 Pond | www.pondco.com