Pond Ingenuity Winter 2021

„ Create an analytical 3D model of the existing structure based on as-built structural drawings and field investigations. „ Account for non-structural components not included in the analytical model by adding point masses where they connect to the building lateral-force resisting system. (At Hangar 3052, a RISA model was created for the main framing systems in the primary and secondary directions, which included all components of the lateral-force resisting system.) „ Determine existing material strengths. If existing material strengths are not available, lower- bound material strengths are permitted based on common material types used in the specific construction era. (At Hangar 3052, existing material strengths were not available for all components, so common material strengths used in the construction era were used.) „ For each component of the lateral- force resisting system, develop and classify component actions; components are classifiedas force- or deformation-controlled, based on the expected failure mode. (The analysis for Hangar 3052 was organized by component types of truss members and their connections, columns and their base connections, chords and collectors and their connections, transfer ties, horizontal roof braces and their connections, vertical braces and their connections, and diaphragms.) „ Compare force- and deformation- controlled actions to acceptance criteria to determine compliance; if the component meets the acceptance criteria, no retrofit is required. Non-structural components also are required to be evaluated as part of the ASCE 41 process. Only verification of compliance with the Tier 1 checklists applies for these items. At Lewis- McChord, deficiencies in some of these items were noted, such as the lack of anchorage of concrete masonry unit partition walls. COMPLETING THE RETROFIT For some existing buildings, the evaluation process alone may determine that the building is in full compliance, and that no retrofit measures are required. When a retrofit is required, however, it may include, as described in ASCE 41, “stiffening or strengthening the structure, adding local components to eliminate irregularities or tie the structure together, reduce the seismic demand on the structure through the use of seismic isolation or energy dissipation devices, and reducing the height or mass of the structure.” At Hangar 3052, a combination of retrofit measures met the required building performance objectives. Horizontal roof braces were replaced in their entirety; existing concrete piers were enlarged to allow for new anchor bolts; and roof girders had stiffener plates added. Non-structural members such as partition walls were provided new brace points along their height. Any retrofit should be detailed within complete construction documents. SCOPING PROJECTS EARLY Identifying early what Seismic Design Category and Risk Category a building will fall under is important to understanding what seismic upgrade triggers will be required. For higher Seismic Design Categories C and D, many renovation projects will trigger an ASCE 41 evaluation at a minimum. For Seismic Design Category D, many buildings will require seismic retrofits in addition to the evaluation. As resiliency of buildings continues to be a push on government projects, incorporating these critical requirements will lead to both project and mission success. Betsi Whiteman, PE, Structural Engineer Eric Adami, PMP, PE Senior Project Manager system consists of long-span steel trusses, and the secondary lateral force-resisting system consists of concentrically braced frames, which was not a common building type. The Tier 3 evaluation process includes several steps and requirements. „ Develop site-specific response criteria that are used to determine seismic forces in each of the two orthogonal directions. (At Hangar 3052, a linear dynamic analysis was performed to determine these criteria.) 3 TIERS OF EVALUATIONS The Tier 1 evaluation process requires field investigation, aided by the structural engineer’s review of existing building structural drawings, to identify deficiencies based on provided checklists in ASCE 41. These checklists look at the adequacy of existing building components to determine if they are compliant, noncompliant, or unknown in regard to the specific requirement listed. The Tier 2 evaluation only must look at the components identified as noncompliant or unknown in the Tier 1 evaluation. Each of these must be shown to meet the acceptance criteria described in the Tier 3 process. This Tier 2 process may require a full building analysis if component forces and deformations cannot be determined by other methodology. The Tier 3 evaluation process requires a complete building system analysis to determine that all members in the structural lateral- force resisting system are adequate. Pond Ingenuity | Winter 2021 13