SEISMIC SERVICES
Seismic evaluations of building structures is one the most important aspects of the structural engineering profession, providing important information to building owners, tenants, and other stakeholders, such as lenders, to make informed decisions about seismic risk and ultimately investment decisions.
Hohbach-Lewin has provided hundreds of seismic evaluations for buildings of all shapes, sizes and construction types in California, Oregon, Nevada, Washington, and Illinois. Utilizing multiple evaluation methodologies including, ASCE 41, , FEMA P-58, FEMA P-807, ASTM E2026 and Thiel-Zsutty coupled with different levels of seismic hazards, Hohbach-Lewin can provide seismic performance prediction with respect to identified seismic performance goals and devise retrofit schemes to improve a building’s seismic performance.
Using these evaluation findings, Hohbach-Lewin also computes scenario loss ratios when calculating Probable Maximum Losses (PML) from earthquakes, such as Scenario Expected Loss (SEL) and Scenario Upper Loss (SUL).
Evaluation approach
Our evaluation is typically based on the ASCE 41-17 Seismic Evaluation and Retrofit of Existing Buildings methodology. This methodology provides a comprehensive framework for assessing the seismic performance of structures and determining effective retrofit strategies.
Performance Objectives and Seismic Hazard Level
A Performance Objective consists of selecting a Seismic Hazard Level and defining the expected post-earthquake damage states. Figure 1 illustrates commonly used discrete expected post-earthquake damage states, while Table 1 outlines commonly employed Seismic Hazard Levels. The selection of Performance Objectives and Seismic Hazard Level is project-specific, taking into account the unique characteristics and requirements of each project.
Typical seismic objectives are summarized in Table 1 (below).
Seismic Hazard Level
Probability of Exceedance in 50 years
Mean Return Period (years)
“BSE-2N”
2% *
2,475 *
“BSE-1N”
10% *
475 *
“BSE-2E”
5%
975
“BSE-1E”
20%
225
Table 1 Seismic Hazard Levels
SOURCE: ASCE 41-17
*approximate
Evaluation approach
Our evaluation is typically based on the ASCE 41-17 Seismic Evaluation and Retrofit of Existing Buildings methodology. This methodology provides a comprehensive framework for assessing the seismic performance of structures and determining effective retrofit strategies.
Performance Objectives and Seismic Hazard Level
A Performance Objective consists of selecting a Seismic Hazard Level and defining the expected post-earthquake damage states. Figure 1 illustrates commonly used discrete expected post-earthquake damage states, while Table 1 outlines commonly employed Seismic Hazard Levels. The selection of Performance Objectives and Seismic Hazard Level is project-specific, taking into account the unique characteristics and requirements of each project.
ASCE 41-17 offers three tiers of analysis to evaluate the susceptibility of a structure to damage and identify the most suitable retrofit strategies.
Tier 1 Screening
The Tier 1 analysis serves as a screening phase, utilizing checklists to rapidly evaluate a building and identify potential deficiencies that can lead to significant damage during an earthquake. This screening process is particularly valuable for assessing compliance with modern detailing standards and gaining a general understanding of the building’s strengths and weaknesses. However, it does not explicitly address the performance of the structure under anticipated seismic loading.
Tier 2 Deficiency-Based Evaluation and Retrofit
In the Tier 2 analysis, anticipated seismic loading is considered. This evaluation focuses only on the deficiencies identified in the Tier 1 screening, employing simplified linear analysis methods. The primary objective of this tier is to identify buildings that do not require extensive rehabilitation.
Tier 3 Systematic Evaluation and Retrofit
The Tier 3 analysis involves a detailed evaluation of the lateral system using both linear and nonlinear provisions of ASCE 41-17. This approach offers increased accuracy and reduced conservatism compared to previous tiers. Consequently, it may determine that a building or component previously identified as deficient in Tier 1 and/or Tier 2 evaluations is actually satisfactory.
ASCE 41-17 mandates a Tier 3 evaluation for buildings meeting certain criteria, including specific number of stories and construction types. This thorough assessment ensures a comprehensive understanding of the structure’s seismic performance and facilitates the development of appropriate retrofit strategies.
By following the ASCE 41-17 methodology, we ensure a systematic approach to seismic evaluation and retrofitting, enabling us to deliver effective solutions tailored to the unique needs of each project.
Discover a comprehensive compilation of typical seismic retrofit projects in Table 2 below.
Table 2 Seismic Studies and Retrofits
Seismic Studies and Retrofits
Description
More Information
Voluntary
Building codes and guidelines broadly permit voluntary seismic improvements, provided that the improvements do not make a building more seismically vulnerable. As a result, individual occupants, building owners, and the larger community benefit from reduced seismic risk.
Recognized retrofit standards include IEBC (International Existing Building Code) Appendix A3 Prescriptive Provisions. Some localities have retrofit incentives.
CEA (California Earthquake Authority) and EBB (Earthquake Brace+Bolt) offer incentives towards a seismic retrofit to helps homeowners strengthen their home and protect against earthquake damage.
Recognized retrofit standards include IEBC (International Existing Building Code) Appendix A3 Prescriptive Provisions
Mandatory Seismic upgrade due to addition and alterations
Mandatory seismic upgrade are sometimes “triggered” due to an addition and/or alteration per the Existing Building Code
See IEBC (International Existing Building Code) section 805.3 and section 1103.2 for more information
Municipal Seismic Ordinances
Some jurisdictions have seismic ordinances that require the evaluation and/or retrofit of specific building typesthat are expected to be particularly vulnerable to seismic events. These ordinances outline minimum requirements to mitigate a specific seismic vulnerability, while not addressing all potential seismic vulnerabilities.
Existing Federally Owned and Leased Buildings
Public Law 101-6141 mandates standards for assessing and enhancing the seismic safety of existing buildings constructed for or leased by the Federal Government. Evaluation and Retrofit design are usually based Standards of Seismic Safety for Existing Federally Owned and Leased Buildings: ICSSC Recommended Practice 10, hereafter referred to as RP 10 (superseded RP 8) or Standards, is to support federal agencies in maintaining and enhancing the seismic safety of their building portfolio.
Table 2 Typical Seismic Studies and Retrofits
Typical Deficiency
Description
Ordinance
Non-Ductile Concrete Buildings
Non-ductile concrete buildings are structures constructed using concrete that lacks the ability to deform and redistribute stress during seismic events. These buildings typically have rigid frames and insufficient reinforcement, making them vulnerable to catastrophic failure under seismic forces. Due to their inability to absorb and dissipate energy, non-ductile concrete buildings are prone to sudden collapse, posing a significant risk to occupants and neighboring structures during earthquakes.
Pre-Northridge Steel Moment Frame Buildings
Pre-Northridge steel moment frame buildings refer to structures constructed before the 1994 Northridge earthquake in Southern California. These buildings were designed using outdated standards and lacked adequate seismic resilience.
Unreinforced Masonry (URM) Buildings
Unreinforced Masonry (URM) buildings are structures constructed primarily with brick, stone, or concrete blocks without the use of reinforcing steel. These buildings are vulnerable to damage and collapse during earthquakes due to their limited ability to resist lateral forces. The lack of reinforcement makes URM buildings prone to cracking, separation, and failure of masonry walls during seismic events, posing significant risks to occupants and surrounding areas.
Reinforced Concrete and Reinforced Masonry Buildings with Flexible Diaphragms
Existing reinforced concrete and reinforced masonry wall buildings with flexible diaphragms refer to structures that have been constructed using these materials and have a design feature of flexible diaphragm systems. These diaphragms, typically made of materials such as plywood or metal decking, serve to distribute lateral forces across the building. The deficiency lies in the inadequate design or implementation of the flexible diaphragm system, which compromises the building’s ability to effectively distribute and transfer lateral forces during seismic events or high winds.
Wood Soft-Story Buildings
Wood soft-story buildings are multi-story structures typically characterized by an open parking or commercial space on the ground floor, with residential units above. These buildings are constructed with wood framing and often feature a large, unobstructed opening on the ground level, such as a garage door or storefront. The “soft-story” refers to the lack of sufficient lateral strength and stiffness in the ground floor, making it susceptible to structural failure during earthquakes or strong winds.
Table 3 Seismic Ordinances
With our expertise and commitment to quality, we deliver tailored solutions that mitigate risks and enhance seismic performance. Trust Hohbach-Lewin for exceptional seismic evaluation and retrofitting services that safeguard your assets. Contact us today to partner with industry leaders in seismic engineering. We invite you to explore our projects to witness our successful implementations.
VIEW FEATURED PROJECTS
Capuchino High School
Stanford University, Crothers Memorial Hall
Historic Hoover Middle School
Marin Executive Center
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Stent Hall, Menlo School
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Phelan Building
Safeway Expansion and Renovation
