How Seismic Retrofits Protect Cities From Earthquakes

Key Concepts

• Seismic Vulnerability: The susceptibility of structures to damage from earthquakes.
• Soft Story: A building with a significantly weaker or more flexible story, typically the ground floor used for parking, making it prone to collapse during an earthquake.
• Unreinforced Masonry (URM): Buildings constructed with brick or block walls that lack steel reinforcement, making them highly vulnerable to seismic forces.
• Non-Ductile Concrete: Concrete structures with insufficient steel reinforcement, leading to brittle failure rather than ductile deformation under seismic stress.
• Retrofit: The process of strengthening or improving an existing structure to withstand seismic forces.
• Anchorage: The connection between a building's structure and its foundation, crucial for preventing movement during an earthquake.
• Mandate: A legal requirement for building owners to perform seismic retrofits.
• Public-Private Partnerships: Collaboration between government entities and private companies to achieve common goals, such as seismic preparedness.
• Life Safety Return per Dollar: A metric to evaluate the effectiveness of a retrofit in terms of lives saved relative to the cost.
• Functional Recovery: The ability of a building or business to resume operations after an earthquake.

Structural Vulnerability and Underestimated Barriers to Resilience

Despite decades of seismic research and warnings, many cities remain structurally vulnerable to earthquakes. Joe Demers, a civil engineer with 18 years of experience in structural repair and design at Alpha Structural, identifies the rarity of major earthquakes as the most underestimated barrier to action. He notes that in Los Angeles, the last significant earthquake was 30 years ago, leading people to underestimate the risk for buildings that have stood for 50-100 years. The perception is that if a building has been fine for so long, it's not an immediate priority to invest heavily in retrofitting.

This sentiment is amplified by the fact that major earthquakes often occur in less populated areas, or their impact is not widely publicized if they don't directly affect major urban centers like Los Angeles or San Francisco. While buildings in the US generally perform better than in many other countries due to better construction standards, the Northridge earthquake in 1994 highlighted critical deficiencies, with bridges and some houses collapsing. The goal is to prevent such failures altogether.

Identifying Seismic Risk: Red Flags and Quick Checks

Joe Demers outlines several "red flag" building types and quick checks for assessing seismic risk:

• Soft Stories: These are easily identifiable, often seen in apartment buildings with parking on the ground floor. The entire side of the building is open with minimal support columns, creating a weak story prone to collapse. Alpha Structural has extensive experience repairing these.
• Single-Family Residences: For smaller homes, the primary concern is adequate anchorage between the house and its foundation. While most houses built in the last 50 years have sufficient anchorage, older homes may require work. This can be quickly assessed by inspecting the crawl space.
• Unreinforced Masonry (URM): Old brick buildings are a significant risk. A key indicator of a retrofit is the presence of small, square metal plates along the top of the wall, tying it to the roof. If these are absent, the building may need retrofitting.
• Non-Ductile Concrete: These are buildings made of concrete with insufficient steel reinforcement. They are difficult to diagnose and retrofit, often affecting larger buildings.

Retrofit Strategies and Cost-Effectiveness

The most effective retrofit strategy in terms of "life safety return per dollar" is often the soft story retrofit. These can be performed while residents remain in the building, with minimal disruption to living spaces, primarily affecting the parking area. This efficiency is why cities like Los Angeles are mandating them.

However, challenges arise with other types of retrofits:

• Non-Ductile Concrete buildings are particularly difficult, requiring lengthy construction periods (six months to a year) and often necessitating building closure, leading to significant costs for businesses and residents. Los Angeles has a mandate for these, but implementation is expected to be challenging.
• Unreinforced Masonry buildings can also present challenges, especially if occupied.

Mandates, Incentives, and Owner Behavior

Mandates have significantly increased the number of seismic retrofits. Before Los Angeles's soft story mandate, voluntary retrofits were rare, as most owners were unaware of the need or unwilling to incur high costs without a clear requirement. The mandate led to a surge in retrofits and also prompted owners of buildings in other cities without mandates to voluntarily retrofit their similar properties. Other cities in LA County are now also considering or implementing mandates.

Regarding financial assistance:

• Single-family homes can benefit from programs like the Earthquake Brace and Bolt program, which provides funding for retrofits.
• For soft stories, while direct payment programs are not common, owners can often spread the cost over 10 years by slightly increasing rent, a mechanism facilitated by the mandate. This makes the financial burden more manageable.

Decision Frameworks: Homeowners vs. Commercial Portfolios

The decision-making process for retrofitting differs between individual homeowners and commercial portfolio owners:

• Individual Homes: The risk is largely considered personal. Single-family wood homes are generally flexible and light, performing well in earthquakes. The primary concern for homeowners is often the risk of items falling from shelves or cabinets rather than structural collapse. Bolting a house to its foundation is a relatively simple and effective fix for older homes.
• Apartment Buildings/Commercial Properties: The risk extends to other people's safety and property. This is why mandates are more common for larger buildings, as it's not solely up to the owner's discretion. The potential for business interruption and significant financial loss due to downtime is a major consideration for commercial portfolio owners.

Phasing and Temporary Works in Retrofit Projects

Minimizing disruption during retrofit projects is crucial:

• Single-family home bolting/bracing: This is typically done under the house with minimal impact on residents, often completed in a day or two.
• Soft story retrofits: While parking may be temporarily unavailable, work is often done during the day, with areas cleaned up overnight. Plywood can cover new foundations, allowing vehicles to drive over them at night. For larger buildings, retrofits can be phased, addressing one area at a time to minimize parking loss.

Evolving Codes and Disincentives for Voluntary Work

Building codes have evolved significantly, particularly after major earthquakes like Northridge. The deficiencies identified were not necessarily due to poor craftsmanship but rather a lack of full understanding of seismic requirements at the time of construction.

A significant disincentive for voluntary retrofits is the potential for work done prior to a mandate to not be fully credited. If a city later implements a mandate with specific requirements, previously completed voluntary work might not meet the new standard, requiring additional work. This can lead owners to wait for mandates rather than proactively improving their properties.

Involvement in Code Development and Voluntary Retrofits

While Joe Demers is not directly involved in code committees, he acknowledges the crucial role of organizations like the Structural Engineering Association of Southern California (SEAOSC) and their task forces in developing and updating seismic codes. These volunteer efforts are essential for establishing mandates and ensuring buildings are built to adequate standards.

Convincing clients to undertake voluntary seismic retrofits often involves highlighting the business continuity aspect. For businesses, the cost of prolonged downtime due to earthquake damage can far outweigh the cost of a proactive retrofit. In some cases, when performing other renovations (like building a deck), engineers will identify and recommend addressing other structural deficiencies, even if not mandated, to improve safety and property value.

Advice for Structural Engineers and Students

Joe Demers offers the following advice for aspiring and practicing structural engineers:

• Learn as much as possible early in your career.
• Embrace all extra work and opportunities offered.
• Get involved with professional organizations like SEAOSC.
• Network and meet many people in the field.
• Work hard.

He expresses his passion for his job, finding building and site inspections fascinating.

Conclusion

The conversation highlights that while seismic research has advanced, the primary obstacle to widespread structural resilience is the perception of low risk due to the rarity of major earthquakes. Soft story retrofits offer a good life safety return per dollar and are increasingly mandated. However, challenges remain with other building types, and financial incentives and clear credit for voluntary work are crucial for encouraging proactive preparedness. The evolution of building codes, driven by lessons learned from past earthquakes and the dedication of structural engineers, is vital for improving the seismic performance of our built environment.