Dealing With The Danger On Our Doorstep
In the summer of 2016, citizens of Los Angeles got a wake-up call about the silent danger threatening their community. Thomas Jordan, Director of the Southern California Earthquake Center, reminded them that earthquake preparedness through soft-story retrofit could not wait. His research revealed strong indications that fault conditions were poised for a big one. And his remarks made headlines when he added, “these conditions could generate a large earthquake imminently.”
Want to read the full newspaper report on his comments?
Jordan’s dire warning supported actions LA Mayor Eric Gareitti enacted for earthquake preparedness a year earlier. He authorized a comprehensive review of upgrades to protect the city from catastrophic damage during a large seismic event. A highly-regarded team of engineers and safety consultants summarized their findings in the Resilience by Design report. And when Gareitti shared it with community leaders, they also called for immediate action.
One initiative from the report targeted at least 13,000 soft-story buildings for seismic upgrades. These structures house nearly 60,000 people and are scattered throughout almost every neighborhood.
Making Soft-Story Buildings Safer
LA is now requiring building owners to meet minimal soft-story retrofit design parameters. Affected owners are now receiving mandate notices. Their next step is selecting the right retrofit plan, but it’s not an easy choice. Soft-story retrofits can cost from $60,000 for minimal first-floor safety upgrades to $175,000 for improvements protecting the entire building.
Owners have significant challenges with their retrofit decision and the clock is running. First, keeping their tenants safe with a non-intrusive and durable repair. Next, justifying investment cost with expected earthquake damage. And finally, choosing a construction method that meets minimal city design expectations and preserves their long-term investment beyond a big quake.
The good news is, owners now have reliable resources to help them decide. Manufacturers have developed factory-made, prefabricated seismic upgrade devices that replace plywood and nails on these vital repairs. Using performance design criteria, structural engineers can design a retrofit plan that makes sure the building can survive a big one. And best of all, these products can be installed quickly, with minimal disruption to tenants. “Property owners can benefit from these new products,” says Randy LoFranco, a Product Sales Specialist, who serves as White Cap’s retrofit community liaison.
Is It Just A Ground Floor Problem?
The LA ordinance only requires a structural engineer to submit a retrofit design for the ground floor of a soft-story building. The reasoning is that current code only requires the building owner to minimally protect the building from severe damage (There’s no consideration of the structure’s habitability following the earthquake.) In their 2012 report Seismic Evaluation and Retrofit of Multi-Unit Wood-Frame Buildings With Weak First Stories (FEMA P-807), FEMA engineers reported for most soft-story buildings, a first-story retrofit addresses its weakest link.
But the Seismic Engineers Association of Southern California advises owners to take a broader view. Choosing a retrofit scheme that only meets the code may not be the best long-term investment strategy since it doesn’t prevent all seismic event damage.
The group is warning that upper stories might have deficiencies that would fail during a quake. And in some circumstances, a first-story retrofit could even make upper stories more vulnerable. They point out that when the first story is strengthened, the quake’s energy could be transferred, causing upper stories to experience greater forces and drifts.
Consider The Building And The Site
All of these considerations suggest that an owner’s best return on investment with a soft-story retrofit can be more accurately calculated when the entire site is reviewed – inside and out. And of course, many contractors provide engineering review as part of their bid process, but experts agree that it’s best to find an independent reviewer.
The SEAOSC addressed this topic in a presentation at the recent Los Angeles Seismic Retrofit Resource Fair. Annie Kao, an engineer from Simpson Strong-Tie, advised owners to step back and perform a thorough review of their building and its environment. And even though contractors are often able to provide a quick review of the work, they can only advise about the immediate task at hand. “It’s best to hire a structural engineer to help you analyze your needs,” said Kao.
Kao reminded owners that structural engineers are licensed professionals trained to look beyond minimal repairs that only address first floor damage from a quake.
They can also give building owners a complete assessment of their property by answering five important questions:
What are the building site’s soil conditions?
What is the site’s level of seismicity?
Are there potential hazards from neighboring structures?
What performance level must the retrofit meet?
Is the building vulnerable to other types of nonstructural damage?
A Continuous Load Path Makes Buildings Safer
The focus of a soft-story retrofit mandate is primarily the first floor, but structural engineers will advise owners to strengthen the entire frame with a “continuous load path”. This term describes a repair that ties the structure together, from the roof to the foundation. A continuous path of support helps the structure respond to the earthquake as a unit instead of as individual sections.
FEMA’s research shows that if all building components aren’t tied together, the individual sections move independently. And when sections move separately, they can pull apart, causing partial or total collapse.
Fortunately, creating this path of strength is now easier than ever before. Structural engineers can design a system using factory-made, prefabricated products like metal connectors, fasteners, shear walls and moment frames to connect this load path throughout a building.
Owners Must Decide On A Level Of Acceptable Risk
The LA mandate doesn’t require a continuous path. Its only concerned with risk of damage on the ground floor – so there’s an implied acceptance of risk when an owner only meets minimum requirements with their retrofit. But while the risk of sustaining unrepairable damage during a big quake may be acceptable to one owner, it may be totally unacceptable to many others.
Owners can choose their own level of risk by working with the structural engineer. Together, they can match the upgrade’s damage prevention level to the owner’s acceptable level of risk from seismic damage. Every risk analysis starts by defining the structure’s risk category. A building’s risk category is based on risk to human life, health and welfare from potential damage or failure of the building.
The four categories are:
Buildings and structures that are normally unoccupied, such as barns and storage sheds, and would likely result in minimal risk to the public in the event of failure.
All buildings and structures that are not classified by the other categories. This includes a majority of residential, commercial, and industrial buildings.
Buildings and structures that house a large number of people in one place, and buildings with occupants having limited ability to escape in the event of failure. Such buildings include theaters and schools.
Buildings and structures designated as essential facilities, such as hospitals and fire stations.
What Retrofit Performance Level Is Best?
In the next step of the analysis, owners select a retrofit design that meets their desired performance level. Engineers define four performance levels in terms of acceptable levels of damage to the building, its contents and occupants during an earthquake.
The four performance levels are:
Mild Quake: There’s no structural damage and the building is safe for occupants. Injuries are few and not serious plus non-structural systems needed for normal use or emergency operations are fully functional. Damage to contents is light and cost is low.
Medium-Sized Quake: There’s moderate, repairable structural damage and some delay in occupants returning. Nonstructural systems needed for normal use are fully operational, but some cleanup and repair may be needed.
Stronger Quake: There’s significant damage to structural elements, but no large falling debris is expected. It’s possible to repair structural damage, but occupants can expect significant delays in returning. Non-structural systems needed for normal use are significantly damaged and inoperable.
Severe Quake: There’s substantial structural damage and repair may not be technically possible – though all significant structural components continue to carry gravity load demands. The building isn’t safe for occupants because they could cause collapse. Non-structural systems necessary for normal use may not be functioning and emergency systems may be substantially damaged and inoperable.
Is Reviewing Three Approaches Better?
Thanks to the LA soft-story ordinance provision allowing structural engineers to choose from among three different engineering approaches, owners can make an informed choice about their levels of risk and performance. This more comprehensive review might cost more than just one design that meets minimal retrofit requirements. But evaluating the individual circumstances of each building will help owners decide on risk and performance. And there’s a good chance it will shed light on a better design that keeps construction costs low. The three-part design review might even call for more compact structural elements or prefabricated seismic materials that can create the continuous load path that protects the whole building.
Many contractors may only consider one method that shows compliance with the ordinance. So, it’s best to let the structural engineer study how the three procedures apply to the project.
Why Choose Prefabricated Frames?
While the threat of a big quake should be motivation enough to do a retrofit, it’s often tough to make the financial commitment because of economic considerations. Owners need to keep loss of rent revenue and tenant disruption down while during repairs. And another issue could be pressure on project timelines due to an impending sale requiring inspection compliance.
Fortunately, owners can rely on prefab frames for construction solutions that limit disruption to the rental income during initial installation and after a quake. They’re an effective tool for enhancing performance too.
Manufacturers can quickly fabricate frames using the engineer’s design. Most finished frames can ship to the jobsite less than three days from when drawings are received. Many custom designs can ship within a week.
Using prefab frames may eliminate some special inspection delays like eliminating the need for on-site weld inspections.
Shipping frame assemblies directly to the jobsite saves time and delivery costs.
There’s less resident disruption and no need to evacuate because of exposure to welding fumes. There’s also less concern about fire.
Is There A Variety Of Prefabricated Designs?
Since there are so many different building designs and cost-to-benefit analyses to consider for these soft-story retrofits, structural engineers need lots of options. The good news for property owners is, there’s a wide range of prefab structural products that can perform the task while increasing performance.
How Can Building Owners Reduce Liability?
By asking their engineer to include prefab frames in designs, the building owner adds a margin of assurance that field-welded frame assemblies just can’t match. Off-site fabricated frames also remove concerns of product liability associated with custom-made members. Documentation of quality assurance comes directly from manufacturer to engineer.
Owners and their insurance agents can be sure of product quality. Frames are manufactured in a production environment with comprehensive quality-control measures and field-bolted connections eliminate quality concerns about field welds. All specialty tension-controlled bolted connections are performed in the factory under third-party witnessed inspection and all field-bolted connections are snug-tight. Documentation that frames are code-listed under jurisdictions like the ICC Evaluation Service, ANSI, and the City of Los Angeles regulation RR25957 comes directly from the manufacturer.
Is A One-Shop Design Approach The Best Idea?
Manufacturers of these frames also offer a full range of seismic strengthening devices that building owners can use to enhance their building’s performance. Engineers can incorporate a product’s design parameters and details, simplifying the creation of installation drawings and drafting project specifications. This one-shop approach to design can also simplify specification of products for repairs that might be needed prior to the retrofit.
When these products are procured through a local distributor like White Cap, who carries prefab seismic retrofit products from top manufacturers, owners can be confident that installation will go according to specifications.
Do Prefabricated Frames Increase Jobsite Safety?
Prefabricated frames are 100% bolted-in-place during installation. Owners won’t be hearing complaints from tenants since these products eliminate the need for on-site welding – a leading cause of jobsite fires in wood structures. And there won’t be any unhealthy fumes or gases that can require displacement of occupants either.
Is Quake Recovery Easier With Do Prefabricated Frames?
When prefab frames are used, buildings can return to service faster after a quake. Structural engineers can help in their rapid inspection and contractors will need less time to complete repairs after a significant seismic event. And if the frame suffers deformation or damage, manufacturers can quickly reproduce an exact replacement. Some manufacturers also include a design element in their frame assembly that bears the brunt of seismic forces. This sacrificial component reduces risk of collapse by isolating the damage.
As owners begin to consider their options regarding the actions taken by some Southern California communities, it’s likely that prefab seismic upgrade products will become a popular choice for a large number of retrofit projects. And as these mandates and requirements spread across at-risk zones, their use should play a big part in helping to preserve building owners’ investments when a significant quake strikes.