Expanding on Tall Buildings Safety Strategy
West Coast cities are releasing Tall Buildings Safety Strategy (TBSS) reports to guide building owners on mitigating potential earthquake damage. For example, the City of San Francisco’s TBSS report outlines recommendations for the seismic safety of new and existing high-rises. Recommendations include actions for reducing seismic risk pre- and post-earthquake, and improving the City’s understanding of its tall building stock.
While TBSS reports lay out a road map for more resilient high-rise structures, they can omit strategies that deserve discussion. We’re shedding light on additional safety measures for tall building owners to consider alongside City-issued recommendations. They apply for tall building owners in any earthquake zone.
Have questions on preparing your tall building for an earthquake?
Reach out to Principal Dan Bech.
Pre-Northridge Buildings: Compliance by Analysis
Pre-Northridge moment frame buildings are steel moment frame structures constructed prior to the 1994 Northridge earthquake. They have a reputation for brittle failures of critical beam-to-column connections often concealed behind architectural finishes. For this reason, TBSS reports may recommend that steel high-rise towers that have withstood major earthquakes be inspected for brittle failures using FEMA 352 post-earthquake inspection guidelines.
Pre-Northridge high-rises may contain 3,000-6,000 moment connections. If FEMA 352 guidelines were strictly adhered to, this would require the inspection of hundreds of connections. The cost of a full FEMA 352 inspection would likely amount to hundreds of thousands of dollars. There are many factors that contribute to the expense of these inspections which include the removal of asbestos containing fireproofing, lead paint, and the impact on occupants.
Holmes has analyzed numerous pre-Northridge steel moment frame high-rises using Performance-Based Engineering (PBE). For these assessments, we benchmarked a given building’s structural performance through an analytical model that was subjected to a recorded seismic event. Models typically did not predict any damage caused. For this reason, authorities should consider augmenting TBSS recommendations for post-earthquake inspection with an allowance for “compliance by analysis” in lieu of detailed, costly inspections. Once an analytical model of the building is prepared, it can also be used to complete a FEMA P58 seismic loss assessment or USRC building rating. These assessments provide building owners and Cities with estimates of financial loss, repair time, and collapse vulnerability within a probabilistic framework.
Investing in Egress Stairs for Public Safety
The 2011 Canterbury earthquakes in New Zealand have highlighted the risk that egress stair failure can pose to building occupants. Notable stair failures occurred in Christchurch buildings, including the Clarendon Tower and the Forsyth Barr building where egress stairs collapsed and one person was killed in the stairwell. Occupants were forced to await rescue, stranded as aftershocks rumbled through the city. Fortunately, rescue by crane was possible. Many modern high-rises are much taller and rescue by crane may not be feasible in the event of a high-magnitude earthquake.
We have observed that egress stairs in many existing high-rises are not designed to accommodate seismic displacement between floors and therefore are vulnerable to failure during a moderate or larger earthquake. Cities should consider augmenting TBSS reports to include a pre-earthquake evaluation of egress stairs within existing high-rise structures, so that a list of buildings with vulnerable stairs can be made available to first responders. Further, Cities should encourage building owners to retrofit egress stairs on a voluntary basis. This would protect public safety and the first responders that would have to navigate stairwells after an earthquake.
Sometimes earthquakes trigger building fires. From a fire and life safety standpoint, Holmes helps high-rise owners achieve conditions where it is safe to get everyone evacuated, protect the property, and provide a reasonably safe platform for firefighters to operate from. Every occupied floor must provide at least two exit paths out of the building, often involving egress stairs. In high-rise buildings, we recommend pressurized egress stairs that are enclosed to prevent smoke spread across levels. Holmes also consults building owners on safety plan development, taking into account egress paths for occupants with disabilities.
Instrumentation Speeds Post-Earthquake Recovery
Seismic monitoring measures building displacements during an earthquake through accelerometers installed at strategic locations within a building. Lateral displacements caused by seismic events are the primary cause for structural damage. Data gathered from these instruments can be used with analysis models to evaluate a building’s likely performance and locations of potential damage. With this information, post-earthquake inspectors can more efficiently target and prioritize inspection locations for tall buildings, ultimately speeding recovery efforts.
Questions regarding structural integrity can significantly impede speed of recovery. Many can be answered through the results of seismic monitoring. Owners of tall buildings should consider installing seismic monitoring equipment to speed the post-earthquake recovery of their buildings and reduce impact on tenants. Some cities only require the instrumentation of newly constructed high-rise buildings through administrative bulletins; we recommend cities require instrumentation of existing high-rises, too. Some precedent exists here as the City of Los Angeles requires the instrumentation of select existing high-rise buildings in LA Building Code.
Narrative of being stuck when egress stairs drop out during an earthquake.
Holmes won an award for retrofitting 40-story, pre-Northridge egress stairs.
We know we’re onto something when the engineering becomes a thing of beauty.