Seismic bracing design in low to moderate seismicity regions may get delegated from the design team to a construction team member, or it may get completely neglected. While significant seismic events are rare in these regions, seismic bracing of critical components in essential facilities is important to keep the facilities operational during smaller events that may cause non-structural damage.
How big of a deal are earthquakes in Ohio (or other regions of low to moderate seismicity)?
According to the Ohio Department of Natural Resources Geological Survey, some of Ohio’s faults could generate a 6.0-7.0 magnitude earthquake. This magnitude event would be similar to the recent earthquake in Amatrice, Italy or the 1994 Northridge Earthquake. They both resulted in significant casualties and property damage. But the fact that this is “possible” doesn’t really tell us much about seismic risk. It doesn’t tell you how likely it is for that quake to occur, or how violent the ground motions would be at a specific site. There is no evidence that an event of this magnitude has occurred in the last 10,000 years. On the other hand, if you visit the United States Geological Survey’s (USGS) earthquake map, you’ll see there are tiny “measurable”, if not humanly perceptible, “events” in the Ohio region on a monthly basis.
It’s a pretty good bet that your building will feel the effects of some seismic activity over its lifetime. It’s a better bet that over the geologically short time frame of a building’s life, these forces, and the risk of significant structural damage, will be small. The historical record of seismic damage in Ohio is documented and has been primarily limited to “non-structural” damage such as toppled chimneys, falling cornices, cracked plaster walls, and broken organ pipes. There have been approximately a dozen earthquakes in the past 200 years that have resulted in damage.
Why is seismic bracing important in areas of low to moderate seismicity?
Toppled chimneys, and broken organ pipes may seem quaint, but the USGS’s historical record of structural damage in Ohio only covers about 200 years. During some of the larger recorded events, much of Ohio was less populated, so less damage was reported. In a more modern urban infrastructure, similar non-structural damage could include toppled server racks and broken gas lines.
The takeaway is that in relatively recent history, Ohio has been subject to seismic events that could cause the type of non-structural damage that would impact an essential facilities ability to provide services to its community. And unlike other localized hazards, earthquakes can affect an entire region which means neighboring facilities could be effected by the same types of damage, and may not be in a position to provide relief.
There are several reasons why we seismically brace non-structural components. In areas of high seismicity, the code requires bracing all heavy components as a means to protect the occupants from falling hazards during a seismic event. In areas of low to moderate seismicity, the code requires bracing of critical components for “position retention”. We want to protect critical components from damage to themselves during a seismic event.
ASCE 7-10 generally identifies these critical components as being:
1. Components required to function for life safety purposes after an earthquake, including fire protection sprinkler systems.
2. Components containing hazardous materials.
3. Components in an essential facility that are needed for the continued operation of the facility.
Failure of a fire sprinkler system, gas lines, or loss of emergency backup power would make an essential facility unusable, even if the building structure is not damaged.
Who is responsible for designing, detailing and installing seismic bracing for non-structural components?
Where seismic bracing of nonstructural components is required, the code states that the design be shown in the construction documents and prepared by a registered design professional. This could be accomplished by:
1. Having each consultant (Mechanical, Electrical, Plumbing, Medical Equipment, etc.) provide engineered anchorage details of the non-structural components they specify,
2. Having the Engineer of Record provide anchorage details for all the consultants, or
3. Engaging a 3rd party specialty engineer to design and coordinate bracing and anchorage details.
Typically, in areas where the Authority Having Jurisdiction (AHJ) doesn’t strongly enforce seismic bracing requirements, none of the above are done, and instead:
4. The project specifications call for bracing design and detailing to be delegated to the contractor, or
5. Seismic bracing and anchorage don’t get provided at all for some or all components.
The design team may elect to delegate seismic bracing design to the contractor because they didn’t ask for or receive any fee for the work. They may choose to delegate portions of the bracing design that are standardized and prescriptive in nature. In many cases, it isn’t efficient for the design team to provide these details up front because many of the non-structural components still have to go out to bid or are designed by the installing subcontractor; anchorage details can’t be finalized until the final equipment is selected and laid out by the appropriate subcontractors and equipment suppliers.
If the bracing design and detailing isn’t provided by the design team, or adequately delegated to the contractor, then it risks not getting done at all. In regions of low to moderate seismicity, the AHJ may not enforce the requirements, and there is no urgency for the owner to make it a priority (not a perceived risk). Often there are too many systems/components to keep track of and no one entity is coordinating the efforts.
So, who is responsible for designing, detailing and installing seismic bracing for non-structural components?
There are several ways seismic anchorage of non-structural components can effectively be provided, but the best course of action is for the owner, users, design team and construction manager (CM) to strategize together. They need to identify what needs bracing, who is designing the bracing, who is installing the bracing and who is following up to make sure it gets done.
Beyond obvious critical components (emergency generators, fuel lines, switchgear, etc.), the decision to brace other components may be a function of the hospital operations. Early participation from the owner and user groups helps to identify which components are essential to the continued operation of the facility after a seismic event. Can the facility function without data? HVAC? Elevators? Nurse Call? The design team can then expand upon this list to include other supporting components.
Once the component list is established, the design team can work with the CM to determine which component bracing can and should be delegated, and which should be included as part of the construction documents. Finally, someone should be given the responsibility of coordinating the work, and making sure it gets done.
FEMA has put together sample seismic bracing responsibility matrices for various seismic design categories which can serve as a good starting point for this coordination.
Seismic bracing is an important part of facility planning for essential facilities in areas of moderate seismicity, even where risk of structural failure is low. If you have questions about how to incorporate seismic bracing in your facility’s design and construction process, include a registered structural engineer early in the discussion. He/she can help determine if bracing is required, and in some cases recommend additional geotechnical testing which may reduce or eliminate the bracing requirements. For more information on how to integrate seismic bracing into the design process, review “FEMA E-74 – Reducing the Risks of Nonstructural Earthquake Damage”.
Marshall Carman serves on the Seismic Code Advisory Subcommittee of the National Council of Structural Engineers Associations.