No one wants their building to collapse, but constructing buildings to withstand overly-harsh loading conditions can make them so expensive that they never get built in the first place. But how can engineers decide which buildings are mission-critical, and which ones are a little less important?
“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”
– Unknown
Not every building needs to remain intact after a huge natural disaster, but critical functions do need to be maintained. The Seattle fire station in the image above needs to be able to send out trucks and coordinate responses after even the largest earthquakes, but if the small apartment building on the left side of the image has some repairable damage after “the big one”, that’s generally seen as okay.
What is a Risk Category?
In order to help define and communicate how important a given building is, engineers work together with architects and building owners to assign a Risk Category to buildings early in the design process.
According to American Society of Civil Engineers (ASCE) Standard ASCE 7-16 “Minimum Design Loads and Associated Criteria for Buildings and Other Structures”:
“Buildings and other structures shall be classified, based on the risk to human life, health, and welfare associated with their damage or failure by nature of their occupancy or use… …for the purposes of applying flood, wind, snow, earthquake, and ice provisions.”
ASCE 7-16 §1.5.1
Risk Categories range from Risk Category I up to Risk Category IV, with more important buildings getting higher category numbers. These categories then drive the magnitude of the loads used in the design process, whether directly multiplying the load’s intensity through an “Importance Factor“, or by changing the underlying drivers.
Wind load, for example, is calculated from wind speed maps provided in ASCE 7-16, or viewable for any given location on the ASCE 7 Hazard Tool. The wind speed used for a particular building depends on the historical wind speed data for the building site, as well as the risk category of the building.
- Risk Category I buildings use the mapped 3-second wind gust with a “mean recurrence interval” (MRI) of 300 years, corresponding to a 15% probability of exceedance in an assumed 50-year design life of a given building.
- Risk Category II buildings use a 700-year MRI, for a 7% probability of exceedance in 50 years
- Risk Category III buildings use a 1700-year MRI, for a 3% probability of exceedance in 50 years
- Risk Category IV buildings use a 3000-year MRI, for a 1.6% probability of exceedance in 50 years
Risk Category I – “Low Risk”
Per ASCE 7-16, Risk Category I is meant for “Buildings and other structures that represent low risk to human life in the event of failure”.
Low risk to human life is typically interpreted as structures that are seldom occupied by humans, so things like storage facilities and barns, though it should be noted that in most of the country farm buildings are not subject to traditional building codes.
Another critical note is that any outbuilding providing a critical service to a building in a higher risk category itself becomes subject to that higher risk category.
An example of a “risk category upgrade” would be a pump house for a well servicing a rural hospital. Though the pump house itself is unoccupied, it provides a critical function to the main hospital building, and would therefore be upgraded to a Risk Category III or IV structure, depending on the category of the main building.
As a practicing engineer, I’ve never designed anything that fell in Risk Category II.
Risk Category II – “The Default”
Officially defined as “All buildings and other structures except those listed in Risk Categories I, III, and IV”, Risk Category II is where the vast majority of commercial buildings fall.
Office buildings, condos, apartments, hotels and mixed-use buildings that don’t have large assembly spaces, they’re all Risk Category II.
Do note, however, that if a building has ANY large assembly space that would trigger a higher risk category, the entirety of the structure is then moved to that higher risk category.
Risk Category III – “Substantial Risk”
“Buildings and other structures, the failure of which could pose a substantial risk to human life”
Two things trigger Risk Category III:
- “…potential to cause a substantial economic impact and/or mass disruption of day-to-day civilian life in the event of failure”
- “Buildings and other structures not included in Risk Category IV… …containing toxic or explosive substances where the quantity of the material exceeds a threshold quantity established by the Authority Having Jurisdiction and is sufficient to pose a threat to the public if released”
Basically, if the structure poses a threat to a large number of people’s safety, economic status, or day-to-day civilian life, but doesn’t quite rise to the level of being a Risk Category IV structure, it lands in Risk Category III.
Historically, prior to ASCE 7-10, the standards laid out specific examples of building types, and places like primary schools with large assembly areas, universities with large lecture halls, and hotels with large conference rooms were included.
Much of that specific language has been stripped out from ASCE 7-10 and onward, but the Commentary still enumerates that theatres, lecture halls, schools, prisons, small healthcare facilities, and the like represent examples of having enough people concentrated in one place.
It goes on to mention that some critical infrastructure, such as redundant water treatment and power generation facilities may be included here, but note the key word being redundant. If a structural failure at one of these facilities will plunge thousands or tens of thousands of citizens into darkness, or leave them without potable water, it does escalate to the level of Risk Category IV.
Risk Category IV – “Essential Facilities”
Risk Category IV is the highest designation available in ASCE 7-16, and represents the most stringent design requirements. Things like wind loads based on a 3000-year mean recurrence interval 3-second wind gust.
These structures are built like tanks, but that comes at a price: they’re expensive to build. As such, this classification is reserved for the most mission-critical facilities in a community.
Think hospitals, emergency services (fire, police, ambulance), water treatment plants, and storage facilities for the nastiest things we make and use. Chemical production plants with dangerous acids and caustic gases, hazardous waste facilities, and the like.
Remember, any structure that provides a critical service to a Risk Category IV building becomes itself Risk Category IV. Hosptial central utility plants are one such example that I actually often see overlooked on this.
Summary
By assigning Risk Categories, the building code calls out higher minimum requirements for buildings with the potential to greatly impact large numbers of people. Engineers and architects can use those same Risk Category designations to help communicate the design needs throughout the entire design team, ensuring everyone is on the same page.
Keep in mind that these code definitions are always just the minimum legal requirement. Some owners, insurers, or other stakeholders may insist on upgrading the design of a building beyond the minimum requirements.
Factory Mutual (an insurer) even has its own entire design guide series, and compliance with it can help building owners save massive amounts on their insurance premiums over the life of a building, much like a safe driver discount for car insurance.
Remember to always consult an expert engineer on any building project, and engineers, remember to reach out to an expert whenever you stray from your main area of expertise.
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