Risk Category – What is it and why is it important in the design of my building?Aug 09 2017 · 0 comments · Nishkian Chamberlain, Technical notes ·0
Building codes require that buildings be classified based on the risk to human life, health, and welfare associated with their damage or failure. Minimum design loads, maximum allowable story drift criteria, and lateral force resisting system limitations are derived based on this classification. Building codes in the U.S. generally reference the ASCE 7 provisions for appropriate building classification criteria.
The idea of designing different types of buildings to different seismic force levels based on their “risk” is not new. The Building Code utilized increased Importance Factors for schools and hospitals for many years to provide a greater degree of resilience in certain structures. In the early 2000’s the first edition of ASCE 7 utilized the term “Occupancy Category” to define a buildings classification. However, the term “occupancy” is primarily used with fire/life safety issues and only implicitly defined risks associated with structural failure of a building. Consequently, the 2010 version of ASCE 7-10, introduced the term “Risk Category” in lieu of “Occupancy Category” to distinguish between the two considerations. Per commentary section C1.5.1 in the ASCE 7-10:
“The Risk Categories in Table 1.5-1 are used to relate the criteria for maximum environmental loads or distortions specified in the ASCE 7 to the consequence of the loads being exceeded for the structure and its occupants.”
Table 1.5-1 the ASCE 7 defines four distinct Risk Categories:
Risk Category I
Structures that are normally unoccupied and would result in negligible risk to the public should they fail. These include structures such as barns and storage shelters.
Risk Category II
This category contains all buildings and structures not specifically classified as conforming to another category. The majority of structures such as residential, commercial, and industrial buildings are included in this category.
Risk Category III
This category includes buildings and structures that could pose a substantial risk to human life in case of damage or failure. Structures under this category include:
- Buildings that house a large number of persons in one place such as theaters, lecture halls, and dining halls.
- Buildings where persons have limited mobility or ability to escape to a safe haven in the event of failure such as grade schools, prisons, and small healthcare facilities.
- Buildings associated with utilities required to protect the health and safety of a community. Such as power-generating stations, water treatment, and sewage treatment plants.
- Buildings housing hazardous substances, such as explosives or toxins which if released in quantities determined by the authority having jurisdiction could endanger the surrounding community.
- Buildings and structures not included under Risk Category IV but have a potential to cause a substantial economic impact and/or mass disruption of day-to-day civilian life.
Careful assessment of the Risk Category for a new project is required prior to design. Minimum design loads for snow, ice, and seismic considerations are greatly influenced by the importance factors defined in Table 1.5-2 of the ASCE 7 for different Risk Categories:
Additionally, buildings located in regions with high seismicity are particularly sensitive to Risk Category classifications. Per Chapters 11 and 12 of the ASCE 7 Risk Category selection has major impacts on:
- The minimum design loads as seismic loads must be amplified by the Importance Factor defined in Table 1.5-2.
- The Seismic Design Category of a building which is used to place limits on the lateral force resisting systems allowed for design. (ASCE 7 Table 12.2-1)
- The maximum allowable story drift per Table 12.12-1.
For this reason it is important to note that changing a buildings occupancy can result in significant changes to gravity (in snowy/icy regions) and lateral designs. Careful consideration must be given to projects involving existing structures whose occupancy change triggers a bump from a lower Risk Category level to a higher one. The existing lateral and gravity systems may require retrofits to accommodate stricter structural system limitations, increased load demands and stricter allowable drift criteria.
In addition to ASCE 7, individual states have further defined and clarified Risk Categories for different buildings and each state’s Building Code should be considered and referenced when determining a buildings Risk Category. It is also helpful to work with a design professional such as an Architect when determining number of occupants in complex buildings made up of multiple occupancies and where total number of occupants may require different Risk Categories. In fact different Risk Categories can be specified within the same building structure in special conditions.
The Nishkian team has years of experience with thousands of projects across all Risk Category types. Should you have any questions on an upcoming or current project, please do not hesitate to contact any of our offices.