ASCE 7 Risk Category Background
Building codes require that buildings be classified by their level of importance in determining the risk taken with safety factors against their failure under critical design loads. Calculations for the structure’s overall stability, flexure, and fatigue are all based on the assumption of a given risk category design level. ASCE 7 references risk category factors typically referenced in building codes (table 1.5.1 ASCE 7-10, ASCE 7-16, ASCE7-22).
ASCE 7 officially defines risk category as:
“A categorization of buildings and other structures for determination of flood, wind, snow, ice, and earthquake loads based on the risk associated with unacceptable performance.”
The Lamens theory regarding risk categories is that a structure with a risk category I should theoretically fail before a II, III, or IV. Morally, a risk category I structure should not cause undue damage to its neighboring structures, but barring that moral issue, they should fail before the neighboring habitable & important structures. An example of structures designable as category I structures would be a freestanding sign which could ‘list’ or break over and technically fail but cause no damage to neighboring buildings. Other examples of risk category I structures include the screen and patio-sunroom enclosures, tool sheds, greenhouses, etc. A more complete list and description are below from IBC/FBC building codes:
Risk Categories are used to find an appropriate design wind velocity for determining corresponding design pressures to design structures and building components. The process is RISK CATEGORY > WIND VELOCITY > WIND PRESSURES > FORCES > DESIGN.
To see wind speeds for your area based on respective risk categories, click here for our article on the ASCE & ATC wind council tools.
Also see our article on Wind Speed vs. Wind Pressure for more information on that topic.
ASCE 7 TABLE 1604.5
RISK CATEGORY OF BUILDINGS AND OTHER STRUCTURES
| RISK CATEGORY | NATURE OF OCCUPANCY |
| I | Buildings and other structures that represent a low hazard to human life in the event of failure, including but not limited to:
• Agricultural facilities.
• Certain temporary facilities. • Minor storage facilities.
|
| II | Buildings and other structures except those listed in Risk Categories I, III, and IV. |
| III | Buildings and other structures that represent a substantial hazard to human life in the event of failure, including but not limited to:
• Buildings and other structures whose primary occupancy is public assembly with an occupant load greater than 300.
• Buildings and other structures containing Group E occupancies with an occupant load greater than 250.
• Buildings and other structures containing educational occupancies for students above the 12th grade with an occupant load greater than 500.
• Group I-2 occupancies with an occupant load of 50 or more resident care recipients but not having surgery or emergency treatment facilities.
• Group I-3 occupancies.
• Any other occupancy with an occupant load greater than 5,000.a
• Power-generating stations, water treatment facilities for potable water, wastewater treatment facilities, and other
public utility facilities are not included in Risk Category IV.
• Buildings and other structures not included in Risk Category IV containing quantities of toxic or explosive materials that:
Exceed maximum allowable quantities per control area as given in Table 307.1(1) or 307.1(2) or per
outdoor control area in accordance with the California Fire Code; and Are sufficient to pose a threat to the public if released.b
|
| IV | Buildings and other structures designated as essential facilities, including but not limited to:
• Group I-2 occupancies having surgery or emergency treatment facilities.
• Fire, rescue, ambulance, and police stations and emergency vehicle garages.
• Designated earthquake, hurricane, or other emergency shelters.
• Designated emergency preparedness, communications, operations centers, and other facilities required for emergency response.
• Power-generating stations and other public utility facilities are required as emergency backup facilities for Risk Category IV structures.
• Buildings and other structures containing quantities of highly toxic materials that:
Exceed maximum allowable quantities per control area as given in Table 307.1(2) or per outdoor control area in accordance with the California Fire Code; and
Are sufficient to pose a threat to the public if released.b • Aviation control towers, air traffic control centers, and emergency aircraft hangars.
• Buildings and other structures have critical national defense functions.
• Water storage facilities and pump structures are required to maintain water pressure for fire suppression.
|
- For purposes of occupant load calculation, occupancies required by Table 1004.1.2 to use gross floor area calculations shall be permitted to use net floor areas to determine the total occupant load.
- Where approved by the building official, the classification of buildings and other structures as Risk Category III or IV based on their quantities of toxic, highly toxic or explosive materials is permitted to be reduced to Risk Category II, provided it can be demonstrated by a hazard assessment in accordance with Section 1.5.3 of ASCE 7 that a release of the toxic, highly toxic or explosive materials is not sufficient to pose a threat to the public.
Quick Reference Guide to Wind Velocities & Risk Categories (Vult, MPH) in South Florida
(Table also refers to any area with corresponding wind velocities, shown as an example only of how wind velocity changes with risk category.)
| Risk Category III | Risk Category II | Risk Category I | Location Example |
| 175 | 160 | 145 | Ft. Myers, FL |
| 180 | 170 | 150 | Palm Beach/Broward, FL |
| 186 | 175 | 165 | Miami-Dade, FL |
| 200 | 180 | 175 | Monroe County, FL |
The formula to convert Vultimate to Vasd (allowable stress) is Vasd= sqrt (0.6)* Vult
Vultimate, or Vult, is the actual upper limit of force. It is used with factored loads to provide a safety factor. Vasd, or Vallowable, is the already factored design load used with allowable loads. Consult a design professional before using load cases, wind pressures, and any information herein before permitting & construction.
Last Update: February 22, 2024