ASCE Risk Categories Explained

ASCE Risk Categories Explained

 

Risk 1

Storage Sheds, Temporary Structures

Low Occupancy,

Low Risk Structures

Risk 2

Homes, Businesses,

(Most Structures) Standard Occupancy

Risk 3

Schools, Gathering Places, Public
Utilities,

High Occupancy

 

Risk 4

Hospitals,

Emergency Utilities,

Sensitive Occupancy

High Risk Structures

Building codes require that every designed structure be classified by its level of importance against its possible failure under extreme conditions. In this way, important structures that experience high or sensitive occupancy (such as schools and hospitals), are more conservatively designed than structures that put human life less at-risk (such as storage sheds and businesses).

The ASCE 7-16 and IBC (2018) use the concept of “Risk Categories” to determine acceptable design criteria and factors of safety when calculating Wind, Snow, Flood, Ice, and Seismic forces that structure may experience throughout its lifespan. This loosely means that a structure with a risk category of I should theoretically fail before a II, III, or IV. Morally, structures with lower risk categories should not cause undue damage to their neighboring structures, but they should otherwise fail before any nearby habitable & important structures.

ASCE & IBC Risk Category Definitions
Below is table 1604.5 from the International Building Code (2018) which provides the general guidelines of which structures should be
considered for each risk category. It is important to note that although other codes & standards may have similar definitions or reference back to this table, it is always up to the discretion of the designing engineer & building department official to determine and agree upon the design criteria and risk categorization of a structure. 

Generally, these risk categories can be broken down as:

Risk 1: Storage Sheds. temporary structures – low occupancy, low risk

Risk 2: Most habitable structures such as homes & businesses – standard occupancy & risk

Risk 3: Schools, gathering places, nonessential public utilities – high occupancy, reasonable risk

Risk 4: Hospitals, emergency utilities – sensitive occupancy, high risk


How Risk Category Influences Design

Once determined, a structure’s Risk Category can be used to find appropriate conditions and safety factors for design. This means that if a home (Risk 2) were to be built near a hospital (Risk 4), the hospital may need to be designed considering higher wind speeds and safety factors than the home, despite feeling the same effects of extreme weather.

The design process varies for different extreme weather situations, but here is a basic outline of how Risk Categorization affects the most common design scenarios:

Wind Design:

As used in ASCE 7-16, Risk Category is used to determine the ultimate wind speed that is used to be in the design of any job across the country. The ASCE provides charts to find the ultimate wind speed anywhere in the Unites States for each Risk Category. However, the ATC also provides an interactive tool that can look up wind speeds, snow loading, and seismic design criteria by searching an address:




ASCE 7-16:
Risk II Wind Speeds

ASCE 7-16:
Risk IV Wind Speeds

Hazards.ATCouncil.Org


Notice that for higher risk categories, ultimate wind speeds increase. By using these higher wind speeds, the ASCE avoids the need to increase safety factors elsewhere in calculation of pressures and loading. Wind calculations do often require one more site categorization called “Exposure Category” (Which we cover in a separate article here). With those 2 pieces of information, you are ready to calculate wind pressures on all sorts of structures using the guidelines of ASCE 7-16 or one of our intuitive design nm calculators.