Shoveling Through a Blizzard of Confusion
Understanding the Snow Load Changes in ASCE 7-22 and How they Affect Component Manufacturers
by Greg Greenlee, P.E.
SBCA Technical Director
When reviewing the 2024 version of the International Building Code (IBC) and International Residential Code (IRC), you may notice that the ground snow load figures are different than those in previous versions. So, what has changed and why? The answer is multifaceted and involves evolving design methodologies, more site-specific climatic research and data, and a more refined analysis of the ground snow dataset.
The loads imposed on a structure can consist of dead loads, live loads, wind loads, seismic loads, or snow loads. Wind, seismic, and snow loads are considered ‘environmental loads’ and are calculated based on factors such as a structure’s location, topographic features, surrounding structures, building configuration, and the building material type. ASCE/SEI 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures is the standard referenced in the building codes primarily used to determine loads, including environmental loading on structures.
Ground snow loads are the starting point used in determining roof snow loads for structural design. This is done by modifying the ground snow loads using equations and factors corresponding to the structures type, location, and configuration. To ensure the ground snow load is properly applied when designing roof trusses, it is important for the truss technician and the truss designer to understand the snow load design requirement that is being provided by the building designer.
The latest version of ASCE 7 is the 2022 version which includes changes to the snow load provisions, which is the subject of this article. The table below shows what versions of ASCE 7 are referenced in the different versions of the IBC and IRC.
Understanding Design Methodologies
To understand the rationale for the snow load changes, it is first necessary to understand the two different design methodologies used: Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD). Both methods are aimed at ensuring the structural member’s strength exceeds the load it is designed to support, with some safety margin factored in. Both design methods are acceptable and will ultimately provide a similar design.
Allowable Stress Design (ASD).
ASD is a traditional structural design method that compares service level loads to allowable capacities. A ‘service level load’ describes a load or force that a structure or component is designed to support without factoring in safety margins. Allowable stresses are determined by dividing the ultimate capacity by a safety factor. The ‘ultimate capacity’ describes the point at which a structural component fails and can no longer support the applied load(s). An important distinction is that ASD uses a single safety factor based on the type of load (axial, shear, moment, torsion). ASD is more efficient when the loads are well known and predictable.
Load and Resistance Factor Design (LRFD).
LRFD compares reliability-targeted (strength based) ground snow load values (ultimate loads) to a maximum strength. Loads are multiplied by different load factors that vary with different load combinations, and the material strength is reduced based on uncertainties in the resistance of a structural member and the associated risk of failure under extreme events. LRFD is more efficient when there are uncertainties in the design, such as with dynamic loads like wind, seismic, or snow.
When designing a structural member, a designer will analyze its behavior under various loading conditions, applying different load combinations to cover a range of situations. Ultimately the designer is checking to ensure that the effects from any of the load combinations do not exceed the capacity of the member, with some type of safety margin incorporated.
Changes in ASCE 7-22
Several changes related to snow loads are introduced in ASCE 7-22. These include changes to the ground snow load values, introduction of ultimate snow loads, and changes to the snow load calculations. In the following, we will discuss each of these individually.
Changes to Ground Snow Load Values
Previous editions of ASCE 7 were based on a ground snow load dataset from the 1950s through the 1990s. Since then, procedures for estimating snow loads have improved and additional snow load data have been developed and incorporated into the ground snow dataset. The result is that the ground snow load for some locations has increased and for some it has decreased. On average, there has been a slight increase in design ground snow loads across the country.
Introduction of Ultimate Ground Snow Loads
Historically, the snow load provisions in ASCE 7 were based on service level loads. To incorporate this updated dataset, ASCE 7-22 provides ground snow loads as ultimate loads facilitating a reliability-targeted approach using the LRFD methodology. The statistical approach and reasoning behind these revisions is beyond the scope of this article. The article, Ground Snow Loads for ASCE 7-22 in the February 2022 edition of Structure Magazine contains a comprehensive discussion about the theory behind the changes.
In ASCE 7-16 and earlier editions, snow load calculations were adjusted through an Importance Factor, Is, which was applied directly to the ground snow load values to account for the varying consequences of structural failure based on the building’s use and occupancy. In ASCE 7-22, this system is replaced with individual ground snow load maps for each Risk Category. There are four Risk Categories, I through IV, based on the impact to human life and infrastructure if the structure were to fail. Risk Categories have been used for handling design loads in other areas of building safety, such as wind and seismic loading. Applying this concept to snow loads aligns with broader building code changes. See below for a summary of each Risk Category. More detailed explanations are provided in ASCE 7-22.
- Risk Category I: Low occupancy, low risk (storage sheds, temporary structures).
- Risk Category II: Standard occupancy and risk (most habitable structures including homes and businesses).
- Risk Category III: High occupancy, reasonable risk (schools, gathering places, nonessential public utilities).
- Risk Category IV: Essential facilities, high risk (hospitals, emergency utilities).
As mentioned previously, ASCE 7-22 now includes four separate ground snow load maps, one for each Risk Category. This allows designers to apply appropriate snow load values based on the building’s classification and location. As the Risk Category increases, so does the design ground snow load on each map as shown in the table below.
In some areas of the country, case studies are required to establish ground snow loads because of extreme local variations. They include Alaska, parts of Colorado, New Hampshire, Idaho, Montana, New Mexico, Oregon, and Washington. ASCE 7-22 provides tables for each of these areas.
As an alternative to using the new ground snow load figures, ASCE has developed a web-based mapping tool, referred to as ‘ASCE Hazard Tool,’ that allows users to quickly retrieve precise data for a specifical location, including the ground snow load. It is available at https://ascehazardtool.org/.
It should be reinforced that even though ASCE 7-22 provides ultimate snow loads, using service level loads with ASD is still a permissible methodology. The IRC and all its prescriptive design tables are still based on ASD in the 2024 version.
Snow Load Calculation Changes
ASCE 7-22 includes several new snow loads calculations. These may be visible in the software used by truss technicians and truss designers and include the following.
- Thermal Factor, Ct (Revised in ASCE 7-22). A new table has been added to determine the Thermal Factor based on the roof insulation R-value that applies to most structures. This update captures the influence of heat loss through the roof as a function of the insulations R-value. These new values are provided in ASCE 7-22, Table 7.3-3 which is referenced in the revised Table 7.3-2.
- Winter Wind Parameter, W2 (New to ASCE 7-22). The new Winter Wind Parameter more accurately estimates unbalanced snow loads and snow drift heights. The parameter is provided in ASCE 7-22, Figure 7.6-1 and is based on geographical location. The parameter can also be determined using the ASCE online hazard tool.
Changes to the Building Codes
Because ASCE 7 is the referenced standard for determining loads in both the International Building Code (IBC) and International Residential Code (IRC), the revised snow load provisions in ASCE 7-22 had notable implications for the snow load provisions in the 2024 versions of both codes. These changes, however, are not consistent between the codes.
International Building Code. The IBC has added the four new ground snow load figures [Figures 1608.2(1) through 1608.2(4)] from ASCE 7-22. This means that the ground snow load provided in the IBC figures, designated as pg, will be an ultimate load based on the Risk Category of the structure. If needed, the IBC provides an equation to convert the ultimate ground snow load (LRFD) to a service level ground snow load (ASD), designated as pg(asd). To determine the design roof snow load, the IBC points to the provisions in ASCE 7-22.
International Residential Code. Many of the provisions and tables in the IRC use allowable stress design (ASD) values. Therefore, the new ground snow load figure in the IRC [Figure R301.2(3)] provides service level loads (ASD), which are identified as pg(asd). The figure in the 2024 IRC was developed by converting the ultimate ground snow loads in the new figure in ASCE 7-22 for Risk Category II from ultimate loads (LRFD) to service level loads (ASD) by multiplying them by a factor of 0.7.
It is also important to remember that the IRC further simplifies snow load design by equating the ground snow load to the roof snow load. This is different than the IBC, which utilizes a factor of 0.7 to convert ground snow loads into roof snow loads. This factor should not be confused with the 0.7 factor used to convert ultimate loads (LRFD) to service level loads (ASD).
What Does all this Mean?
These changes complicate the snow load design process. Between the two codes, there are potentially five different ground snow load figures that may be used, providing either service level loads (one figure in the IRC), or ultimate loads based on risk categories (four figures in the IBC). Furthermore, the new Winter Wind Parameter (W2) and the revised Thermal Factor (Ct) were added, which are based on geographical location and the structure’s thermal condition, respectively. For the roof trusses to be properly designed, it is imperative that snow load design information provided by the building designer is understood and properly applied.
Fortunately, updated software will automate many of these new calculations. However, it is important that the user understands the information that is being populated and how the information is being used in the design. For example:
- If an ultimate ground snow load (LRFD) is provided in the construction drawings and is entered into the software as a service level load (ASD), the resulting load on the truss will be greater than intended, resulting in an overly conservative design.
- If a service level load (ASD) is provided and it is entered into the software as an ultimate load (LRFD), the loading on the truss will be less than intended, resulting in a dangerous unconservative design.
- When the software accounts for the building code version used, it becomes more complicated. If an ultimate ground snow load (LRFD) is provided in the construction drawings for an IBC-24/ASCE 7-22 project, and the truss technician or truss designer incorrectly enters the ultimate ground snow load (LRFD) and specifies the 2024 IRC as the code, the applied roof loads will be too high.
The following is a checklist of items to consider and understand to ensure the snow loading is properly being used in the software.
Conclusion
ASCE 7 is used to determine loads, including environmental loading, such as snow loads on structures. It is the standard referenced in the building codes throughout the United States. In addition to ASD, snow load provisions in ASCE 7-22 now include a reliability-targeted strength design methodology (LRFD). This approach considers the fact that the probability of extreme snow events varies significantly across different geographic regions and building types. ASCE 7-22 is referenced in the 2024 versions of both the IRC and IBC.
It is extremely important that truss technicians and truss designers understand these snow load changes, ensuring that the proper type of load (service level vs. ultimate) is being used in the truss design, as well as understanding how the ASCE 7-22 utilizes Risk Categories, Thermal Factors, and Winter Wind Parameters.
In the coming months and years, jurisdictions will begin adopting all or part of the 2024 IBC and IRC, and therefore ASCE 7-22 will be adopted, as well. Accordingly, the software used to design metal plate connected wood trusses will be updated to accommodate these new code provisions. As these updates are released, component manufacturers must work with their software providers to understand how snow loads are delivered and applied.
A special thanks to the following for their contributions to this article: Kathleen Anne Wills Rittenburg, P.E., Alpine, an ITW Company; Matt Vinson, P.E., Eagle Metal; Andrew Johnson P.E., MiTek; Larry L. Messamer, P.E, Simpson Strong-Tie; and John Teems, Simpson Strong-Tie.