ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures is the recognized US standard for design loads. This standard is referenced by many of the ASME and API equipment design standards used by industry. The latest edition of the standard, ASCE 7-22, was recently published, replacing the previous edition, ASCE 7-16. Several significant changes have been made in this latest revision. This article will provide an overview of these changes and the potential impact on ASME and API standards that reference ASCE 7. The next edition of the International Building Code (IBC) will be published in 2024, and this code will adopt all significant ASCE 7-22 changes discussed in this article.
Summary of Significant Changes in ASCE 7-22
Seismic Load (Chapters 11 through 23)
ASCE 7-22 is described by code committee members as a “correction” to 7-16, which generally amplified seismic loads relative to ASCE 7-10 but also required site-specific studies for many seismic applications. ASCE 7-16 has been referred to as the “geotechnical engineer employment act” in some circles since its publication. ASCE 7-22 will see an increase in seismic loads in some cases and a decrease in others compared to ASCE 7-16, but the overall magnitude of seismic loading is similar to ASCE 7-16. Many of the issues identified late in the 7-16 review cycle that prompted requirements for site-specific studies have been incorporated in the updated spectral acceleration curve procedures in 7-22.
The two-point design spectral acceleration curve based on SS and S1 is no longer used in 7-22. The new design spectrum uses 22 points and is a smooth curve (i.e., no more “plateau” region that occurs in the two-point method). As a result, seismic loads can no longer be calculated by hand, and there is a need to use the (free) online ASCE 7 hazard tool. The ASCE 7 committee is strongly encouraging the use of this online tool to calculate all loads, and the data it contains are officially published as part of ASCE 7 for the first time in the 2022 edition. The two-point seismic spectrum is still in the ASCE 7-22 text and is provided by the online hazard tool, but this is only to be used if there is insufficient data for the new multi-point spectrum.
Factors Fa and Fv have undergone several revisions in the past couple of ASCE 7 editions. Industry standards, such as API 650, have slowly kept up with these changes. These factors are now accounted for in the spectral acceleration dataset, and their effect varies with respect to period instead of uniformly scaling the spectrum, which is more physically realistic.
Intermediate site classes BC, CD, and DE have been added to reduce the severity of step changes between classes. The site classes can now only be determined by shear wave velocity, whereas they previously could be determined using blow counts or shear wave velocity. A few of the other changes include:
- Lateral forces can now be calculated using the equivalent lateral force procedure (ELFP) for any type of structure without restriction. The ELFP calculations are still pegged to “SDS” and “SD1,” but those parameters are to be taken from the new design spectrum at 0.2s and 1s periods, respectively (instead of the old two-point spectrum).
- The equation for non-structural component seismic loads has changed and now includes dynamic characteristics of the structural system to which the component is attached.
- Concrete tabletop structures are now included as a lateral force resisting system type. In the petrochemical industry, this would apply to support structures for equipment such as coke drums.
Wind Load (Chapters 26 through 31)
The wind maps have been updated and are available via the online hazard tool. Mapped wind speeds have increased along hurricane-prone coastal areas (for example, the Gulf Coast), and most of the special wind regions that used to require a site-specific wind study now include mapped values.
The directionality term, kd, has been moved from calculation of velocity pressure (q) to wind pressure (p). This is a technical change with no impact on loading in this edition; however, it paves the way for future editions which are expected to use separate kd factors for interior and exterior pressures based on recent research. A few of the other changes include:
- The tabular “simplified” wind procedure has been removed.
- New provisions were added for wind loads on elevated structures (e.g., homes in flood-prone regions).
- Some changes have been made to roof pressure coefficients and zone determinations.
Tornado Loads (Chapter 32)
Tornado loads are a new required consideration in the base ASCE 7-22 standard, where past editions of the standard only addressed these loads in the code commentary. Tornado loads are detailed in the new Chapter 32, which uses a layout, variable names, and equation structures similar to those for basic wind loads to minimize difficulties in learning this new requirement. Tornadic loads require consideration anywhere east of the Rocky Mountains for Risk Category III or Category IV structures where the mapped tornado wind speed exceeds 60 mph (96 km/h). Design according to Chapter 32 does not provide a “tornado shelter” level of safety (see Appendix G for guidance on this type of design) but is intended to improve reliability (which, in turn, reduces both the risk to human life and the cost of damage) for this hazard. Tornado loads are risk-targeted loads with the same return periods (MRI) for Risk Categories III and IV as wind: 1,700 and 3,000 years MRI, respectively. Some other considerations with tornadic loads include:
- Mapped tornado speeds are often lower than basic wind speeds; however, tornado loads can still be higher than basic wind loads due to differences in how this velocity is used to calculate pressure. ASCE 7-22 Figure 32.1-2 provides a flowchart that will indicate if basic wind controls over tornadic wind, thus eliminating the need to calculate tornado loads.
- Tornado loads are also a function of the footprint area of the building (calculated using the smallest convex polygon that encloses the building, or, more conservatively, a rectangle that encloses the building).
- An interesting fact about tornado winds is that they are higher near the ground than at higher elevations, so the profile looks very different from standard wind loads which increase over height.
Snow Load (Chapter 7)
Snow maps have been updated in ASCE 7-22 for the first time in approximately 30 years. Snow is now a risk-targeted load and uses a load factor of 1.0 in the strength design load combination (3a in ASCE 7-22), which maximizes roof loads; this is similar to what was done with wind in ASCE 7-10 and seismic loads some years prior. The importance factors for snow have been removed. Additionally, 90% of the snow regions that required a site-specific case study have been removed and are now mapped in the online hazard tool.
Snow drift load calculations have been updated significantly using physics-based simulations, where previous data was based on historical observations. The thermal roof factors have been updated, and there is a new mapped variable that accounts for “winter windiness,” which affects drift heights. Areas with the same ground snow load can have very different winds during the winter, resulting in different drift heights.
Rain Loads (Chapter 8) and Ice Loads (Chapter 10)
Generally, rain and atmospheric ice loads have minimum impact on most of the equipment and structures designed and analyzed in our industry. A few minor changes have been made in each of these chapters. Chapter 8 was completely rewritten for rain loads; however, the loads are unchanged except for the addition of rain head due to ponding. Similarly, the changes to Chapter 10 for ice loads are minimal. Importance factors on ice loads have been removed. Wind-on-ice mapped values are now also available on the online hazard tool.
Some significant changes have been made to ASCE 7 in the 2022 edition, and IBC 2024 will soon adopt these changes as well. In terms of petrochemical industry RAGAGEP, equipment codes and standards are generally an edition or two behind on adoption of ASCE 7 rules (some still referencing ASCE 7-10, or even ASCE 7-05). ASCE has noted this in communications with some of the equipment standards committees. Regardless of pace, change is inevitably coming to load requirements for design and analysis for all industries. E2G is participating in ASCE activities and on various equipment standards committees to ensure our Consulting, Engineering Practices, and Software stay up to date with the appropriate design loads. If you have any concerns or questions regarding the changes to ASCE 7, please complete the form below and Joel or Derek will follow up with directly.
With the many changes to ASCE 7-22, designers and analysts should rely on the online ASCE 7 hazard tool provided by ASCE. The tool is now free and includes loads from several previous editions. In some cases, the tool is now the only way to calculate design loads since some maps and hand calculation procedures were removed from the document text. Engineers are encouraged to familiarize themselves with this tool.
If you have any questions, please fill in the form below.