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This quarterly article addresses some of the questions received about structural standards developed by the Structural Engineering Institute (SEI) of the American Society of Civil Engineers (ASCE). Questions from engineers, building officials, and other design professionals are often considered to develop future editions.
These topics and more are discussed on the ASCE Peer-to-Peer Standards Exchange Forum. ASCE/SEI members can ask and answer questions in the forum. Visit https://collaborate.asce.org/standards-exchange/home to learn more and read about other topics.
Enclosure Classification
When calculating the total area of openings in a wall that receives positive external pressure, does failure potential of the glazing need to be considered, e.g. a window breached by wind-borne debris?
Answer: Windows that meet the protection requirements for glazed openings, or that are permitted to be unprotected per ASCE 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures Section 26.12.3, do not need to be considered as openings.
Seismic Requirements for Unanchored Storage Racks
Does ASCE 7-22 Chapter 15, Seismic Design Requirements for Nonbuilding Structures, apply to movable, unanchored, free-standing storage rack systems?
Answer: No. Chapter 15 only applies to anchored storage racks.
Differential Settlement Threshold
Section 12.13.9, Requirements for Foundations Subject to Seismically Induced Soil Displacement or Strength Loss, of ASCE 7-22 references Table 12.3-3 for the differential settlement threshold for foundations subject to seismically-induced soil displacement or strength loss. Should the table reference actually be 12.13-3, Differential Settlement Threshold?
Answer: Yes. This table reference was corrected in Errata 1 for ASCE 7-22, issued in September 2024. Errata are available for free download at asce.library.org. A full list of available errata and formal interpretations for ASCE/SEI standards can be accessed at https://www.asce.org/communities/institutes-and-technical-groups/structural-engineering-institute/asce-7-and-sei-standards.
Ice Dams and Icicles Along Eaves
ASCE/SEI 7-22 Section 7.4.4, Ice Dams and Icicles Along Eaves, states that the uniformly distributed load of 2pf applies to unventilated roofs with a thermal factor, Ct, less than or equal to 1.1. Is there a similar requirement for ventilated roofs as there was in ASCE 7-16?
Answer: No. From a thermal standpoint, modern ventilated roofs are equivalent to unheated roofs and are not subject to ice damming.
Ice Loading
Why does ASCE/SEI 7-22 Section 10.4.1, Ice Load, allow for a larger reduction in the volume of ice, Vi, for horizontal plates than for vertical plates (0.6 versus 0.8)?
Answer: The trajectory of freezing raindrops is determined by their vertical fall speed and the horizontal wind speed. The wind direction is conservatively assumed to be perpendicular to the face of the vertical plate. Because freezing raindrops tend to be small, their fall speed is typically smaller than the wind speed, resulting in more ice on vertical plates than on horizontal plates.
For a flat plate, why is the surface area, As, used to calculate the volume of ice the area on only one side of the plate?
Answer: The ice is assumed to form on the top of horizontal plates and on the windward side of vertical plates and angled plates.
Does ice accumulation depend on the type of surface? For example, a horizontal sunshade with aluminum airfoil blades vs a horizontal sunshade with terracotta blades?
Answer: No. If the air and surface temperature are both at or below 32F, the impinging precipitation will accumulate as ice, independent of the material.
NAVD 88
The ASCE Hazard Tool provides elevation with respect to NAVD 88. What does that mean?
Answer: NAVD 88 is the North American Vertical Datum of 1988 used as the reference for zero elevation in North America. NAVD 88 is not equivalent to Mean Sea Level (MSL). Every Flood Insurance Rate Map (FIRM) and Flood Insurance Study (FIS) is referenced to a specific vertical datum. Prior to the mid-1990s, the National Geodetic Vertical Datum of 1929 (NGVD 29) was used as the reference for zero elevation. FEMA requires that all new flood maps and updates within the continental United States be referenced to NAVD 88.
The 100-year flood elevations shown in the overlay on the ASCE Hazard Tool come from the FEMA National Flood Hazard Layer and are with respect to NAVD 88. ■
This article’s information is provided for general informational purposes only and is not intended in any fashion to be a substitute for professional consultation. Information provided does not constitute a formal interpretation of the standard. Under no circumstances does ASCE/SEI, its affiliates, officers, directors, employees, or volunteers warrant the completeness, accuracy, or relevancy of any information or advice provided herein or its usefulness for any particular purpose. ASCE/SEI, its affiliates, officers, directors, employees, and volunteers expressly disclaim any and all responsibility for any liability, loss, or damage that you may cause or incur in reliance on any information or advice provided herein.
If you have a question you want to be considered in a future issue, please send it to sei@asce.org with FAQ in the subject line. Visit asce.org/sei to learn more about ASCE/SEI Standards.
About the Author
Jeannette Torrents, PE, SE, F.SEI, is the Technical Director of ASCE’s Structural Engineering Institute.
