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 Snow Loading with ASCE 7-22

  • ASCE Standards
  • ASCE7-22
  • Snow and Rain Loads
Justin Fields's profile image
Justin Fields posted 03-27-2023 11:44 PM

We are trying to help owners understand the new changes in codes, as well as follow best practices, even with the current code jurisdiction not requiring ASCE 7-22.  Based on my understanding of the latest ASCE 7-22, we are seeing a significant increase in the loading when considering the balanced snow condition with rain on snow surcharge.  For instance in TN, this has increased nearly 8psf for a roof (3psf for rain on snow + 5psf for  flat roof snow) from ASCE 7-16.  I know this increase is accounting for new data and studies.  Am I missing something, or is it really increasing this much on average? 

Secondly, when considering the Lu for drift, I'm not seeing a max length.  With the significant increase in drift values from the rise of the pg (though still not having to consider this in combination with rain/minimum), we are seeing continued issues with these increases when checking existing buildings for proposed loading.  I know the IEBC can be considered with this, but these values are drastic.  Please provide any input as you can.

Ronald Hamburger's profile image
Ronald Hamburger posted 04-06-2023 07:17 PM

Justin, I referred these questions to John Dunteman, Chair of the ASCE 7 Rain and Snow Loads task committee.  He offers the following response:

1-  the 5 to 8 psf change in rain on snow is because of the change in load factor in ASCE 7-22. The adoption of reliability-targeted design ground snow loads in 7-22 represents a significant change from ASCE 7-16 and prior editions, which previously used ground snow loads with a 50-year mean recurrence interval (MRI). Reliability-targeted loads are adopted to address the nonuniform reliability of roofs designed according to the 50-year snow load in different parts of the country, due to climatic differences. With the change to reliability-targeted values, the load factor on snow loads has also been revised from 1.6 to 1.0 to appropriately represent the reliability basis of the values.  This will result in more stringent snow design criteria in some locations and less stringent criteria in others, depending on the local climate.  This change is similar to changes made in the wind and seismic chapters in past years.

2- Professor Michael O'Rourke, the former chair of the Snow Loads committee offers the following response to question 2, based on an FAQ in the ASCE 7 22 Design Guide for Snow:"

.  A new building in South Carlina has an upwind fetch of 2000 feet. 

Is there an upper limit for the fetch distance?

The ASCE 7-16 provisions wherein the drift load was a function of the ground snow load pg and the upwind fetch lu, there was a specific

note in the figure for determination of the drift height hd (Figure 7.6-1 in 7-16) which stated “if lu > 600 ft, use equation”.  In ASCE 7-22, hd is

now also a function of the wind parameter W2, and the old 7-16 Figure 7.6-1 along with the specific note have been removed. 

However there is still no upper limit for the upwind fetch. 

As noted in Chapter 7 of this Guide, the drift height relation in ASCE 7-22 was based upon simulated drifts for upwind fetches of 100, 200

and 500 feet.  As such, an upwind fetch of 2000 ft is four times larger than the largest fetch (500 ft) upon which the new drift relations were

based.  The reasonableness of the ASCE 7-22 relation for large fetch distances will be discussed below.

Based upon Equation 7.6-1, an increase in fetch from 500 to 2000 would result in an increase in the drift surcharge height of

That is, increasing the fetch by a factor of 4 increases the drift height

by a factor of only 1.62.