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ASCE705S.xls

The section pertaining to rain-on-snow surcharge, Section 7.10 of the ASCE 7-05 Code, has been revised from how it read in the ASCE 7-02 Code. At first, I believed that there was room for interpretation of ASCE 7-05 Section 7.3, because of the way it has been revised and worded. It states basically that the flat roof snow load shall be computed per Equation 7.1 as pf = 0.7*Ce*Ct*I*pg, but not less than the minimum pf = I*pg when pg <= 20 psf nor pf = 20*I when pg > 20 psf. Section 7.10 states that the rain-on-surcharge, when applicable, is to be added to the balanced snow load case. So to me, there in lied the interpretation issue. Does the balanced snow load case referred to in Section 7.10 mean "pf" as calculated solely per Equation 7.1, or does one follow through literally the complete provisions of Section 7.3, where the minimum value of "pf" is also to be considered? In version 1.0 of the "ASCE705S.xls" workbook, I originally opted to take the latter interpretation, which would result in higher (conservative?) design balanced snow loads.

With clarification received and passed on to me from one of the guys who posed these questions to the ICC, they stated that there was really no intent in the revisions made to the ASCE 7-05 Code to increase the balanced snow loading when considering rain-on-snow surcharge from what the ASCE 7-02 produced. An excerpt of that question and answer clarification received from the ICC is shown below.

Q1. The 2006 IBC requires snow loads to be determined using Chapter 7 of ASCE 7. I am unclear on how to apply Section 7.10 Rain-on-snow Surcharge Load. In the code section, it merely states that the ROS surcharge is to be added when pg is 20 psf or less, but not zero on roofs with slopes less than W/50 (degrees). It could be interpreted to add the 5.0 psf to the minimum design snow load, which, without ROS being considered, would be the larger of pf calc (Eq. 7-1) or p min (I x pg). But in the commentary section, it gives an example of evaluating ROS surcharge where the calculated ps = 14 psf, which is pf calculated for a sloped roof when Cs = 1. The minimum load is pmin = I x pg, which in this case is 18 psf. In applying the ROS surcharge, the 5.0 psf is only considered to be added to the calculated snow load from EQ. 7-2, or in the case of a lower sloped roof, EQ. 7- 1. This results in comparing calculated ps + ROS (14+5=19) to pmin =18, yielding a controlling design snow load of 19 psf.

A1. It is our understanding that, in this case, the ASCE 7 commentary does represent the intended application of the standard. A recently balloted change to ASCE 7 will, among other revisions, clarify Section 7.10 to state that the ROS surcharge need not be used in combination with drift, sliding, unbalanced, minimum or partial loads.

Q2. Was there any intent between ASCE 7-02 and ASCE 7-05 to increase the snow load by changing the language of Section 7.10?

A2. Not to our knowledge.

In summary, it appears that the only real change in snow loading using ASCE 7-02 versus ASCE 7-05 is the roof slope criteria for when to consider rain-on-snow surcharge. In ASCE 7-02, it applied only to roof slopes of less that 1/2" per foot, whereas in ASCE 7-05 it applies to roof slopes of less than W/50 degrees.

I have made the necessary revisions, and the "'ASCE705S.xls" workbook is now version 1.1.

It was brought to my attention that in the "ASCE705S.xls" workbook on snow loading, I inadvertently had the user input cell for "Type of Roof" locked, so that the input pick box could not be utilized. Thus, you were stuck with the "Monoslope" roof selection.

I have made the correction and the workbook is now version 1.2.

It was discovered in all 3 snow loading workbooks (ASCE705S.xls, ASCE702S.xls, and ASCE798.xls) that the sketch at the bottom of the calculation page as well as the sketch off of the calculation page to the right were displaying the calculated snow drift length, "w", when they should have been displaying the actual snow drift length to be used, "w(use)". The actual snow drift length to be used is the minimum of either the calculated value "w" or the maximum value "w(max)" per the Code.

At some point I will just drop the ASCE798.xls workbook, since most folks are probably using either the ASCE 7-05 or ASCE 7-02 Codes in their work.

For now I have made the corrections to the sketches in all three workbooks, and they are all now version 1.3.

Useful Web Resource
I was given a couple of websites which you might find convenient for determining the basic wind speed and ground snow load
for any location in the continental United States per the ASCE/SEI 7-05 Code, so I thought I'd pass them along.

The website links are as follows:
www.windspeedbyzip.com/
www.groundsnowbyzip.com/

You must register to make use of either website, but you only have to register at one, not both of the websites.
Once you register at one of them, you will automatically have access to information from the other one.

The two websites allow you to input either the street/city/state address or just the zip code.
The resulting location latitude and longitude are given. Regular (street), satellite, and hybrid maps are available as well.

In the "ASCE705S.xls" workbook, it was brought to my attention that the comment box in cell C36 in the "Snow Load" worksheet pertaining to rain-on-snow loading was an apparent "leftover" from the version of this workbook for the ASCE 7-02 Code. What I typically do when a new version of a particular code comes out is use the workbook for the previous code version and edit it as required. Obviously, I forgot to edit this particular comment box. The programming logic did reflect the change in the code on that subject.

I have corrected that comment box, and the current version of this workbook is now version 1.4.