7 th Edition Ground Snow Loads: Determination of Allowable Roof Rafter Spans
- The 6 th edition contains rafter span tables for each ground snow load value of 25, 30, 35, and 40 at three different deflection restraints L/180, L/240 and L/360.
- The 7 th edition 780 CMR specifies new snow load values across the Commonwealth. The new ground snow load values are 35, 45, 55, and 65 psf. See Table 5301.1.2(5).
- The 7 th edition 780 CMR contains rafter span tables for ground snow load values of 30, 50 and 70 psf at two different deflection restraints, L/180 and L/240. See Tables 5802.5.1(3) to (8).
- The 7 th edition 780 CMR does not contain span tables for the four ground snow load values noted in the 2 nd bullet above. In addition it does not have a table for the deflection restraint of L/360.
How does someone design, or check a design of, a roof rafter, using the new ground snow loads that the 7 th edition promulgates, if there is no span table for that specific snow load value?
This 7 th Edition Guideline for Rafter Spans with Snow Loads :
- Allows for the use of industry accepted methodologies for determining rafter spans, specifically those utilized by National Design Specifications (NDS) and the American Wood Council (AWC) as reflected in the Wood Frame Construction Manual-2001 Edition and the online AWC Span Tool: http://www.awc.org/calculators/span/calc
- Allows for linear interpolation of the 7 th edition span tables, 5802.5.1, to determine rafter spans for the specified ground snow load value. If use of the Span Tool is not used and interpolation is required, a worksheet is provided for this use. It is found at the end of the Guideline.
Question 1: The 6 th edition of 780 CMR had everything I needed to design a roof rafter for the specific snow load that applied to my locale. The 7 th edition doesn't even have a table for my new ground snow load, and as I understand it, one will not be provided in the future. Why the change?
Answer 1: BBRS recognizes that this is a significant change which, for some constituents, may create more work to either design, or check allowable spans for the new snow loads. However, for the general population BBRS sees a potential significant benefit in productivity in the design or design checking process with the use of the Span Tool. This tool uses the most current mathematical algorithms (found in the National Design Specifications and the AF&PA/AWC Wood Frame Construction Manual) in the calculation of allowable clear spans. It has other benefits as well, including but not limited to:
- It is a web-based tool so it can be used with any computer with internet access.
- It has a wide range of input variables that can be independently changed, including load, deflection restraint, species and grade of wood and with output of "allowable span". In addition, the user can input a "required" span and wood species (Doug Fir, for example) and get output of the different grades, and their section properties, that meet the requirements.
- The allowable clear span, for a given set of design conditions, is available immediately and all the input data and results can be printed on one sheet or saved electronically for record keeping purposes.
Question 2: I have constituents who don't have a computer or easy access to a computer, and some that do not have the skills to interpolate the tables. What are their options?
Answer 2: They have two options:
- They can use a more conservative snow load and use the spans as shown in the table for that more conservative snow load value. For example if the rafters will be installed in a building in a 45 psf ground snow load zone then they may use the span tables for 50 psf.
- They can use the one page "Linear Interpolation Worksheet" which is found at the end of this Guideline. The only tool that would be helpful to use to complete the worksheet is a calculator.
Question 3: The tables contain spans for deflection restraints of L/180 and L/240. What if I have a design that requires the use of a veneer plaster ceiling attached to the rafters, how do I get the rafter span for this case, which is L/360?
Answer 3: The Span Tool will compute the allowable span for any of those deflection restraints. If the Tool can not be used then BBRS permits the use of Wood Frame Construction Manual (WFCM). The Commentary, 2001 Edition, has several examples (pages 45 to 51) that can be used as an aid for determining the allowable span for the specific case that you have. If at a point in the future, BBRS gets feedback that this approach is not meeting the needs of our constituents then another method may be provided. This may involve the publication of a limited amount of prescriptive allowable spans for this deflection restraint. However, as the Span Tool becomes more widely known, BBRS expects that our constituents will rely on it as their preferred method of getting the job done.
Question 4: I just used the Span Tool to check a roof rafter design. To feel more comfortable with the answer that it provided I compared the span to that found in the 7 th Edition code, for the exact same design conditions, and there was a difference of several inches in a span of approximately 16 feet. Why the difference? Which number is correct?
Answer 4: Both numbers are "accurate" and both are deemed acceptable because they were each determined by accepted methods as defined by this guideline for the 7 th Edition 780 CMR approved by the Board of BBRS on December 12, 2006. However, the span value computed by the Span Tool is more "precise" than the Table because it uses a more refined approach to the "rounding" of numbers used in the calculations. Often there will be exact agreement of the two methods, but also one or the other method may produce a slightly larger or smaller allowable clear span, as a result of these rounding differences.
Question 5: I calculated the span using the method outlined in the NDS and compared the answer to the Span Tool, which supposedly uses the same mathematical equations, and there was a difference of about three inches. Can you comment on why there is a difference?
Answer 5: The two methods do use the same mathematical equations. The difference is probably due to the fact that the NDS span values are as measured from center line of bearing surface to center line of bearing surface. The Span Tool and Tables produce values of clear span. This is done so that building inspectors can easily check the installed condition by measuring from stud face to stud face for example.
Question 6: How can we be assured that the Span Tool will always be there?
Answer 6: AF&PA is a highly regarded and long standing organization that provides design and construction guidelines that are in use throughout the US and are required by the IBC Chapter 23. AF&PA first published Span Tables in 1922. AF&PA regularly updated those tables as design values or design equations were updated. The last 'hard copy' span table for all species groups in the NDS was published in 1996. The Span Tool was developed to replace the Span Tables. The Span Tool site is well known at this point in time, with approximately one half million hits per month (as of December, 2006) and increasing . The tool is maintained by the AWC within AF&PA with the same dedication as was shown to the hard copy span tables.
If in the unlikely event the tool is no longer available the Board can review other available technology and direct constituents accordingly, via interpretation on the issue if it may arise.
Question 7: How can we be assured that the Span Tool gives the correct answer?
Answer 7: The Span Tool uses the same mathematical algorithms used in NDS and that were used to develop the span tables in IRC 2003.
Question 8: I am a novice computer user. Is there training material available to help me understand how to access and use this tool?
Answer 8: Yes, you may find a file at this link " Powerpoint file.ppt file size 1MB ". Using this file you may work your way through a couple examples, step-by-step.
Question 9: If I have questions or see that there might be a problem with this new tool, how can I get my issue resolved?
Answer 9: It would be very helpful to access the DPS website and submit your question, comments, or other issue to the DPS Info. BBRS would appreciate it if you can take a little extra time to outline in some detail as much as you can in your submittal so that we can quickly understand the issue and get back to you with our response in a timely manner.