Southern Pine Design Values: Comments

Now that some of the dust has settled after ALSC's decision to postpone its ruling on proposed southern pine design values, the debate can begin with all sides in the mix.  While the debate continues about the need for new design values, what does the industry do in the mean time to protect itself from inadequate design?  An engineer from Florida recently asked me this question and it definitely bears some consideration.  An edited version of my response is as follows: 

At this time, I am not sure how I will design wood while waiting on the possible new design values.  There are several issues to consider before making this decision.   

First, I have yet to see the actual proposed new design values.  While I do not doubt the testing, I would like to see the test data.  Researchers at Mississippi State have conducted some tests that show a decrease in lumber strength but not enough to justify the amount of design value reduction proposed by SPIB.  

Second, while the proposed new values cover each size of southern pine, only 2x4's have been tested.  I do not think we can assume a correlation between the design values of each size based on the In-Grade testing performed 20 years ago.  Too much has changed in the timber industry.  

I think the easiest thing to do is limit the stress ratio of members we design to maybe 0.80 instead of 1.0.  This would be easier than trying to adjust design values at this time and it may not make a significant difference in the overall design.  

This was a short and quick answer to a complex problem.  I have two new wood projects on my desk, including a multi-story apartment building, that will give me a chance to create and implement solutions for ensuring an appropriate design.  I will post about my findings in the next few weeks as I complete the projects. 

Also, I have requested a copy of the SPIB proposal.  The recent comments from the SLMA has a link to request the proposal for those who are interested.   

Another good commentary regarding the industry's stance (or lack thereof) on what to do between now and January 5th when the ALSC rules on the design changes is here. 

Southern Pine Design Values: Updates

While it may be difficult to navigate the different sources of information regarding the proposed southern pine design values, one source I have found continually provides excellent coverage.  Just remember that the coverage is from the perspective of a group that does have a stake in the outcome. 

SBCA Lumber News

I would also like to add that I receive weekly emails from SBCA regarding news that affects the lumber industry as a whole.  I recommend anyone with an interest in the lumber and housing market to sign up for the emails.

SBCA Magazine News

Southern Pine Design Values: Update

According to a memo sent out by the American Lumber Standards Committee, the Board of Review has postponed making a decision regarding the SPIB's proposed changes to Southern Pine design values for 60 days. This postponement is to allow interested parties to review and comment on the technical aspects of SPIB's proposal. See the link below for the memo. ALSC Memo

Southern Pine Design Values

As some of you may already be aware, on October 20th, 2011 the Southern Pine Inspection Bureau will be proposing new allowable design values for southern pine lumber to the Board of Review of the American Lumber Standard Committee.  The proposal will lower design values for all grades and sizes of visually graded southern pine by approximately 25-30%.  This will have a major impact on all aspects of the lumber industry. 

While the testing for these changes began in 2010, it appears as though many groups have only recently become aware of the proposed changes.  One point of contention for many industry representatives is the limited sample size of testing performed to provide the basis for the changes.  To my knowledge, approximately 400 southern pine #2 2x4 were tested.  No other sizes or grades were tested.  Further contention lies in the lack of collaboration with other industry groups to insure that everyone affected has a chance to aid in the process of determining the new design values. 

Much more information can be found in the following links:

SBCA News Page

Southern Pine Inspection Bureau Updates

Anthony Forest Products News Page


Another Note:  While there appear to be several written items about the changes, I have not found a table of the proposed design values to compare with the current design values. 

Building Codes change in Joplin and Pennsylvania

Building Codes change in Joplin aimed at creating safer homes - KOAM TV 7 Joplin and Pittsburg

Build It Right: Bracing Walls for Wind

It is interesting that as one City increases the requirements of the Building Code one State is decreasing its requirements. Decreasing the wall bracing requirements could have a negative impact on a homes resistance to a tornado. That may be the difference between a City that has seen first hand the damaging effects from a tornado and a State that has not.

The article below highlights findings by a National Science Foundation sponsored research team.  Code Authorities should look to studies such as this to aid in decision making when planning for code changes, especially when reducing code requirements. 

 

Structures on Storm’s Edge Could Benefit Greatly from Improved Engineering, According to UA-Involved Study

Deck Design Do's and Don't's: Part 2

For Part 2 (click here for Part 1) of Deck Design Do's and Don't's, I want to address guard post attachments.  Based on research at Virginia Tech, we now know that typical attachments used in deck construction do not meet code required design loads.  One such instance can be seen in the picture below. 

The 4x4 post has been notched and nailed to the exterior rimboard of the deck.  As you can see, the nails in the thin portion of the member have created a split in the wood along the grain.  Similar notched and un-notched connections with bolts also fail to meet the code design loads.  

Use of hold-down anchor brackets, as shown in the picture below, are required to be installed at guard posts to meet the design loads.  The primary method of failure for typical bolted connections stems from the rotation created in the rimboard.  The rimboard attachment to the joists must support that rotation.  Hold-down anchors provide a means of transferring the load out of the rimboard and into the floor system.  

Photo Courtesy of Simpson Strong-tie.

For more information on deck design, including links to deck design guides, you can look at my previous post here.  For more information about guard post connections you can look here for details and here for design.   

2010 Wood Handbook

The Forest Products Laboratory (FPL) has recently released the printed edition of the 2010 Wood Handbook: Wood as an Engineering Material.  While you can download the entire book from the FPL website (see below), I encourage anyone serious about wood design to buy the printed edition as a desk reference.  At a cost of $60, this may be one of the least expensive engineering reference books you will find. 

If you are looking for a textbook on wood design, such as how to design beams, etc., this is not the book for you.  (I would recommend Design of Wood Structures by Breyer, Fridley, et.al.)  This book focuses on the physical and mechanical properties of wood and wood composites and provides information on wood preservation, fastening, finishing, and adhesives.  

This is taken from the FPL website:

The Wood Handbook is one of the most widely used documents in the wood literature. The 2010 edition is the first update of the 21st century and includes several new chapters. The original handbook was first published in 1935 by the USDA Forest Products Laboratory (FPL).
The Forest Products Society (FPS) is pleased to cooperate with the FPL to present this newly updated edition in hard copy. This 2010 edition is your one-stop source for information on wood as an engineering material and offers content such as:

• Properties of wood and wood-based products
• Design information for architects and engineers
• Wood and non-wood composites
• Wood-moisture relationships and wood durability

...plus NEW CONTENT on:
• Wood as an environmentally responsible, sustainable building material
• Heat-treating and sterilization procedures for wood infected by invasive insect species
• A special COLOR chapter on low-magnification micrographs of cross sections of commercial wood species 

You can order your hard copy of the 2010 Wood Handbook here or you can download content or sample the material that is in the book here.