Design Considerations for Sawn Lumber Wood Studs

I am excited to say that my first publication for Structure Magazine just came out in the May issue.  I wrote the article about wood stud design after seeing inconsistencies in the way that different engineers perform their analysis.  I know of several engineers that use Enercalc's wood column feature for stud design.  Using a column analysis for wood studs tends to alter the way adjustment factors are applied, specifically the repetitive member factor. 

Another point of contention between some engineers is the allowable deflection for wood studs supporting brick veneer.  While the L/600 limit is applicable to cold formed steel stud framing, the same limit is not applied to wood studs. 

The article can be found here if you would like to read it and learn more about the design of sawn lumber wood studs. 

Southern Pine: Status Update

Click here for a summary of the current status of the proposed changes to southern pine design values.  I imagine that within the next 5 1/2 months (prior to implementation of new values) there will still be much debating on this issue.  The primary point of concern boils down to this: change the lumber design values or change the lumber grading process.  Timber Products Inspection has stepped up to the plate to develop a revised lumber grading process.  Click here for more information from TPI.  

Southern Pine Span Tables - Proposal

The Southern Forest Products Association (SFPA) is kind of like a customer service department for Southern Pine.  They provide assistance to end users in understanding Southern Pine grading rules and design values while also marketing lumber products.  The SFPA website at http://www.southernpine.com/ provides information for end users regarding green building, pressure treatment, and product applications in addition to providing span tables for joists, rafters, headers and beams. 

The SFPA is closely involved with the Southern Pine Inspection Bureau's (SPIB) proposal of new Southern Pine design values.  While SFPA is not directly involved with testing of lumber or the establishment of design values, they will be the group that helps end users understand and adapt to any changes in Southern Pine use.  The SFPA has published a sample span table for joists and rafters based on the proposed design values.  You can find the tables here. 

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.

Residential Deck Design

In the last 6 weeks I have been researching and reading about deck and guardrail design and construction.  I am currently working on a project to develop two observation platforms for air monitoring equipment.  These platforms are stand-alone wood frames structures.  While they are not residential decks, they are very similar in many ways with the exception of design loads. 

For an introduction to deck design, I recommend Design for Code Acceptance-6: Prescriptive Residential Wood Deck Construction Guide published by the American Wood Council.  This guide covers design and detailing for several conditions including deck-ledger attachment, post-beam connections, and guard post attachments.

The guard post attachments are interesting because recent testing shows the inadequacy of many standard connections.  The International Residential Code requires that residential deck railings be designed for a live load of a 200lb concentrated load in any direction located at any point along the rail.  Upon reviewing local construction practices of guard-post attachments, many of which appeared questionable for meeting code requirements, researchers at Virginia Tech began testing these connections.  The results, which have been published in Wood Design Focus and the Journal of Light Construction, were somewhat surprising.  Typical bolted and lapped/bolted connections were unable to meet the code requirements. 

After attempts to redistribute load using typical connections were unsuccessful, the Researchers began testing Simpson HD2A anchors attached to deck joists.  This method of connection for guard-posts has subsequently been included in DCA-6.  While the 200lb concentrated load has been a code requirement for some time, until now there have been no prescriptive guidelines for meeting the code.  Further details for guard post attachment can be found here. 

Further Reading:

Safe and Durable Coastal Decks

Professional Deck Builder

Gable-End Wall Design

Tuesday morning I received my Winter 2010 issue of Wood Design Focus.  For those not familiar, Wood Design Focus is a quarterly journal published by the Forest Products Society and is a membership benefit to people who join the American Wood Council.  This issue is especially important to myself thanks to the inclusion of my first article.  The link to the online journal is http://www.forestprod.org/wdf/ for anyone who wants to check it out.  Here is the abstract:

Improperly constructed gable end walls are a known weakness in residential structures in hurricane prone regions.  With many of today’s architectural designs using increased wall heights and roof slopes, gable end walls can even be a concern in buildings with wood roof framing systems in areas of low wind design speeds or seismic activity.    There are several references available for prescriptive design of these structural components for residential structures.  Although these references are subject to limitations, they provide a basis of design that can be used for any structure.  This article will discuss general design considerations, two basic methods for bracing gable end walls, prescriptive design methods available and their limitations, and provide recommended details for complete bracing of a gable end wall.  Balloon framing of gable end walls is outside the scope of this article. 

Follow-Up: June 30, 2011.  This article was picked up in the Spring 2011 issue of The Scope, a newsletter for the Birmingham Chapter of the Construction Specifications Institute.  You can link to the newsletter from my About page.   

OverSTRESSED

What an interesting week.  I have spent the last three days out of the office in full long-john gear, two pairs of socks and gloves and a boggin.  For the last two days I climbed into, through, and around 20 attics.  Most of the projects we see are under construction.  I learned a lot of things the last two days by observing roof trusses and framing that have been in place over a decade.  Particularly, I learned the importance of proper truss member bracing.  As shown in the picture, weak axis bending is a limiting factor in truss design.  The truss manufacturer relies on the installation of bracing to prevent this mode of failure.  When the bracing is not installed, the web members are overstressed and, as shown, can fail. 

Relocating Hold Down Anchors

Here is a common issue that can arise in the placement of hold down anchors.  In this picture, two cast in place anchors interfered with the installation of the hold down anchors.  The easiest fix from the Framer's perspective is to move the hold down to the nearest stud for a clean installation.  This presents several items that the Engineer needs to consider prior to approving this change. 

• As you can see in the picture, the fasteners provided with the anchor are too long for one stud.  Hold down anchor manufacturers generally require an attachment to a minimum of two studs.  Additional studs may be required to meet the manufacturer's specifications.
• If the hold down anchor is relocated away from the edge of the panel, does the shear wall diaphragm boundary element change? The tension load needs to be transferred into the stud pack with the hold down anchor.  I recommend requiring the specified edge nailing be used to fasten the sheathing to the new stud pack at the relocated hold down anchor.  
• By relocating the hold down anchor, the length of the shear wall changes.  This may be critical in a high wind or seismic area.  The design of the shear wall should be verified based on the field changes.

Merry Christmas and Happy New Year.