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. 

USDA Study of the Environmental Benefits of Wood

The USDA recently released a study of the science of the environmental benefits of using wood and wood products in construction.  In a previous post here, I wrote about the USDA's decision to support wood construction.  This recent study is a continuation of the USDA's path to officially supporting wood construction for public entity structures. 

USDA Report

Comments from SFI

Q&A with the President of AISC

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.

Deck Design Do's and Don't's: Part 1

I recently visited a wood framed observation platform that I am replacing with new designs.  The owner wants to keep a small section of the platform that is newer while replacing the older portion that is failing in some areas.  I will post some of the failure items over the next few weeks so as not to show everything at once.

For Part 1, we can discuss the importance of post caps when bearing a girder on a beam.  See the photograph above.  Reasons to use a post cap for this condition include:

1. Increases lateral strength of the connection.
2. Eliminates use of toe-nails, which when improperly installed can split the top of the post.
3. Contains movement of the wood girders relative to the top of post that results from the shrinking, swelling and twisting of wood as it reaches its equilibrium. 

Ultimately, you do not want the condition in the photograph to result in the photograph below. 

For more information about deck design, please see my previous post 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

Repetitive Member Factor: An Introduction

An increase in the reliability of wood members when placed into assemblies with repetitive use components has been recognized for many years by the wood industry.  The term assembly is typically defined as 3 or more members spaced at 24” on center or less and attached with load distributing elements capable of supporting the design loads.  Examples include floors, walls, roofs and three-ply beams with load distributing elements of wood structural sheathing or transverse mechanical fasteners. 

Floors and walls are typically solid-sawn members attached with plywood or OSB sheathing.  Roofs, however, are solid-sawn members or prefabricated trusses that have the top chord and/or bottom chord connected with structural wood sheathing.  The sheathing creates load sharing between the members of the assembly.  Three-ply beams share load through the mechanical fastening system used to build the member. 

The capacity of the assembly benefits from an increase in lateral stability of the members, partial composite action where the load is distributed lengthwise along the member, and load sharing where the load is distributed across parallel members that have different stiffnesses.  The parallel members having different stiffnesses result in varying amounts of deflection under uniform loads.  The load distributing element reduces the effects of various deflections by distributing the load away from more flexible members to stiffer members.  Members that are more flexible tend to be weaker than stiffer members, resulting in load being distributed away from weaker members and into stronger members.

Load sharing is accounted for through the use of repetitive member factors.  Since 1968, the National Design Specification for Wood Construction (NDS) has specified a repetitive member factor of 1.15 for bending design values of dimension lumber.  Also, since 1970, ASTM D245 has recommended a load sharing factor of 1.15 for bending stresses in multiple member systems.

Lessons from a Tornado

The April 27 tornadoes left many opportunities to study structural performance of both commercial and residential buildings.  One area that I have observed was a residential development located just out of the path of an EF3 tornado that made its trail not far from my home.  With the exception of one building's collapse, damage to each home generally consisted of a portion of roof missing, as seen in the photo below. 

A close up inspection of the rafter heel condition at the top plate highlighted the failure of the roof system.  A birds-mouthed rafter with (3) toe-nails into the wall top plate is a common framing detail used in residential structures.  This detail was used throughout this development.  There appear to be two realistic modes of failure for this detail as highlighted in the pictures below.  The first mode is failure of the 2x rafter at the birds-mouth.  This failure mode occurred in a small portion of rafters witnessed. 

The second mode of failure is at the toe-nail connection.  Although this connection works reasonably well for lateral shear, the typical limit state for uplift loading is nail withdrawal from the wall top plate.  The nails in the picture below appear to have seen very little stress during failure.     

Athough this is a common detail, I avoid detailing a rafter/truss to top plate connection with toe-nails. A Simpson H2.5A anchor, available at local hardware stores, is inexpensive and provides a significantly improved connection capacity. This syle of clip results in a more consistent installation and brings a different connection failure mode that does not include nail withdrawal. For a small home with a simple framed roof, such as homes in this development, a small investment in H2.5A clips can have a major impact on structural performance.

Cross-Laminated Timber News

Techniker Blog - Cross-Laminated Timber Construction for Schools

Structurlam Brings CrossLam to North America

Investment in CLT Lumber in Europe Continues

Tuscaloosa, Alabama Tornado Meets Wood Truss

I pulled the picture below from The Birmingham Business Journal's coverage of the April 27, 2011 tornado damage.  I have been slow to look at online pictures and videos of the event for a myriad of reasons.  I lived in Tuscaloosa for several years while studying for my BS and MS.  Even though many people have seen the devestation via TV, Web, or first hand, I could not resist sharing this picture here on the blog. 

A close look at the stick of wood embedded in the bumper of this car, and the wood lying on the ground in front of the car, reveals that this "stick" is actually from a metal plate connected wood truss.   I do not know the context of this picture to understand how this may have happened or the location of adjacent buildings from where this truss may have lived.  Whatever the process, the end result is amazing.  

Tornadoes and Home Construction

USA Today - Making a tornado-proof home is tough

I saved this post to publish this week. Not sure how it feels now. Thankfully, my family has made it through yesterday's tornadoes. Many families have not been as fortunate. Our thoughts and prayers are very important, but please don't forget to volunteer if possible or send supplies.

China Timber Deficit

Forecasts for China's Timber Supply Deficit

Hebei, China - A Growing Market

Clearwater Times - Hebei gets third truss plant, wood-frame research centre
Further growth in the Asian markets for wood construction.

Expected Increase in U.S. Exports

Japan's wood imports sure to rise
This may help our timber industry begin to put people back to work as demand rises.

U.S. Lumber Exports on the Rise

U.S. Lumber gets an Asian boost
 Home Channel News
Good to see that while we import many things from Asia, we can benefit from exports to the area as well.

USDA to Support Wood Construction

USDA to Support Wood Construction

It is nice to see recognition given to the sustainability if wood framing. By allowing alternate rating systems, such as Green Globes, portions of our industry certified under forest management programs not named FSC will benefit from this new direction.

Follow Through!

I spent April Fool's Day travelling to two different sites with wood frame structures.  While I observed several items that will eventually make this blog, one that stood out the most is shown below.  The anchor bolt installation was consistently inconsistent, with about 1 in 3 installed correctly.  From cut-off anchor bolts cast outside the edge of the sill plate to anchor bolts embedded into wood studs to the 1/2" diameter in lieu of 5/8" diameter, it was a free-for-all for picture taking.  But please, when everything comes together and all you have to do is place the nut on the bolt, just do it.  That nut has threads just waiting to be twisted onto the end of the bolt. 

Six-story Wood Framing Less Expensive than Steel

BTY Group cost analysis finds substituting wood for steel in building construction cuts costs – Daily Commercial News

As our economy is recovering ever so slowly, switching to wood framing may become more common. Similar stories have been published regarding construction here in the U.S. As good as this is for the wood industry, we need to ensure that qualified framers are used on commercial projects.

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.   

Cross-laminated Timber Production Aided by Government Investment

Cash injected into forest industry

Cross-laminated timber production is on the rise in Canada. I am looking forward to this product becoming an option for construction here in the U.S.

New Market Potential for Lumber

Lumber industry poised for post-earthquake rally
A comparison of Japan and Haiti is a quick way to see the benefits of wood construction used in high-seismic areas. An increase in lumber demand around the world will help our wood industry be less reliant on U.S. markets, which is good considering our current housing demand.

Wood First Legislation Grows Wood Demand

Quesnel Cariboo Observer - Wood First to grow demand

What a great way to show support for wood framing and local industry.

BC Housing Supports Wood-Frame Construction

It is both good and disheartening to see how well our neighbors to the North support their wood industry.  Unfortunately, while Canada pours significant money into wood research, especially with new products, the U.S. is more interested in the publically exciting research such as sustainability and green jobs.  We already have a market demand for wood products while the market for being green is still developing.  Investment in wood products is an investment in both the wood industry and sustainability thanks to woods inherent green characteristics. 

It Happens!

I find myself behind on this beautiful Monday morning so I decided to make a quick post with a bit of humor.  Without a good foundation (and maybe some hold-down anchors), everything else can go to crap real quick.  Please remember this the next time some tells you:

"Hold-down anchors?  We don't have hurricanes or earthquakes here!" 



Wood Truss Test Burn


After visiting a burned out apartment yesterday, I decided to post this video that I recently found.  This is an interesting test to show differences in wood truss framing and traditional stick framing.  This highlights the importance of fire stops and firewalls in buildings.  Limiting the fire's access to certain portions of the structure can greatly impact the overall fire resistance. 

I can say that I was impressed with the wood performance at the apartment complex I visited.  As you can see above, several studs were burned through and no longer supporting the top plate.  The top plate was also severely charred.  The trusses were still in place.  I would not recommend being in the area without shoring the trusses, but at least they did not fall. 

Timber Market Effects from Earthquake in Japan

http://www.sbcmag.info/kb/KB_SBC_NewsDetails.php?KBID=17249&prid=769&prin=2146170345

North American engineered wood products and other lumber materials may be vital to rebuilding damaged structures and homes in Japan.

AWC/NAHB Fire Resistance for N.C. Building Code

http://www.sbcmag.info/kb/KB_SBC_NewsDetails.php?KBID=17193&prid=768&prin=2146170345
Interesting topic with respect to engineered wood products versus sawn lumber. As we explore more applications of wood construction, fire resistance will become a larger portion of the equation.

Skyscrapers Next Challenge

Tall timber skyscrapers workable, report says

Believe it or not, a wood skyscraper could be in our future, especially with the BC government actively supporting its own forest industry.  Mr. Green's comments about context are dead on.  Today's context is not tomorrow's context.  We should allow new knowledge to expand our existing way of thinking.  Can't wait to see what happens next.

Sustainable Wood High-Rise in Australia

http://www.theage.com.au/victoria/wooden-highrise-to-bring-cheers-to-old-brewery-site-20110227-1ba1p.html


The use of wood as a sustainable structural material is increasing as the industry moves toward the use of life-cycle assessment to determine a materials 'green' properties. For this structure, wood will ultimately be responsible for insulating the building and providing a power source, in addition to being the material of choice for the structural framing.

The Mullions are just not that bad.

In our last installement, we looked at some rather unfortunate mullions framed in a stairwell.  Let's take a close look at the framing around the wall.  The upper portion of the wall is gable framed above trusses that bear at the top plate.  This is good because it allows the trusses to brace the top plate for out-of-plane wind resistance.  To lessen the axial load on the wall, a three-ply beam spanning the width of the stairwell was placed approximately 3-4ft inside the exterior wall.  Somehow, someone missed a few inches of framing, which resulted in the trusses NOT bearing on the exterior wall.  If the trusses do not bear on the wall, how can they brace the wall to resist out-of-plane wind pressure? 

Mulligan for Mullions

It has been a while since I looked at this picture, but I remember this condition quite well.  As some context, this condition occurs at the fourth floor level of a stairwell.  This wall condition spans from the roof framing down to the intermediate landing between the third and fourth floor.  If we look a bit closer, as in the photo below, we can see that only two studs run the full wall height at each side of the window.  This project was located in a region with a wind speed of 100mph.  As is, this condition required remediation to the mullions and the header-mullion connections to satisfy design requirements.  Had the mullions between the windows been constructed full height, instead of spanning between the headers, the load path would be easier to follow and require less remediation.  

 

Unbearable!

I do feel bad about not getting a second post for January.  I had a slight delay due to the birth of my first child.  My little girl weighed in at 6lb 11oz and made it into this world 13 days before her due date, which coincidentally was today.  I will try to make up for the lapse in posting with two pictures in one post.  (Yes I know, it is cheating when it shows the same thing.)

As the title hints, the corner of this wall is not actually bearing on the slab.  Since this corner is at a shearwall (hopefully the hold down anchor was installed after this visit), at some point during its life this stud pack will see a compression load.  Will this cause the building to fail?  Most likely not.  It will cause the interior gypsum board to buckle.  That would sure be a mess to clean up.  

A second picture below better indicates the true gap between the sill plate and the slab.  You can also see separation between some studs and the sill plate.  If someone were to tighten the anchor bolts to cinch the sill plate to the slab, more separation could occur, which does not correct the problem.  You could fill the gap with an epoxy grout.  This building uses prefabricated walls which should be fabricated to the correct height.  If you fill the gap, what happens at the top of the wall?  



  

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.