Friday, June 1, 2012

New Southern Pine Design Values

Today is the day.  June 1, 2012 is the effective date for the new design values for No. 2 Dense and lower grades of visually graded Southern Pine lumber.  The changes are for lumber that is 2"-4" thick and 2"-4" wide.  The Addendum to the National Design Specification for Wood Construction by the American Wood Council can be found here.  Please note that this is only for visually graded lumber and does not affect machine stress rated lumber. 

For engineers, the severity of the change in design values for 2x4's will have less impact than what may be expected. (Not true for truss manufacturers.)  We typically only use 2x4's as wall studs; and for many projects we are using 2x6 wall studs as directed by an Architect.  This results in 2x4's being a small portion of a typical project. 

Moving forward, is it only a matter of time before the other grades and sizes of Southern Pine have reduced design values.  If you want to see what those other design values may look like, click here for information from the Southern Pine Inspection Bureau. 

I have already been asked the question of changing from Southern Pine to Spruce Pine since the new design values have changed. If you allow the change, you will need to make adjustments to all of the connection designs to account for the lower specific gravity of Spruce Pine.  This makes a change much more complicated than just checking the stud strength. 

Friday, May 11, 2012

Deck Design Do's and Don'ts: Part 3

Sawn lumber, unless protected, is highly susceptible to shrinkage, swelling, twisting, and bending when exposed to moisture.   For an outdoor deck, lumber is exposed to rain and changes in humidity, the latter of which can be a nightmare in Alabama.  The deck in the picture below shows signs of joist failure.  One saving grace for the deck is the fact that the two test stations that have toppled over are constructed of thin, hollow metal and are much lighter than they may appear.  

The second picture, shown below, shows how the joists are being held up by the deck.  The 1x ledger board used to support the joists is still in place.  Unfortuntaly, the beam has rotated away from the deck, allowing the ledger to slide out from under the joists.  The beam's rotation and deflection were exacerbated by the effects of weather exposure.  The few nails that connected the beam to the joists were nailed from the beam and into the end grain of the joist, leaving very little resistance to lateral separation.  The beam's lack of resistance to moving laterally allowed the effects of loading and weather to combine; ultimately causing failure of the deck without failure of the beam or joists.   

Small changes in the construction of the above deck, such as toe nails or joist hangers, could have made a big impact on the decks performance.  Thankfully, the employees working at the facility were unharmed. 

Statistically, deck collapses are responsible for more people being injured or killed than any other part of their residence.  Taking the time to consider low cost, high impact changes in all aspects of construction should be at the forefront of every builder.   

Click here for Deck Design Do's and Don'ts Part 2 and here for Part 1. 

Thursday, May 3, 2012

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. 

Tuesday, April 17, 2012

Possible World Record Log Truss

Nothing tops this truss - Black Hills Pioneer: Local News: DEADWOOD — There are log trusses and then, well, there's the 80-foot behemoth log truss recently handmade over a 10-week period by the blood, …

Very interesting project for both log cabin folks and structural building component folks.  I would be interested to know the amount of design load the truss will carry and what type of support reaction it will produce.  Following the load path for that monster would be a fun task.

Monday, March 12, 2012

Monday, February 6, 2012

Kerfing a Bowed Stud

Wood studs are not perfect and never will be.  Neither will any other material, plant, etc. that is made in nature.  Through experience, science, and understanding, we learn how to manipulate natural materials to be suitable for our needs.  Trees, which have branches in many aspects of our lives (pun intended!), are a top renewable resource.  After centuries of harvesting tress, and thus lumber, to build shelters, etc., you would think our understanding of the material would be very thorough.  But as some say, it can be hard to see the forest through the trees.  In other words, sometimes you need to see the big picture.  Below is an example of an instance where the big picture reveals much more than what is in front of you 

It is common for carpenters to kerf a wood stud to remove a bow in the stud to allow for the wall finish to be flat.  Depending on the function of the wall, such as load bearing, the kerf in the stud may be reinforced with a section of 2x material lapped and fastened to the full height stud.  Regardless of how the stud is kerfed, reinforced, etc. the most important piece of knowledge involved is the function of the stud.  Knowing what the stud supports can result in an appropriate repair solution. 

These 2x6 studs have a kerf that leaves just over an inch of solid material. 

Now, back to that forest and tree stuff.  The worker who cut these studs could only see the bowed stud (tree) in front of him.  Had he looked at the big picture (forest), he might have realized that kerfing a first floor stud in a four story building might not be a good idea.  And yes, this wall does carry floor trusses from each floor above. 

Tuesday, January 17, 2012

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.