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. 

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.

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.   

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

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.