Tuesday, June 28, 2011

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.

Friday, June 10, 2011

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: