What Can We Tell from This Photo? (Case Study 6)

Posted by in Simple Forensic Methods, Wood | April 06, 2015

 

During our moisture source evaluations we sometimes observe other conditions that we bring to the attention of the owner.  Such is the case for this week’s topic.

Figure FT-1 Fractured web member of wood truss.

In the featured photo and Figure FT-1 we see a fractured web member in a wood truss.  What can we tell from the photographs of this fracture?

  1. The fracture occurred at a knot that extended across the full width of the web member.
  2. The knot in the fracture indicates that the member was severely weakened by the knot.  It is possible that the fracture initiated at the knot from shrinkage.
  3. The variation in the discoloration of the fracture indicates that it was a progressive failure, that is, it didn’t completely fracture at one point in time, but over a period of years.
  4. The four different degrees of discoloration indicate that the fracture progressed significantly on four different occasions which were separated by a significant period of time (see the featured photo).
  5. The relatively heavy accumulation of dust on the fracture indicates that the fracture initiated years ago.
  6. The discoloration on the cleaner fracture surfaces indicates that the latest progression of the fracture occurred months to years ago.
  7. The relatively uniform pattern of raised fibers along the far edge of the fracture indicates that the fracture was also affected by a lateral bending or buckling (see the featured photo).
  8. The relatively long web has no lateral bracing and is susceptible to lateral buckling from a compressive load or excessive lateral bending from being pulled or pushed on by a grasping hand for temporary support during access in the attic.

Figure FT-1 Close-up of fracture in web member of wood truss.

A fracture of this kind can often be repaired by sandwiching the fractured web with two 2×4’s that are properly fastened with nails screws,  or bolts on each side of the fracture.  We recommend that the details of repair be overseen by the truss manufacturer, a structural engineer, or possibly a qualified contractor experienced in the repair of damaged trusses.  If there is any doubt or concern, we recommend that a structural engineer be consulted to review the conditions and offer repair recommendations.  A poor repair could lead to another fracture and additional damages.

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