An Oversight Makes a Rafter More Susceptible to Fracture

Posted by in Construction & Shortcuts, Roof, Structural Analysis, Wood | December 07, 2015

In the featured photograph we see where the rafter fractured at its end.  This fracture was due to vertical tension in the rafter.  That is, the weight of the roof framing and the snow were pushing downward along the length of the rafter and the bearing plate at the end was with an equal and opposite force pushing upward.  The pushing down along its length and the pushing upward at the small area of bearing at the end of the rafter caused the rafter to be stretched across its depth.

The greatest magnitude of the stretching occurred where the end of the rafter begins to rest on the bearing plate (see Figure FR-1).  The fracture initiated at the point of the greatest magnitude of stretching and over time the fracture extended across the length of the rafter as the point of the greatest magnitude moved down along the length of the rafter.

Fracture at end of rafter starts at edge of board bearing plate.

Figure FR-1 Fracture at end of rafter starts at edge of board bearing plate.

If the area of the bearing at the end of the rafter had been longer, the magnitude of the stretching would have been less.  That is, the equal and opposite force pushing upward would have been spread out over a wider area, thus reducing the stress on individual wood fibers.

Since, in the subject case, the bearing area is about one-sixth of the full depth of the angled cut at the end of the rafter, the stretching stresses at the fracture were about six times greater than they would have been had there been a full depth bearing.  Safety factors in lumber design, inherent excess strength in the lumber, and less than design snow loads over the life of the roof have helped to reduce this overload of a factor of six during the service life of the rafter.  However, when heavy snow and ice accumulations exceeded the design snow load in the subject case, the rafter was severely over-stressed and it fractured.  If the rafter been installed with a proper bearing, it probably would have even withstood the excessive weight of ice and snow.

For the want of full end bearing a roof was lost.

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