Residential Framing, Maximum Allowable Spans

This spreadsheet is derived from the previous example which listed the allowable wood stresses for various member sizes.  To know how far each member can span we must first describe the loads that are to be supported.  Use your scroll buttons to move to the right (past column "H").  You'll see that I have added two areas: "Loading Data" and "Loading Examples".

The example dead loads (weights of construction materials & finishes) are expressed in terms of pounds per square foot of floor (or roof) area.  Individual loads are listed in the "Loading Data" area.  You will need to overwrite the example loads when you work on your project.  The total of all dead loads is automatically updated as you complete each entry.

Project live loads (loads related to occupancy or use) are entered next. Live loads can include temporary roof loads during construction, snow loads, or residential floor loads (40 psf).

  • Tributary Width refers to the spacing of the members (in feet).

  • Stress Increase is always equal to 1.0 for floors, but can be greater than 1.0 for certain roof loading conditions.

  • Deflection Criteria are limits set for member deflection under dead (D) and dead plus live (D+L) loading situations.  For example:

  1. Roof joists that do not support a rigid ceiling - Hold L/180 max. (D+L)

  2. Joists or rafters supporting a gypboard ceiling - Hold L/240 max. (D)

  3. Joists supporting a cement plaster (stucco) soffit - Hold L/360 max. (D)

  4. Floor joists - Hold L/360 for dead loads and L/300 for dead + live loads.  In addition, live load deflection should be less than 1/2" total, regardless of span.

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Once all values are input, you can use the scroll buttons to move back to the far left of the window then downwards.  You will see that all of the members now have a maximum span listed based on the input loading.  But where did these come from?

Scroll back to the right.  In the Calculation Region you'll see that four checks are made:

  1. Against the listed bending strength.

  2. Against the listed shear strength.

  3. Against the desired dead load deflection limit.

  4. Against the desired dead plus live load deflection limit.

The smallest span of the four results becomes the maximum allowable span for that member.  With some bookkeeping, I also preserve the reason for the limiting condition - Strength, Area, Dead Deflection or D+L Deflection.  These are listed under Reason to the right of the Maximum Span values.

Knowing the maximum span for a member is helpful when checking existing construction, but does not really help us when designing new construction.  I'll begin to address this in the next example.

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