Rational Method

In a separate article entitled “Accurate Flitch Beam Design Made Easier with Software” there was an allusion to the difficulty associated with designing the connection between the solid sawn members and the steel members of a flitch beam.  In this article there will be a more in depth discussion on the methodology for attaching the different materials of a flitch beam so that all the materials act as one solid member.

Flitch beams must be connected together to appropriately transfer loads to the wood and steel portions of the beam in proportion to the relative stiffness of each material.  Most structural engineering software packages don’t provide this calculation; two sample methods are provided below for determining this connection.

Empirical Method

The first method is an empirical method, which is purely based on what has worked well in the past.  An example of a regular bolting pattern might be 1/2 inch diameter or 5/8 inch diameter bolts spaced 16 inches on center.  Stagger the bolts and make sure the bolts are placed a minimum of 2 1/2 inches from the edge of the beam.

Rational Method

The alternative to the empirical method is the rational method.  Using the rational method the load transfer between the steel and wood members is actually calculated.  The first step in the rational method is determining the percentage of load that is carried by both the steel and wood portions of the beam.  If structural engineering software was used to size the flitch beam then somewhere within the software there should be a display of the load transfer percentages.  If the flitch beam was sized by hand, then the load transfer percentages can be determined from the modular ratio that was calculated.  The load carried by the steel plate can then be determined by multiplying the percentage of load carried by the steel plate by the total load on the beam.  After the load has been determined bolts can then be sized by using tables found in the National Design Specification.

Example Calculation

Now, determine capacity of 5/8 inch diameter bolts for loads traveling perpendicular to the grain of the wood.  For simplicity, use table 11B of the National Design Specification.  This is a table for single shear bolt capacities.  This is conservative since the flitch beam being sized actually has bolts in double shear.  Higher values can be calculated using the six yield equations.

End bolts required to transfer steel plate load to wood members for bearing are required unless the steel plate bears on a steel bearing plate.

Final Considerations

This is just one example of how to design the bolting for a flitch beam; there are certainly other valid methods and assumptions that will provide an adequate design.  When doing any kind of beam design, especially a flitch beam using structural design software will greatly ease the entire process of calculating adequacy.  There are several different engineering design software packages available for beams, columns, or foundation design.  StruCalc, Enercalc, Risa, and BeamChek are all examples of such software.beam design using structural design software will greatly ease the entire process of calculating stresses.  There are several different engineering design software packages available for beams, columns, or foundation design.  StruCalc, Enercalc, Risa, and BeamChek will all take in to account normal and shear stresses when doing any kind of beam design.

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