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.

James DiNardo, P.E.
Josh Parker, E.I.T.
Cascade Design Group

Normal Stress
A normal stress is a stress that occurs when a member is loaded by an axial force.  The value of the normal force for any prismatic section is simply the force divided by the cross sectional area.

A normal stress will occur when a member is placed in tension or compression.  Examples of members experiencing pure normal forces would include columns, collar ties, etc.

Bending Stress

When a member is being loaded similar to that in figure one bending stress (or flexure stress) will result.  Bending stress is a more specific type of normal stress.  When a beam experiences load like that shown in figure one the top fibers of the beam undergo a normal compressive stress.  The stress at the horizontal plane of the neutral is zero.  The bottom fibers of the beam undergo a normal tensile stress.  It can be concluded therefore that the value of the bending stress will vary linearly with distance from the neutral axis.

Calculating the maximum bending stress is crucial for determining the adequacy of beams, rafters, joists, etc.

Shear Stress
Normal stress is a result of load applied perpendicular to a member.  Shear stress however results when a load is applied parallel to an area.  Looking again at figure one, it can be seen that both bending and shear stresses will develop.  Like in bending stress, shear stress will vary across the cross sectional area.

Calculating the maximum shear stress is also crucial for determining the adequacy of beams, rafters, joists, etc.

Final Considerations
When doing any kind of 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.

Josh Parker, E.I.T.
Cascade Design Group

• Determine the loads
• Calculate the stresses
• Check the allowable stresses against the actual stresses.

Determine the Loads
The first step in the structural analysis of a beam is determining the amount of load, or weight the beam is going to support.  There are two major categories of loads:

Live Loads - A live load is a type of load that is temporarily placed on a structure (i.e. loads from snow, wind, vehicles, etc.).  The magnitude of live loads will be defined or referenced in a local building code.

Dead Loads - are loads permanently attached to a structure (i.e. loads from building materials, furniture, etc.).  Sometimes the weights of materials are exactly known and can be added together to determine the total dead load.  More often the dead load is assumed and given an approximate weight.

Calculating the Stresses
There are two types of stresses that are typically calculated when performing a beam design: bending stress and shear stress.  A more complete definition of both bending stress and shear stress can be found here.  In order to calculate the bending and shear stresses it will be first necessary to calculate the maximum bending moment and maximum shear that occurs in the beam.

The maximum moment and shear will most likely occur at different locations, and the process used to determine their value will be defined in a separate article.  The other two pieces of information needed to determine the stresses will be the section modulus and cross sectional area of the beam being used.  The section modulus and cross sectional area can be calculated, or in most cases can be looked up in tables (like in the National Design Specification (NDS) for wood beams, or the AISC Steel Manual for steel beams).  Once all the information has been tabulated the following equations can be used to determine the nominal maximum bending stress and nominal maximum shear stress:

Compare Actual Stresses against Allowable Stresses
In most cases the allowable stresses are tabulated in a design manual of some sorts (like in the NDS for wood, or the AISC Steel Manual for steel).  Once the allowable stresses have been located determining the adequacy of a beam is simply a matter of comparing the actual stresses to the allowable stresses.  So, a beam is adequate if the following is true:

Other Considerations
One major consideration not discussed in this article is that of deflection, or sag in the beam.  A beam might be strong enough structurally, but might deflect so much that it effects the actual performance of the beam.  Deflection is a calculation that is very important and will be addressed in a separate article.

Another consideration when doing any kind of beam design is that of using structural design software.  There are several different engineering design software packages available for beams, columns, or foundation design.  StruCalc, Enercalc, Risa, and BeamChek are a few examples of those structural design software packages.

Josh Parker, E.I.T.
Cascade Design Group