Flitch Beam Design: How to Calculate & Design Bolting for Flitch Beams

How to Calculate & Design Bolting for Flitch Beams

Designing the connection between the solid sawn members and the steel members of a flitch beam can be challenging, so in this article, we will review the methodology for attaching the different materials of a flitch beam so that all the materials act as one solid member.

What is a Flitch Beam?

A flitch beam is a steel plate pinned in place between two wood beams by bolts. Flitch beams are useful because they increase strength without the full cost of a steel beam. However, the use of flitch beams has waned due to the rise of composite wood, which can offer similar benefits at lower prices. 

Despite their diminishing use, there are still occasions where a flitch beam will be preferred over alternatives. In these cases, you need to know the calculation to appropriately transfer loads to the wood and steel portions of the beam in proportion to the relative stiffness of each material. There are two primary methods to do this: empirical and rational.

Flitch Beam Calculations

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. While this method takes more time, it provides more confidence in the design for a given structure. Since flitch beams are being used less often these days and used in more particular builds, the rational method is generally advisable.

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

Next, determine the 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.

You will need end bolts to transfer the steel plate load to wood members for bearing, 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.