How to Design a Wood Flitch Beam

A wood flitch beam is a composite beam containing some amount of steel (usually steel plates) to increase its strength. Flitch beams are typically built from a combination of visually graded lumber and a steel plate. The most common configuration for this type of beam is to sandwich a long, narrow steel plate between two or more pieces of lumber. Adding steel to a wood beam can significantly increase its bending and shear capacity. While flitch beams are becoming less common due to widespread availablity of engineered wood and other materials, there are still a few cases where reinforced visually graded lumber makes sense, including: asthetics, cost, availablility and servicability (reinforcing existing beams).

For a starting size, the flitch beam span table gives the longest span of 2x8, 2x10 and 2x12 flitch beams with common plate thicknesses, and the flitch beam calculator checks one for your own span and load.

Regardless, flitch beam analysis and design is not that difficult. The key to this type of analysis is to transform the steel plate properties to equivalent wood properties, as shown in the image below.

3pedals, CC BY-SA 4.0, via Wikimedia Commons

Much like other materials and shapes, flitch beams can be designed in just a few steps.

  1. Select a size: pick the lumber and the number of plies (usually 2 for a flitch beam).
  2. Add the steel: add the steel plate to the lumber cross-section.
  3. Choose a material: pick the wood species and grade.
  4. Add spans and supports: enter the distance the beam spans.
  5. Add loads: enter loads by type and load case.
  6. Select a design method: in the United States, the National Design Specification (NDS) from the American Wood Council is common.
  7. Review the bending, shear and deflection checks.

In this tutorial we'll use the beam designer to work through each of these steps. When we're done, we'll have a simply supported flitch beam carrying a uniform load, checked for bending, shear and deflection.

If you'd like to follow along, open the beam designer in a separate tab. Each panel at the top of the page opens one step of this tutorial: material, shape, design specification, spans, loads and bracing. Plies and steel plates need a Pro account; the free flitch beam calculator checks a simple-span flitch beam without one.

The beam designer with its six panels, the beam drawing and the design checks

Let's get started

For this example, we'll assume the following:

  • Our flitch beam spans 11 feet and supports 2x8 joists at 16" spanning 11 feet on each side.
  • The maximum depth of the beam is 7 ¼".
  • The load is 40 psf dead load and 10 psf live load.
  • Lumber is Douglas Fir-Larch, Select Structural.

Select the Lumber and Plies

Our requirements state that the depth cannot exceed 7 ¼", which is the depth of a standard 2x8. This is the size we'll start with.

Click the Shape panel to open the Select a Shape dialog. Click Dimensional Lumber, set Number of Plies to 2, and click the 2x8 row.

The Select a Shape dialog with Dimensional Lumber, 2 plies and a 2x8 picked

The panel on the right shows the shape and turns red when it fails under the current loads. Two 2x8s on their own aren't enough for this beam, so we'll add a steel plate.

Add the Steel Plate

A very common layout for flitch beams is a single steel plate sandwiched between two pieces of lumber. A symmetric cross-section is preferable.

In the same dialog, check Add steel plates (Flitch Beam). Four inputs for the steel plate appear. Enter:
  • Plate thickness, t = 0.375 in
  • Plate height, h = 7.25 in
  • Number of plates = 1
  • Steel yield strength, Fy = 50 ksi (the default; use 36 for A36 plate)
The steel plate is assumed to have E = 29,000 ksi. Its bending contribution is 0.6·Fy·Zx per plate (Zx = t·h2/4) and its shear contribution is 0.6·Fy·t·h per plate. To take the steel out, uncheck Add steel plates (Flitch Beam).

The steel plate inputs: 0.375 in thick, 7.25 in high, 1 plate, 50 ksi

With steel plates added, the beam designer transforms the steel properties to equivalent wood properties. It first works out a transformation factor from the elastic (Young's) modulus of each material:

n = Esteel/Ewood

This factor is multiplied by the shape properties of the steel plate (Ix, Sx and A). The transformed properties are added to the wood properties, giving the total composite shape properties used for the design.

Close the dialog. The Shape panel now says 2x8 Flitch (277.034 in4). The number in parentheses is the transformed moment of inertia about the strong axis.

The Shape panel reading 2x8 Flitch (277.034 in4)

Choose a Material

Next, pick the species and grade of the lumber. For this example we're using Douglas Fir-Larch, Select Structural.
Click the Material Grade panel to open the Dimensional Lumber Species and Grade dialog. Open Douglas Fir-Larch and click Select Structural.

The species and grade dialog with Douglas Fir-Larch Select Structural picked

Add Spans and Supports

Our beam spans 11 feet and, as is common for wood beams, is simply supported at both ends.

Click the Spans panel to open the Edit Spans dialog and set Span 1 to 11'. A new beam starts with two spans; remove the second one so only one is left. Add Left Span and Add Right Span add more.

The Edit Spans dialog with one 11 ft span

To change a support, click it in the drawing. Each click moves it to the next of three support types: Pinned, Fixed or Free.

For our example both supports should be Pinned. Click them in the drawing until both are.

Configure Bracing

Because joists frame into our beam, we'll assume it is continuously braced. You can change this to custom or equally spaced braces in the Bracing panel.

Continuous bracing is the default. Check that the Bracing panel says Continuous Bracing.

Add Loads

The beam designer handles the common load cases and load types for wood beams. For more about load cases and load types, see the steel beam tutorial.

Our beam carries 40 psf dead load and 10 psf live load from 5.5 feet of floor on each side, 11 feet in all. As loads along the beam, that's:

  • Dead load = 40 lb/ft2 × 11 ft = 440 lb/ft = 0.44 k/ft
  • Live load = 10 lb/ft2 × 11 ft = 110 lb/ft = 0.11 k/ft
Click the Loads panel to open the Loads dialog. It lists the beam's loads; click one to edit it, or use the buttons at the top to add a uniform, linear, point or moment load.

The Loads dialog with one uniform load

Click the uniform load to open Edit Uniform Load. Click Left End and Right End so the load covers the whole beam. Check Show All Load Cases (Design Mode), enter 0.44 for Dead Load (D) and 0.11 for Live Load (L), then click Save. If the beam has any other loads, open each one and click Delete Load.

Edit Uniform Load with 0.44 k/ft dead load and 0.11 k/ft live load over the full 11 ft

Your model should now look like this:

The finished model: a 2x8 flitch beam spanning 11 ft with the uniform load

Select a Design Method

When you choose a wood shape, the beam designer switches the design specification to NDS. You can choose Load and Resistance Factor Design (LRFD) or Allowable Stress Design (ASD) load combinations, or plain capacity checks. We'll use NDS with ASD combinations, which is common for wood.

Click the Design Specification panel and click ASD. Keep the default deflection limits: L/360 for live load and L/240 for dead plus live load.

The Design Specification dialog set to NDS 2024 ASD with its deflection limits and ASCE 7-22 load combinations

Load Combinations

The beam designer factors your loads and checks every load combination in the list at the bottom of the dialog.

Review the Results

The beam designer checks the beam again after every change, so there's nothing to press. It runs a finite element analysis of the beam, including the steel plate, and shows the results under the drawing. Green means the bending, shear and deflection ratios are all under 1.0.

If you entered everything correctly, your results should look like this

Bending 0.75, shear 0.41 and deflection 0.64, all passing, for the 2x8 flitch beam

Scroll down to see the deflection, shear and moment diagrams for each load combination and service case.

And that's it! Wood flitch beam design in just a few clicks.

If a check is red, its ratio is over 1.0 and the beam doesn't meet the design criteria. You need a bigger wood shape, more plies or more steel. For example, make the plate thicker, add a plate, or add a ply of 2x8 to get a larger moment of inertia. The results update as you change it.

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