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flitch beam

Types of Beam Design

Types of Beam Designs

There are many different types of beam designs and materials to choose from when designing a structure. Engineers can choose from various shapes, sizes, construction materials, and construction techniques. Deciding on the proper beam design for a particular structure can be a complicated process. Structural engineers and builders have many different beam designs and materials to choose from when attempting to create a sound structural design.

Cantilever Beams

Cantilever beam designs create a suspended effect. These beams allow the creation of a bay window, balconies, and some bridges. In cantilever beam designs, the weight load is distributed back into the main beams of the structure, allowing a portion of the structure to extend beyond the supported perimeters of the structure’s foundation.

Steel I Beam

Steel I beams are very popular choice in construction. The I beam is shaped like a capital I also know as a W shape. The I beam design is the most efficient use of structural steel since it moves the bulk of the steel into the portions of the beam actually resisting the loads. The I beam design is the most common foundational beam design found in commercial structures but can be used in Residential design.
Flitch Beam

Flitch beam designs are composite beams made from layering steel and wood to create a lightweight beam with adequate strength. The addition of wood elements allows the beams to be nailed to existing wooden structures.  Flitch beams are less expensive than solid steel beam designs. They are used to support heavy vertical loads while maintaining a strict construction budget.  Flitch beams are also very useful when adding additional load carrying capacity to an existing beam.
Hip Beam Designs

Hip beam designs are popular in roofing designs. A hip beam provides support for other load bearing beams branching off at symmetrical angles. This design is often used in residential construction.
Beam Materials

The various types of beams may be constructed of various materials as well as a mixture of shapes and sizes. Some beams are made of pre-stressed concrete, poured concrete, iron, wood, glulams, and other composite materials.
Beam designs vary so greatly that making the correct choice when designing a structure can be challenging. Structural engineering software can help take the guesswork out of the design process. Structural engineering software can also help an engineer decide if the desired beam designs are appropriate for the structure.

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Elements and Examples of Beam Design

The elements of beam design is a topic of great interest for structural engineers and contractors. Beam design is integral in the design and construction of a structure. Most structural beams are comprised of wood, steel or concrete. Each of these construction materials reacts differently under the stress of a load. Each also has its own unique advantages.

Elements and Examples of Beam Design: Concrete Beams

Concrete beams are most often seen in commercial construction, such as in the erection of multi-level parking decks, hospitals, and large hotels. Concrete beams are also commonly used as bridge and highway supports.  Some concrete beams are used in conjunction with steel beams to provide added strength. Newer concrete beams may also contain a hybrid material of traditional concrete mixed with Glass Fiber Reinforced Polymer (GFRP) or Carbon FRP.

Concrete is a strong building material, but it is susceptible to water damage and cracking.  Iron bars are often included in the beams to add strength and stability over areas prone to greater stress. Concrete beams area also desirable for their ability to absorb sound and vibration.

Elements and Examples of Beam Design: Steel Beams

One very common type of steel beam is the I-beam. These I shaped beam are strong and moderately affordable. Steel beams are capable of supporting heavy loads without experiencing great amounts of deflection by distributing the load of the structure over the flange of the beam. Steel beams may be treated to prohibit corrosion and oxidation, especially when used near or under water, such as in bridge construction.

Elements and Examples of Beam Design: Wood Beams

Wood beams are common in residential structures. Wood beams may be notched or jointed together for added strength. Wood beams are inexpensive and easy to alter to a builder’s specifications. However, they are also susceptible to rot and insect infestation.  Specially treated wood beams are now available that resist decomposition, moisture and insects, making them an attractive choice in beam materials for most homeowners.

Elements and Examples of Beam Design: Flitch Beams

Flitch beams are specially constructed beams that join a steel plate with adjacent wood panels to form one composite structural beam. These flitch beams are strong, yet less expensive and lighter than solid steel beams.  The construction of a flitch beam results in a reduction of the overall size of the beam, and the wooden exterior also allows the builder to nail the beam to other existing wooden structures in the home.

Elements and examples of beam designs are plentiful. Beam design and selection are an important part of the construction process and the wide variety of beams to choose from allow a builder to meet the needs of each project more easily.

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Flitch Beam Bolting

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

 Flitch Beam Bolting

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.

 Flitch Beam Bolting Bolt

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.

Flitch Beam Bolting Number of Bolts

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

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Flitch Beam Design & Software

Accurate Flitch Beam Design Made Easier with Software

Flitch beam design software is a useful tool for architects, engineers, designers, and builders. Flitch beams are a common type of composite construction. Composite construction materials are formed by combining two or more materials in a way that allows them to function as a single component structurally. Flitch beams are created by layering wood beams with steel plates or plywood in order to form a wider, lighter structural beam. Bolts hold the layered components together to form a single unit.

These advantages make flitch beams a desirable and attractive choice in light frame construction projects:

  • they can support heavier loads over longer distances
  • are thinner than solid wood or steel beams with similar load-bearing qualities
  • can be nailed to other components of wood structures during construction
  • are much lighter than solid beams.

Using computer software to design a flitch beam can greatly improve the cost effectiveness of a project by allowing for a more exact and efficient design.  Software packages precisely calculate the needed thickness, depth, and length of each beam much easier than any kind of hand calculation.

Utilizing flitch beam design software eliminates the possibility of using beams that are too thick or too closely spaced together.  This can drastically reduce construction costs by allowing each beam to be more fully utilized to its capacity.

One of the more difficult calculations associated with a flitch beam is that of the Deflection the beam will undergo.  Software packages will carefully calculate the deflection of flitch beams.  Properly constructed flitch beams ensure that all of the components deflect by exactly the same value. The relative stiffness value of steel and wood is vastly different.  When used correctly, structural analysis software will accurately determine the proper interaction of multiple materials.

Bolt size and Spacing in the construction of flitch beams is crucial. A separate article will briefly discuss a simple way of determining the bolt spacing for a flitch beam.

Flitch beam design software is a must-have tool for the careful architect, engineer, or designer.

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