A flitch beam (or flitched beam) is a compound beam used in the construction of houses, decks, and other primarily wood-frame structures. Typically, the flitch beam is made up of a vertical steel plate sandwiched between two wood beams, the three layers being held together with bolts. In that common form it is sometimes referenced as a steel flitch beam. Further alternating layers of wood and steel can be used to produce an even stronger beam. The metal plates within the beam are known as flitch plates. [1] Flitch beams were used as a cost-effective way to strengthen long-span wooden beams, and have been largely supplanted by more recent technology.
"Flitch" originally referred to a slab of bacon, which was cut into strips lengthwise. [1] Similarly, a wooden beam was flitched by cutting it lengthwise; one half was then rotated 180 degrees both longitudinally and laterally to ensure that any defects were separated. In the 18th century, before the availability of steel beams, pine beams were flitched with hardwood such as oak. [2]
With the availability of affordable steel, flitch beams became a way to strengthen long-span wooden beams cost-effectively while taking up less space than solid wood. An 1883 article from The American Architect and Building News compares three alternatives in a hypothetical railway station "in which the second story is devoted to offices, and where we must use girders to support the second floor of 25-foot span, and not less than 12 feet on centres if we can avoid it. This would give us, to be supported by the girder, a floor area of 12' x 25' = 300 square feet" [3] and 31,500 pounds of load. After performing calculations the beams compare as follows:
Beam Material | Size | Cost |
---|---|---|
Pine | 14" x 17" | $25.75 |
Iron | Two 12" girders | $83.32 |
Flitch-plate | 12" x 14" | $70.70 |
The article also cites the fire-retardant character of the flitch beam, "in case of a fire would not probably affect the iron until the wooden beams were badly burned." [3]
With the advent of high-strength engineered lumber, the advantages of flitch-beams disappeared. For example, comparing the capacity of 2 beams spanning 18 feet:
Beam Composition | Allowable Applied Load In Pounds Per Linear Foot |
---|---|
3½ʺ × 11¼ʺ laminated veneer lumber (LVL) member | 398 |
2 × 12 flitch beam constructed of ½ʺ steel plate with two laminations of #2 SPF | 386 |
Additionally, use of this type of beam has greatly declined due to the high cost of labor. Engineered lumber can be cut to length and installed much like sawn lumber; the flitch requires shop fabrication and/or field bolting. This, coupled with a much increased self-weight of the beam (11.4 pounds (5.2 kg) for engineered wood vs. 25.2 pounds (11.4 kg) for a flitch beam), decreases the viability of the system.
Flitch beams are currently mainly used in historic renovations, where they can be used to reinforce aged lumber supports, or for aesthetic purposes, where exposed beams with the appearance of wood and the strength of steel are required. An adaptive use project in the UK, changing stables into offices, required cutting the beam supporting a floor down its entire length, and then inserting a similarly sized steel plate. The resulting flitched beam was then secured with resin and bolts, preserving appearance while providing strength. Flitch beams were used as columns in a two-story new construction. Glulam beams were used to support the second floor and the roof. This allowed the appearance of wooden columns, while providing the necessary strength. [2]
The method for calculating the size of a flitch beam to be used in construction is straightforward, using the transformed-section method. The steel plate is treated as an equally stiff piece of wood, with its width modified by the ratio of their moduli of elasticity. This allows the deflection of the entire beam to be calculated as if it were entirely made up of wood. [4] [5] [6]
There is modest business activity involving flitch beams with The Timber Research and Development Association (TRADA) developing a new flitch beam, [7] a construction software program offering calculation for flitch beam designs, [8] [9] and at least one firm offering pre-fabricated flitch beams in various configurations. [10]
Lumber is wood that has been processed into uniform and useful sizes, including beams and planks or boards. Lumber is mainly used for construction framing, as well as finishing. Lumber has many uses beyond home building. Lumber is referred to as timber in the United Kingdom, Europe, Australia, and New Zealand, while in other parts of the world the term timber refers specifically to unprocessed wood fiber, such as cut logs or standing trees that have yet to be cut.
Engineered wood, also called mass timber, composite wood, human-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of wood, together with adhesives, or other methods of fixation to form composite material. The panels vary in size but can range upwards of 64 by 8 feet and in the case of cross-laminated timber (CLT) can be of any thickness from a few inches to 16 inches (410 mm) or more. These products are engineered to precise design specifications, which are tested to meet national or international standards and provide uniformity and predictability in their structural performance. Engineered wood products are used in a variety of applications, from home construction to commercial buildings to industrial products. The products can be used for joists and beams that replace steel in many building projects. The term mass timber describes a group of building materials that can replace concrete assemblies.
A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure.
A joist is a horizontal structural member used in framing to span an open space, often between beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joists serve to provide stiffness to the subfloor sheathing, allowing it to function as a horizontal diaphragm. Joists are often doubled or tripled, placed side by side, where conditions warrant, such as where wall partitions require support.
Glued laminated timber, commonly referred to as glulam, is a type of structural engineered wood product constituted by layers of dimensional lumber bonded together with durable, moisture-resistant structural adhesives so that all of the grain runs parallel to the longitudinal axis. In North America, the material providing the laminations is termed laminating stock or lamstock.
A plate girder bridge is a bridge supported by two or more plate girders.
A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck, roof covering and its associated loads. A pair of rafters is called a couple. In home construction, rafters are normally made of wood. Exposed rafters are a feature of some traditional roof styles.
Framing, in construction, is the fitting together of pieces to give a structure, particularly a building, support and shape. Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called mass wall construction, where horizontal layers of stacked materials such as log building, masonry, rammed earth, adobe, etc. are used without framing.
A tie, strap, tie rod, eyebar, guy-wire, suspension cables, or wire ropes, are examples of linear structural components designed to resist tension. It is the opposite of a strut or column, which is designed to resist compression. Ties may be made of any tension resisting material.
Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework supports the shuttering molds. In specialty applications formwork may be permanently incorporated into the final structure, adding insulation or helping reinforce the finished structure.
A plate or wall plate is a horizontal, structural, load-bearing member in wooden building framing.
A girder is a beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing flanges separated by a stabilizing web, but may also have a box shape, Z shape, or other forms. Girders are commonly used to build bridges.
A girder bridge is a bridge that uses girders as the means of supporting its deck. The two most common types of modern steel girder bridges are plate and box.
The Saltwater River Rail Bridge is a large steel arch truss railway bridge completed in 1858 and crossing the Maribyrnong River on the Melbourne to Footscray railway in Melbourne, Victoria. It had the longest span of any bridge in Victoria for thirty years.
A Howe truss is a truss bridge consisting of chords, verticals, and diagonals whose vertical members are in tension and whose diagonal members are in compression. The Howe truss was invented by William Howe in 1840, and was widely used as a bridge in the mid to late 1800s.
Composite construction is a generic term to describe any building construction involving multiple dissimilar materials. Composite construction is often used in building aircraft, watercraft, and building construction. There are several reasons to use composite materials including increased strength, aesthetics, and environmental sustainability.
Cross-laminated timber (CLT) is a subcategory of engineered wood panel product made from gluing together at least three layers of solid-sawn lumber. Each layer of boards is usually oriented perpendicular to adjacent layers and glued on the wide faces of each board, usually in a symmetric way so that the outer layers have the same orientation. An odd number of layers is most common, but there are configurations with even numbers as well. Regular timber is an anisotropic material, meaning that the physical properties change depending on the direction at which the force is applied. By gluing layers of wood at right angles, the panel is able to achieve better structural rigidity in both directions. It is similar to plywood but with distinctively thicker laminations.
In structural engineering, the open web steel joist (OWSJ) is a lightweight steel truss consisting, in the standard form, of parallel chords and a triangulated web system, proportioned to span between bearing points.
American historic carpentry is the historic methods with which wooden buildings were built in what is now the United States since European settlement. A number of methods were used to form the wooden walls and the types of structural carpentry are often defined by the wall, floor, and roof construction such as log, timber framed, balloon framed, or stacked plank. Some types of historic houses are called plank houses but plank house has several meanings which are discussed below. Roofs were almost always framed with wood, sometimes with timber roof trusses. Stone and brick buildings also have some wood framing for floors, interior walls and roofs.
A plyscraper, or timber tower is a skyscraper made of wood. They may alternatively be known as mass timber buildings.