Flitch beam
Encyclopedia
A flitch beam is a compound beam used in the construction of house
s, decks
, and other primarily wood-frame structures. Typically, the flitch beam is made up of a 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 plate(s) within the beam are known as flitch plates.
Flitch beams can also be created from existing in-situ joist
s or beams permitting easier renovations.
For example compare the capacity of 2 beams spanning 18 feet:
the allowable strength of a 3½ʺ × 11¼ʺ laminated veneer member has a tabulated allowable applied load of 398 plf (pounds a linear foot);
compare this to a 2 × 12 flitch beam constructed of ½ʺ steel plate with two laminations of #2 SPF with an allowable applied load of 386 plf
The engineered lumber is cut to length and installed similar to sawn lumber; the flitch requires shop fabrication and/or field bolting.
This coupled with a much increased self weight of the beam (11.4 lbs for engineered wood vs. 25.2 lbs for flitch) decreases the viability of the system.
House
A house is a building or structure that has the ability to be occupied for dwelling by human beings or other creatures. The term house includes many kinds of different dwellings ranging from rudimentary huts of nomadic tribes to free standing individual structures...
s, decks
Deck (building)
In architecture, a deck is a flat surface capable of supporting weight, similar to a floor, but typically constructed outdoors, often elevated from the ground, and usually connected to a building...
, and other primarily wood-frame structures. Typically, the flitch beam is made up of a steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
plate sandwiched between two wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
beams, the three layers being held together with bolts
Screw
A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...
. 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 plate(s) within the beam are known as flitch plates.
Advantages
Because flitch beams are significantly stronger than wood alone, they:- require less depth than a wood-only beam of the same strength
- are much lighter than a steel beam of the same size
- can still be attached (e.g. nailed) to the rest of a wooden structure
Flitch beams can also be created from existing in-situ joist
Joist
A joist, in architecture and engineering, is one of the horizontal supporting members that run from wall to wall, wall to beam, or beam to beam to support a ceiling, roof, or floor. It may be made of wood, steel, or concrete. Typically, a beam is bigger than, and is thus distinguished from, a joist...
s or beams permitting easier renovations.
Disadvantages
Due to the high cost of labor, use of this type of beam has greatly declined. The advent of high-strength engineered lumber which uses modern adhesives and lower cost wood fibers has rendered this system largely obsolete.For example compare the capacity of 2 beams spanning 18 feet:
the allowable strength of a 3½ʺ × 11¼ʺ laminated veneer member has a tabulated allowable applied load of 398 plf (pounds a linear foot);
compare this to a 2 × 12 flitch beam constructed of ½ʺ steel plate with two laminations of #2 SPF with an allowable applied load of 386 plf
The engineered lumber is cut to length and installed similar to sawn lumber; the flitch requires shop fabrication and/or field bolting.
This coupled with a much increased self weight of the beam (11.4 lbs for engineered wood vs. 25.2 lbs for flitch) decreases the viability of the system.