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A bar joist is a structural component used to frame floors or roofs. The bar joist consists of a pair of parallel chords connected by a series of intersecting supports known as "webs." Bar joists feature a similar design to trusses, including the same triangular web pattern, but are generally smaller and more lightweight than most trusses. The design of these joists helps distribute the weight of a floor or roof structure to the walls or foundations at either end of the joist. A bar joist may be made from wood, steel, or composite materials depending on application and loads.
Each bar joist must be carefully selected based on the loads it will support. This includes the weight of building materials, as well as people and furnishings. A structural engineer or architect can help installers select these joists using information such as required span, building design, and local building code requirements.
Many manufacturers and industry personnel use an alpha-numeric identification system, such as 12K8, to categorize different types of bar joist. The joist often contains a stamp or label containing this code, which can help buyers and installers identify the joist. The code starts with a number, which represents the depth of the joist in inches. Each depth measurement is followed by a group of letters, which represents the joist design. A "K" indicates a standard joist, while "CS" is used to identify joists designed to support concentrated loads. Other specialty designations are used to identify joists that are very long, deep, or oversized.
At the end of each code is a second number of set of numbers. This figure identifies the type of chords used in each joist. Each particular chord size and thickness corresponds to a matching number, which is constant across different joist materials.
Bar joists offer several advantages over traditional beam or girder construction. Most joist systems are pre-manufactured in factories, and arrive on job sites ready for erection. This helps to speed up the building process and reduce labor costs. A pre-manufactured bar joist is also more precisely constructed than most structures that are framed in the field. This improves building safety and results in a more durable and stable structure. Joist construction also reduces overall waste, which helps reduce disposal and transportation costs.
One of the primary drawbacks to bar joist construction is the high cost of the joist compared to stick framing. While this cost is often offset by labor and disposal savings, stick framing is still cheaper in terms of upfront costs. Because joists must be pre-manufactured to specific design criteria, there is also the risk of time lost to design and engineering.