Different Types of Joists?
What are the different types of joists?
WHAT ARE JOISTS?
In this series we've introduced different building components and in our last video we talked about the building foundations and how they affect our system design in the Fire Protection industry.
Today we're going to spend some time talking about different types of joists.
As a quick refresher, a joist is a horizontal structural member that takes a vertical load directly from a roof or floor and transitions that load horizontally. A joist frames into girders or bearing walls. Joists are usually never by themselves. They are numerous in quantity, consistent in size for each structural bay that they serve, and are usually constructed in a way that is structurally strong, lightweight, and makes a very efficient use of materials.
So what types of joists are most common?
A bar joist is a term that we use to describe open web steel joists. These joists are mostly open and can use angle iron (still steel) or Tubes in the webbing. These are relatively lightweight, built up engineered trusses, which are mostly open if you're looking at a bar joist in a profile or section view. A bar joist a more casual term that is synonymous with “open-web steel joist”. One of the most common identifiers for a bar joist is a number K designation on structural plans, such as 14K1. This describes a joist that is part of a K series, or a particular type of joist. SJI, or the Steel Joist Institute, creates designations for types of joists. While the case series is the most common, other designations for open web steel joists include KCS, LH, and DLH. The K and KCS series can be up to 60 feet with depth ranging from 10 inches to 30 inches. The LH series can span up to 96 feet, with depths between 18 inches and 48 inches. The DLH can span up to 240 feet, with depths ranging from 52 to 120 inches.
For our purposes, if you look on structural framing plans for a floor or a roof and you see a label with a number, letter description, and a number, you can then copy and paste that into Google and get all the dimensions and product data for that type of joist. When we're laying out sprinkler systems, this is usually the workflow to get us the information that we need about the structure that's going into a building.
A solid wood joist or sometimes just called a wood joist, is a solid wood member that has a rectangular cross section. These are usually nominally 2 to 4 inches wide and up to about 14 inches deep. These are not built up, usually just a solid member of wood that is dimensionally cut. In the United states, solid wood joists were common in residential construction up through about the 1970s.
COMPOSITE WOOD JOISTS
A composite wood joist is a wood beam of an “I” shape that has a solid web section made from engineered wood products. A composite wood joist will have wood flanges or a cord on the top and bottom and a consistently solid web. In residential construction in the 1970s, homeowners were inspired by open floor plans which required floor loads over long clear spans. Solid lumber could come longer than 20 feet, but it was expensive, hard to find and lacked the capacity to support loads over such long spans. Composite wood joists used deep plywood webs with strong wood flanges that could accommodate these larger spans. It wasn't until about the 1990s during a timber crisis when the prices of this engineered wood became stable and began to become more popular in mainstream residential wood construction. The center webbing or the plywood more commonly became osb or oriented strand board, which is less expensive and stronger than plywood.
Wood joists didn't stop there though. Open-web wood joists or essentially a wood truss that supports floor ceiling assemblies, was invented in Canada in the 1980s and has become more popular in residential construction today. These can be constructed with all wood, or can be combined with metal plates at the joints. These open web wood joists make efficient use of wood and reduces its weight, while still maintaining a strong stiffness that reduces deflection across the span. While these trusses are often deeper than a composite wood joist, it has a major advantage in that duct work, plumbing, electrical, and Fire Protection now have some pathways that they can use when we have to cross perpendicular to these open web wood joists. These have become fairly popular in larger residential construction in the US, but certainly introduce new fire challenges as they open up large combustible areas where fire can spread horizontally quickly.
There are a few common joist types that each affect the design of our systems. They impact sprinklers with both heat collection and obstructions, they impact how we route pipe and conduit, and they impact fire alarms with our ability to respond to smoke and heat.
In our next segment, we’ll talk about the different pieces of a joist and how they affect our work.
I'm Joe Meyer, this is MeyerFire University.
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