![Joist Floor](https://upload.wikimedia.org/wikipedia/commons/c/c9/Joist_Floor.jpg)
An engineered wood
joist
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 su ...
, more commonly known as an I-joist, is a product designed to eliminate problems that occur with conventional wood joists. Invented in 1969, the I-joist is an
engineered wood
Engineered wood, also called mass timber, composite wood, man-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 woo ...
product that has great strength in relation to its size and weight. The biggest notable difference from dimensional lumber is that the I-joist carries heavy loads with less lumber than a dimensional solid wood joist.
[Vogt, Floyd. Carpentry. 4th ed. Clifton Park, NY: Thompson Delmar Learning, 2001] As of 2005, approximately 50% of all wood light framed floors used I-joists. I-joists were designed to help eliminate typical problems that come with using solid lumber as joists.
The advantage of I-joists is they are less likely to bow, crown, twist, cup, check or split as would a dimensional piece of lumber. I joists' dimensional soundness and little or no shrinkage help eliminate squeaky floors. The disadvantage is very rapid structural failure when directly exposed to fire, reducing the time available for residents to escape and increasing the danger to firefighters.
Design and manufacture
An I-joist has two main parts, the web and
flange
A flange is a protruded ridge, lip or rim, either external or internal, that serves to increase strength (as the flange of an iron beam such as an I-beam or a T-beam); for easy attachment/transfer of contact force with another object (as the f ...
. The web is sandwiched between a top and bottom flange, creating the “” shape. The flange can be made from laminated
veneer
Veneer may refer to:
Materials
* Veneer (dentistry), a cosmetic treatment for teeth
* Masonry veneer, a thin facing layer of brick
* Stone veneer, a thin facing layer of stone
* Wood veneer, a thin facing layer of wood
Arts and entertainment
* ' ...
lumber or solid wood
finger-joint
A finger joint, also known as a comb joint, is a woodworking joint made by cutting a set of complementary, interlocking profiles in two pieces of wood, which are then glued. The cross-section of the joint resembles the interlocking of fingers ...
ed together for ultimate strength. It is grooved on one side to receive the web. The web is typically made from
plywood, laminated veneer lumber, or
oriented strand board
Oriented strand board (OSB) is a type of engineered wood similar to particle board, formed by adding adhesives and then compressing layers of wood strands (flakes) in specific orientations. It was invented by Armin Elmendorf in California in 1963 ...
. After sizing the webs and flanges, they are assembled with water-resistant
glue by pressing the web into the top and bottom flange. After assembly, the I-joist is end-trimmed and heat-cured or left at room temperature to reach approximately equilibrium moisture content. Sizes vary according to the I-joist's intended load and span. Depths can range from and reach up to in length, although is more common. The intended use for an I-joist is for floor and roof joists, wall studs and roof rafters in both residential and commercial construction.
Installation
I-joists require correct installation. The most common mistake is misplacing or improperly sizing holes in the web, which can compromise the joist's strength, potentially leading to structural failure.
Common mistakes made with installing I-joists include cutting or chiseling the flange, improperly sized joist hangers, improper nailing and wrong-sized nails. The rim joist depth must match the I-joist size. Mismatches can strain the joist.
A similar situation occurs where the I-joist crosses a main beam. Installing squash blocks (2×4 materials higher than the I-joist) alongside the I-joists transfers the load from the I-joist onto the beam. Missed nails and glue setting too fast can lead to an uneven or squeaky floor.
Safety
The lightweight nature of I-joists makes them more vulnerable to
fire
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
At a certain point in the combustion reaction, called the ignition point, flames a ...
than dimensional lumber. A report by
Underwriters Laboratories
The UL enterprise is a global safety science company headquartered in Northbrook, Illinois, composed of three organizations, UL Research Institutes, UL Standards & Engagement and UL Solutions.
Established in 1894, the UL enterprise was founded as ...
found that structural assemblies composed of I-joists
fail
Failure is the state or condition of not meeting a desirable or intended objective, and may be viewed as the opposite of success. The criteria for failure depends on context, and may be relative to a particular observer or belief system. One ...
significantly sooner under
fire conditions than those composed of dimensional lumber.
Fire-induced failures of lightweight
trusses
A truss is an assembly of ''members'' such as beams, connected by ''nodes'', that creates a rigid structure.
In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assembla ...
and I-joists have led to the deaths of several firefighters. In order to use i-joists in a fire-rated assembly, additional detailing is required to ensure building safety.
See also
*
I-beam
An I-beam, also known as H-beam (for universal column, UC), w-beam (for "wide flange"), universal beam (UB), rolled steel joist (RSJ), or double-T (especially in Polish, Bulgarian, Spanish, Italian and German), is a beam with an or -shape ...
References
{{DEFAULTSORT:I-Joist
Building materials
Engineered wood