How Does i joist beam Work?

What are I-joists?

Timber I-joists are commonly used throughout the world for residential and commercial floor structures. Wood I-joists are lightweight, with excellent strength to weight ratios and spanning capabilities.

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I-joists consist of two main parts: the flanges, and the webs.

The flanges are located at the top and the bottom of the I-joists. The flanges do most of the work in an I-joist. Therefore they are usually made out of high strength timber, which in the case of Wesbeam's innovative e-joist, our laminated veneer lumber (LVL).

The top and bottom flanges are joined by a vertical web. The role of the web is essentially to hold the bottom and top flanges apart, much like the chords or metal webs used in a truss. The webs consists of a thinner timber product; typically Plywood or Oriented Strand Board (OSB).

What is an I-joist used for?

I-joists are primarily used as floor joists or roof rafters in residential and commercial construction. I-joists have structural flanges on the top and bottom, connected by a vertical, structural web. These joists offer several advantages over traditional timber beams, including longer available lengths, lighter weight per meter than solid beams, increased straightness, and the ability to cut large service penetrations through the structural web. This makes them the ideal choice for residential floor construction.

How do they work?

If we look at a single spanning I-joist with weights spread across the top (much like a floor joist), it deflects downwards just like any other beam. When we look a little closer at what the different parts are doing, the top flange is pushing in on itself, and is in &#;compression&#;. As the bottom of the I-joist stretches outward, the bottom flange is put into &#;tension&#;.

These two flanges are then connected by the structural web, which has forces running up-and-down known as &#;shear&#;.

The forces in the top and bottom flanges are quite consistent along the length of the I-joist. This is why it is so important not to cut or damage these flanges. Just like a truss, damaging the tension and compression members will compromise the integrity of the I-joist.

What about penetrations?

With the shear forces in the web, these are much higher at the support points, where all of the vertical weight on the beam is essentially supported. At these locations, the web can be stiffened if needed and should not be cut without consulting an engineer.

What this does mean is that in the middle of the beam, the web is not doing a whole lot! The forces are low and the top and bottom flanges are being well supported from the ends. This allows large penetrations and holes to be cut into the web without compromising the integrity of the joist.

These holes can be used to pass through plumbing pipes or air conditioning ducts. For rules regarding what size and where holes can be drilled in Wesbeams e-joists, refer to our new Hole Creation - Quick Reference.

A High-Performance Advantage

I-Joist Basics

I-joists are strong, lightweight, "I" shaped engineered wood structural members that meet demanding performance standards. I-joists are comprised of top and bottom flanges, which resist bending, united with webs, which provide outstanding shear resistance. The flange material is typically laminated veneer lumber (LVL) or solid sawn lumber, and the web is made with plywood or OSB. The robust combination of structural characteristics results in a versatile, economical framing member that is easy to install in residential and light commercial projects.

Common I-Joist Applications

I-joists are used extensively in residential floor and roof framing. They are ideal for long spans, including continuous spans over intermediate supports. Because I-joists are straight and true, it's easier for builders to avoid crowning and maintain a level framing surface. In addition, wood I-joists are typically stocked and readily available in depths that meet the needs of residential framing.

Common Depths

9-1/2", 11-7/8", 14", 16". Flange widths vary from 1-1/2" to 3-1/2"; long lengths are readily available.

Common Uses

I-joists are popular for floor and roof framing. Many APA member-manufacturers have software design tools; contact the manufacturer (at right) for more information. 

APA PRI-400

APA Performance Rated I-joists (PRIs) are manufactured in accordance with APA PRI-400, Performance Standard for Residential I-joists. This Performance Standard provides an easy-to-use table of allowable spans for applications in residential floor construction, allowing designers and builders to select and use I-joists from various member manufacturers using just one set of span tables. APA PRIs are recognized through ICC-ES ESR-.

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The APA Mark of Quality

APA trademarks appear only on products manufactured by APA member mills. The mark signifies that product quality is subject to verification through APA audit&#;a procedure designed to assure manufacture in conformance with APA performance standards or the standard shown in the mark.

I-Joist Publications
 

Includes information on span ratings, installation details, cantilever designs, architectural specifications and engineering design properties for APA Performance Rated I-Joists.

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Resource Library

Access a complete list of APA publications about I-joists in APA's Resource Library.

Fire Protective Assemblies for Wood I-Joist Floors

Learn about several options that enhance the fire performance of I-joist floor systems and comply with the International Residential Code (IRC) by visiting the I-Joist Fire Assemblies page.

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Technical Topics TT-015: Wood I-Joist Floors, Firefighters and Fire

A discussion of the durability and safety of wood I-joist floor assemblies in structure fires. Includes APA&#;s recommendation to increase the fire endurance of all floor-joist assemblies based on the International Building Code.

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Technical Topics TT-126: Applicability Of APA Trademarks On Engineered Wood Products Surface-Coated With Fire Protective Coatings

Contains Q&A concerning fire protective coatings. Certification and inspection of products treated with fire-protective coatings are outside the scope of APA.

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White Paper: Basis of IRC Membrane Protection Provisions

APA, AWC and WIJMA jointly developed this paper that describes the history, background and intent of the IRC provisions for fire protection of floors as well as the methods used for establishing equivalence and compliance with these fire protection requirements.

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White Paper: Code Requirements for Prefabricated Wood I-Joists

APA, AWC and WIJMA jointly developed this paper that describes the basic code recognition for prefabricated wood I-joists and the need for product certification in compliance with the codes.

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White Paper: Fire Protection of I-Joist Floors with Factory-Applied Solutions

APA developed this paper that describes the basic fire protection requirements for prefabricated wood I-joists with factory-applied fire protection solutions for compliance with the codes.

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