Construction problem: I-shaped joist-fire engineering

2021-12-14 07:39:02 By : Ms. Linda Hu

Articles and photos by Gregory Havel

Since the 1970s, artificial wood I-beams have been in widespread use. They are advertised as providing the same or higher strength as the size of timber joists, while using less material and lighter weight. They are also advertised as being straighter than dimensional wood and therefore more suitable for horizontal floors and ceilings. These statements about this material are correct under normal conditions, but not in structural fires.

The top and bottom chords of the manufactured I-joists were originally 2 x 4 or 2 x 3 sized wood, with grooves cut on one face of each wood to connect the webs with glue. The webs of the manufactured I-shaped joists were originally plywood, with a thickness of 1/4 inch or 3/8 inch (6 mm or 9 mm), depending on the weight to be supported.

Photo 1 shows a close-up of the upper chords and webs of today's I-shaped joists, which are still bundled after being delivered to the construction site. The top and bottom strings are laminated veneer lumber (LVL), just like plywood, but the texture in all laminates is parallel to the long axis of the joist and is cut into a certain size of wood instead of boards. The fiber web is oriented strand board (OSB), which is a product made from wood shavings and glue that can be pressed into sheets and is advertised as stronger than plywood. Both LVL and OSB are cured under pressure and heat. The adhesive used to manufacture OSB and LVL and assemble Type I joists is usually urea-formaldehyde resin or phenolic resin.

Photo 2 (left) shows an I-shaped joist with LVL chords and OSB webs on top of the wall of a 2 x 6s frame (providing space for insulation required by building codes). These I-beams will support the floor of the upper half of the house and the gypsum board ceiling of the room below. The roof rafters shown are sawn-size timber. Please note that I-shaped joists are sometimes used for rafters instead of size wood, usually with LVL, size wood or plywood spine boards.

Photo 3 shows the I-shaped joists that will support the first floor of this house. Please note that the short section of the I-shaped joist between the ends of the top joist of the steel beam is not fireproof. They are required barriers for the I-joist system and provide lateral support to prevent the I-joist from twisting.

Also note that the one-inch plate shown in the picture is bolted to the top flange of the steel beam, which is common in this construction method; it allows the I-joist to be fixed in place.

See also photo 3 The splicing in the web of the central I-shaped joist blocking part. Since OSB boards are usually only 8 feet long and I-shaped joists can be many times longer, OSB boards are trimmed to serrated or fan-shaped edges, interlocked and glued together when assembling the joists. Expect to see several of these joints in the webs of the long I-shaped joists.  

In all cases, the floor (plywood or oriented strand board) will be glued and nailed (or screwed) to the top of the joist, and the floor finish (carpet or tile) will be applied. In all cases, the gypsum board ceiling (usually 1/2 inch) will be screwed under the joists, unless the code allows the joists to be exposed in the basement or cellar.

The I-shaped joist can be part of the fire-resistant floor ceiling assembly. In this case, the ceiling will be a 5/8-inch X-type gypsum board, which is connected to the joists according to the list specifications in the Underwriters Laboratories catalog, including the number, length, and spacing of the screws. The fire rating is only tested on one side of the floor and ceiling components, not on the inside. Fires in fire protection components, especially man-made wood products, will cause them to fail quickly and catastrophically. Since the adhesives used to make I-joists are cured by heating, they will begin to weaken when heated by fire, even if the temperature may not be enough to ignite them.

For a detailed comparison of the behavior of sawn timber, I-beams, and wood trusses under fire conditions, please check the Underwriters Laboratory's online short course at http://www.ul.com/fire/structural.html or http:/ /www.ul.com/fire/structural.html /www.ul.com/global/eng/pages/offerings/industries/buildingmaterials/fire/structural. According to UL testing, manufactured wood I-beams fail faster than wood trusses assembled with metal gussets under fire conditions. No matter what type of frame is used (sawn timber, manufactured joists or wooden trusses) and what type of ceiling is connected (whether fireproof components are created or not), the finished product looks the same: carpet or tiled floor on top, and Put plaster on the underside. If there is a fire in this building, the only way you will know what might support you is to make a pre-fire plan for the structure based on any notes you made during construction.

Gregory Havel is a member of the Burlington (WI) Fire Department; a retired deputy chief and trainer; and a 30-year fire brigade veteran. He is a Wisconsin-certified second-level fire instructor and second-level firefighter, a part-time instructor of the Gateway Technical College fire service program, and the safety director of Scherrer Construction Co., Inc. Havel holds a bachelor's degree from St. Norbert College; has more than 30 years of experience in facility management and building construction; and has taken FDIC courses

 Topic: Fireman building construction, I-shaped joist