In the past two decades, the fire department has had to deal with the increasing use of lightweight structural components. During that time, our main focus was on small-sized truss floors and roofs. But with the building boom in most parts of the country in the past few years, concerns about small-size timber must now also include other types of engineered timber. One of the main issues we must focus on now is the wooden I-beam (photo 1).
Timber I-beams are called multiple terms in the wood products industry and fire services, including wood I-beams, composite wood joists, plywood I-beams, TJI® and silent floor joists. From the fire service perspective, these joists have a history of risk of collapse when exposed to fire. The collapse seemed to be sudden, with almost no warning. So one question must be asked: Will these silent floor joists become silent killers of firefighters when they are hit by fire?
The wooden I-beam consists of upper and lower horizontal parts called upper and lower flanges and vertical parts called "webs" between the flanges. These three components are considered engineering products. The flange size can range from 1 5/16 to 1 1/2 inches thick, and from 1 1/2 inches to 3 1/2 inches wide. They are made of laminated veneer wood or laminated structural wood.
The net is usually made of Oriented Strand Board (OSB) and has a thickness of 3/8 inch or 7/16 inch, depending on the residential installation. OSB is a structural board made of multiple layers of thin rectangular wooden strips, produced by a cutting machine called a stranding machine. The wood thread is mixed with the adhesive and then bonded to the desired panel thickness under heat and pressure. The strands in the exposed layer of the panel form a right angle with the strands in the core layer, thereby strengthening and strengthening the panel. 1 The panel can be made into a sheet, such as plywood, and can be used as a structural sheath. In this article, we will study its use as an I-type joist net.
The top and bottom edges of the web are cut to match the corresponding grooves cut into the top and bottom flanges. Then glue the webs, insert the flanges, and press them together to assemble the wooden I-joists. 2
The depth of the I-shaped joist may also vary, and is usually 9 1/2 to 16 inches in most residential installations, although the depth of commercial buildings may be as high as 48 inches. Note that when the I-beam was first introduced, plywood webs and sawn timber flanges were used; therefore, they may appear in some older installations. In addition, some manufacturers still provide mixed sawn timber flanges and OSB webs (photo 2).
Wooden I-beams are used because they can withstand the same load as the size of wood, can span a large span within the structure, and are light in weight. They are usually manufactured uniformly, which means that the vast majority are accepted rather than rejected at the construction site. They do not warp or shrink, thus eliminating floor squeaks that occur when size wood shrinks or warps. The joists are usually printed by the manufacturer with the date and time of production, which can be seen if the property is visited and inspected before the gypsum board is installed (photo 3).
Timber I-beam performance standards are included in American Society for Testing and Materials (ASTM) D 5055, a standard specification used to establish and monitor the structural capabilities of prefabricated timber I-beams.
These engineering I-shaped joists are widely used in wood frame and mixed wood frame construction: residential and commercial land, new and existing building restoration projects, where floors and roofs are being added or replaced. One of the most concentrated areas of I-shaped joists is residential buildings, including multiple houses, townhouses and single-family houses.
But their existence can also be found in countless commercial real estate and ordinary buildings. Wooden I-beams may not be found throughout the project; what you see in one area may not represent what you can find in other parts of the building. For example, it is obvious from the street that sawn-size timber is being used for the roof structure. However, a closer inspection may reveal that the I-shaped joist is being used in the construction of the floor of the same building. I-beams may be used on the roof, but the structure of the floor may be different (photo 4-8).
Some commercial buildings may have truss overhangs and wooden I-beams on most of the roof (photo 9). The additional part of the existing structure may contain I-shaped joists, while the original part may be constructed using dimensional timber (photos 10, 11). Pre-planning direction drills should be a common practice so that members can become familiar with construction practices, especially when woodworking joists and trusses can be seen stacked on the job site. Timber beams should only be used where they can be protected from the weather in dry use applications.
Although it is possible to make holes in the web (rather than the flange) for the passage of utilities, it should be carried out in accordance with the manufacturer's installation instructions. Some nets may have indicated perforation knockout for this type of use. The I-joists produced by some manufacturers already have large holes (up to 20 inches) in the joists and are specifically designed to meet certain projects.
The building inspector should be notified of a chipped or damaged flange, or an oversized or oddly shaped hole punched in the web for utility operations. The manufacturer may need its engineers to evaluate this type of damage, and certain repairs are required to ensure the stability of the joists (photo 12-14). The manufacturer’s recommendations must be followed, including its specified penetration chart, which shows how to cut the I-beam. These charts are different for end supports, intermediate supports, and cantilever support joists, so contractors want to follow the correct guidelines. The information in the chart will also vary according to the joist model and depth. The space required between the holes may also vary, depending on whether the opening is square or round.
The allowed hole chart contains too much information to be covered here, but there are several guidelines that should be followed, and the degree of compliance with these guidelines should be considered during site inspections. They include the following: There should be at least 1/8 inch of web at the top and bottom of each opening; no hole should be less than 12 inches from any end, middle, or cantilever support; cantilever steel bars should not have holes Pass them; the flange should not be chipped or cut (photo 15).
As long as there are no other holes in the joist, some joists may allow a simple span of 5 feet or more (without center support) and a largest round hole in the center of the I-joist. For example, a residential I-shaped joist with a depth of 16 inches may allow a maximum opening of 13 inches, as long as the joists are uniformly stressed and meet the standards of the installation guidelines. 3 Others may have designed larger openings among them. Determine whether the holes have been designed when the joists are delivered, or whether these holes are made on site. Ask the contractor, talk to the building inspector, and view the installation guide (usually available from the manufacturer's website) to obtain information about these products.
Because wooden I-beams can span long distances, many times a single joist will extend from the load-bearing wall to the load-bearing wall. However, sometimes longer area distances require intermediate support, usually steel I-beams or engineered laminated wood beams, which are placed on top of the support column in turn.
The joist hanger is nailed to the wood placed on the steel I-beam web, or directly nailed to the laminated wood beam. Then place one end of the wooden I-beam in the joist hanger, and the other end on the outer wall, inner load-bearing wall or additional rated support, depending on the length to be covered (photo 16).
Other installation practices may move the wooden I-beam over the steel I-beam and fix it to the wood on top of the steel. When supported by beams, the I-shaped joists need to be fixed in place vertically. When the joist is fixed vertically, the full strength of the I-type joist is brought into play. I-type joist webs are not designed to transfer extremely concentrated loads. If they are not properly supported in a vertical position, they may bend or crack. One way to get the correct support is to use a joist hanger. The other is to increase the stiffness of the web near the end or intermediate support point of the I-beam by using blocking. (2, 5) Usually these "web stiffeners" are 2 × 4 lumber or additional OSB web cutting pieces, to be installed between the flanges of the joists, load-bearing and fixed to the web, or 2 × 4 Saw dowels are nailed to the flanges and the webs on both sides of the adjacent joists (photo 17). The second method is called "squash blocking". When there is a load-bearing wall above it, it can be found anywhere along the length of the I-shaped joist.
The end of each compartment needs lateral support to fix the in-situ I-joist. The connection to the supporting end wall is one way to achieve this. The extra cut part of the I-shaped joist can be fixed at right angles between the longer I-shaped joists to help lateral support. In many cases, the contractor told me that these smaller cut pieces between the joists are "fireproof", even if there are obvious gaps during the installation of the product, which cannot meet the correct definition of fireproofing. A sheath or deck nailed to the front four feet of the joist can also be used as a support. A plank of at least 1 x 4 inches should be nailed to each joist from the supporting wall or sheath area. Before this is completed, the I-shaped joists can be bent or turned over under minimal load, including workers walking on the joists. (3, 5)
When a sprinkler system needs to be installed and there is a problem of blocked sprinkler heads, the fire protection issue comes into play. In the National Fire Protection Association (NFPA) 13, Sprinkler System Installation Standard, Section 8.6.4.1.2 (4) [Applies to "open" I-beams, such as those found in basements without suspended ceilings] Read, "Install the deflector in a horizontal plane from 1 inch to 6 inches below the composite wood joist, with a maximum distance of 22 inches below the ceiling/roof deck, which is equivalent to Web structure, so a single channel area does not exceed 300 feet (27.9 m).” Simply cut the smaller I-shaped joists and place them between the ends of the longer composite joists instead of using rated Fire caulk fills the gaps between these components, which is not enough to prevent the fire from spreading (photo 18). Some contractors may use 1/2-inch plywood, cut it into strips equal to the depth of the web, and fix it on the surface of a shorter I-joist to close these gaps.
Considering that multiple references to wooden I-beams and hindered combustible structures can be found throughout NFPA 13, and nozzle placement regulations will vary depending on the nozzle type. Fire protection requirements may also change. Extended coverage and early suppression-fast response sprinkler-standard does not allow installation under combustible obstacle structures. A quick guide on this topic can be downloaded from the website of the US Fire Department/National Fire Protection Academy, and the location of sprinkler deflectors under obstructed construction. 4 Please also note that section 8.14.1.4 of NFPA 13 stipulates that closed composite wood joist passages are required when the joist passage contains heating devices and a concealed space of no more than 160 square feet (when the ceiling is connected to the I-joist) , There is water spray protection.
The last important component of the I-joist is the adhesive used to hold the components together. In the wooden I-beam, the flange and the web are glued parts. The adhesive used in the construction of the I-beam is synthetic, and the wood products industry classifies it as an adhesive for structural products (as opposed to adhesives for indoor non-structural products). Adhesives in this category include phenol formaldehyde, resorcinol, phenol resorcinol, polymeric MDI, polyurethane polymers, and several others. Such adhesives are used for wood products that require structural strength immediately after assembly, and they risk exposure to moisture. 5
In the manufacturing process of wooden I-beams, various types of adhesives may be used. The OSB mesh portion can include phenolic or polymeric MDI binders, or both. The web can be glued to the flange with resorcinol or polymeric isocyanate adhesive. Laminated veneer wood flanges can be bonded with phenolic adhesives used in hot presses. The finger joints on the flange can be assembled using phenol resorcinol formaldehyde or melamine adhesives. (5, 4)
Although timber I-beams have only recently played a greater role in modern construction projects, they have been in use for more than 35 years. Although it is mainly related to the floor structure, it can also be used for roof components. Most timber I-beam roof components are flat, but these joists have also been used for pointed roofs. Walkthrough inspections during construction will enable you to discover these assembly problems (photo 19).
Our main concern is the reaction of timber I-beams under fire conditions. Test burns and real-world history indicate that operations in buildings with wooden I-beams should cause great concern. The timber industry would say that as a protected component, wooden I-shaped joists should not pose a major threat in accordance with ASTM E-119, Standard Test Method for Fire Tests of Building Structures and Materials. But real-life experience shows that fires start and may spread to "protected" collections. In addition, in many residential and commercial structures under construction, the first floor joists in the basement are not covered with gypsum board.
In 1986, the Illinois Fire Department of the University of Illinois tested five types of floor systems to determine their structural stability. This test included a wooden I-beam placed 24 inches in the center. In all flooring systems, the wooden I-beam first fails at 4 minutes and 40 seconds. In their report, the author wrote:
In May 1981, the Los Angeles (CA) Fire Department tested several unprotected components, including wooden I-shaped joists with 3/8-inch webs. Fuel load includes paint thinner and tray; no live load is applied. The test time starts from the ignition of the fuel; the time limit is six minutes. The wooden I-beam covers a 12-foot span with a center-to-center spacing of 32 inches, and is clad with 1/2-inch CDX plywood. The report stated that the assembly failed within 1 minute and 20 seconds. 7
The test results show that the failure time is faster than the truss, so when it comes to fire, the wooden I-beam needs to be regarded as a serious threat to firefighters. During the construction of the fire department, the late Francis Brannigan referred to two case studies in which timber I-beams caused early collapse during a structural fire. 8
Due to the extensive use of wooden I-beams in new buildings in my area over the past few years, I decided to conduct my own non-scientific test burning. In the first burning, two four-foot-long I-beams were supported at both ends, with a plasterboard nailed to them, and caught fire. The fuel load includes shredded wooden pallets. An I-type joist suddenly failed about 13 minutes after ignition, but this was only 7 minutes and 11 seconds after the flame hit the joist flange.
The second test burn consisted of four OSB wood beams 10 feet long and 9 1/2 inches deep. The joists are supported at both ends and span a 103-inch opening. The joists are covered with 5/8 inch CDX plywood and fixed with a compressed gas nail gun; there are no openings in any nets. A licensed professional engineer believes that the component has the capacity to carry 720 pounds. A load of only 171 pounds consisting of concrete blocks was placed on the top and spread on the center bracket. Likewise, a fire load consisting only of wooden pallets is ignited under the assembly. Partial collapse occurred 7 minutes and 15 seconds after ignition. At 9 minutes, a second crash occurred; and all four network members were burned (photos 20-23).
So what seems to happen to these I-beams in these fires? In the several burns I participated in, similar flame patterns seemed to be forming. First, the fire starts to ignite the flange, and the flame quickly moves upward across the surface of the center of the web. The fuel load in the net seems to make the fire burn faster and maintain its growth. When the fire moves upward through the web and top flange, it touches the deck on the top of the shell. This action allows the fire to continue along the bottom of the deck until it meets the surface of the adjacent I-shaped joist and pushes down along it. In many cases, when the flame quickly fills the bay area between the joists, a distinct flame vortex will be seen. The fire will burn through the center of the net, allowing the fire to enter the next compartment on each side. When the flame burns on the front and the entire length of the web, it will destroy the structural integrity of the joist, even though the flange may still appear to be intact. Soon thereafter, with the destruction of the web between them, the flange collapsed with little or no sagging or other warnings.
On December 4, 2006, a notice from the International Association of Fire Chiefs warned that the I-beam composite floor system in fires across the country has caused several firefighters to be injured and may cause the deaths of the incumbents. It also includes the following recommendations:
But when these products are exposed to fire, what does the engineered wood industry think about these products? The two publications that address this issue are the resource guide for I-beams and the resource guide for adhesives used in engineered wood products. These guidelines were developed through an agreement between the U.S. Fire Department and the American Forest and Paper Association. Regarding the ignition of glue, the adhesive guide states:
But for firefighters, does it really matter whether the wood or the adhesive catches fire first, especially if the fire temperature difference may only be 26°C? As for the fact that burning adhesive does not produce a "significantly" higher flame spread rate, it must be concluded that a slightly higher flame spread rate has been found. This guide also states
The statement supports the fact that a nominal 2 inch lumber is more durable than a 3/8 inch or 7/16 inch thick I-beam net.
In the section titled "General Thermal Degradation Information," the guide reports that the thermal degradation of phenolic adhesives can be divided into three stages. 10 But from a fire protection point of view, there may be some problems with these defined stages. For example, the guide states the following:
The Wood I-Joist Resource Guide is more clear in its description of the fire test of I-joist:
It also reported that “Once the web is consumed, the bottom flange is no longer connected to the joists and falls off the system.” Under the “Fire Incident” section, the following is written:
Following this paragraph is the warning: “When a fire reaches flashover conditions in any area directly under the unprotected floor or roof joists, firefighters [yes] stay out, stay in and stay alive in any structural fire. ”(2, 8)
Forintek Canada Corporation, an organization that claims to be the Forestry and Government Wood Products Research Institute, also tested I-beams. These tests also showed that the web burned through first, followed by the lower flange. Report continues
But again, these tests raise some questions. What is the superimposed load used for the test, and is it comparable to the water hose or search and rescue team operating above the fire field? How long does the fire burn before the web burns through and the lower flange fails? What is the time difference between the failure of the I-shaped joist and the failure of the sawn joist? What does the "obvious deflection" of the top flange and deck look like, and will it cause firefighters to slip into the burned opening?
The report pointed out that with regard to I-beams: “On average, the maximum deflection of the floor is about 320 mm one minute before the final failure.” For traditional wooden joists, the same report said: “On average, when the structure fails In the last minute before, the maximum deflection of the floor was about 90 mm."
No one will feel comfortable working above or below two of the completely destroyed structural components in any three parts; the fact that the top (fire-damaged) flange may remain until the floor burns down is almost incomfortable, this It may be the key reason why the floor looks or feels good, until it seems to have suddenly and severely collapsed, giving the frequently described situation such as "no warning."
Another view on the deflection of floor slab components can be found in the "National Engineering Light Building Fire Protection Research Project Technical Report: Literature Search and Technical Analysis", which contains an excerpt from the report "Comparative Fire Test of Unprotected Engineering Wood Components" . In this section, the report summary states
The last page of the Timber Beam Resource Guide contains the most persuasive information about fire services. It lists 7 accidents (6 in the United States and 1 in Canada) that occurred between February 1995 and December 2003, in which the structure collapsed and resulted in the death of firefighters, and the floor frame was made of unprotected wooden I-joists composition.
Firefighters must be aware of the hazards associated with wood I-beams and all engineered wood products. Although these products have been on the market for some time, their rapidly increasing use has increased the hazards to firefighters, which may be as serious or even greater than the hazards posted by light trusses.
It is necessary to take a familiar tour of the work site of the new and repaired structure. If your state or municipality wishes to introduce legislation to mark buildings that contain trusses, consider also including markings for buildings that contain I-joists. You can also look for information from the industry, not only to understand how products are used, but also to understand how they react under fire conditions.
Some of this information is presented in terms that might make the joists seem safer than them. You may read things like "Adhesives will not ignite; they "pyrolyze" and the burn-through area is the pyrolysis zone" or "Composite wood products will not start to decompose when heated, but will'randomly break the chain '." Regardless of the term used, the end result is the same: fire damaged, collapsed, engineered wood products. Try to find out where these products are in your response area; pass on this information; expect to extinguish fires in structures containing wood beams, and train for this. When you encounter these products, if necessary, please change your strategy accordingly. Don't let the silent floor be a silent killer.
1. Resource Guide for Panels Used in Wood Structures, American Forest and Paper Association, 2, 3. Website: www.woodaware.info/PDFs/Struc_Panels.pdf.
2. Woodaware Resource Guide, American Forest and Paper Association 2. Website: www.woodaware.info/PDFs/I-joists.pdf.
3.ilevel?? Trus Joist® TJI® Specifier's Guide TJ-4000, October 2006, 11. Website: www.ilevel.com/literature/TJ-4000.pdf.
4. U.S. Fire Department/National Fire Academy Tea Break Training, No. 2006-46. November 14, 2006. www.usfa.dhs.gov/downloads/pdf/coffee-break/cb-2006-46.pdf
5. Adhesive Resource Guide, American Forest and Paper Association, 2. Website: www.woodaware.info/guideadhesives.html
6. Straeske, Jim & Charles Weber, Charles. "Testing the Floor System", Fire Command Magazine, June 1988, 47.
7. Mittendorf, John. "Lightweight structural testing has opened up the fire department's vision to special hazards." Sponsored by the Los Angeles (CA) Fire Department. May 1981.
8. Brannigan, Frank, Fire Department Building Construction, Third Edition, National Fire Protection Association (1992), 552-553.
9. The website of the International Association of Fire Chiefs: www.iafc.org, article 32101.
10. Knop, Andre and Louis A. Pilato. "Phenolic resins: chemistry, applications and properties-future directions." 1985. Springer Press.
11. Richardson, Leslie R. "Failure of floor components constructed using wood joists, wood trusses, or I-joists during the fire test." Forintek Canada
12. Grundahl, Kirk, PE National Engineering Light Building Fire Protection Research Project, National Fire Protection Research Foundation. (October 1992), 79-80.
JAMES KIRSCH is a 24-year fire veteran and a lieutenant of the Bergenfield (New Jersey) Fire Department. As a former volunteer fire chief, he also served as the logistics manager of the New Jersey Task Force 1 (NJ-TF1) US&R team. He has been a classroom lecturer at FDIC and holds a master's degree in public administration.