Truck Module Page 1 of 7 LIGHTWEIGHT CONSTRUCTION METHODS Lightweight building and roof construction is currently very popular with architects and building contractors across the country, and for good reason. Considering the present cost of labor, equipment, and building materials, it is not economically feasible to construct buildings the same as 50 years ago. Ease of installation and utilization of lightweight building materials have become the standard. Heavy timber, laminated beams, and 1- X 6-inch decking have been replaced by 2 X 4 s and half-inch plywood, OSB, or particle board regardless of building size. As modern architects reduce the size of what there is to burn, today's fire departments are losing one of their most valuable fire ground factors--time. This section will focus on the four major types of lightweight roof construction: panelized, metal gusset plate trusses, open web construction, and wooden "I" beams. NOTE: This type of construction may also be utilized in floors, walls, etc. Panelized Roof System Description: This roof can be found on structures constructed of wood, masonry, or concrete tilt-up. This roof consists of four major components: a. Laminated beams b. Purlins c. 2 X 4 inch rafters d. 1/2 or 5/8-inch plywood decking Panelized roofs do not have a space between the ceiling and underside of the roof occupied by the familiar trussed joist construction. The main structural support of the roof is accomplished by the use of laminated beams of various sizes (6 X 36 inches are common). The beams are supported at their ends by pilasters or posts and additional posts may be supporting the beam along the span. The beams will be spaced from 12 to 40 feet apart and may be bolted together to create lengths well in excess of 100 feet. Wooden purlins are installed with metal hangers on 8-foot centers. The common size of purlins is 4 X 12 inches with the length depending on the spacing between beams. Purlins may also be MGP truss or I beam (wood or metal) in newer tilt ups. Joists measuring 2 X 4 inches X 8 feet are installed with metal hangers on 2-foot centers between the purlins and run parallel to the beams. Sheets of plywood, OSB, or particle board (4 X 8 feet X 1/2 inch) are nailed over this framework. They are then covered with composition roofing material. A three-layer insulation paper is stapled to the underside of the roof between the beams and purlins. This paper offers no protection to the 2 X 4 inch joists and 1/2 inch sheets of plywood. Insulation paper consists of tar impregnated kraft paper covered on either side by thin aluminum foil.
Truck Module Page 2 of 7 Strengths: The strengths of this roof are beams, purlins and the perimeter of building where roof ties into the exterior walls. Hazards: Span supports for beams of 4-inch hollow steel pipe may be found. Expect weakening and/or collapse of these supports with failure of large portions of the roof under heavy fire conditions. When the insulation paper is subjected to fire, the foil will peel away from the middle layer of tar-impregnated paper. This paper will then begin to give off flammable gases that build up between the insulation paper and plywood decking. When ignition temperature is reached, the gases will flash resulting in heavy char to the wood and burning insulation dropping to the floor below. Fire is then able to expose the 2- X 4- inch joists and 1/2-inch plywood, OSB, or particle board decking which offer little resistance to fire. When walking across a panelized roof, utilize the beams or purlins. Any other section of this roof is comprised of 2 X 4 s and 1/2-inch plywood that will rapidly fail when exposed to fire. Metal Gusset Plate Trusses Description: Rough carpentry wood trusses used in commercial and residential applications utilize 2 X 4 s held together by metal gusset plate connectors (See illustration next page). This truss system is enjoying widespread use in roof, floor, rough window, and rough door openings. Trusses for roofs are constructed in a wide variety of forms. Regardless of form, these trusses share common features. Trusses are constructed of top chords, bottom chords, and webbing (supports between the top and bottom chords are referred to as "webbing"). Metal gusset plate connectors hold the trusses together. Metal gusset plate connectors vary in size, thickness, and depth of penetration; however, 18-gauge steel plates with prongs that produce 3/8-inch penetration are common and used in a wide variety of applications. Utilizing 2 X 4 s, spans of up to 55 feet will be found. A point of interest with this type of construction (also open web construction and wooden "I" beams) is the fact that trusses are supported at their outside edges only (unless used as a cantilever truss). Interior partition walls do not support the truss at any point along the bottom chord. Eighteen gauge "roof truss clips" may be found at various chords. These clips are nailed to the bottom chord and to the top plate of the interior wall. Roof truss clips provide some lateral stability for partition walls. In this configuration, interior partition walls could be classified as "free standing." Common on-center spacing for this construction is 2 or 4 feet and may be covered with 1/2-inch plywood. This method of construction is used in floor and roof systems. Strengths: Consider the area where the trusses cross or are tied into the outside bearing walls as strong areas.
Truck Module Page 3 of 7 Hazards: Extensive use of 2- X 4-inch trusses with metal gusset plate connectors equals short burning time and early failure and collapse. These trusses are under tension and compression and when the bottom chord or webbing fails, either from connector plates that have pulled out or from deep char, the truss will fail. When the metal connector plates and surrounding wood are exposed to fire, the plates will fail in a short period of time by pulling out of the wood. The bottom chord of the truss has replaced the 2- X 6- inch or larger ceiling joist of conventional construction. Coupled with the fact that these bottom chords do not rest on the interior walls, which offer additional support, expect total collapse of portions or the entire roof in a short period of time. As in other truss-type construction, depth of cuts for ventilation purposes is critical so as not to reduce the structural integrity. An additional hazard is identification of the construction of the roof you are about to ventilate. A simple flat roof may be constructed from different types of joists or trusses, yet there is a wide range of strength factors inherent in different types of construction.
Truck Module Page 4 of 7 Open Web Construction (Trussed Joist Construction) Description: Open web construction consists of bottom and top parallel wooden supporting beams called chords that are cross-connected by steel tube web members. The top chord (supported) and under-load offer a bridging effect causing the top chord member to be in compression and the bottom chord member (unsupported) to be in tension. Open web construction is prefabricated at the factory before installation with either parallel chords laid on edge or with flat laid chords. The steel tube web members are prefabricated from 1- to 2-inch cold rolled steel tubing with the ends pressed flat into a semicircular shape and a hole punched through each end. These flattened ends are then inserted into slots in the chords. Steel pins (up to 1 inch) are then driven through the chord members and through each end. These flattened ends are then inserted into slots in the chords. Steel pins (up to 1 inch) are then driven through the flattened ends of the web members completing the assembly. When the prefabricated joists are installed, top chord members are secured to the top of bearing walls with bottom chord members remaining unsupported away from the wall. Spans to 70 feet are possible using a single 2 X 4 or two 2 X 3 s as top and bottom chord members. Two-by-fours exceeding lengths of 20 feet are made possible by joining different lengths of 2 X 4 s in glued mitered "finger joints". Normal on-center spacing is 2 or 4 feet. This method of construction is used in floor and roof systems. Strengths: Consider the perimeter of the building where the roof ties into the exterior wall as a strong area. Hazards: The hazards, of this roof are many. Basically, this roof is constructed of 2 X 4 s or 2 X 3 s under tension and compression and 1/2-inch plywood decking. These components offer minimum resistance to fire. Some structures leave the chords exposed to the interior of the structure, which increases the exposure hazard and assists in larger areas of the roof being exposed to fire. Expect to find a lack of fire stops in this construction. Due to the size of lumber and chord members in tension and compression, expect rapid failure of this construction. When 2 X 4 s are laid flat as chords (1 1/2-inch thick) with 1/2-inch plywood decking, fire fighters only have to cut 2 inches deep (when cutting for ventilation purposes) to cut through the chord members. This may cause partial roof failure of the area supported by the severed chords. Shown above are typical bearing conditions one might encounter with truss construction. Note how some extend well past the bearing wall. If a fire were to burn through the truss on the inside, the remainder of the truss could topple over. Beware of this situation! Truss joists are lighter in weight than most other systems. This feature cuts erection costs, speeds construction and makes possible the use of less costly footings, foundations and bearing walls. For these reasons, truss construction is very popular in commercial areas. A truss may span up to 25 feet in most floor systems and up to 40 feet in many roofs. Multiple spans are possible up to 70 feet total length.
Truck Module Page 5 of 7 Wooden "I" Beams Description: Consists of three main components: 1. Top Chord 2. Bottom chord 3. 3/8-inch plywood, OSB, or particle board stem The plywood, OSB, or particle board is joined to the top and bottom chords by a continuous glued edge joint. Two by fours are used as chords, but 2- X 3-inch chords are very common. Some chords may resemble wood because of laminations. However, the laminations (trade name of Micro Lam) or veneers run horizontally in the chords. Micro Lam differs from plywood where every other veneer is 90x to the preceding veneer. The Micro Lam veneers are stronger than solid sawn lumber. In the 2- X 3-inch configuration, spans of up to 40 feet will be found. Until adequately braced and the plywood decking is nailed down, this construction is very unstable. Common on-center spacing for this construction is 2 feet. Half-inch plywood is utilized for the decking. Used in floor and roof systems.
Truck Module Page 6 of 7 Strengths: Consider the perimeter of the building where the roof ties into the exterior walls as a strong area. Hazards: The material there is to burn consists of a 3/8-inch plywood stem and 2 X 3 or 2 X 4 inch chords. It will take little time for the 3/8" plywood to burn, weaken and cause collapse of the chords and the roof or floor. Buildings will be found with open and unprotected chords. Common practice is to run heating and air conditioning ducting of various sizes through the stems, which removes a good percentage of the stem and gives fire horizontal access to adjacent "I" beams. As with other lightweight roofs, depth of cuts for ventilation purposes is critical. Non Corrugated Metal Roofs -Standing Seam/Stone Coated Description: There are a number of metal roofing products on the market today. These products are available for both residential and commercial applications. The two most widely used are Standing Seam and Stone Coated. Steel is the most widely used substrate, however Aluminum and Zinc are also used. Understanding how the product is applied is more important than what it is made of. Stone Coated panels are installed on new roof decking or over pre-existing roof coverings. Some building codes allow for the installation over combustible roofs such as cedar shake/shingles. They are typically made of 28-gauge steel and are roughly four feet wide by 15 inches deep and stamped to look like different styles of tile roofing. The panels are attached to a wood batten grid system built over the existing roof/deck. Once completely fastened to the batten system, these panels are very difficult to remove. An opening may be created by completely cutting through the panels, batten system and underlying roof decking. Standing Seam panels are typically seen on commercial buildings, however their use in residential applications is becoming more popular. These panels are roll formed from
Truck Module Page 7 of 7 various gauge steel and may be 12-24 inches wide. They are typically applied over solid decking. The two types that are most often encountered are mechanical seam and snaplock seam. As the name implies, mechanical seam panels are crimped mechanically to hold them together. Snap-Lock panels are held together by pushing a female leg over a male leg. When mated together, they are held in place by friction. These panels run from ridge to eave in one continuous piece. They are fastened to the roof deck by placing a fastening clip every 12-24 inches on the male leg. Due to the high quantity of fastening clips, the panels are difficult to pry up. Strengths: Stone Coated panels are predominately used in residential re-roofing. The Batten system used to support the panels is applied over the existing roof joists, thereby strengthening the entire structure. Standing Seam panels run continuously from eave to ridge and offer additional shear strength which can help reduce collapse potential. Hazards: (Stone Coated) Some building codes allow for the installation of Stone coated panels over combustible roofs such as cedar shake/shingles. If the installation is done in this manner, there will be additional overhaul work to be done to ensure there is no fire in the void space. A rotary saw with a carbide tipped blade, chainsaw with a Terminator style chain or axe will work. Once the structural components and decking are severed, the panel can be louvered. Firefighters should be prepared to use multiple cutting devices due to the heavy toll taken by cutting through the overlapping panel sections. (Standing Seam) roofs present a number of challenges that must be addressed if vertical ventilation is to be attempted. The panels are extremely slippery when wet and even a very low pitched roof is likely to be difficult to walk on. Due to the thickness of the material, cutting through the seams or vertical legs will be a challenge. Once the panel section is cut and removed, the decking will need to be cut and louvered. When attempting to ventilate these roofs, cut through the panels with a rotary saw, remove the panels by prying them off the retaining clips and then louver the decking. Attempting to cut through the panels and the decking simultaneously may be done if the proper blade is used on the rotary saw. Attempting the same cut with a chainsaw, regardless of chain type will destroy the chain as the combined overlap thickness of metal is too great. Finally, caution must be exercised when handling cut panels as the edges are extremely ragged and sharp. Due to the complexity of this operation, a risk v/s gain analysis should be completed and other ventilation methods considered. If the determination is made to ventilate through this material, be prepared for an operation that is longer in duration than with traditional roof coverings.