BY GREGORY HAVEL
Lightweight steel framing is a common construction method that includes the open-web girders and joists, the floor or roof decks that they support, and the structures that support them. The framing and deck assembly is designed to support itself and permanently attached equipment (dead load), plus the building contents, people, and wind and snow loads (live load) with a calculated amount of deflection. Lightweight steel framing is a system: All of the parts depend on the other parts for structural strength and integrity and on the connection of each component to the others (photo 1). For example,
• A joist may depend on two girders for support.
• The girders may each depend on two columns or walls for support.
• The walls or columns depend on the girders to brace them against the wind and other lateral forces.
• Each of these depends on the riveted, welded, or bolted connections to the others to hold it in place.
• The roof or floor deck depends on the joist and girder assembly to hold it up.
• The joist and girder assembly depends on the roof or floor deck and its welded or pinned connections to the joists to help hold it square.
• Each girder or joist depends on the quality of its welds to hold its many parts together and to perform as designed by the engineer.
(2) This ‘big box’ department store is 100,000 square feet of lightweight steel framing with load-bearing exterior masonry walls.
Although many of the components discussed are also used in other types of construction, this article will be limited to their use in lightweight steel framing. This type of steel framing can be used in more than one type of construction. Steel joists spanning two masonry walls as floor or roof supports typically are classified as a form of “noncombustible” construction, which can include multiple spans of girders and joists supported by walls and columns. These buildings can be single or multistory, can be “noncombustible” or “protected noncombustible,” and may or may not have automatic fire sprinkler systems. Every fire department response area has these buildings: clear span spaces like gymnasiums and auditoriums; “big box” stores (photo 2); schools; factories and warehouses; office buildings (including the World Trade Center); medical buildings; and residential buildings, including apartments, dormitories, and elder care facilities.1
Lightweight steel framing has been around for more than 75 years and has evolved and become more common as better materials, connections, and designs were developed. Although the fire service sometimes regards architects and engineers as insensitive to its needs, they do learn from their mistakes and from accidents and injuries just as quickly as firefighters do. Some existing buildings are substandard by today’s practices, either because of age, poor workmanship, or remodeling by owners with low budgets (and little understanding of engineering principles under normal conditions, let alone in a fire). Today’s buildings use steel joists and decking as a building material or method in its own right rather than as a “fireproof” substitute for wood joists and deck. Today, architects and engineers usually use the following accepted engineering practices and consensus standards developed over the years:
• Standard for Cold-Formed Steel Framing2;
• Standard Specification and Weight Tables for Joist Girders3;
• Standard Specification for Open Web Steel Joists, K-Series4;
• Manual of Construction with Steel Deck5;
• Structural Welding Code-Steel6;
• Standard Test Methods and Definitions for Mechanical Testing of Steel Products7;
• Standard Specification for Carbon Steel Bolts and Studs, 60,000 PSI Tensile Strength8; and
• OSHA’s Steel Erection Standard.9
This article is based on construction that is compliant with the first four of these standards, which reference the others, and by other standards produced by other organizations.
All structural steel components, including columns, beams, girders, joists, decking, bolts, rivets, welds, and other fasteners, have data tables that show how each can be expected to perform under different conditions. These data are from laboratory testing, from analyses of problems discovered in the field, and from computer modeling. When these data are used in the design of a building, the resulting structure will stand and perform reliably for years under the conditions for which it was designed. These conditions do not usually include tornado, earthquake, fire, or terrorist activity.
Most structural steel components have a protective coating to prevent rust during shipment, storage, and assembly. Usually, this is a high-quality metal primer that also serves as the base for the finish paint if the component will be exposed and as a skid-resistant coating if it is used as a walking surface during erection.10 This primer requires touch-up after assembly on abrasions and welds. Some roof decking is galvanized (zinc coated) or phosphatized on one or both sides.
The stability of the structure depends on the following:
• Axial loading of columns. Eccentric or torsional loads require additional strength or bracing.
• Axial loading of girders and joists. Eccentric or torsional loads require additional strength or bridging.
• The strength and quality of the connections. Poor welds or the wrong grade or size of bolts can seriously weaken a connection, creating a point of potential collapse within the structure.
Loading. When a column is loaded axially, the line of force runs through the center of the column from the top to the bottom, and all parts of the column carry the load evenly. When it is loaded eccentrically, the line of force runs down one side of the column rather than down the center, placing more stress on that side of the column than in the center or on the other sides, which can cause the column to bow to the stressed side. When a column is loaded torsionally, it is being twisted, which affects the way the line of force runs from the top to the bottom of the column. Columns, girders, and joists that are to be loaded other than axially must be designed for these loads, or they may fail and collapse.
(3) A steel joist welded to its bearing on a steel girder.
Connections. Welded connections depend on good metal-to-metal contact, the proper welding rod, the proper amperage (electric current), the proper size of weld, the proper technique, and a skilled welder (photo 3). Bolted connections depend on good metal-to-metal contact, the proper size of hole, the proper size and type of bolt and nut, and the proper torque applied when tightening the connection (photo 4).
(4) Girders and joists bolted to a column.
Pinned connections depend on good metal-to-metal contact, the proper size and length of drive pin, and the proper size of powder charge, or proper air pressure, to drive the pin through the layers of steel and seat the head against the top layer (photo 5). Screwed connections depend on good metal-to-metal contact-the proper size and length of screw, the proper size of hole (accomplished by making each screw self-drilling), and the proper amount of torque applied when tightening the connection.
5) A steel floor deck connected to the joist by powder-driven pins.
Riveted connections depend on good metal-to-metal contact, the type of riveted joint, the proper size and length of rivet, the proper size and shape of each hole, the proper temperature of each rivet while being set, and the proper technique in seating and drawing down each rivet. This type of joint is no longer used in structural steel, since bolted and welded connections are less expensive, require less labor, and are of more uniform quality. However, there are still many older buildings that are supported by this type of connection.
Load-bearing walls are designed to carry the weight of the building and transfer it to the ground. In older buildings, the masonry in these walls may or may not be reinforced for stability, and the steel girders and joists may bear directly on the masonry. Today, many supporting walls have welded wire reinforcement laid in the horizontal mortar joints (photo 6) and steel reinforcing rods grouted into the vertical cores of the concrete block, or they are precast structural concrete panels (photo 7). Joist girders and joists that support floors or the roofs rest on steel-bearing plates that are grouted into pockets cut in the masonry or that are grouted onto the top of the wall or set into the face of the precast concrete wall panel. After the girders and joists are positioned on the bearing plates, they are welded into position.
(7) A steel joist resting on a steel bearing plate grouted into a pocket in a precast structural wall panel.
Columns are designed to carry the weight of the building and transfer it to the ground. In one-story buildings, they reach from the foundation to the roof; in multistory buildings, each column section may be two or three stories high. In multistory buildings, 18 feet between floors is not uncommon; the space between the ceiling and the upper floor is occupied by the depth of the supporting joists, air-conditioning ducts, pipes, electrical equipment, and communications and data cables.
(8) This steel H-column has been bolted to its foundation. The space between the foundation and the column flange will be packed with cement grout to give it more compressive strength.
In single-story buildings like factories, “big box” stores, warehouses, and gymnasiums, floor-to-ceiling heights often exceed 35 feet. Columns are connected to both the foundation and the roof and to other members at each floor level. The H column (its cross-section resembles the letter H) is the most common. Lighter-duty columns may be square, rectangular, or round tubing. In older buildings, flanges and other attachments were riveted to the column.
Today’s columns are fabricated by welding and are built for connections that are bolted, welded, or both. They are flanged at top and bottom and at intermediate floor levels for bolted connections. Since 2001, they require at least four bolts per top and bottom flange, (9) which was already standard practice for many steel fabricators and erectors (photo 8).
(10) Close-up of a bolted joint between a girder and a column (three more bolts will be added when it has been plumbed and leveled) and the slip-joint between the bottom chord of a joist and the column.
Girders/joist girders. A girder is a horizontal load-bearing member made of steel that supports other structural members, such as joists, and transfers the weight they carry through the columns and bearing walls to which they are connected to the ground. A joist girder is a girder with an open-web design (truss) made of steel.
In older buildings, the parts of the girder are connected by rivets or welds and may bear directly on the masonry. Today’s girders are usually welded; bolted connections are found only where the parts of a long span were field-assembled. Ends of girders bearing on masonry or concrete are lapped at least six inches onto steel bearing plates that are grouted into the masonry or concrete and are bolted (two 3⁄4-inch bolts) or welded (two 1⁄4-inch by two-inch fillet welds). Those bearing on structural steel are lapped at least four inches onto the steel and are connected by bolts. The bottom chord of a joist girder is restrained from lateral movement and from overturning by a slip-joint connection at the wall or column (to allow the girder to deflect under load) (photos 9, 10) and by properly connected steel joists.
By today’s standards, a joist girder may not have a span that exceeds 24 times its depth. (A joist girder spanning 100 feet will have a top chord to bottom chord dimension not less than 1⁄24 of 100 feet, or four feet, two inches.)
(11) This 2005 bar joist has its web made of rolled sheet-metal C-channels welded between angle irons.
Joists. A bar-joist is a horizontal load-bearing member made of steel that spans two structural supports (columns, girders, or walls) and that supports a deck that is part of a floor or roof assembly. It is called a bar joist because the early versions were made from a square or round zigzag steel bar and welded between angle irons on each edge to form a truss. More recently, the webs of bar joists were constructed of pieces of tubular steel. Presently, many bar joist webs are made of pieces of angle iron or sheet metal rolled into a channel with a “C” cross section, flattened on the ends, and welded between angle irons that form the chords (photo 11). This gives the strength of tubular steel in the web with less cost and less weight.
(12) The X-brace between two steel joists stabilizes them and distributes any lateral forces carried by the bridging between the top and bottom chords of the trusses and to the end wall.
All of the components of a joist are welded together. Connections between joist and girders or bearing plates are welded (two 1⁄8-inch by one-inch fillet welds) or bolted (two 1⁄2-inch bolts) (photo 3). (According to news stories and documentaries following the collapse of the World Trade Center towers on September 11, 2001, each steel joist was connected to the girder at each end with two 1⁄2-inch bolts.) The span of a joist does not usually exceed 24 times its depth. Spacing between joists varies from two to eight feet.
Bracing and bridging. Bracing and bridging are provided to prevent the lateral movement that causes joist girders and joists to twist or overturn. Joist girders are usually sufficiently braced by the slip-joint connections at the bottom chords and by properly connected joists. Joists require additional bracing and bridging to prevent twisting and overturning. These are usually angle iron laid through the truss webs and welded to each bottom and top chord and fastened to the end walls to transfer nonaxial loads from one joist to many others and to the walls. “X” bracing between the chords of adjacent joists may also be used to balance these loads between the top and bottom chords of the joists (photo 12).
Design loading. The top chord of a joist or girder is in compression. The bottom chord of a joist or girder is in tension. These trusses are designed for axial loading. If loads are eccentric or torsional, extra strength and bracing will be needed to offset these stresses. Joist girders and joists are designed to deflect under load and have a camber (upward curve) until they are installed and loaded, at which time they should be straight. The standard design deflection of a joist girder supporting a floor is 1⁄360th of the span (3.25 inches in a 100-foot span). The standard design deflection of a joist girder supporting a roof is 1/240th of the span (about five inches in a 100-foot span).
Most steel girders and joists are designed to carry the load on the top chord, which is in compression. A load suspended from the bottom chord of a bar joist that is designed for top loading only could cause the failure of that joist, since the tension in the bottom chord with the additional load would no longer balance the compression in the top chord. Suspension of pipes, ducts, and other equipment from the bottom chords of girders and joists usually requires the approval of the design engineer. Concentrated loads such as machinery or rooftop air-conditioning units must be designed into the roof or floor system and could require heavier or deeper joists or girders.
(13) The manufacturer’s label from a bundle of decking.
Decking for floors and roofs is sheet steel pressed into parallel-ribbed panels that are laid on steel joists with the ribs perpendicular to the joists and attached to them. The sheets are usually no longer than 40 feet for ease in transportation and packaged in bundles of at least 4,000 pounds, with manufacturer, job, and Underwriters Laboratories/Factory Mutual labels on each bundle (photo 13). Decking sheets may be bare steel, galvanized, painted, or phosphatized for rust protection.
(14) These bundles of steel decking bear on four joists spaced at eight feet. They are designed for end lap joints at least two inches wide directly over a joist.
The sheets come in widths from 12 inches to 36 inches. Each sheet of decking must span at least three joists, and each end of each sheet must bear at least two inches on a joist (photo 14). Sheets of decking may be butted at the ends on a joist and fastened independently or lapped on a joist and fastened through the lap to the joist, depending on the design. These sheets of decking are designed for a “side-lap connection,” with each sheet lapping the next at a rib. Decking is attached to each joist at each lapped or butted joint by arc puddle welding, self-drilling screws, or pins (powder- or air-driven) (photo 5), with the connections capable of resisting a lateral force of at least 300 pounds.
Additional sheet-to-sheet fastening (“stitching”) is also required if joist spacing exceeds three feet and may be done by welding, crimping, or self-drilling screws. This method of laying out and fastening decking sheets not only reliably holds down the roof or floor deck but also adds rigidity to the building frame.
Cantilevered sheets are rarely permitted. Where openings must be cut in the steel deck for ducts, pipes, or any other purpose, most must be reinforced to maintain the strength of the deck.
• An opening that removes no more than one rib per sheet (two webs maximum) usually needs no reinforcement.
• A hole up to 13 inches round or square needs a reinforcing plate screwed or welded to the top of the deck around the opening. Additional reinforcing may also be required.
15) A two-foot-square duct opening in a steel roof deck with a steel frame bearing on steel joists spaced at eight feet.
• Openings larger than 13 inches must be reinforced with a steel frame that bears on adjacent joists (photo 15). (5)
(16) This composite floor is ready to pour: galvanized steel floor decking with embossments to lock to the concrete and with welded wire mesh reinforcement.
Floor decking. Floor decking sheets serve as the form for the concrete topping (two-inch minimum) that is applied. These sheets have rolled-in embossments (photos 5, 16, 19) or some other means of locking the concrete to the steel and adding strength besides that provided by reinforcing steel or wire mesh. These decking sheets are installed and fastened with an unpainted side up (in contact with the concrete) and must provide a working platform with a minimum capacity of 50 pounds per square foot after installation and fastening. This part of the system is sometimes called a “composite floor deck,” since the finished deck is made up of several dissimilar materials (steel deck sheets, concrete, reinforcement) that work together to make a component that is stronger than the individual materials. In addition to providing strength, the steel deck is also a working platform, the concrete form, and a stabilizer for the building frame.
(17) Acoustical steel deck before the insulation board and roof membrane were applied. The darker area to the rear is a conventional steel deck.
Roof decking. Sheets support the roofing system chosen for the building. These sheets are painted, galvanized, or phosphatized on both sides and have wider ribs than floor decking, to provide better support for thermal insulating board. After installation and fastening, a steel roof deck must provide a working platform with a minimum capacity of 30 pounds per square foot.
(18) The same acoustical deck after the membrane roof was complete. The rust around each hole will be reprimed and painted and will be the exposed ceiling in a school lobby. Note the roof drain pipe supported from the top chord of the bar joist.
The finished roof is sometimes called a “composite roof deck,” since the finished roof is made up of several dissimilar materials (steel deck sheets, insulation board, roofing material, ballast) that work in a way that none of the individual materials can. In addition, the steel deck is also a working platform, the support for the roofing material, and a stabilizer for the building frame. An acoustical deck is a variety of roof decking that uses the same type of ribbed sheets as the conventional deck but with hundreds of small holes punched through the webs so that sound waves can penetrate and be absorbed by the softer roofing materials above (photos 17, 18). These holes can also allow heat to transfer above the steel deck by convection into the voids formed by the ribs in the decking during a structure fire, in addition to heat transfer by radiation and direct flame contact.
A common opening in a roof deck is a skylight-a glass panel or plastic dome in a frame to let natural light into a large enclosed space. Although the Occupational Safety and Health Administration requires protection of employees from falling through skylights by screens, guardrails, or other means,11 this required fall protection is not often provided in building designs because it is not required by building codes. With vision impaired by smoke or darkness, firefighters have fallen through skylights.
A lightweight steel-framed building may be classified as noncombustible. (1) If the steel structure is protected by an approved sprayed-on insulation or by fire-rated ceiling assemblies and column enclosures, it could achieve a fire rating of one hour or better and be classified as “Protected Non-combustible.” (1) However, even with the best fire-rated materials installed to protect the steel, the building still has a lightweight steel frame, and there is no guarantee that the integrity of the fire-rated materials, assemblies, and enclosures has been maintained during the life of the building and its renovations. Thus, there is no guarantee that a “protected” building will react to a fire differently than if it were unprotected.
(19) Composite floor deck and bar joists protected by an automatic sprinkler system. Note the embossments on the deck to bind it to the concrete.
Many of these buildings are protected by automatic fire sprinkler systems (photo 19), usually because they are large enough that building codes require them or because the property insurance carrier insists on it. Although the sprinkler system may have been designed to protect the structural steel and a specified amount of combustibles in an office building, it may not be sufficient to protect the structural steel if the building’s use is changed to retail space or another use with larger amounts of combustibles.12
Since lightweight steel framing is a “system” of building construction that depends on the strength of its connections, and since every part of the building frame is connected to other parts, failure of one connection or part will lead to other failures and even to the collapse of the whole structure. This is nearly certain if there is a fire in the building.
Since today’s design of lightweight steel-framed buildings is based on computer models of components under load, there is a much lower safety factor in the design than there was 20 years ago. Although the engineering proposes that the structure will carry the loads for which it is designed, and past performance of these buildings under “normal” conditions proves that it will, we must remember that the design does not allow for abnormal conditions like the heat of a fire reducing the strength of structural members and that the past performance of these buildings under fire conditions shows that they will collapse.
A change in the use of a multistory building with a lightweight steel frame can affect its stability even if there is no fire. If the floor deck was designed to support the 50 pounds per square foot of a data processing office and it is converted to file storage, warehouse, or manufacturing space requiring 100 or 150 pounds per square foot without redesigning and reinforcing the structure, it is unstable on a good day. If there is a fire, it will collapse. Because of the lateral stability given to the structure by a properly attached roof deck, cutting a vertical ventilation opening could significantly weaken the structure and decrease the time before its collapse. In addition, because of the distance between joists, a vertical ventilation opening cut by firefighters may leave them working on sheets of cantilevered roof deck which will sag under their weight and collapse, dropping them into the fire below. (12) This type of structure will fail earlier in a fire than we have been used to allowing in a building of ordinary or mill construction. Vincent Dunn (retired deputy chief, Fire Department of New York) states: “A steel bar joist system may collapse after only five or ten minutes of exposure to fire. When a fire building has a steel bar joist roof … horizontal window ventilation should be preferred to vertical roof ventilation, which requires firefighters to operate on the roof above a fire.”13 Considering that it takes more than five minutes from the receipt of the alarm by the dispatcher to the arrival of many fire departments’ first apparatus, we have little time to safely conduct interior or roof operations in one of these buildings, and unless they are multistory, many of these buildings have no windows.
Lightweight steel-framed buildings are a fact. They have already been built and are in use, and more will be built and used in the future. This style of construction is popular because it allows for large open spaces at a relatively low construction cost. Many new fire stations use this type of framing for precisely that reason. However, we do need to protect ourselves and other firefighters when we are called to work in one of these buildings.
• This type of building fails early in the life of a fire, and the failure involves a large area or the entire building. To delay the structural failure, let’s continue to promote the installation of automatic fire sprinkler systems in these buildings as they are built and as existing ones are remodeled or renovated.
• Let’s expand our public fire education efforts beyond elementary school and senior citizen programs and involve the business community, code-writing and enforcement people, building owners, and public officials. This is already being done by national organizations but needs more attention regionally and locally.
• To ensure that a change in building use will not add more combustibles than the sprinkler system is designed for, let’s include a check on that in our periodic fire inspections-and let’s be sure that our inspectors have the education to take care of this.
• Let’s establish a working relationship with the building inspector’s office and provide information on our concerns. These are the people who issue permits for remodeling and who can keep us informed about what is happening to the buildings in our response areas. If we are willing to work with them, they may be willing to work with us and call us during their plan review so that modifications to the automatic sprinkler system can be included in the permit requirements if the use of the building is changing.
• Even though the concentrated load of a rooftop air conditioner or a manufacturing machine may have been designed into the steel framing system, the extra weight of the machinery will compensate for the extra strength in the design. In a fire, this reinforced area will probably collapse as soon as the rest of the floor or roof.
• In this type of building, there can be as much space above the ceiling as below, or the underside of the roof may be exposed 40 feet above the floor. Let’s make sure that our pump operators are trained to provide the volume and pressure required by the nozzles we use so that we get hose streams with the volume and effective reach we need in these buildings. This is critical when automatic nozzles are used, since they adjust to the pressure provided to shape a nice stream that may not have enough volume or reach to do the job we require.
• Let’s include in our fire inspections and preplans notes on skylights and whether they protect firefighters from falling through.
• This type of building fails early in the life of a fire, and the failure involves a large area or the entire building. Let’s revise our operating procedures and policies so that we will not perform interior or roof operations as often or for as long as we do now. Although we prefer to save a building if possible, none of us wants to be under it or riding the roof down as it collapses. ■
1. Essentials of Fire Fighting (fourth ed.), Chapter 3, “Building Construction.” Oklahoma City OK: IFSTA, 1998. See also NFPA 220, Standard on Types of Building Construction (1999 ed.). Quincy, MA: National Fire Protection Association.
2. The Standard for Cold-Formed Steel Framing (2001 ed.). Washington, DC: The American Iron & Steel Institute. Web site: www.steel.org.
3. Standard Specification and Weight Tables for Joist Girders (2005 ed.). Myrtle Beach, SC: The Steel Joist Institute. Web site: www.steeljoist.org.
4. Standard Specification for Open Web Steel Joists, K-Series (2005 ed.). Myrtle Beach, SC: The Steel Joist Institute. Web site: www.steeljoist.org.
5. Manual of Construction with Steel Deck (rev. 2000). Fox River Grove, IL: The Steel Deck Institute: Web site: www.sdi.org.
6. Structural Welding Code-Steel (AWS D1.1/D1.1M). Miami, FL: The American Welding Society. Web site: www.aws.org.
7. Standard Test Methods and Definitions for Mechanical Testing of Steel Products, ASTM A370 (2005 ed.). West Conshohocken, PA: ASTM International. Web site: www.astm.org.
8. Standard Specification for Carbon Steel Bolts and Studs, 60,000 PSI Tensile Strength, ASTM A307, (2004 ed.). West Conshohocken, PA: ASTM International. Web site: www.astm.org
9. Steel Erection Standard: Safety and Health Regulations for Construction: 29 CFR 1926 Subpart R. Washington DC: U.S. Department of Labor, Occupational Safety and Health Administration (OSHA). Web site www.osha.gov. Subpart R includes sections 29 CFR 1916.750-761, plus Appendices A through H.
10. 29 CFR 1926.754(c)(3), “Slip-Resistance of Skeletal Structural Steel,” part of Steel Erection Standard: Safety and Health Regulations for Construction: 29 CFR 1926 Subpart R. Washington DC: U.S. Department of Labor, Occupational Safety and Health Administration (OSHA), Web site: www.osha.gov. See Appendix B for approved tests for slip-resistance.
11. 29 CFR 1910.23(a)(4) and 1910.23(e)(8), part of Safety and Health Regulations for General Industry: 29 CFR 1910 Subpart D, “Guarding Floor and Wall Openings and Holes.” Washington DC: U.S. Department of Labor, Occupational Safety and Health Administration (OSHA), Web site: www.osha.gov
12. Brannigan, Frank. Building Construction for the Fire Service (third ed.). Quincy MA: National Fire Protection Association, 1992, 255-323; especially 299-300.
13. Dunn, Vincent. Collapse of Burning Buildings: A Guide to Fireground Safety. New York: Fire Engineering Books, 1988, 125; 136-139.
■ GREGORY HAVEL is deputy chief and training officer with the Burlington (WI) Fire Department and a 28-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for the Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College and has 30 years of experience in facilities management and building construction.