STEEL STRUCTURAL SYSTEMS

Transcription

1 STEEL STRUCTURAL SYSTEMS Steel elements are of two basic types: Structural steel shapes are formed into their final shapes by hot-rolling. This method produces such common elements as wide flange sections, angles, channels, bars, and plates. Lightweight steel members are cold-formed from thin sheets or rods. Such elements include roof and floor decking and a variety of light framing members such as channels, studs, and joists. STRUCTURAL STEEL FRAMING Conventional hot-rolled structural steel is a versatile, strong material that has applications ranging from single-story structures to the tallest buildings. The high level of prefabrication normally used with structural steel results in a system that is precise and fast to erect. Structural steel elements are normally configured as a post and beam frame, with other materials or systems added to make a complete building. The slab system most commonly used with structural steel framing is a sitecast concrete slab poured over corrugated steel decking. Other sitecast or precast concrete systems are also used. Steel frames can support a great variety of cladding systems, with curtain walls of steel, aluminum, glass, masonry, and stone being the most common. Due to the rapid loss of the strength of steel at elevated temperatures, special measurements must be taken in most circumstances to protect the structural elements in a steel frame from the heat of fire. The required fire-resistive assemblies or coatings may have a significant impact on the architectural use of structural steel. See pages for more information on the requirements for fire protection of structural steel. LIGHTWEIGHT STEEL Lightweight steel framing finds applications in low-rise structures where the light weight and ease of assembly of these elements are an advantage. Many of the details of this system and the sizes of the structural elements are similar to those used in Wood Light Frame construction, a system lightweight steel framing often competes with. However, the noncombustibility of steel allows this system to be used in building types where wood construction is not permitted. (See pages and 314 for more information on building types permitted using lightweight steel framing.) The small size of the individual structural elements and the reliance on on-site fabrication and erection also make this system a good choice where buildings of irregular or unusual form are desired. 87

2 STEEL LIGHTWEIGHT WALL STUDS 88 This chart is for curtain wall studs nonbearing studs resisting wind loads only. For light loads, close stud spacings, or heavy-gauge studs, read toward the top in the indicated areas. For heavy loads, wide stud spacings, or light-gauge studs, read toward the bottom. For brittle facings such as a masonry veneer which requires increased stiffness, read toward the bottom in the indicated areas. Typical stud spacings are 12, 16, and 24 in. (305, 406, and 610 mm). Stud widths vary from l to in. (25 to 64 mm) or more. Actual stud size is usually equal to nominal size. Availability of sizes varies with the manufacturer. Stud heights may be increased with the addition of intermediate bracing perpendicular to the wall plane. FIRE-RESISTANCE RATINGS FOR LIGHTWEIGHT STEEL FRAMING Lightweight steel construction may be used without fire protection in Unprotected Noncombustible construction, but this is seldom practical, because an interior surface material generally must be attached to the studs and joists to stabilize them against buckling. Gypsum board or gypsum veneer plaster base, the most common interior finishes, can be applied in thicknesses sufficient to achieve up to a 4- hour fire-resistance rating.

3 STEEL LIGHTWEIGHT WALL STUDS The top chart is for lightweight steel loadbearing studs up to 9 ft (2.7 m) tall between floors. For light loads, close stud spacings, or heavy-gauge studs,read toward the top in the indicated areas. For heavy loads, wide stud spacings, or light-gauge studs, read toward the bottom. Typical stud spacings are 12, 16, and 24 in. (305, 406, and 610 mm). Stud widths vary from l to in. (25 to 64 mm) or more. Actual stud size is usually equal to nominal size. Availability of sizes varies with the manufacturer. Total loaded width is the width of one floor supported by the wall multiplied by the number of floors and roof above the wall. For stud walls taller than 9 ft (2.7 m) between floors, read both charts on this page. Use the larger of the two sizes indicated. For light loads, close stud spacings, or heavy-gauge studs, read toward the top in the indicated areas. For heavy loads, wide stud spacings, or light-gauge studs, read toward the bottom. Stud heights may be increased with the addition of intermediate bracing perpendicular to the wall plane. 89

4 STEEL LIGHTWEIGHT FLOOR JOISTS FIRE-RESISTANCE RATINGS FOR LIGHTWEIGHT STEEL FRAMING Lightweight steel construction may be used without fire protection in Unprotected Noncombustible construction, but this is seldom practical, because an interior surface material generally must be attached to the studs and joists to stabilize them against buckling. Gypsum board or gypsum veneer plaster base, the most common interior finishes, can be applied in thicknesses sufficient to achieve up to a 3-hour fireresistance rating. 90

5 STEEL LIGHTWEIGHT FLOOR JOISTS This chart is for lightweight steel floor joists. For light loads, close joist spacings, or heavy-gauge steel, read toward the right in the indicated areas. For heavy loads, wide spacings, or light-gauge steel, read toward the left. Actual size is equal to nominal size less from 0 to 3 4 in. (19 mm), depending on the manufacturer. Typical joist widths are 1 5 8,2,and in. (41, 51, and 64 mm).

6 STEEL COLUMNS COLUMN LAYOUT All columns at the perimeter of a building should be oriented with their flanges facing outward to facilitate the attachment of cladding to the structural frame of the building. Elsewhere, columns should be oriented with their webs parallel to the short axis of a building whenever possible. This permits the maximum contribution from the columns to the stability of the building in the direction in which the building is most susceptible to lateral forces. Columns above and below each other at the perimeter of a multistory building are also often aligned on their outer faces. Despite the misalignment of column centers that occurs as the column size reduces on upper floors, this arrangement is desirable for the consistent curtain-wall fastening detail that it produces. See pages for additional information on the sizing of column bays. 92 FINAL DIMENSIONS OF STEEL COLUMNS The finish dimension of a steel column must be increased from the actual size of the section to account for applied fireproofing, protective cover or other finishes, and the added depth of connecting plates and protruding bolt heads where column sections are joined. The total finish dimension may range from 2 to 8 in. (50 to 200 mm) greater than the actual column size. FIRE-RESISTANCE RATINGS FOR STEEL COLUMNS Exposed steel columns may be used in Unprotected Noncombustible construction. Fire-resistance ratings of up to 4 hours are easily achieved with applied fireproofing.

7 STEEL COLUMNS The top chart is for steel wide flange section columns up to 12 ft (3.7 m) tall between floors. For normal loads, read high in the solid areas. For heavy loads, read lower in the solid areas. For light loads, read in the open areas. Approximate actual column sizes are shown to the sides of the bars. For high-strength (50 ksi or 345 MPa) steel columns, sizes W8 or larger, increase the indicated tributary area by 30%. For columns that are at the perimeter of a building, or that are part of a rigid frame system, select one nominal column size larger than shown by this chart. W14 sections are the largest standard rolled sizes commonly used as columns. Larger built-up sections capable of carrying greater loads may be shop-fabricated. Total tributary area is the total area of roofs and floors supported by the column. The bottom chart shows the maximum height permitted for each nominal column size. The tributary areas for average loads, at the maximum height and at 12 ft (3.7 m), are shown next to the bars. (The area that can be supported decreases with increasing column height). For intermediate heights, the tributary area may be interpolated between these two values. Maximum column height must be decreased for columns that are part of a rigid frame system. Column height may be increased with the use of intermediate bracing or with rigid end connections that restrain buckling. 93

8 STEEL TUBE COLUMNS STRUCTURAL STEEL TUBING Standard shapes for structural steel tubing include square tubes, rectangular tubes, and round pipes. Compared to wide flange sections or other shapes of similar size, tubes and pipes are more resistant to buckling forces, making them good choices for columns and compressive struts in all types of steel systems. They are employed as columns in long-span steel structures for their greater efficiency, and because they are available in lighter weights than other standard shapes, they are frequently used in one- or two-story steel structures as well. Tube and pipe sections are popular choices for use in the fabrication of steel trusses and space frames, and their high torsional resistance makes them excellent choices for single post supports such as for signs or platforms. The simple profiles and clean appearance of steel tubes and pipes also make them popular for use where the steel may remain visible in the finished structure, or for structures exposed to the weather where the absence of moisture- and dirt-trapping profiles and ease of maintenance are desirable characteristics. 94 SIZES FOR STEEL TUBES AND PIPES Tubes and pipes are generally available in whole-inch (25-mm) sizes up to 6 or 8 in. (152 or 203 mm). Greater sizes are available in even-inch (51-mm) increments. Width of Tube Shape or Diameter of Pipe Thickness of Wall Square tubes 3" 3" 16" 16" 0.188" 0.625" (76 76 mm mm) (5 16 mm) Rectangular tubes 3" 2" 16" 12" " (76 51 mm mm) (5 16 mm) Pipes 3" 12" 0.216" 0.875" ( mm) (5 22 mm) FINISH DIMENSIONS OF STEEL COLUMNS The finish dimension of a steel column must be increased from the actual size of the section to account for applied fireproofing, protective cover or other finishes, and the added depth of connecting plates and protruding bolt heads where column sections are joined. The total finish dimension may range from 2 to 8 in. (50 to 200 mm) greater than the actual column size.

9 STEEL TUBE COLUMNS This chart is for square tube steel columns up to 12 ft (3.7 m) tall between floors. For normal loads, read high in the solid areas. For heavy loads, read lower in the solid areas. For light loads, read in the open areas. For columns at the perimeter of a building, or that are part of a rigid frame system, select one nominal column size larger than shown by this chart, or consider rectangular tubes larger in one axis only. Actual column size is equal to the nominal size. Total tributary area is the total area of roofs and floors supported by the column. Tributary area will be less for taller columns. 95 FIRE-RESISTANCE RATINGS FOR STEEL TUBE AND PIPE COLUMNS Exposed steel columns or other framing elements may be used in Unprotected Noncombustible construction. Fire-resistance ratings of up to 4 hours are easily achieved with applied fireproofing.

10 STEEL FLOOR AND ROOF DECKING STEEL FLOOR DECKING Corrugated steel floor decking with a sitecast concrete topping is the slab system most commonly used over structural steel framing. Typical span ranges for steel floor decking when used with structural steel framing are from 6 to 15 ft (1.8 to 4.6 m). Longer spans or shallower depths than those indicated on the chart on the facing page may be possible, although increased construction costs may result from the need for additional temporary shoring of the decking during erection. 96 CELLULAR FLOOR DECKING The use of cellular decking to provide protected spaces within the floor slab for the running of electrical and communications wiring may influence the overall framing plan for the building. The layout of such a distribution system can determine the direction in which the decking cells will run in various areas of the building plan. The orientation of the beams or joists carrying the decking will be determined from this in turn, as in all cases these elements must run perpendicular to the cells in the decking. See page 187 for additional information on the planning of such systems. When reading from the chart for cellular deck on the facing page, read toward the bottom in the indicated area. STEEL ROOF DECKING Steel roof decking may have a sitecast concrete or gypsum topping or may be covered directly with a variety of board or roofing products. A common and economical configuration for roof decking is in. (38-mm) decking spanning up to approximately 8 ft (2.4 m). Many proprietary metal roof decking systems, with a wide variety of performance characteristics, are also available. For information on such systems, consult individual manufacturers FIRE-RESISTANCE RATINGS FOR STEEL DECKING Steel roof decking without a concrete topping may be used in Unprotected Noncombustible construction. The fire resistance of roof or floor decking with a concrete topping varies with the configuration of the decking and the thickness of the topping. Though resistance ratings of as high as 3 hours may be possible, for preliminary design, assume that decking must be protected with applied fireproofing or an appropriately fire-resistive ceiling to achieve ratings of more than l hour.

11 STEEL FLOOR AND ROOF DECKING The top chart is for corrugated or cellular steel floor decking with concrete slab topping. For light loads, read toward the bottom in the indicated areas. For heavy loads, read toward the top. Total depth of slab is the depth of the decking and the concrete topping. Approximate sizes for the steel decking alone are shown within the chart. Deeper deck sections with spans of up to approximately 25 ft (7.6 m) may be available from some manufacturers. 97 The bottom chart is for corrugated steel roof decking. For light loads, read toward the right in the indicated areas. For heavy loads, read toward the left. Deeper deck sections with spans of up to approximately 25 ft (7.6 m) may be available from some manufacturers.

12 STEEL BEAMS AND GIRDERS Structural steel is a versatile building material. While it can be used in a great variety of ways, consider the following guidelines for what is most economical in common practice. FLOOR AND ROOF FRAMING The most economical span range for conventional steel floor and roof framing is from 25 to 40 ft (8 to 12 m). Individual column bays should be approximately 1,000 square feet (95 m 2 ) in area, and rectangular in shape, with the long side 1.25 to 1.5 times as long as the shorter side. Above spans of approximately 40 feet (12 m), consider open-web steel joists for their lighter weight and greater economy (see pages ). The spacing between individual beams depends on the applied loads and the decking system. Spacings from 6 to 15 ft (1.8 to 4.6 m) are common with corrugated steel and concrete slab decking. Spacings up to approximately 8 ft (2.4 m) are typical for roof decking systems. 98 BEAM AND GIRDER CONFIGURATION The orientation of beams and girders in a floor or roof framing system may depend on a variety of factors. In relation to the building at large, it may be advantageous to run girders parallel to the building s shorter axis, the direction most susceptible to lateral forces. In this way, these stronger members can contribute additional lateral resistance to the building through rigid frame action. Within individual column bays, it is usually more economical to run girders in the shorter direction of a rectangular bay, allowing the lighter beams to span the longer way. However, when cellular decking is used as part of a wiring system, beam and girder directions may be set so that the wire conduits within the decking run in preferred directions as required by communications or power distribution plans (see page 187). COMPOSITE BEAMS In composite construction, shear studs are added to the top of the floor beams. This causes the concrete deck and steel framing to act as a unified structural element and results in reduced beam depths. Composite construction can be more economical, particularly at longer spans. However, a thicker concrete deck may be required. In some cases partial composite design, where fewer studs are used and less than full composite action is achieved, proves to be the most economical solution. For economical framing of steel bays, the lighter beams should span 1.25 to 1.5 times the span of the heavier girders. Bay area should equal approximately 1000 ft 2 (95 m 2 ).

13 STEEL BEAMS AND GIRDERS FIRE-RESISTANCE RATINGS FOR STEEL BEAMS AND GIRDERS Exposed steel beams and girders may be used in Unprotected Noncombustible construction. Fire-resistance ratings of as high as 4 hours are easily achieved with applied fireproofing or an appropriately fire resistive ceiling. Some building codes also allow reduced fire protection or exposed steel for roof structures that are 15 to 25 ft (4.6 to 7.6 m) or more above the floor. This chart is for steel wide-flange beams and girders. For average and light loads, read toward the right in the indicated areas. For heavy loads, read toward the left. For beams acting as girders or as composite beams, read in the open areas indicated. Beams or girders also acting as part of a rigid frame for lateral stability may be deeper than indicated by this chart. Typical widths of beams and girders range from approximately one-third to one-half the depth of the member. Heavy sections used for heavy loads or to conserve depth may be wider. Depths of up to 36 in. (914 mm) are available as standard rolled sections. Greater depth beams capable of longer spans may be shop fabricated. 99

14 STEEL OPEN-WEB JOISTS OPEN-WEB JOIST FRAMING The light weight of open-web steel joists makes them an economical alternative to conventional structural steel members for spans greater than 30 to 40 ft (9 to 12 m). Where significant concentrated loads exist, open-web joists may need to be supplemented with additional structural members. Girders used with open-web joists may be joist girders (a heavier version of an open-web joist) or conventional structural steel members. For greater loads and spans, heavy steel trusses may also be used. For rectangular bays, the joists usually span the longer direction. (See pages for structural steel beams and girders and pages for heavy steel trusses.) A variety of proprietary composite systems are also available. Such systems are particularly effective at overcoming the excessive flexibility sometimes encountered with long-span joist systems. 100 FIRE-RESISTANCE RATINGS FOR OPEN-WEB STEEL JOISTS Exposed open-web joists and joist girders may be used in Unprotected Noncombustible construction. Fire-resistance ratings of as high as 3 hours are easily achieved with applied fireproofing or an appropriately fireresistive ceiling. The fire-resistive ceiling is used more commonly, due to the difficulty of applying fireproofing directly to the complex surfaces of an open-web joist. Some building codes also permit reduced fire protection or exposed steel for roof structures that are 15 to 25 ft (4.6 to 7.6 m) or more above the floor.

15 STEEL OPEN-WEB JOISTS This chart is for open-web steel joists and joist girders for floors and roofs. For light loads or close joist spacings, read toward the right in the indicated areas. For heavy loads or large joist spacings, read toward the left. Joist spacings range from 2 to 10 ft (0.6 to 3.0 m) or more, depending on the floor loads and the decking system applied over the joists. Joists generally come in depths of 8 to 32 in. in 2-in. increments (from 203 to 813 mm in 51-mm increments) and from 32 to 72 in. in 4-in. increments (from 813 to 1829 mm in 102-mm increments). Availability of sizes varies with the manufacturer. Joist girders come in depths of 20 to 96 in. in 4-in. increments (from 508 to 2438 mm in 102-mm increments). 101

16 SINGLE-STORY RIGID STEEL FRAMES RELATED DIMENSIONS FOR SINGLE-STORY RIGID STEEL FRAMES For the span ranges indicated on the chart on the facing page, the following dimensions may be used: Wall Height Depth at Base Roof Pitch 8' 30' ( m) 7" 21" ( mm) 1:12 4:12 Typical spacing of frames is 20 or 25 ft (6.1 or 7.6 m). For variations on the rigid frame system, or for sizes outside the range of those shown in the chart, consult with individual manufacturers. 102 FIRE-RESISTANCE RATINGS FOR SINGLE-STORY RIGID STEEL FRAMES Exposed steel frames may be used in Unprotected Noncombustible construction. Fire-resistance ratings of as high as 4 hours are easily achieved with applied fireproofing or an appropriately fire-resistive ceiling. Some building codes also allow reduced fire protection or exposed steel for roof structures that are 15 to 25 ft (4.6 to 7.6 m) or more above the floor.

17 SINGLE-STORY RIGID STEEL FRAMES This chart is for single-story rigid steel frame structures. For heavy loads, read toward the top in the indicated areas. For light loads, read toward the bottom. Spans as great as 200 ft (61.0 m) or more may be available from some manufacturers. Greater spans are also available with the use of intermediate columns. 103

18 STEEL TRUSSES ECONOMICAL SPAN RANGES FOR PARALLEL CHORD TRUSSES Parallel chord trusses are most economical for spans up to 120 to l40 ft (35 to 45 m), due to the increased difficulty of shipping elements greater than 12 ft (3.7 m) deep. Triangular and bowstring trusses can be shipped at slightly greater depths.trusses spanning 300 ft (90 m) or more may be fabricated on-site. FIRE-RESISTANCE RATINGS FOR STEEL TRUSSES Exposed steel trusses may be used in Unprotected Noncombustible construction. Fire-resistance ratings of as high as 4 hours are easily achieved with applied fireproofing or an appropriately fire-resistive ceiling. Some building codes also allow reduced fire protection or exposed steel for roof structures that are 15 to 25 ft (4.6 to 7.6 m) or more above the floor. 104

19 STEEL TRUSSES 105 This chart is for steel trusses fabricated from structural steel members. Because these trusses are custom designed and fabricated, a great variety of shapes and configurations are possible.