Accordingly, a means of egress is a continuous, unobstructed path for evacuees from any point in a building to a public way. Its three parts are: Exit accessthat portion that leads to an entrance to an exit Exitthe portion that is separated from all other building spaces by construction or equipment required to provide a protected path to the exit discharge Exit dischargethe portion that connects the termination of an exit to a public way Means of egress may be provided by exterior and interior doors and enclosed horizontal and vertical passageways, including stairs and escalators. (Elevators and exterior fire escapes are not generally recognized as reliable means of egress in a fire.) Exit access includes the space from which evacuation starts and passageways and doors that must be traversed to reach an exit. Types of Exits. Building codes generally recognize the following as acceptable exits when they meet the codes safety requirements: Corridorsenclosed horizontal or slightly inclined public passageways, which lead from interior spaces toward an exit discharge. Minimum floor-to-ceiling height permitted is generally 80 in. Minimum width depends on type of occupancy and passageway (Table 3.7 and Art. 3.5.11). Codes may require subdivision of corridors into lengths not exceeding 300 ft for educational buildings and 150 ft for institutional buildings. Subdivision should be accomplished with noncombustible partitions incorporating smokestop doors. In addition, codes may require the corridor enclosures to have a fire rating of 1 or 2 hr. Exit passagewayshorizontal extensions of vertical passageways. Minimum floor-to-ceiling height is the same as for corridors. Width should be at least that of the vertical passageways. Codes may require passageway enclosures to have a 2-hr fire rating. A street-floor lobby may serve as an exit passageway if it is sufficiently wide to accommodate the probable number of evacuees from all contributing spaces at the lobby level. Exit doorsdoors providing access to streets or to stairs or exit passageways. Those at stairs or passageways should have a fire rating of at least 3/4 hr. Horizontal exitpassageway to a refuge area. The exit may be a fire door through a wall with a 2-hr fire rating, a balcony providing a path around a fire barrier, or a bridge or tunnel between two buildings. Doors in fire barriers with 3- or 4-hr fire ratings should have a 11/2-hr rated door on each face of the fire division. Walls permitted to have a lower fire rating may incorporate a single door with a rating of at least 11/2 hr. Balconies, bridges, and tunnels should be at least as wide as the doors providing access to them, and enclosures or sides of these passageways should have a fire rating of 2 hr or more. Exterior-wall openings, below or within 30 ft of an open bridge or balcony, should have at least 3/4-hr fire protection. Interior stairsstairs that are inside a building and that serve as an exit. Except in one-story or two-story low-hazard buildings, such stairs should be built of noncombustible materials. Stairway enclosures generally should have a 2-hr fire rating. Building codes, however, may exempt low dwellings from this requirement. Exterior stairsstairs that are open to the outdoors and that serve as an exit to ground level. Height of such stairs is often limited to 75 ft or six stories. The stairs should be protected by a fire-resistant roof and should be built of noncombustible materials. Wall openings within 10 ft of the stairs should have 3/4-hr fire

C  C      (4.4) eq 6 20 10 50 10 40 where Ni  nickel content, % Cr  chromium content, % Mo  molybdenum content, % V  vanadium content, % Cu  copper content, % Carbon equivalent appears to be related to the maximum rate at which a weld and adjacent base metal may be cooled after welding without underbead cracking occurring. The higher the carbon equivalent, the lower will be the allowable cooling rate. Also, the higher the carbon equivalent, the more important use of lowhydrogen electrodes and preheating becomes. The processing of steels after conversion of pig iron to steel in a furnace has an important influence on the characteristics of the final products. The general procedure is as follows: The molten steel at about 2900
F is fed into a steel ladle, a refractory-lined open-top vessel. Alloying materials and deoxidizers may be added during the tapping of the heat or to the ladle. From the ladle, the liquid steel is poured into molds, where it solidifies. These castings, called ingots, then are placed in special furnaces, called soaking pits. There, the ingots are held at the desired temperature for rolling until the temperature is uniform throughout each casting. Ideally, an ingot should be homogeneous, with a fine, equiaxial crystal structure. It should not contain nonmetallic inclusions or cavities and should be free of chemical segregation. In practice, however, because of uneven cooling and release of gases in the mold, an ingot may develop any of a number of internal and external defects. Some of these may be eliminated or minimized during the rolling operation. Prevention or elimination of the others often adds to the cost of steels. Steel cools unevenly in a mold, because the liquid at the mold walls solidifies first and cools more rapidly than metal in the interior of the ingot. Gases, chiefly oxygen, dissolved in the liquid, are released as the liquid cools. Four types of ingot may resultkilled, semikilled, capped, and rimmeddepending on the amount of gases dissolved in the liquid, the carbon content of the steel, and the amount of deoxidizers added to the steel. A fully killed ingot develops no gas; the molten steel lies dead in the mold. The top surface solidifies relatively fast. Pipe, an intermittently bridged shrinkage cavity, forms below the top. Fully killed steels usually are poured in big-end-up molds with hot tops to confine the pipe to the hot top, which is later discarded. A semikilled ingot develops a slight amount of gas. The gas, trapped when the metal solidifies, forms blowholes in the upper portion of the ingot. A capped ingot develops rimming action, a boiling caused by evolution of gas, forcing the steel to rise. The action is stopped by a metal cap secured to the mold. Strong upward currents along the sides of the mold sweep away bubbles that otherwise would form blowholes in the upper portion of the ingot. Blowholes do form, however, in the lower portion, separated by a thick solid skin from the mold walls. A rimmed ingot develops a violent rimming action, confining blowholes to only the bottom quarter

Dimensional tolerances for precast members and tolerances on fitting of precast members vary for type of member, type of joint, and conditions of use. See PCI TABLE 9.26 Minimum Reinforcement Cover for Precast Members, in Concrete exposed to earth or weather: Wall panels: No. 14 and No. 18 bars 11/2 No. 11 bars and smaller 3/4 Other members: No. 14 and No. 18 bars 2 No. 6 through No. 11 bars 11/2 No. 5 bars, 5/8-in wire and smaller 11/4 Concrete not exposed to weather or in contact with the ground: Slabs, walls, joists: No. 14 and No. 18 bars 11/4 No. 11 bars and smaller 5/8 Beams, girders, columns: Principal reinforcement: Diameter of bar db but not less than 5/8 in and need not be more than 11/2 in Ties, stirrups or spirals 3/8 Shells and folded-plate members: No. 6 bars and larger 5/8 No. 5 bars, 5/8-in wire and smaller 3/8 Design Handbook, and Design and Typical Details of Connections for Precast and Prestressed Concrete, Precast / Prestressed Concrete Institute; and Standard Specifications for Tolerances for Concrete Construction and Materials, ACI 117, American Concrete Institute. For strength and durability, precast concrete members require adequate curing. They usually are given some type of accelerated curing for economic reuse of forms and casting space. At atmospheric pressure, curing temperatures may be held between 125 and 185F for 12 to 72 h. Under pressure, autoclave temperatures above 325F for 5 to 36 h are applied for fast curing. Casting temperatures, however, should not exceed 90F. See Fig. 9.5. (Standard Practice for Curing Concrete, ACI 308; Accelerated Curing of Concrete at Atmospheric PressureState of the Art, ACI 517.2R, American Concrete