Greasy ducts are serious fire hazards and should be cleaned periodically. There are on the market a number of automatic fire-control systems for greasy ducts. These systems usually consist of fusible-link fire dampers and a means of flame smotheringCO2, steam, foam, etc. FIGURE 13.2 Double hood for exhausting heat. Figure 13.2 shows a double hood. This type collects heat more efficiently; i.e., less exhaust air is required to collect a given amount of heat. The crack area is arranged to yield a velocity of about 1000 ft /min. A curtain of high-velocity air around the periphery of the hood catches the hot air issuing from the range or heat source. Canopy hoods are designed to handle about 50 to 125 ft3 /min of exhaust air per square foot of hood. The total amount of ventilation air should not yield more than 60 changes per hour in the space. Where hoods are not practical to install and heat will be discharged into the room, the amount of ventilation air may be determined by the following method: Determine the total amount of sensible heat generated in the premiseslights, people, electrical equipment, etc. This heat will cause a temperature rise and an increase in heat loss through walls, windows, etc. To maintain desired temperature conditions, ventilation air will have to be used to remove heat not lost by transmission through enclosures. qv  1.08Q(Ti
To ) (13.24) where qv  heat, Btu/hr, carried away by ventilation air Q  flow of ventilation air, ft3 /min Ti  indoor temperature to be maintained To  outdoor temperature With Eq. (13.24), we can calculate the amount of ventilation air required by assuming a difference between room and outdoor temperatures or we can calculate this temperature gradient for a given amount of ventilation air. The same method may be used to calculate the air quantity required to remove any objectionable chemical generated. For example, assume that after study of a process we determine that a chemical will be evolved in vapor or gas form at the rate of X lb /min. If Y is the allowable concentration in pounds per cubic foot, then Q  X/Y, where Q is the ventilation air needed in cubic feet per minute. Where moisture is the objectionable vapor, the same equation holds, but with X as the pounds per minute of moisture vaporized, Y the allowable concentration of moisture in pounds per cubic foot above outdoor moisture concentration. Once, the amount of ventilation air is determined, a duct system may be designed to handle it, if necessary. Ventilation air may be provided by installing either an exhaust system, a supply

Cold-formed shapes invariably cost more per pound than hot-rolled sections. They will be found to be more economical under the following circumstances: 1. Where their use permits a substantial reduction in weight compared to hotrolled sections. This occurs where relatively light loads are to be supported over short spans, or where stiffness rather than strength is the controlling factor in the design. 2. In special cases where a suitable combination of standard hot-rolled shapes would be heavy and uneconomical. 3. Where quantities required are too small to justify the investment necessary to produce a suitable hot-rolled section. 4. In dual-purpose panel work, where both strength and coverage are desired. Cold-formed shapes are usually made from hot-rolled sheet or strip steel, which costs less per pound than cold-rolled steel. The latter, which has been cold-rolled to desired thickness, is used for thinner gages or where, for any reason, the surface finish, mechanical properties, or closer tolerances that result from cold-reducing is desired. Manufacture of cold-formed shapes from plates for use in building construction is possible but is done infrequently. 8.1.1 Plate, Sheet, or Strip The commercial distinction between steel plates, sheet, and strip is principally a matter of thickness and width of material. In some sizes, however, classification depends on whether the material is furnished in flat form or in coils, whether it is carbon or alloy steel, and, particularly for cold-rolled material, on surface finish, type of edge, temper or heat treatment, chemical composition, and method of production. Although the manufacturers classification of flat-rolled steel products by size is subject to change from time to time, that given in Table 8.1 for carbon steel

T  0.1N (5.318) a where N is the number of stories. The base shear V is distributed vertically to the n stories as lateral forces F: F C V (5.319) x vx w hk C  x x (5.320) vx n  w hk i i i1 where Cvx  vertical distribution factor, wi and wx  the portion of the total gravity load of the building, W, located or assigned to level i or x, hi and hx  the height (ft) from the base to level i or x, and k  a distribution exponent related to the building period as follows: For buildings having a period of 0.5 seconds or less, k  1 For buildings having a period of 2.5 seconds or more, k  2 For buildings having a period between 0.5 and 2.5 seconds, k shall be 2 or shall be determined by linear interpolation between 1 and The seismic design story shear in any story, Vx is n V   F (5.321) x i i1 Rigid Diaphragms. For rigid diaphragms the seismic design story share, Vx shall be distributed to the various vertical elements of the seismic force-resisting system in the story under consideration based on the relative lateral stiffness of the vertical force resisting elements and the diaphragm. For flexible diaphragms, seismic design story shear, Vx shall be distributed to various vertical elements based on the tributary area of the diaphragm to each line of resistance. For the purpose of this section, the vertical elements of the lateral force-resisting system are permitted to be considered to be in the same line of resistance, if the maximum out-of-plane offset between each of the elements is less than 5% of the building dimension perpendicular to the direction of lateral load. Torsion. Where diaphragms are not flexible, the design shall include the torsional moment Mt, resulting from the difference in locations of the center of mass and the center of stiffness. Also where diaphragms are not flexible, in addition to the torsional moment, the design shall include accidental torsional moments Mta, caused by assumed displacement of the center of mass, each way from its actual location, by a distance equal to 5% of the dimension of the building perpendicular to the direction of the applied forces. Dynamic Amplification of Torsion. For a structure in Seismic Design Category C, D, E, or F, where Type 1a or 1b plan torsional irregularity exists, effects of torsional irregularity shall be accounted for by multiplying the sum of Mt plus Mta at each level by a torsional amplification factor, Ax, determined from the following