Jigs, patterns, templates, stops, or other suitable means should be used for all complicated and multiple assemblies to ensure accuracy, uniformity, and control of all dimensions. All tolerances in cutting, drilling, and framing must comply with good practice and applicable specifications and controls. At the time of fabrication, tolerances must not exceed those listed below, unless they are not critical and not required for proper performance. Specific jobs, however, may require closer tolerances. Location of Fastenings. Spacing and location of all fastenings within a joint should be in accordance with the shop drawings and specifications, with a maximum permissible tolerance of 1/16 in. The fabrication of members assembled at any joint should be such that the fastenings are properly fitted. Bolt-Hole Sizes. Bolt holes in all fabricated structural timber, when loaded as a structural joint, should be 1/16 in larger in diameter than bolt diameter for 1/2-in and larger-diameter bolts, and 1/32 in larger for smaller-diameter bolts. Larger clearances may be required for other bolts, such as anchor bolts and tension rods. Holes and Grooves. Holes for stress-carrying bolts, connector grooves, and connector daps must be smooth and true within 1/16 in per 12 in of depth. The width of a split-ring connector groove should be within 0.02 in of and not less than the thickness of the corresponding cross section of the ring. The shape of ring grooves must conform generally to the cross-sectional shape of the ring. Departure from these requirements may be allowed when supported by test data. Drills and other cutting tools should be set to conform to the size, shape, and depth of holes, grooves, daps, etc., specified in the National Design Specification for Wood Construction, American Forest & Paper Association. Lengths. Members should be cut within 1/16 in of the indicated dimension when they are up to 20 ft long, and 1/16 in per 20 ft of specified length when they are over 20 ft long. Where length dimensions are not specified or critical, these tolerances

Fan-Coil Terminal Units. A fan-coil terminal device consists of a fan or blower section, chilled-water coil, hot-water heating coil or electric-resistance heating elements, filter, return-air connection, and a housing for these components with an opening for ventilation air. The electric-resistance heating coil is often used with two-pipe systems to provide the performance of a four-pipe system without the cost of the two extra pipes for hot water, insulation, pumps, etc. Fan-coil units may be floor mounted, ceiling mounted-exposed or ceiling mounted-recessed, or ceiling mounted-recessed with supply- and return-air ductwork. When furnished with heating coils, the units are usually mounted on the outside wall or under a window, to neutralize the effects of perimeter heat losses. Built-in centrifugal fans recirculate room air through the cooling coil. Chilled water circulating through the coil absorbs the room heat load. Ventilation air that is conditioned by another remote central plant is ducted throughout the building and supplied directly to the room or room terminal devices, such as a fan-coil unit. A room thermostat varies the amount of cooling water passing through the cooling coil, thus varying the discharge temperature from the terminal unit and satisfying the room thermostat. Induction Terminal Units. These units are frequently used in large office buildings. The units are served by a remote air-handling unit that provides high-pressure conditioned air, which may be heated or cooled and is referred to as primary air. It is distributed to individual induction units that are located on the outside walls of each room or zone. At the terminal induction unit, a flow of high-pressure primarily air through several nozzles induces a flow of room air through the heating or cooling coil in the unit. Room air mixes with the primary air to provide a mixedair temperature that satisfies the thermal requirements of the space. In most systems, the ratio of induced air to primary air is about 4 to 1. FIGURE 13.40 Control circuit for an air-conditioning system in which the thermostat controls compressor operation. The induction system is a large energy consumer because of the extra power required to maintain the high pressure necessary to deliver the primary air to the room induction nozzles and induce room air to flow through the unit coils. Also, the induction terminal unit operates simultaneously with heating and cooling, wasting energy as in a terminal-reheat type of operation. Air-water systems generally have substantially lower installed and operating costs than all-air systems. They do not, however, provide as good control over room temperature, humidity, air quality, air movement, and noise. The best control of an air-water system is achieved with a fan-coil unit with supplemental ventilation air from a central, primary-air system that provides ventilation air. (H. E. Bovay, Jr., Handbook of Mechanical and Electrical Systems for Buildings, and N. R. Grimm and R. C. Rosaler, Handbook of HVAC Design, McGraw-Hill Publishing Company, New York.) One commonly used system has a thermostat wired in series with the compressor holding-coil circuit (Fig. 13.40). Thus the compressor will stop and start as called for by room conditions. The high-low pressure switch in series with the thermostat is a safety device that stops the compressor when the head pressure is too high and when the suction pressure approaches the freezing temperature of the coil. A liquid solenoid will shut off the flow of refrigerant when the compressor stops, to prevent flooding the coil back to the compressor during the off cycle. This valve may be eliminated when the air-handling unit and compressor are close together. A second type of control is the pump-down system (Fig. 13.41). The thermostat shuts off the flow of the refrigerant, but the compressor will keep running. With the refrigerant supply cut off, the back pressure drops after all the liquid in the coil vaporizes. Then, the high-low pressure switch cuts off the compressor. Either of these two systems is satisfactory. However, the remainder of this discussion will be restricted to the pump-down system. Additional safety controls are provided on packaged units to reduce compressor burnouts and increase the average life of these units. The controls include crankcase FIGURE 13.41 Pump-down system for control of air conditioning. heaters, motor-winding thermostats, and nonrecycling timers. These controls are usually prewired in the factory. The manufacturer supplies installation and wiring instructions for interconnecting the various components of the air-conditioning

Fu and Fxx are defined as for arc spot welds (Art. 8.16.4). Minimum edge distances also are defined as for arc spot welds. If measurements indicate that a given weld procedure will consistently give a larger effective width de or larger average diameter da, as applicable, these values may be used to calculate the maximum allowable load on an arc seam weld, if that welding procedure will actually be used. 8.16.6 Fillet Welds These are made along the edges of sheets in lapped or T joints (Fig. 8.10d). The fillet welds may be made in any position and either sheet to sheet or sheet to thicker steel member. The shear load Pn, kips, on a fillet weld in lapped or T joints should not exceed the value of Pn computed from Eqs. (8.33) to (8.34). For longitudinal loading along the weld: P
(1  0.01L/ t)tLF L/ t  25 (8.33) n u P
0.75tLF L/ t 25 (8.34) n u where t
smaller thickness of sheets being welded, in L
length, in, of the fillet weld Fu
specified tensile strength of base steel, ksi For loading transverse to the weld: