1.0 for normal-weight concrete
0.85 for sand-lightweight concrete
0.75 for all-lightweight concrete Precast-concrete members are assembled and fastened together on the jobsite. They may be unreinforced, reinforced, or prestressed. Precasting is especially advantageous when it permits mass production of concrete units. But precasting is also beneficial because it facilitates quality control and use of higher-strength concrete. Form costs may be greatly reduced, because reusable forms can be located on a casting-plant floor or on the ground at a construction site in protected locations and convenient positions, where workmen can move about freely. Many complex thinshell structures are economical when precast, but would be uneconomical if cast in place. Design of precast-concrete members under the ACI 318 Building Code follows the same rules as for cast-in-place concrete. In some cases, however, design may not be governed by service loads, because transportation and erection loads on precast members may exceed the service loads. Design of joints and connections must provide for transmission of any forces due to shrinkage, creep, temperature, elastic deformation, gravity loads, wind loads, and earthquake motion. (Design and Typical Details of Connections for Precast and Prestressed Concrete, 2d ed., Precast / Prestressed Concrete Institute.) Less concrete cover is required for reinforcement in precast-concrete members manufactured under plant control conditions than in cast-in-place members because the control for proportioning, placing, and curing is better. Minimum concrete cover for reinforcement required by ACI 318-99 is listed in Table 9.26. For all sizes of reinforcement in precast-concrete wall panels, minimum cover of 3/4 in is acceptable at nontreated surfaces exposed to weather and 3/8 in at interior

Power roof ventilators are usually roof mounted and utilize either centrifugal or axial blade fans. Both types are generally used in low-pressure exhaust systems for factories, warehouses, etc. They are available in capacities up to about 30,000 ft3 /min. They are, however, limited to operation at a maximum pressure of about 1/2 in water gage. Powered roof ventilators are also low in first cost and low in operating costs. They can provide positive exhaust ventilation in a space, which is a definite advantage over gravity-type exhaust units. The centrifugal unit is somewhat quieter than the axial-flow type. Fans vary widely in shapes and sizes, motor arrangements and space requirements. Fan performance characteristics (variation of static pressure and brake horsepower) with changes in the airflow rate (ft3 /min) are available from fan manufacturers and are presented in tabular form or as fan curves. Dampers. Dampers are mechanical devices that are installed in a moving airstream in a duct to reduce the flow of the stream. They, in effect, purposely produce a pressure drop (when installed) in a duct by substantially reducing the free area

The preceding data indicate that, because of the low modulus of elasticity, aluminum members have good energy absorption. Where stiffness is important, however, the effect of the low modulus should be taken into account. Specific data for an application should be obtained from the producers. Weldability and brazing properties of aluminum alloys depend heavily on their composition and heat treatment. Most of the wrought alloys can be brazed and welded, but sometimes only by special processes. The strength of some alloys TABLE 4.22 Finishes for Aluminum and Aluminum Alloys Types of finish Designation* Mechanical finishes: As fabricated M1Y Buffed M2Y Directional textured M3Y Nondirectional textured M4Y Chemical finishes: Nonetched cleaned C1Y Etched C2Y Brightened C3Y Chemical conversion coatings C4Y Coatings: Anodic General A1Y Protective and decorative (less than 0.4 mil thick) A2Y Architectural Class II (0.40.7 mil thick) A3Y Architectural Class I (0.7 mil or more thick) A4Y Resinous and other organic coatings R1Y Vitreous coatings V1Y Electroplated and other metallic coatings E1Y Laminated coatings L1Y *Y represents digits (0, 1, 2, . . . 9) or X (to be specified) that describe the surface, such as specular, satin, matte, degreased, clear anodizing or type of coating. depends on heat treatment after welding. Alloys heat treated and artificially aged are susceptible to loss of strength at the weld, because weld is essentially cast. For this reason, high-strength structural alloys are commonly fabricated by riveting or bolting, rather than by welding. Brazing is done by furnace, torch, or dip methods. Successful brazing is done with special fluxes. Inert-gas shielded-arc welding is usually used for welding aluminum alloys. The inert gas, argon or helium, inhibits oxide formation during welding. The electrode used may be consumable metal or tungsten. The gas metal arc is generally preferred for structural welding, because of the higher speeds that can be used. The gas tungsten arc is preferred for thicknesses less than 1/2 in. Butt-welded joints of annealed aluminum alloys and non-heat-treatable alloys have nearly the same strength as the parent metal. This is not true for strainhardened or heat-tempered alloys. In these conditions, the heat of welding weakens the metal in the vicinity of the weld. The tensile strength of a butt weld of alloy 6061-T6 may be reduced to 24 ksi, about two-thirds that of the parent metal. Tensile yield strength of such butt welds may be only 15 to 20 ksi, depending on metal thickness and type of filler wire used in welding. Fillet welds similarly weaken heat-treated alloys. The shear strength of alloy 6061-T6 decreases from about 27 ksi to 17 ksi or less for a fillet weld. Welds should be made to meet the requirements of the American Welding Society, Structural Welding CodeAluminum, AWS D1.2. Aluminum connections also may be bolted or riveted. Bolted connections are bearing type. Slip-critical connections, which depend on the frictional resistance of joined parts created by bolt tension, are not usually employed because of the relatively low friction and the potential relaxation of the bolt tension over time. Bolts may be aluminum or steel. Bolts made of aluminum alloy 7075-T73 have a minimum expected shear strength of 40 ksi. Cost per bolt, however, is higher than that of 2024-T4 or 6061-T6, with tensile strengths of 37 and 27 ksi, respectively. Steel bolts may be used if the bolt material is selected to prevent galvanic corrosion or the steel is insulated from the aluminum. One option is use of stainless steel. Another alternative is to galvanize, aluminize, or cadmium plate the steel