However, if the water towers and evaporative condensers are designed for 75
F WB, then at a wet-bulb temperature above 75, the equipment capacity would be decreased and the compressor head pressure would build up too high, overload the motor, and kick out on the overload relays. So we use 78
F WB for the design of heat rejection equipment. Now, on a 78
F WB day, the compressor head pressure would be at design pressure, the equipment will be in operation, although maintaining room conditions a little less comfortable than desired. At 80
F WB, the compressor head pressure will build up above design pressure and the motor will be drawing more than the design current but usually less than the overload rating of the safety contact heaters. Water condensers are sized for 104
F refrigerant temperature and 95
F water leaving. The amount of water in gallons per minute required for condensers is subject to the type of heat rejection equipment used and manufacturers data should be used. A rule of thumb is 3 gpm/ton per 10
F water-temperature rise. When choosing a condenser for water-tower use, we must determine the water temperature available from the cooling tower with 95
F water to the tower. Check the tower manufacturer for the capacity required at the design WB, and 95
F water to the sprays, and the appropriate wet-bulb approach. (Wet-bulb approach is equal to the number of degrees the temperature of the water leaving the tower is above the wet-bulb temperature. This should be for an economic arrangement about 7
F.) Thus, for 78
F, the water leaving the tower would be 85
F, and the condenser would be designed for a 10
F water-temperature rise, i.e., for water at 95
F to the tower and 85
F leaving the tower. Evaporative condensers should be picked for the required capacity at a design wet-bulb temperature for the area in which they are to be installed. Manufacturers ratings should be checked before the equipment is ordered. For small- and medium-size cooling systems, air-cooled condensers are available for outdoor installation with propeller fans or for indoor installation usually with centrifugal blowers for forcing outdoor air through ducts to and from the condensers. Compressor capacity decreases with increase in head pressure (or temperature) and fall in suction pressure (or temperature). Therefore, in choosing a compressor-motor unit, one must first determine design conditions for which this unit is to operate;

TABLE 6.16 General Factors That Control the Stability of the Excavation Slopes Construction activity Objectives Comments Dewatering In order to prevent boiling, softening, or heave of the excavation bottom, reduce lateral pressures on sheeting, reduce seepage pressures on face of open cut, and eliminate piping of fines through sheeting. Investigate soil compressibility and effect of dewatering on settlement of nearby structures; consider recharging or slurry wall cutoff. Examine for presence of lower aquifer and need to dewater. Install piezometers if needed. Consider effects of dewatering in cavity-laden limestone. Dewater in advance of excavation. Excavation and grading (also see Art. 6.10) Utility trenches, basement excavations, and site grading. Analyze safe slopes or bracing requirements, and effects of stress reduction on overconsolidated, soft, or swelling soils and shales. Consider horizontal and vertical movements in adjacent areas due to excavation and effect on nearby structures. Keep equipment and stockpiles a safe distance from the top of the excavation.

where Asw
area of reinforcing steel required to develop compression strength of FIGURE 9.15 Stresses and strains in a T-beam at ultimate load: (a) beam crosssection; (b) strain distribution; (c stress distributions in web; (d ) block distribution of flange compression stresses. web and As
area of reinforcing steel required to develop compression strength of overhanging flanges. The reinforcement ratio for balanced conditions is given by bw 0.85c
1 87,000 As 
   (9.37) b b 87,000  b d y y w The depth to the neutral axis c can be found in the same way as for rectangular beams (Arts. 9.46.1 and 9.46.2). Design at a section of a reinforced-concrete flexural member with factored shear force Vu is based on V  V
(V  V ) (9.38) u n c s where 
strength-reduction factor (given in Art. 9.44.1) Vu
factored shear force at a section Vc
nominal shear strength of concrete Vs
nominal shear strength provided by reinforcement Except for brackets, deep beams, and other short cantilevers, the section for maximum shear may be taken at a distance d from the face of the support when the reaction in the direction of the shear introduces compression into the end region of