A counterweight usually is made up of cut steel plates set in a steel frame. Moving up as the car moves down and down when the car moves up, the counterweight is kept in a fixed vertical path by upper and lower guide rollers that are attached to its frame and roll along a pair of guide rails. 16.9.7 Roping for Elevators The American National Standard Safety Code for Elevators, Dumbwaiters, Escalators and Moving Walks, ANSI A17.1, requires that a car be suspended from at least three hoisting ropes for traction-type machines and two ropes for windingdrum machines. At least two ropes are needed for a counterweight. All these ropes should be at least 1/2 in in diameter. A wire rope for an elevator installation comprises a group of steel strands laid helically around a hemp core. Each strand, in turn, consists of steel wires placed helically around a central wire and has a symmetrical cross section. For a given weight of a car and load, the method of roping an elevator has a considerable effect on car speed and loading on the hoisting ropes machine bearings, and building structural members. The simple arrangement of hoisting ropes, cars, and counterweight shown in Fig. 16.14a, for example, is called 1:1 roping, because car speed equals rope speed. The ropes are attached to the top of the car frame, wind around the driving sheave, bend around a deflector sheave, and then extend downward to the top of the counterweight. This rope arrangement is also known as single-wrap roping because the ropes pass over the driving sheave only once between the car and the counterweight. The 1:1 single-wrap roping often is used for high-speed passenger elevators. For single-wrap roping, the rim of the driving sheave is given wedge-shaped or undercut grooves, to obtain sufficient traction. The sheave grips the ropes because of a wedging action between the sides of the grooves and the ropes. The pinching, however, tends to shorten rope life. For good traction with less rope wear, double-wrap roping (Fig. 16.14b), is frequently used for high-speed passenger elevators instead of single-wrap. For double-wrap roping, the ropes are attached to the top of the car frame, wind twice around the driving sheave and secondary sheave, and are then deflected down to the counterweight by the secondary sheave. Because of the double wrap, less grip is needed at the driving sheave. As a result, its rim may be given U-shaped or round-seat grooves, which cause less rope wear. The 1:1 double-wrap roping shown in Fig. 16.14b, however, applies twice the load to the driving sheave for the same weight of car and counterweight as does the single-wrap roping in Fig. 16.14a and requires a heavier design for affected components. For the double-wrap roping shown in Fig. 16.14c, rope speed is twice the car speed. The arrangement, called 2:1 roping, is suitable for heavily loaded, slow freight elevators. For this arrangement, a higher-speed, less costly motor can be used for a given car speed than with 1:1 roping. The ropes in this case are not attached to the car and counterweight as for 1:1 roping. Instead, the ropes wind around idler sheaves on car and counterweight, and the ends of the ropes are anchored at the top of the hoistway at beams. As a result, the load on the driving and idler sheaves is only about one-half that for 1:1 roping. In most buildings, driving machines are located in a penthouse. When a machine must be placed in a basement (Fig. 16.14d), the load on the overhead supports is increased, rope length is tripled and additional sheaves are needed, adding to the cost. Other disadvantages include higher friction losses and a larger number of rope bends, requiring greater traction between ropes and driving sheave for the same elevator loads and speeds; modestly higher power consumption; and potentially

Mcr
cracking moment based on the modulus of rupture (Art. 9.51) bw
width of web pc
compressive stress in the concrete, after all prestress losses have occurred, at the centroid of the cross section resisting the applied loads, or at the junction of web and flange when the centroid lies within the flange Vp
vertical component of effective prestress force at section considered In a pretensioned beam in which the section h/2 from the face of the support is closer to the end of the beam than the transfer length of the tendon, the reduced prestress in the concrete at sections falling within the transfer length should be considered when calculating Vcw. The prestress may be assumed to vary linearly along the centroidal axis from zero at the beam end to the end of the transfer length. This distance can be assumed to be 50 diameters for strand and 100 diameters for single wire. (PCI Design Handbook, Precast / Prestressed Concrete Institute.) Three- or seven-wire pretensioning strand should be bonded beyond the critical section for a development length, in, of at least L
(  2/3 )d (9.134) d ps se b where dp
nominal diameter of strand, in ps
stress in tendons at nominal strength, ksi se
effective stress in tendons after losses, ksi (The expression in parentheses is used as a constant without units.) Investigations for bond integrity may be limited to those cross sections nearest each end of the member that are required to develop their full strength under factored load. When bonding does not extend to the end of the member, the bonded development length given by Eq. (9.134) should be doubled. Minimum Bonded Reinforcement. When prestressing steel is unbonded, the ACI 318 Building Code requires that some bonded reinforcement be placed in the precompressed tensile zone of flexural members and distributed uniformly over the tension zone near the extreme tension surface. The amount of bonded reinforcement that should be furnished for beams, one-way slabs, and two-way slabs, except for two-way flat plates, is A
0.004A (9.135) s For two-way flat plates, where tension stress in the concrete under service loads is not greater than 2

, bonded reinforcement is not required in positive moment c areas. When the tension stress in the concrete under service load exceeds 2

, c the minimum amount of bonded reinforcement provided in positive moment areas

The disadvantages are: It is dependent on estimates from management of percentage of completion and cost to complete. Retained percentages and accounts receivable not yet billed are taken as income although they may not be collected until a much later date. It does not permit the deferral of income taxes, a procedure that may be available with the cash or completed-contract method. Responsibility for job safety rests initially with the superintendent. Various safety manuals are available giving recommended practice for all conceivable types of construction situations: for example, see Manual of Accident Prevention in Construction, Associated General Contractors of America, Inc., 1957 E St., NW,Washington, Because of wide diversity in state safety laws, the Federal government in 1970 passed the Occupational Safety and Health Act (OSHA) (Title 29Labor Code of Federal Regulations, Chapter XVII, Part 1926, U.S. Government Printing Office). Compared with state safety laws of the past. the Federal law had much stricter requirements. For example, in the past, a state agency had to take the contractor to court for illegal practices. In contrast, the Occupational Safety and Health Administration can impose fines on the spot for violations, despite the fact that inspectors ask the employers to correct their deficiencies. OSHA enforcement, however, may eventually be taken over by the states if they develop state regulations as strict as