16.9.2 Elevator Control The system governing starting, stopping, direction of motion, speed, and acceleration and deceleration of the car is called control. Multivoltage control (also known as variable-voltage control) or rheostatic control has been commonly used for electric elevators, due largely to the relative simplicity of controlling the dc motor. The advent of larger power transistors has resulted in control systems known as VVVF control, that can be applied to ac motors to produce smooth starting and stopping equal to the classic dc elevator control system. Multivoltage control usually is used with driving machines with dc motors. For elevator control, the voltage applied to the armature of the motor is varied. Because buildings usually are supplied with ac power, the variable voltage generally is obtained from a motor-generator set that converts ac to dc. This type of control commonly is used for passenger elevators because it combines smooth, accurate speed regulation with efficient motor operation. It also permits rapid acceleration and deceleration and accurate car stops, with low power consumption and little maintenance. But multivoltage control costs more initially than rheostatic control. Variable-voltage, variable-frequency control is a means used to produce smooth acceleration, deceleration, and stopping of common ac motors at nonsynchronous speeds. VVVF control offers much higher efficiency than that realized through dc motors and is gradually replacing the various means used to control dc motors, along with the dc motor. 16.9.3 Car Leveling at Landings Elevator installations should incorporate equipment capable of stopping elevator cars level with landings within a tolerance of 1/2 in under normal loading and unloading conditions. Because changing car loads vary the stretch of the hoisting ropes, provision should be made to compensate for this variation and keep the car platform level with the landing. Most elevators employ automatic leveling. Automatic leveling controls the driving motor to level the car. Elevators typically employ a two-way, automatic leveling device to correct the car level on both overrun and underrun at a landing and hold the car level with the landing during car loading and unloading. 16.9.4 Terminal Stopping Devices For safety, provisions should be made to control car movement as it approaches a terminal landing and to keep it from passing the terminal. For the purpose, special speed-limiting and stopping devices are needed. An emergency terminal speed-limiting device is required to reduce car speed automatically as the car approaches the terminal landing. This should be done independently of the functioning of the operating device, which actuates the elevator control, and of the normal terminal stopping device if it should fail to slow the car

DSU. Data service unit, electronic equipment that interfaces a digital service network such as a T1 carrier to the DCE (data communications equipment). DSUs are usually located in the main communications closet on the wall or in a rack. DTE. Data terminal equipment, the source or destination of signals on a network. Data terminals or display terminals that had no integral computing power were used as the destination of signals from mainframe computers. Data terminals became known as dumb terminals as personal computers were developed. PCs are smart DTEs. Edge Equipment. Electronic equipment that interfaces ATM and Ethernet networks. EIA. Electronic Industries Alliance, a trade organization representing the United States high-technology community. Electronic Industries Association. A standards-writing body. ELAN. Emulated LAN on an ATM network. Entrance Facility. The entrance to the building for communications cables, including the point at the wall or floor and the conduit or path to the buildings main communications room. Entrance Facility, Fiberoptic. The box in a buildings main communications room where the outside-plant cable is spliced or connected to the buildings

5.7.6 Local Buckling A column may not only fail by buckling of the member as a whole but as an alternative, by buckling of one of its components. Hence, when members like I beams, channels, and angles are used as columns or when sections are built up of plates, the possibility of the critical load on a component (leg, half flange, web, lattice bar) being less than the critical load on the column as a whole should be investigated. Similarly, the possibility of buckling of the compression flange or the web of a beam should be looked into. Local buckling, however, does not always result in a reduction in the loadcarrying capacity of a column. Sometimes, it results in a redistribution of the stresses enabling the member to carry additional load. 5.7.7 Behavior of Actual Columns For many reasons, columns in structures behave differently from the ideal column assumed in deriving Eqs. (5.88) and (5.91). A major consideration is the effect of accidental imperfections, such as nonhomogeneity of materials, initial crookedness, and unintentional eccentricities of the axial load, since neither field nor shopwork can be perfect. These and the effects of residual stresses usually are taken into account by a proper choice of safety factor. There are other significant conditions, however, that must be considered in any design rule: continuity in frame structures and eccentricity of the axial load. Continuity affects column action in two ways. The restraint at column ends determines the value of k, and bending moments are transmitted to the column by adjoining structural members. Because of the deviation of the behavior of actual columns from the ideal, columns generally are designed by empirical formulas. Separate equations usually are given for short columns, intermediate columns, and long columns. For specific materialssteel, concrete, timberthese formulas are given in Secs. 7 to 10. For more details on column action, see F. Bleich, Buckling Strength of Metal Structures, McGraw-Hill Publishing Company, New York, 1952: S. Timoshenko and J. M. Gere, Theory of Elastic Stability, McGraw-Hill Publishing Company, New York, 1961; and T. V. Galambos, Guide to Stability Design Criteria for Metal Structures, 4th ed., John Wiley & Sons, Inc., Somerset, N.J., 1988. FIGURE 5.45 Addition of forces by (a) parallelogram law; (b) triangle construction; (c) polygon construction. A force may be represented by a straight line of fixed length. The length of line to a given scale represents the magnitude of the force. The position of the line parallels the line of action of the force. And an arrowhead on the line indicates the direction in which the force acts. Forces are concurrent when their lines of action meet. If they lie in the same plane, they are coplanar. 5.8.1 Parallelogram of Forces The resultant of several forces is a single forces that would produce the same effect on a rigid body. The resultant of two concurrent forces is determined by the parallelogram