To be acceptable as a means of egress, a ramp inside a building more than three stories high or a building of noncombustible or fire-resistant construction is required to be of noncombustible construction. The ramp also should be protected by separation from other parts of the building in the same way as other means of egress. There should be no enclosed usable space, such as closets, under the ramp, nor should the open space under the ramp be used for any purpose. (Other enclosed ramps, however, are permitted to be located under the ramp.) For all inside ramps, guardsvertical protective barriersshould be provided along the edges of ramps and along the edges of floor openings over ramps, to prevent falls over the open edges. Requirements for type of construction and minimum height for such barriers are the same as those for stairs (see Art. 16.3). Handrails are required only for Class B ramps. Outside Ramps. A ramp permanently installed on the outside of a building is acceptable as a means of egress if the life-safety requirements for inside egress ramps are met. For outside ramps more than three stories high, however, guards along ramp edges should be at least 4 ft high. Also, for such ramps, provision should be made to prevent accumulations of snow or ice. Powered Ramps. In some buildings, such as air terminals, in which pedestrians have to be moved speedily over long distances, traffic may be transported on a moving walk, a type of passenger-carrying powered device on which passengers stand or walk. In the moving walk, the treadway, guards, and handrails are continuous and travel parallel to the direction of motion, which may be horizontal or on a slope up to 15
. (For greater slopes, an escalator should be used. See Art. 16.4.) Although moving walks can transport passengers at speeds up to 180 ft /min, speeds are generally between 90 and 120 ft /min. Inclined moving walks are classified as powered ramps. Such ramps are acceptable as a means of egress if they meet the egress requirements of stationary ramps. (Moving walks are acceptable if they meet the requirements for exits. See Art. 3.10.5.) Powered ramps, however, must be incapable of operation in the direction opposite to normal exit travel. The installation of moving walks and powered ramps should meet the requirements of the American National Standard Safety Code for Elevators, Dumbwaiters, Escalators and Moving Walks, ANSI A17.1. Basically, moving walks and powered ramps consist of a grooved treadway moved by a driving machine; a handrail on each side of the treadway that moves at the same speed as the treadway; balustrades, or guards, that enclose the treadway on each side and support the handrails; brakes; control devices; and threshold plates at the entrance to and the exit from the treadway. The purpose of the threshold plates is to facilitate smooth passage of passengers between treadway and landing. The plates are equipped with a comb, or teeth, that mesh with and are set into grooves in the treadway in the direction of travel. Their purpose is to provide firm footing and to prevent things from becoming trapped between the treadway and the

Similarly, shearing stresses produced by central shears and twisting moments may be calculated from equations of the form v 
z (5.176) xy t t 3 /12 where D  twisting moment and T  unit shear force along the middle surface. Normal shearing stresses may be computed on the assumption of a parabolic stress distribution over the shell thickness: FIGURE 5.98 Folded-plate structure. V t 2 v 
 z2 (5.177) xz t 3 / t 4 where V  unit shear force normal to middle surface. 5.15.5 Folded Plates A folded-plate structure consists of a series of thin planar elements, or flat plates, connected to one another along their edges. Usually used on long spans, especially for roofs, folded plates derive their economy from the girder action of the plates and the mutual support they give one another. Longitudinally, the plates may be continuous over their supports. Transversely, there may be several plates in each bay (Fig. 5.98). At the edges, or folds, they may be capable of transmitting both moment and shear or only shear. A folded-plate structure has a two-way action in transmitting loads to its supports. Transversely, the elements act as slabs spanning between plates on either side. The plates then act as girders in carrying the load from the slabs longitudinally to supports, which must be capable of resisting both horizontal and vertical forces. If the plates are hinged along their edges, the design of the structure is relatively simple. Some simplification also is possible if the plates, though having integral edges, are steeply sloped or if the span is sufficiently long with respect to other dimensions that beam theory applies. But there are no criteria for determining when such simplification is possible with acceptable accuracy. In general, a reasonably accurate analysis of folded-plate stresses is advisable. Several good methods are available (D. Yitzhaki, The Design of Prismatic and Cylindrical Shell Roofs, North Holland Publishing Company, Amsterdam; Phase I Report on Folded-plate Construction, Proceedings Paper 3741, Journal of the Structural Division, American Society of Civil Engineers, December 1963; and A. L. Parme and J. A. Sbarounis, Direct Solution of Folded Plate Concrete Roofs, EB021D, Portland Cement Association, Skokie, Ill.). They all take into account the effects of plate deflections on the slabs and usually make the following assumptions: The material is elastic, isotropic, and homogeneous. The longitudinal distribution of all loads on all plates is the same. The plates carry loads transversely only by bending normal to their planes and longitudinally only by bending within their planes. Longitudinal stresses vary linearly over the depth of each plate. Supporting members, such as diaphragms, frames, and beams, are infinitely stiff in their own planes and completely flexible normal to their own planes. Plates have no torsional stiffness normal to their own planes. Displacements due to forces other than bending moments are negligible. Regardless of the method selected, the computations are rather involved; so it is wise to carry out the work by computer or, when done manually, in a wellorganized table. The Yitzhaki method offers some advantages over others in that the calculations can be tabulated, it is relatively simple, it requires the solution of no more simultaneous equations than one for each edge for simply supported plates, it is flexible, and it can easily be generalized to cover a variety of conditions. Yitzhaki Method. Based on the assumptions and general procedure given above, the Yitzhaki method deals with the slab and plate systems that comprise a foldedplate structure in two ways. In the first, a unit width of slab is considered continuous over supports immovable in the direction of the load (Fig. 5.99b). The strip usually is taken where the longitudinal plate stresses are a maximum. Second, the slab reactions are taken as loads on the plates, which now are assumed to be hinged along the edged (Fig. 5.99c). Thus, the slab reactions cause angle changes in the plates at each fold. Continuity is restored by applying to the plates an unknown moment at each edge. The moments can be determined from the fact that at each edge the sum of the angle changes due to the loads and to the unknown moments

Bumper. A device other than a buffer for stopping a descending car or counterweight beyond its bottom terminal by absorbing the impact. Car. The load-carrying element of an elevator, including platform, car frame, enclosure, and car door or gate. Car-Door Electric Contact. An electrical device for preventing normal operation of the driving machine unless the car door or gate is closed. Car Frame. The supporting frame to which the car platform, guide shoes, car safety, and hoisting ropes or hoisting-rope sheaves, or the plunger of a hydraulic elevator are attached. Car Platform. The structure on which the car and its floor are mounted. Car Switch. A manual operating device in a car by which an operator actuates the control. Control. The system governing the starting, stopping, direction of motion, acceleration, speed, and retardation of the car. VVVF Control. A method of controlling the smooth starting and stopping of alternating-current motors, utilizing solid-state, variable-voltage, variablefrequency controls. This system is displacing dc motors for medium and highspeed