The treadway may be constructed in one of the following ways: 1. Belt typea power-driven continuous belt. 2. Pallet typea series of connected, power-driven pallets. (A pallet is a short, rigid platform, which, when joined to other pallets, forms an articulated treadway.) 3. Belt pallet typea series of connected, power-driven pallets to which a continuous belt is fastened. 4. Edge-supported belt typea belt supported near its edges by rollers in sequence. 5. Roller-bed typea treadway supported throughout its width by rollers in sequence. 6. Slider-bed typea treadway that slides on a supporting surface. Powered ramps resemble escalators in construction (see Art. 16.4). For example, both types of transporters are supported on steel trusses. The driving machine may be connected to the main drive shaft by toothed gearing, a coupling, or a chain. Movement of the treadway and handrails can be halted by an electrically released, mechanically applied brake, located either on the driving machine or on the main drive shaft and activated automatically when a power failure occurs, when the treadway or a handrail breaks, or when a safety device is actuated. For moving walks and ramps, safety devices required include switches for starting, emergency stopping, and maintenance stopping, and a speed governor that will prevent the treadway speed from exceeding 40% more than the maximum design speed. Balustrades should be at least 30 in high, measured perpendicular to the treadway. Hand or finger guards are installed where the handrails enter the balustrades. The handrails should extend at normal height at least 12 in beyond each end of the exposed treadway, to facilitate entry and exit of passengers from or onto a level

8.21.1 Types of Steel Roof Deck Steel roof deck is commonly available in 24- and 30-in covering widths, but sometimes in 18- and 36-in widths, depending on the manufacturer. Thickness of steel commonly used is 0.048 or 0.036 in, but most building codes permit 0.030-in-thick steel to be used. Figure 8.14d and e shows full-width decking in cross section. Usual spans, which may be simple, two-span continuous, or three-span continuous, range from 4 to 10 ft. The SDI Design Manual for Composite Decks, Form Decks, Roof Decks and Cellular Deck Floor Systems with Electrical Distribution gives allowable total uniform loading (dead and live), lb / ft2, for various steel thicknesses, spans, and rib widths. Some manufacturers make special long-span roof-deck sections, such as the 3- in-deep, Type N roof deck shown in Fig. 8.15, in 24- to 16-ga black and galvanized. FIGURE 8.15 Cross sections of types NS and NI roof deck for 9- to 15-ft spans. The weight of the steel roof deck shown in Fig. 8.14 depends on rib dimensions and edge details. For structural design purposes, weights of 2.8, 2.1, and 1.7 lb / ft2 can be used for the usual design thicknesses of 0.048, 0.036, and 0.030 in, respectively, for black steel in all rib widths, as commonly

Suppose, for example, that the beam is a crane girder with a span of 60 ft. The wheel loads are 20 and 10 kips, respectively, and are spaced 5 ft apart. For maximum shear at the quarter point, the wheels should be placed with the 20-kip wheel at that point and the 10-kip wheel to the right of it. The corresponding ordinates of the influence line (Fig. 5.22c) are 3/4 and 40/45
3/4. Hence, the maximum shear is 20
3/4  10
40/45
3/4  21.7 kips. Figure 5.22d shows influence lines for bending moment at several points on a beam. It is noteworthy that the apexes of the diagrams fall on a parabola, as shown by the dashed line. This indicates that the maximum moment produced at any given section by a single concentrated load moving across a beam occurs when the load is at that section. The magnitude of the maximum moment increases when the section is moved toward midspan, in accordance with the equation shown in Fig. 5.22d for the parabola. 5.5.9 Maximum Bending Moment When there is more than one load on the span, the influence line is useful in developing a criterion for determining the position of the loads for which the bending moment is a maximum at a given section. Maximum bending moment will occur at a section C of a simple beam as loads move across it when one of the loads is at C. The proper load to place at C is the one for which the expression Wa /a  Wb /b (Fig. 5.23) changes sign as that load passes from one side of C to the other. When several loads move across a simple beam, the maximum bending moment produced in the beam may be near but not necessarily at midspan. To find the maximum moment, first determine the position of the loads for maximum moment FIGURE 5.23 .Moving loads on simple beam AB ae placed for maximum bending moment at point C on the beam. FIGURE 5.24 Moving loads are placed to subject a simple beam to the largest possible