Gas-fired unit heaters are often installed in kitchens and other premises where large quantities of air may be exhausted. When a makeup air system is not provided, and the relief air must infiltrate through doors, windows, etc., a negative pressure FIGURE 13.19 Pipe embedded in a concrete slab for radiant heating. FIGURE 13.20 Pipe embedded in a plaster ceiling for radiant heating. must result in the premises. This negative pressure will cause a steady downdraft through the flue pipe from the outdoors into the space and prevent proper removal of the products of combustion. In such installations, it is advisable to place the unit heater in an adjoining space from which air is not exhausted in large quantities and deliver the warm air through ducts. Since propeller fans on most unit heaters cannot take much external duct resistance, centrifugal blower unit heaters may give better performance where ductwork is used. Sizes of gas piping and burning rates for gas can be obtained from charts and tables in the ASHRAE Handbook Fundamentals for various capacities and efficiencies. The efficiency of most gas-fired heating equipment is between 70 and 80%. Radiant heating, or panel heating as it is sometimes referred to, consists of a warm pipe coil embedded in the floor, ceiling, or walls. The most common arrangement is to circulate warm water through the pipe under the floor. Some installations with warm-air ducts, steam pipes, or electric-heating elements have been installed. Warm-air ducts for radiant heating are not very common. A modified system normally called the perimeter warm-air heating system circulates the warm air around the perimeter of the structure before discharging the air into the premises

A stairway consists of a series of steps and landings. Each step consists of a horizontal platform, or tread, and a vertical separation or enclosure, called a riser (Fig. 1.9a). Railings are placed along the sides of the stairway and floor openings for safety reasons. Also, structural members may be provided to support the stairs and the floor edges. Often, in addition, the stairway must be enclosed for fire protection. Escalators, or powered stairs, are installed in such buildings as department stores and transportation terminals, or in the lower stories of office buildings and hotels, where there is heavy pedestrian traffic between floors. Such powered stairs consist basically of a conveyor belt with steps attached; an electric motor for moving the belt, and steps, controls, and structural supports. Elevators are installed to provide speedier vertical transportation, especially in tall buildings. Transportation is provided in an enclosed car that moves along guides, usually within a fire-resistant vertical shaft but sometimes unenclosed along the exterior of a building. The shaft, or the exterior wall, has openings, protected by doors, at each floor to provide access to the elevator car. The car may be suspended on and moved by cables (Fig. 1.9b) or set atop a piston moved by hydraulic pressure (Fig. 1.9c). More information on vertical-circulation elements is given in Sec. 16. Intelligent Buildings. In addition to incorporating the major systems previously described, intelligent buildings, through the use of computers and communication equipment, have the ability to control the total building environment. The equipment and operating personnel can be stationed in a so-called control center or the equipment can be monitored and controlled remotely via a computer, modem and telephone line. Various sensors and communication devices, feeding information to and from the control center, are located in key areas throughout the building for the purposes of analyzing and adjusting the environment, delivering messages during emergencies, and dispatching repair personnel and security guards, as needed. To conserve energy, lighting may be operated by sensors that detected people movement. HVAC may be adjusted in accordance with temperature changes. Ele- vators may be programmed for efficient handling of variations in traffic patterns and may be equipped with voice synthesizers to announce floor stops and give advice in emergencies. In addition, intelligent buildings are designed for ease and flexibility in providing for changes in space use, piping, electrical conductors, and installed equipment. See also Arts. 3.5.12 and 3.7.2. (F. S. Merritt and J. Ambrose, Building Engineering and Systems Design, 2nd Ed., Van Nostrand Reinhold, New York.) As indicated in Art. 1.3, the client in the initial design phase develops a program, or list of requirements. The goal of the designers is to select a system that meets these requirements. Before the designers do this, however, it is advisable for them to question whether the requirements represent the clients actual needs. Can the criteria and standards affecting the design be made less stringent? After the program has been revised to answer these questions, the designers select a system. Next, it is advisable for the designers to question whether the system provides the best value at the lowest cost. Value engineering is a useful procedure for answering this question and selecting a better alternative if the answer indicates this is desirable. Value engineering is the application of the scientific method to the study of values of systems. The major objective of value engineering in building design and construction is reduction of initial and life-cycle costs (Art. 1.6). Thus, value engineering has one of the objectives of systems design, in which the overall goal is production of an optimum building, and should be incorporated in the systemsdesign

at midspan. Then shift the loads until the load P2 that was at the center of the beam is as far from midspan as the resultant of all the loads on the span is on the other side of midspan (Fig. 5.24). Maximum moment will occur under P2. When other loads move on or off the span during the shift of P2 away from midspan, it may be necessary to investigate the moment under one of the other loads when it and the resultant are equidistant from midspan. 5.5.10 Bending Stresses in a Beam To derive the commonly used flexure formula for computing the bending stresses in a beam, we have to make the following assumptions: 1. The unit stress at a point in any plane parallel to the neutral surface of a beam is proportional to the unit strain in the plane at the point. 2. The modulus of elasticity in tension is the same as that in compression. 3. The total and unit axial strain in any plane parallel to the neutral surface are both proportional to the distance of that plane from the neutral surface. (Cross sections that are plane before bending remain plane after bending. This requires that all planes have the same length before bending; thus, that the beam be