Systems with large numbers of variables may sometimes be optimized by a process called simulation, which involves trial and error with the actual system or a model. In simulation, the properties of the system or model are adjusted with a specific input or range of inputs to the system, and outputs or performance are measured until an optimum result is obtained. When the variables are quantifiable and models are used, the solution usually can be expedited by use of computers. The actual system may be used when it is available and accessible and changes in it will have little or no effect on construction costs. For example, after installation of air ducts, an air-conditioning system may be operated for a variety of conditions to determine the optimum damper position for control of airflow for each condition. Suboptimization is a trial-and-error process in which designers try to optimize a system by first optimizing its subsystems. It is suitable when components influence each other in series. For example, consider a structural system consisting only of roof, columns, and footings. The roof has a known load (input), exclusive of its own weight. Design of the roof affects the columns and footings, because its output equals the load on the columns. Design of the columns loads only the footings. Design of the footings, however, has no effect on any of the other structural components. Therefore, the structural components are in series and they may be designed by suboptimization to obtain the minimum construction cost or least weight

Parallel-Chord Trusses. Spans of 100 ft or more are possible with chords and webs of solid-sawn lumber or glulam timbers. Parallel-chord trusses are commonly used in floor systems and long-span commercial roofs. Advantages for this type of truss include ease of installation, relatively long spans, flexibility for installation of ductwork, plumbing, and electrical wiring, and overall economics. Triangular and Scissor Trusses. Spans of 100 ft or more are possible with chords and webs of solid-sawn lumber or glulam timbers. These trusses are used extensively in residential roof construction and long-span applications in commercial roofs. Advantages are the same as those described above for parallel-chord trusses. Truss Joints. Connectors used in truss joints include bolts, split rings, timber rivets, shear plates, lag screws, and metal connector plates, as described in Art. 10.22.1. The type of connector used depends on the type and size of wood truss components, loads to be transferred, fabrication capabilities, and aesthetics. Lag screws, bolts, timber rivets, and shear plates are suitable for field fabrication, whereas metal connector plates are almost always installed in a truss assembly

in H2O per 100 ft Airflow, cfm Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min * Based on data in ASHRAE HandbookFundamentals, American Society of Heating, Refrigerating and Air- Conditioning Engineers. The shape of ducts usually installed as rectangular, because dimensions can easily be changed to maintain the required area. However, as ducts are flattened, the increase in perimeter offers additional resistance to airflow. Thus a flat duct requires an increase in cross section to be equivalent in air-carrying capacity to one more nearly square. A 12  12-in duct, for example, will have an area 1 ft2 and a perimeter of 4 ft, whereas a 24  6-in duct will have the same cross-sectional area but a 5-ft perimeter and thus greater friction. Therefore, a 24  7-in duct is more nearly equivalent to the 12  12. Equivalent sizes can be determined from tables, such as those in the ASHRAE HandbookFundamentals, where rectangular ducts are rated in terms of equivalent round ducts (equal friction and capacity). Table 13.6 is a shortened