A  area of the compression flange, in2 The allowable bending stresses Fb, ksi, for values often used for various grades of steel are listed in Table 7.14. TABLE 7.14 Allowable Bending Stresses, ksi Fy 0.60Fy 0.66Fy 0.75Fy Lateral Support of Beams. In computation of allowable bending stresses in compression for beams with distance between lateral supports exceeding requirements, a range sometimes called laterally unsupported, the AISC ASD formulas contain a moment factor Cb in recognition of the beneficial effect of internal moments, both in magnitude and direction, at the points of support. For the purpose of this summary, however, the moment factor has been taken as unity and the formulas simplified in Fig. 7.31. The formulas are exact for the case in which the bending moment at any point within an unbraced length is larger than that at both ends of this length. They are conservative for all other cases. Where more refined values are desired, see Art. 7.20.2 or refer to the AISC ASD specification for structural

1. A complete balanced financial statement with schedules of the principal items. This is a condition precedent to the approval of any contract bond. Sureties have forms on which a contractor may furnish financial information. They should be completed by the individual responsible for the financial operations of the company and the data taken directly from the companys books. It is preferable to have the financial statement prepared and certified by a public accountant. 2. A report on the contractors organization. The surety is interested in knowing the length of time the contractor has been in business, whether the firm operates as an individual, a partnership, or a corporation, and certain specific details, depending upon the form of organization. 3. A report on the technical qualifications and experience of the individuals who will be in charge of work to be performed. 4. A report on the type of work undertaken in the past, together with information regarding jobs successfully completed. 5. An inventory of equipment, noting value and age of each piece and any existing encumbrance. An inventory of materials will also be helpful. From the construction management point of view, the most important question involving bonding is: What avenues of business are open to the contractor who lacks sufficient bonding capacity to do bonded work? In Art. 17.4 various sources of business are described, and in Art. 17.1 various types of construction companies are discussed. Many of these types of business and construction companies do not require bonds for their work. For example, it is very rare that a bond is required in a construction management contract. When a contractor lacks capacity for bonding, it is well to pursue the lines of work described in those articles for which a bond will not be required. The second question confronting a construction manager is whether or not to require a bond of subcontractors. In general, if the financial capability or experience of a subcontractor is sufficiently doubtful as to require bonding, the job should not be awarded to that company. Exceptions to this can be made to assist young companies in starting and gaining experience. There are alternatives to subcontract bonds. These alternatives include the following: Personal guarantees by the principals of the subcontracting company Personal guarantees of other individuals of substantial worth unconnected with the subcontracting company Posting of a sum of money or of a security, such as a letter of credit, until performance of the subcontractors work has been completed by the subcontractor. The Contractors Trade Payment Breakdown (Fig. 17.20) is usually prepared by the contractor long in advance of purchase of materials or subcontracts. Therefore, the amounts shown may vary from the actual costs of the various items. In addition to this, the contractor usually has start-up costs for such expenses as mobilization, FIGURE 17.20 Contractors Trade Payment Breakdown. temporary structures, temporary utilities, and layout, which may not be reflected completely in the breakdown. To offset both of these conditions, the breakdowns may be unbalanced by giving greater value than true costs to the work of some of the trades done early than to the work of the trades done later. This unbalancing will enable the contractor to receive funds for these early start-up costs and thus to be compensated for having to pay various subcontractors and suppliers greater amounts than anticipated at the time the breakdown was prepared. The unbalancing will also make funds available for paying certain subcontractors and suppliers their retained percentages prior to payment of the general contractors retained percentage. The reason why such payments are made earlier is that direct labor costs and many purchase commitments made by contractors, such as lumber, hardware, millwork, doors, and frames, are not subject to retained percentage. These materials, which are merely delivered and not contracted for to be both delivered and installed, must be paid for in full. Since there is a retained percentage being held on all of the work of the general contractor, funds must be available for the part of the retainage that is paid out by the contractor. The unbalancing of the Trade Payment Breakdown will serve this purpose. Payments to subcontractors and suppliers are made on the basis of payments received by the general contractor on monthly requisitions. The general contractor should pay to these subcontractors and suppliers only the same percentage as approved on the contractors requisition to the owner. In addition, the contractor should pay only extras for which corresponding payments have been received from the owner. In case of disputed extras, the contractor may have to pay a portion of the disputed amount to the subcontractor to keep the job running smoothly, despite the fact that the owner has not paid anything on these disputed claims. Subcontractors should be paid only when the contractor gets paid for the subcontractors portions of the work. Furthermore, subcontractors should be paid only after their subcontracts are signed and insurance certificates have been received. All subcontractors should be required to use the same requisition form. A typical Application for Payment on Account of Contract is illustrated in Fig. 17.21. Both the front and back must be filled out by the subcontractor and signed. Payments to a subcontractor should be shown cumulatively on the subcontractors financial folder, which should be separately set up for each subcontractor. After the first payment is made to a subcontractor, the contractor should verify that the subcontractor is paying suppliers. If suppliers are not being paid, the general contractor should insist on joint checks to suppliers and the subcontractor and require that the subcontractor endorse the check so that the supplier can receive the

It should be at least as wide as the step but not more than 13 in wider than the step. 16.4.3 Safety Devices for Escalators To provide a firm footing, treads are grooved in the direction of travel. The grooves mesh with the combs or teeth of the threshold plates at top and bottom of the escalator. The handrails, which move in synchronization with the steps, should be between 30 and 34 in above the treads. The handrails should extend at normal height at FIGURE 16.8 Limitations on dimensions of a straight escalator. least 12 in beyond the line of points of the combplate teeth. The balustrades carrying the handrails and acting as a guard to prevent passengers from falling off the sides of the moving steps should be designed to resist simultaneous application of a horizontal load of 40 lb / ft and a vertical load of 50 lb / ft, both applied to the top of the balustrades. The driving machine may be connected to the main drive shaft by toothed gearing, a coupling, or a chain. Step movement is halted by an electrically released, mechanically applied brake, located either on the driving machine or on the main drive shaft. The brake is activated automatically when a power failure occurs, when the treadway or a handrail breaks, or when a safety device is activated. Safety devices required for escalators include switches for starting, emergency stopping, and maintenance stopping and an electromechanical speed governor that will prevent the step speed from exceeding the maximum design speed. An emergency stop button, protected against accidental activation, is required to be set in the right-hand (when facing the escalator) newel at the top and bottom landings. 16.4.4 Escalator Speeds and Capacities Escalators typically operate at 90 or 120 ft /min, as needed for peak traffic, and are reversible in direction. Slope of the stairs is standardized at 30
in the United States, although inclines of both 30
and 35
are used in other parts of the world. Standard escalator widths are 32 and 48 in. Manufacturers rate their 90-ft /min units at corresponding capacities of 5000 and 8000 persons per hour, although observed capacities, even in heavy traffic, rarely exceed 2000 and 4000 persons per hour, respectively. Although 120-ft /min escalators will move about 30% more volume, they are rarely specified because of the potential for adverse litigation. 16.4.5 Planning for Escalators The location of moving stairs should be selected only after a careful study of potential traffic flow within the planned project. They should be installed where most attractive to traffic and where convenient for passengers. The facility should be designed and signed in a manner that makes it apparent where the visitor will find the escalator. Since escalators are devices that will fail on occasion, the designer must provide alternative transportation (usually adjacent stairs) for times when the escalator is unavailable for passenger use. More importantly, where escalators will be operating at capacity as a result of specific programmatic considerations, the designer must plan alternative routing for times when one or more escalators is under repair. In retail applications, marketing needs generally motivate selection of escalator locations. In design of a new building, adequate space should be allotted for escalators. Generous areas should be provided at both loading and unloading areas. Special consideration should be given to the possibility of a disaster resulting at a constricted exit from an escalator when pedestrian traffic is restricted below the escalators capacity in the path of travel. Similarly, planning of landing areas should consider both queuing space and what happens when an escalator is stopped for some reason while pedestrian traffic continues. In addition, before stacked escalators are planned for an arena, stadium, or other facility having exit peaks, the potential for pedestrian traffic jams should be carefully weighed. If exiting traffic is very heavy in a stacked escalator system, upper levels can easily fill lower-level escalators, creating a jam at the escalator entries and leaving little space for lower-level