TABLE II.-FOR FLOORS IN RETAIL STORES, (Superimposed load, from 125 to 130 pounds per square foot.) Span, in TABLE III.-FOR FLOORS IN WAREHOUSES. (Superimposed load, from 200 to 210 pounds per square foot.) DISTANCES BETWEEN CENTRES OF BEAMS. It will be seen from these tables that it is more economical to space the beams farther apart, and use as short spans as the conditions of the building will permit. For example, if we have an office floor 48 feet square, to support with iron beams and tile arches, we may either use one girder down the centre, with 12-inch beams, spaced 4 feet apart; or two girders, and 10-inch beams spaced 6 feet apart. In the former case we should require 11 beams the full width of the building, weighing 16,896 pounds, and in the latter 7 beams weighing 8,568 pounds, a saving of nearly 50 per cent. in the steel. From this, however, will have to be deducted something for extra girders and columns, but the total saving would probably equal 25 per cent. In regard to the columns, it will not make much difference in the amount of iron used, whether there are one or two rows, as the total weight to be supported is the same in either case, and if one row of girders is used the columns will be closer and heavier than if two rows are used. Deflection of Rolled I-Beams.-The deflection of rolled iron I-beams can be computed by Formula 1, under the Stiffness of Beams, Chap. XVI. According to the calculations of Mr. C. L. Strobel, C. E., the beams in the foregoing tables will not deflect over one-thirtieth of an inch for every foot of span, under the load which they have been calculated to support. Tie-rods.—Tie-rods from five-eighths to one inch in diameter are ordinarily employed to take the thrust of the brick arches, and to add to the security of the floor. These may be spaced from eight to ten times the depth of the beams apart, and the holes for them should always be punched at the centre of the depth of the beam. The formula for the diameter of the tie-rod for any floor is, Diameter squared W span of arch, in feet 62832 × rise of arch, in feet W denoting weight of floor, and superimposed load resting on the arch half-way between the tie-rods on each side. EXAMPLE. What should be the diameter of the tie-rod to take the thrust of a 4" brick arch, between 10" beams, spaced 5 feet apart; the arch having a rise of 6', and the tie-rods to be spaced 7 feet apart? The superimposed load to be taken at 100 lbs. Of course, where arches abut against each side of a beam, there is no need of rods to take the thrust of the arches; but it is always safer to use them, as the outside bay of the floor might be pushed off sidewise if the whole were not tied through; also, if one of the arches should fall, or break through, the rods would keep the other arches in place. CHAPTER XXIV. MILL CONSTRUCTION.1 In this chapter it is proposed to describe the principal constructive features of what, in the Eastern States, is known as the "Mill Construction," or "Slow-burning Construction." It is a method of construction brought about largely through the influence of the factory mutual insurance companies, and especially through the efforts of Mr. William B. Whiting, whose mechanical judgment, experience, and skill as a manufacturer, have been devoted for many years to the interests of the factory mutual companies and to the improvement of factories of all kinds. Mr. Edward Atkinson, president of the Boston Manufacturers' Mutual Insurance Company, has also done a great deal towards influencing the public in favor of this mode of construction. The desideratum in this mode of construction is to have a building whose outside walls shall be built of masonry (generally of brick) concentrated in piers or buttresses, with only a thin wall containing the windows between, and the floors and roof of which shall be constructed of large timbers, covered with plank of a suitable thickness; the girders being supported between the walls by wooden posts. No furring or concealed spaces are allowed, and nothing is permitted which will allow of the accumulation of dirt, the concealment of fire, or, in short, any thing that is not needed. Mr. C. J. H. Woodbury, inspector for the factory mutual fireinsurance companies of Massachusetts, who has written a very able book on the "Fire Protection of Mills" (published by John Wiley & Sons of New York), has given such concise and clear statements of what does and what does not constitute safe construction for mills and warehouses, that with his permission we quote them verbatim from his work. 1 Cuts 1 to 6 in this chapter are taken from Woodbury's Fire Protection of Mills, and reduced, to conform to the size of the page. 66 Prevailing Features of Bad Construction of Mills and Storehouses. - The experience of the Factory Mutuals has shown that in mill and storehouse construction, where considerations of safety, convenience, and stability are essential, the following prevalent features of bad construction should be omitted: "Bad roofs. "Rafters of plank, eighteen to twenty-four inches between centres, set edgewise. 66 Any roof-plank less than two inches thick (three inches preferred); any covering which is not grooved and splined. 66 66 Any hollow space of an inch or more in a roof. Any and every mode of sheathing on the inside of the roof so as to leave a hollow space. 66 Any and every kind of metal roof, except a tin or copper covering on plank. "Boxed cornices of every kind. 66 'Bad floors containing hollow spaces or unnecessary openings. “Thin or thick floors resting on plank set edgewise, eighteen to twenty-four inches between centres. "All sheathing nailed to the under side of plank or timber, making a hollow floor. "Bad finish, leaving hollow spaces, or flues. "All inside finish which is furred off so as to leave a space between the finish and the wall. 66 Any and all concealed spaces, wooden flues, or wooden ventilators of every kind, in which fire can lurk or spread, and be protected from water. 66 Any and all openings from one floor to another, or from one department to another, except such as are absolutely required for the conduct of the business (all necessary openings should be protected by self-closing hatches or shutters, or by adequate wooden fire-doors covered with tin; automatic doors preferred in many places). 66 · Essential Features for the Safe Construction of Mills and Storehouses. - Solid beams, or double beams bolted near together, eight to ten feet between centres. Not to be painted, varnished, or 'filled' for at least three years after the building is finished, lest dry-rot should ensue. Ends of timbers ventilated by an inch air-space each side in the masonry. "Roof nearly flat. Timbers laid across the tops of the walls to |