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 shail 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 rerbatim from his work.

Cuts 1 to 6 in this chapter are taken from Woodbury's Fire Protection of 1, and reduced, to conform to the size of the page.

project eighteen to thirty-six inches, as may be desired, serving as brackets. Plank laid to the ends of the timbers. Neither gutters nor boxed cornices of any kind. Wooden posts of suitable size, not tapered, unless when single posts turned from the trunks of trees with the heart as a centre, following the natural taper. Cores bored one and a half inches diameter; two half-inch holes transversely through the post near top and bottom for ventilation.

"Floor-planks not less than three inches thick for eight-foot bays, three and a half to four for wider bays. In some cases, beams have been placed twelve feet apart, with four-inch plank for the floor; but in such cases a careful computation of the strength should be made, based upon the load to be placed thereon, before 30 wide a space between beams is adopted, lest there should be excessive deflection. The better method, where the arrangement of the machinery requires such wide bays, is to alter the plan of floortimbers. Top floor one and a quarter inch boards of Southern pine, maple, or some hard wood. The best construction requires this top floor to be laid over three-quarter inch mortar, or two thicknesses of rosin-sized sheathing-paper, certain grades of which are now made especially for this purpose.

"All rooms in which special dangers exist, such as hot drying, to be protected overhead with plastering on wire-lath, following the line of ceiling and timber, thus avoiding any cavity in the ceiling. In such rooms, the wooden posts should also be protected with tin; care being taken to leave the half-inch holes through the poss near the top and base uncovered, so that dry-rot may not take place."

Fig. 1 represents the proper construction of one bay of a threestory mill, each bay being like the others, and the building beir g formed of any number of such bays placed one after the other.

Such a building cannot be considered as fire-proof; but the ma terial is in such a shape that it would not readily take fire, and would burn slowly even then. Moreover, the construction is such. that any part of the building can be easily reached by a stream of water; so that a fire can be readily extinguished before it has gained much headway.

In a brick building no granite should be used, except for steps and underpinning, as it splits badly when exposed to heat, and is therefore unsuitable for sills or lintels, or any work liable to be exposed to any intense heat in case the building should be on fire. The best qualities of brown sandstone may be used for sills, and for other places it would be better to use brick or terra-cotta. Moulded bricks are now manufactured in a great variety of forms. and are well suited for decorative work.