269. Within the past sixteen years it has become quite common to frame the floors of mercantile buildings, and sometimes of office buildings, after the method known as Mill Construction and sometimes called "slow burning" construction. While "mill construction" should be slow burning, it differs widely from "slow burning construction" as now defined in Building Ordinances.
Mill construction applies only to those buildings in which no small timbers are used, and in which the floor beams and girders have a sectional area of at least 72 square inches and the posts (if of wood) a sectional area of at least 100 square inches, while in slow burning construction, floor joists 2 and 3 inches thick are used, and the "slow burning" is obtained by protecting all woodwork with metal latch and plaster, or plaster boards, and placing fire stops wherever practicable. The actual construction of the floor required by Building Ordinances for "slow burning" buildings is the same as for the ordinary construction described in Sections 258-260, although ordinary wood partitions or wood furrings of all kinds are not permitted.
The essential features of mill construction, as regards its resistance to taking fire and retarding the progress of flames, and also in preventing the building from being totally destroyed in case a fire gets under headway, are, that all wooden construction shall be of large dimensions and so arranged that there will be no spaces for flames to pass through, and no opportunity for dirt and dust to collect. The under flooring should be of plank and the floor beams and girders should be self-releasing from the walls; the posts should not depend entirely upon the beams and girders to keep them in position. No furring of any kind (except metal furring for plastering on planks or solid timbers) should be used in the building, and all partitions should be of planks, tongued and grooved together, or of incombustible materials.
Large timbers and thick flooring are slow to take fire and also burn very slowly. The avoidance of air spaces greatly retards spreading of the fire, and enables the firemen to direct a stream of water against all parts of the burning construction.
Mill construction, as the term signifies, was first used in the construction of the large woolen and cotten mills of New England, and it is the only form of construction that can be insured in the Mutual insurance companies, which carry practically all of the insurance on those mills.
Fig. 495 shows the manner in which the floors are constructed and supported in mills built on this principle.
The posts are spaced 8 feet apart endways of the building and 14 or 25 feet the other way. There are no longitudinal girders, the floor beams, which are usually 12x14 hard pine timbers, resting on the post caps as shown. The floor on top of the beams is constructed, first, of 3-inch planks, not over 9 inches wide, planed both sides and grooved on both edges. The grooves are filled by strips of hard pine, called splines, about ¾ of an inch by 1 \ inches. The splines take the place of the tongue in matched boarding.
In nailing the planks it is better to blind nail them, after the manner of nailing matched flooring in dwellings, as this allows the planks to shrink or swell without cracking and without splitting the splines.
The upper flooring is generally of some hard wood, 1 ¼ inches thick, merely jointed. Between the plank and the finished flooring a layer of mortar about ¾ of an inch thick is generally spread. " The layer of mortar preserves the lumber from decay, prevents the floor from becoming soaked with oil, and is so slow-burning that it is more nearly fireproof than any other practical method of construction."*
In the original mill construction the posts were usually made round and tapered, and the posts in the upper stories, instead of resting on the cap of the post below, either rested on the girder or on a cross-shaped pintle cut in between the ends of the girders. These methods have now given place to the square post with chamfered corners and large bearing plates, as shown in the figure. The cap shown at A is the Goetz cap; that shown at B is not patented.
Other forms of post caps are shown in Figs. 465, 466 and 472.
The roofs of these mills are generally flat, and are framed in the Fig.496 same way as the floors, but with lighter timbers and 2 ½ inch roofing plank.
270. In applying this principle of construction to stores and office buildings, it is necessary to deviate considerably from the original mill method of framing.
In stores it is desirable to have as few posts as possible, which necessitates longitudinal girders and very often iron posts. In office buildings it is generally practicable to locate the posts so that they will come in the partitions, and as the rooms or offices are generally about 12 or 14 feet wide, the method of framing usually adopted is that shown in Fig. 496. When the posts are placed over 8 feet apart, so as to necessitate a longitudinal girder, it is more economical to space the floor beams about 4 feet from centres, as this permits the use of 2-inch planks for the under floor.
* Fire Protection of Mills,"by C. J. H. Woodbury, p. 163.
If the building is several stories in height it will also be necessary to use iron or steel posts, as wooden posts would take up too much room. If the posts come in partitions or are to he fireproofed, the square section makes the best shape for a cast iron post. Fig. 496 Fig.497.
shows the manner of casting the bearing plates and arranging the end connections.
Iron posts should always be protected by fireproof materials, as they will not stand as long as wooden posts in a fire.
If the building is not more than three stories high, wooden posts with iron plates can generally be used.
The floor beams and girder should always be flush on top, and the former should be hung to the latter either by stirrup irons or joist hangers, with joint bolts on at least every other beam.
The under flooring, if of 2-inch spruce planks, will be cheaper tongued and grooved than splined. It is also better to lay the flooring diagonally, as this stiffens the building and gives a better surface on which to lay the finished flooring, which should run at right angles to the beams.
A layer of mortar or of some fireproof lining, such us " Salamander," should always be placed between the plank and flooring in all first-class buildings.
When the span of the longitudinal girders does not exceed 14 feet it will generally be practicable to make them of wood, but when the span is 16 feet or more, steel beams should be used. In this form of construction it will probably be better to bolt 3-inch planks to the sides of the steel beams and hang the floor beams in joist hangers, as shown in Fig. 497. The girder should then be covered with metal lath and plaster. As it is practically impossible to give a neat finish to the under side of mill floors without casing or plastering, it is generally customary, when this method of construction is used in office buildings, apartment houses, etc., to cover the beams and under side of the planking with metal lathing and plaster. By taking a little pains in spacing the floor beams the ceilings of the various rooms can usually be divided into panels of equal size, which when plastered and tinted, give a very neat appearance.
The details of the floor and roof framing in this form of construction are usually quite simple, the construction shown in Fig. 497 applying to the whole floor.
In calculating the longitudinal girders the loads should not be assumed as distributed, but as concentrated at the points where the floor beams are supported. Formulae for such conditions of loading are given in the Architects' and Builders' Pocket Book.
Where the beams and girders rest on the columns they should be connected endways, either by notching the ends of the beams over the lug cast on the Goetz cap, or by bolts or wrought iron straps. It is also desirable to have the columns bolted together endways, as shown in the figures.
The method of anchoring and supporting the wall end of the beams and girders requires more consideration in this method of construction than in the ordinary method, as, the beams being larger, they make a greater hole in the wall, bring a greater crushing weight on the bricks, and have a more severe effect on the wall when falling.
The writer believes that the Goetz Box Anchor, shown in Fig. 498, makes probably the best anchor and support for large beams or girders built into the wall, as it provides a sufficient bearing plate, supports the wall above the beam, provides for free circulation of air around the beam, and readily releases the beam in case of fire. The Duplex and Van Dorn Wall Hangers accomplish the same purpose, although the beams are not so well tied to the wall, as only spikes should be used for securing to the hanger, otherwise the falling timber will pull the wall with it. The method of supporting the wall ends of the beams in the original mill construction is shown in Fig. 499. The bricks were carefully built around the end of the beam so as to leave an air space and plenty of room for the beam to fall out without injuring the wall. This method is not patented. Over beams exceeding 12 inches in thickness it would be well to place a flat stone or iron plate to support the wall above the beam end. In no case should the brickwork be built directly on top of the wood.