The floors of small houses are usually constructed of wood, while in larger buildings, especially in factories or warehouses where heavy loads have to be carried, iron is used to a greater extent; while various combinations of iron, terra-cotta, and cement are used for constructing floors which are to be of a fire-resisting nature. This part of the subject is to be treated in a subsequent Volume. The beams which support the floor boards and ceiling of a room are called the Naked Flooring, and it is according to the disposition of these beams that floors are classified under three heads: -

Fig. 265.

1. Single-Joisted Floors.

2. Double-Joisted Floors.

3. Triple-Joisted, or Framed Floors.

1. Single-Joisted Floors. - When a floor is constructed of one series of joists, as in Fig. 265, it is called Single-Joisted Floor, and the joists are called Bridging Joists.

Bridging Joists are usually spaced at intervals of from 12 to 15 feet centre to centre, with their ends resting upon the strongest adjacent walls, i.e. walls containing the least number of openings. It is usually impossible to select walls without openings for supporting the joists, and when there is little to choose in this respect, the joists are made to rest upon the walls which are the least distance apart.

Sizes Of Joists

In order to effect economy in timber, joists should be narrow and deep; but the breadth is governed by practical requirements, for it is found that joists under 2 inches thick are inclined to split when the floor brads are driven into them.

The depth of single joists is usually determined by some readily remembered rule, such as the following: - The depths of singlejoists in inches is found by dividing the span in feet by two and adding two.

This rule will be found quite satisfactory in practice, but where a very sound floor is required Tredgold's formula, given below, should be used. This formula gives larger scantlings than are usually used in practice, and indeed larger than the scantlings exhibited in Tredgold's own tables, a condensed form of which is here given: -

Tables Of Scantlings Of Girders, Binding Joists, Bridging Joists, And Ceiling Joists Of Different Bearings, From 10 To 30 Feet

Based on Tredgold's Tables.

 Length of Bearing. Girders.10 ft. Centre toCentre. 9 to 12 in.Bearing on Walls. Binders.6 ft. Centre toCentre. 4 to 6 in.Bearing on Walls. Joists.1 ft. Centre toCentre. 1 ft. Centre toCentre. Depth. Breadth. Depth. Breadth. Depth. Breadth. Depth. Breadth. Feet. Inches. Inches. Inches. Inches. Inches. Inches. Inches. Inches. 6 .. .. 6 4 6 2 3 1/2 2 8 .. .. 7 4 1/2 7 2 1/2 4 2 1/4 10 9 7 8 5 7 1/2 2 1/2 5 2 1/2 12 10 8 9 5 1/2 8 2 1/2 6 2 1/2 14 11 9 10 6 9 2 1/2 .. .. 16 12 10 11 6 1/2 10 1/2 2 1/2 .. .. 18 12 11 12 7 12 2 1/2 .. .. 20 13 11 13 7 1/2 12 3 .. .. 22 14 12 .. .. .. .. .. .. 24 15 12 .. .. .. .. .. .. 26 16 12 .. .. .. .. .. .. 28 16 13 .. .. .. .. .. .. 30 16 14 .. .. .. .. .. ..

When the timbers are spaced at greater distances than are assumed in this table their sizes must be increased in proportion.

Tredgold's Rules For Sizes Of Floor Timbers

 Bridging joists (12 inch, centre to centre) D= 3√L2/Bx2.2 for fir. or 2.3 for oak. Binders (6 feet apart) D= 3√L2/Bx3.42 " 3.53 " B= L2/D3x40 " 44 "
 Girders(10 feet apart)' D= 3 √L2/B x 4.2 for fir, or 4.34 for oak. B= L2/D3 x 74 " 82 " Ceiling joists (12 inch.centre to centre) D= L/3 √Bx0.64 " 0.67 " Where L = Span in feet. B= Breadth in inches. d= Depth in inches.

Herring-Bone Strutting

When single joists are employed for spans exceeding 8 feet they should be strutted to prevent them twisting or turning sideways. When the span exceeds 12 feet, two rows of strutting should be used, and so on, a row of strutting being added for each additional 4 feet in the span. Sometimes the strutting is more widely spaced, but the distance between the rows should never exceed 6 feet. The strutting usually employed is composed of pieces of timber 2 by 1 1/2 inches, fixed as shown at B, Fig. 266. The advantages of this method of strutting are - (1) that it is not loosened by the shrinkage of the joists; (2) that it acts at the edge of the joist, where its power to resist twisting is greatest. Care should be taken that the struts fit well.

Solid Strutting

Sometimes the joists are strutted by means of pieces of 1 1/4-inch board, not quite as deep as the joists cut to fit, and nailed between the joists as shown at A. The disadvantage of this system of strutting is the difficulty experienced in making the struts abut properly against any warped sides of the joists, while any shrinkage in the width of the joist renders it practically useless. This defect is overcome to a great extent by passing a bolt through the centre of the joists immediately to one side of the strutting, and tightening it up with a nut, so that the strutting is tightly cramped between the joists.

Trimming

When openings for fireplaces, stairs, trap doors occur in floors the joists cannot all be carried on the walls of a building, but are constructed as shown in Figs. 265 and 267, when they are said to be Trimmed. When the joists run parallel to the fireplace a Trimming Joist is carried in front of the chimney breasts, into which Trimmers are framed on either side of the fireplace to carry the trimmed joists, all as is shown in the right-hand back room in Fig. 265. If a trimmed arch is used in connection with this form of construction two anchor bolts are taken through the trimmer and built into the wall to resist the thrust of the arch.

Fig. 266.