This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
As stability is essential to the erection, bracing between each bay longitudinally, and at least every second bay transversely, should be adopted. The runners should be strutted on their under side from the standards (see Fig. 271). If the struts (which should not be less than half the sectional area of standard) are of the same scantling as standard and head, about double the weight can be carried. The cleats which support these struts should, for additional strength, be housed into the standards.
For the purpose of stability and of preventing any lateral movement, cross bracing at ends should be adopted. In cases where perfect rigidity and strength are specially needed, and where the space between each row of standards must be kept open for building purposes, and where no cross bracing could be permitted, strutting to each standard, as shown in Fig. 291, will be needed. The struts are secured by bolting to standards near the top, and to a foot block driven into the ground.
When platforms or gantries are needed, mainly to allow of free passage along a footpath, they are of lighter form than those previously described, inasmuch as strutting is not necessary; but so far as the framing of sides is concerned, the method is similar to that required for travelling gantries (see Fig. 273, and articles on Gantries, pages 150, 151, Volume I.).
Stagings are constructed similarly to travelling gantries. Now that the Scotch derrick system is generally adopted in large buildings, stagings are not so much needed as formerly. It is, however, necessary to mention them for reference.
The construction of the first runner is exactly the same as that of gantries over footpaths (Fig. 273), but as the scaffolding may be required to be carried up higher it will be necessary to lay horizontal pieces across the scaffold over each standard, and to project them for 8, 9, or 10 feet as required beyond the face of runners, and connected longitudinally by transoms (see Fig. 274). It will be seen that the rising tiers of standards are strutted from the projecting part of the beam called the "footing piece." This "footing piece" is supported by struts from the lower standard, and so bolted to the sides of "footing piece" and standards as to allow of the passing of the shores from footing piece to standard. The strutting to the bays formed by the standards and the cross-bracing to top tier are carried out as shown. The rails for travellers are laid on top runners as before described.
Particular care should be taken that all the uprights of the upper tiers should be placed exactly over those of the lower ones, so that no cross strain should occur to the runners. It is very important that the joints of the runners should be immediately over the standards.
In cases where the erection is only required for temporary purposes dog irons may be used for connections, but bolts and straps should be used in permanent structures. Pole Scaffolding. - For a general description of bricklayers' and masons' scaffold, see pages 149, 150, Volume I.
In Fig. 232 of that volume it will be observed that single poles or standards are dealt with. Where no great weight or great height are required single poles are sufficient, but double poles should be used if heavy weights or a considerable height are to be dealt with.
In the case of double poles, the first pair are erected of different lengths. This difference of length permits of a lap in connecting the succeeding poles, the lap being equal to half of the full length of pole. The short pole is called a "puncheon." Where the nature of the earth will permit, the butt-ends of the poles should be set from 2 to 3 feet underground and the earth well rammed round them. If, however, this cannot be, the ends should be placed in barrels and filled in with earth closely and lightly rammed (see Fig. 275).
Should the standard be a single pole the second pole in height should have a lap of 10 to 15 feet, and stand on a putlog close to the first pole (see A, Fig.
276). The inner end of putlog is securely fixed to the scaffold or into the building.
The standards are spliced or "married" together with band ties. A ledger is tied across the standards as a support for the working platform at a height of about 3 feet, to admit of a man working with ease. In cases where a single pole is not sufficiently long to form a continuous ledger it will be desirable to arrange that the putlogs should lie evenly and on the strongest support.
There are various methods of ledger-lapping, but the best and most reliable is shown at B, in Fig. 276. It will be seen that the ledgers' ends butt one another. A short pole is fixed across the two standards, and tying at the standards a double ledger is formed.
Another way is to lap the ledgers horizontally as at C, but although evenness of the putlogs is obtained it is not so strong.
It will happen sometimes that the putlogs cannot be carried into the wall, on account of a window or other opening. In such a case they are supported in one of the ways shown in Fig. 277.
The wedging in of the putlogs to the wall is not absolutely reliable, for although some strength may be imparted to the scaffold, a slight strain might be quite enough to draw the putlog.
Pole scaffolding should be strengthened and stiffened longitudinally by braces tied on the outer side of the scaffold, as shown in Fig. 275, at all crossings, and connected to all main timbers of the scaffolding with which they adjoin. These braces should always be used in exposed positions where the height of scaffolding exceeds 30 feet. Of course, there would be difficulty in carrying out the bracing in cases of irregularly shaped buildings, but although the scaffold may, in such instances, butt or break with return walls, it would be well to adopt it if and where possible.
In order to secure level boarding over the putlogs the boards should butt as shown at A, Fig. 278, when two putlogs should be placed about 4 inches apart. If, however, lapping of the boards has to be adopted, one putlog only would be required, as shown at B. The former method is more satisfactory.