Scaffold or Scaffolding

The two former of this type of structure are now little used. Many more or less permanent timber gantries exist in goods yards and similar places but modern methods of handling and hoisting materials have rendered the old type of traveller gantry almost obsolete.

The form of fixed staging or platform gantry which is used over a public pavement is indispensable for conducting building opera tions in large towns where it is important to keep the footway clear and to safeguard pedestrians who are using the footway in front of the building site. They consist of two sets of standards, sill and head, one set being erected close to the building and the other about 8 or io ft. away. These stages are formed of square timber, framed and braced in a similar manner to gantries de signed to support a traveller, but, instead of external shores or braces the uprights are braced across to each other, care being taken to fix the braces at such a height as to allow free passage beneath them. Joists are placed across from head to head, and a double layer of scaffold boards is laid to form the floor, the double thickness being necessary to prevent materials dropping through the joints upon the heads of passers-by. When the gantry abuts on the road, a heavy timber fender splayed at each end should be placed so as to ward off the traffic. At the level of the platform a fanguard is often thrown out for a distance of about 6 ft. or more and closely boarded to protect the public from falling materials and the workmen from accident.

Derrick Towers.

Derrick "gantries" or "towers" are skeleton towers of heavy timbers erected in a central position on a site to support a platform at such a height as to enable an electric or steam power derrick crane placed upon it to clear the highest portions of the building. The crane revolves upon a base through nearly three parts of the circumference of a circle, and in addition to this the jib of the crane is capable of an "up and down" motion which enables it to command any spot within a radius of three quarters of the length of the jib. For a single crane, a derrick tower with three legs is built, and the crane is placed over one of these, stayed back to the other two and then counterbalanced by heavy weights. Each leg is usually from 6 ft. to 1 o ft. square on plan, the "king" leg (that is, the leg supporting the crane) being larger than the "queen" legs. The three legs are placed from 20 to 3o ft. apart in the form of an equilateral or isosceles triangle. When two cranes are used, as is the case when important opera tions are to be conducted over the entire area of a circle, a f our legged square derrick tower is constructed, and a crane set upon a platform over each of two opposite legs. The ground upon which it is proposed to erect the towers must be well chosen for its solidity, or specially prepared to receive the towers. The founda tion usually consists of a platform of 9-in. by 3-in. deals under each leg. The corner posts may be of three 9-in. by 3-in. deals bolted together, but those for the king leg may advantageously be larger. They are connected at every 8 or 1 o ft. of their height by means of cross pieces or transoms from 7" by 2" to 9" by 3" in size, and each bay thus formed is filled in on all four sides with diagonal bracing of the same or slightly smaller timber. Up the

centre of the king leg, from the bottom to the top, is carried an extra timber to take the weight of the crane. It may be a balk of whole timber, 12 or 14 in. square, or may consist of deals bolted together up to 16 in. square. This central member must be well braced and strutted from the four corners to resist any tendency to bending.

When the towers have reached the desired height the king leg is connected to each of the queen legs by a trussed girder; the two queen legs may be connected with each other either by a similar trussed girder or by a single timber which is supported by struts from the supporting legs to shorten the span and give rigidity. For the connecting girders a balk of timber reaching from king to queen legs is placed on each of the two topmost transoms, which may be from 4 to 8 ft. apart, the depth of the top bays often being modified to the required depth of the connecting beams. Upright struts are fixed at intervals of about 5 ft. between the two balks, which are also connected by long iron bolts and cross braces filled into each bay. The top balks project 6 or ro ft. beyond the king leg and form the support for a working platform of deals. Struts are thrown out from the sides of the leg to support the ends of the balks. Upon the platform are laid two "sleepers" of balk timber extending from beneath the bed of the crane and passing over the centre of each queen leg. The "mast," a vertical member com posed either of a single timber or two pieces strutted and braced, is erected upon the revolving crane bed, and the "jib," which is simi lar in construction to the mast, is attached to the base of the latter by a pivoted hinge. The jib is raised and lowered by a rope fixed near the end of the jib and running to the engine by way of a pul ley wheel at the top of the mast. The rope or chain used for lifting the materials passes over a pulley at the end of the jib and thence to the winch over a pulley at the top of the mast. In the operation of lifting it is obvious that a great strain is put upon the mast and a considerable overturning force is exerted by the leverage of the weight lifted at the end of the jib. To counter-balance this, two timber "stays" or "guys" are taken from the mast head, one to the centre of each queen leg, and there secured. From these points two heavy chains are taken down the centre of each queen leg and anchored to the platform at their bases, which are each loaded with a quantity of bricks, stone or other heavy material equal in weight to at least twice any load to be lifted by the crane. A coupling screw link should be provided in the length of each anchor chain so that it may be kept taut. The coupling screws should be placed in an accessible place near the ground, where they may easily be seen and tightened when necessary. The legs of the structure should be cross braced with each other to resist twisting tendencies due to wind pressure and swinging of the load, either by ties of steel bars with tightening screws, or, as is more usual, with diagonally placed scaffold-pole or squared-timber braces secured to the framework.