Constructing Pentagons

pentagon. At B, erect a perpendicular, and make it equal to the given side; bisect A-B, and from its centre c, with radius c-d, describe the arc d--c. Then from A and B as centres, and A-c as radius, describe arcs intersecting in E. From points A-B-E, with radius A-B, draw intersecting arcs. Join up the points of intersection and the pentagon will be constructed.

To construct an Oblique Projection of a Pentagonal Prism, Fig. 5.—Draw the ground line, and upon it construct one face of the prism, as A'-a-b-B'-A'. Draw the projector c'-E at angle of 45°. We have next to find its length. Set the height of the pentagon C-E (Fig. 4) off on the ground line from C to E', and draw a projector from this point, intersecting the first at E. We have now the diminished height of the pent agon, and as all the points will be similar at the other end of the prism, erect a perpendicular (dotted) at E, and make it equal in length to A'-a. We have next to place the inclined sides in their relative position. We might do this by finding a proportional at right angles to the line c'-E as shown at the top end. and so locating the two side angles, but it is easier to deal with the side in relation to a right angle. Draw a dotted projector from B'. Next draw a perpendicular to B-d, Fig. 4. from the angle F, intersecting B-d in i. Set off B-i along the ground line from B' and draw a projector from i. This gives the diminished length of B-i, and on a parallel to the ground line through the intersection we can locate point /, which represents F in Fig. 4. Points B'-f and E can now be joined up and the two sides drawn. To obtain the opposite angle, draw a horizontal line from /, and intersect it by a projector from h", which is made the same distance from A' that h is from A in Fig. 4.

Shuttering and Forms for Reinforced Concrete Work, page ioq.- -These are the terms applied to the wood moulds or casings, erected to contain and support the concrete which is poured around the iron bars and straps composing the reiniorcement, until the concrete has

FORMS AND SHUTTERS FOR CONCRETE hi

" set." The term " shuttering " is applied to boards which are secured together with ledges, having merely to support the concrete, such as against the face of walls or under sides of ceilings, fronts of galleries, etc. When the duty of the casing is to mould or shape the concrete, poured into it, as in the case of girders and columns, it is termed " forms." Both kinds are shown in the illustration, which is given as an advanced example of diminished oblique projection.

The objects of these constructions are that they shall be economical in cost of construction, shall be readily removable without damage to the green work, and shall be strong enough to support the loads without deflection, and shall not cast or warp on the. introduction of the wet concrete. Variousmetliods and devices are used to meet these ends, and the best must be determined by individual circumstances.

As at the present time almost all that is left for the carpenter to do on modern buildings is the formation of these casings, it may not be out of place to offer a few remarks upon their construction. The wood selected should not be the roughest and commonest that can be obtained. Whilst it is not suggested that first-class joiner's wood should be used, the cost for labour In conversion and subsequent making good of defects on the face of the work will, if any rubbish is used for the forms, soon outweigh the first saving in cost of material. Dry stuff should not be used, as it will swell abnormally 011 the introduction of the wet concrete; shaky stuff should be avoided, but sound knots maj be disregarded ; loose ones will supply undesirable " keys that will prevent removal. The fewer nails used the better. These will rust in and be unremovable, and as a consequence much of the stuif would be useless for conversion. The shuttering should not be in too large pieces for convenient, handling; joints in the stutf do not matter, as the surfaces ot the concrete, it not rendered, are usually rubbed, unless they are out of sight.

Forms for beams should be made so that the sides can be removed without interference with the bottom; it is often ii2 FIXING SHUTTERING

necessary to accelerate the drying by removing the side shuttering as soon as the concrete is sufficiently set, yet not strong enough to support itself, hence the soffit board and centering must be left up. For this reason the " troughs " are not nailed together, but, as shown in the foreground of the drawing, are cither wedged up tightly against the bottom board or where the " panel" is not to be tilled in, until the girders are set, a method often used in heavy floors, " Fox cleats" are formed around the trough, letting the ends of the ledges project slightly and notching them into cross pieces. If a nail is driven into the loose piece to prevent its falling off, the head should stick out for the pincers to grip.

Mouldings or chamfers are generally made on the lower edges of girders ; the. shapes for these should be nailed to the bottom, not to the sides of the forms. Let the ends of the beam troughs rest upon notches cut in the column forms as shown, because the column casing should always be the last removed. A good foreman will keep these up as long as he can, to protect the concrete. The fixed ledges on the " sides " of the column casing should run over the edges of the " faces." This will keep them in right position when setting up whilst the box cleats are being fixed. These cleats are turned out in numbers from the mill, and holes bored in the tenons for the oak pins to secure them. The latter are kept in a box handy to the workman, and can be used many times over. Another form of clamp is shown in Fig. 2. These are nailed together at the angles as they are placed around the forms, but are only suitable for columns of small size or height. Near the bottom of high columns, the clamps should be made of 3 in. x 3 in. stuff bolted at the angles, as the pressure is great at the bottom of a column, especially if it is filled in quickly.

Most of this casing is shown as inch stuff, but the thickness will depend upon the nature and size of the beams, etc. The props must be placed pretty closely together, so that no sagging takes place. The bearers carrying the panels

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