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BRIDGES, TRUSSED WORK AND LIKE STRUCTURES

BRIDGES.--Bridge building is not, strictly, a part of the carpenter"s education at the present day, because most structures of this kind are now built of steel; but there are certain principles involved in bridge construction which the carpenter should master.

SELF-SUPPORTING ROOFS.--In putting up, for instance, self-supporting roofs, or ceilings with wide spans, and steeples or towers, the bridge principle of trussed members should be understood.

The most simple bridge or trussed form is the well-known A-shaped arch.

[Ill.u.s.tration: _Fig. 235._]

COMMON TRUSSES.--One form is shown in Fig. 235, with a vertical king post. In Fig. 236 there are two vertical supporting members, called queen posts, used in longer structures. Both of these forms are equally well adapted for small bridges or for roof supports.

THE VERTICAL UPRIGHT TRUSS.--This form of truss naturally develops into a type of wooden bridge known all over the country, as its framing is simple, and calculations as to its capacity to sustain loads may readily be made. Figs. 237, 238 and 239 ill.u.s.trate these forms.

[Ill.u.s.tration: _Fig. 236._]

[Ill.u.s.tration: _Fig. 237._]

THE WARREN GIRDER.--Out of this simple truss grew the Warren girder, a type of bridge particularly adapted for iron and steel construction.

This is the simplest form for metal bridge truss, or girder. It is now also largely used in steel buildings and for other work requiring strength with small weight.

[Ill.u.s.tration: _Fig. 238._]

[Ill.u.s.tration: _Fig. 239._]

[Ill.u.s.tration: _Fig. 240._]

THE BOWSTRING GIRDER.--Only one other form of bridge truss need be mentioned here, and that is the _bowstring_ shown in Fig. 240.

In this type the bow receives the entire compression thrust, and the chords act merely as suspending members.

FUNDAMENTAL TRUSS FORM.--In every form of truss, whether for building or for bridge work, the principles of the famous A-truss must be employed in some form or other; and the boy who is experimentally inclined will readily evolve means to determine what degree of strength the upper and the lower members must have for a given length of truss to sustain a specified weight.

There are rules for all these problems, some of them very intricate, but all of them intensely interesting. It will be a valuable addition to your knowledge to give this subject earnest study.

CHAPTER XIII

THE BEST WOODS FOR THE BEGINNER

In this place consideration will be given to some of the features relating to the materials to be employed, particularly with reference to the manner in which they can be worked to the best advantage, rather than to their uses.

THE BEST WOODS.--The prime wood, and the one with which most boys are familiar, is white pine. It has an even texture throughout, is generally straight grained, and is soft and easily worked. White pine is a wood requiring a very sharp tool. It is, therefore, the best material for the beginner, as it will at the outset teach him the important lesson of keeping the tools in a good, sharp condition.

SOFT WOODS.--It is also well for the novice to do his initial work with a soft wood, because in joining the parts together inaccuracies may be easily corrected. If, for instance, in mortising and tenoning, the edge of the mortised member is not true, or, rather, is not "square," the shoulder of the tenon on one side will abut before the other side does, and thus leave a crack, if the wood is hard. If the wood is soft there is always enough yield to enable the workman to spring it together.

Therefore, until you have learned how to make a true joint, use soft wood.

Poplar is another good wood for the beginner, as well as redwood, a western product.

HARD WOODS.--Of the hard woods, cherry is the most desirable for the carpenter"s tool. For working purposes it has all the advantages of a soft wood, and none of its disadvantages. It is not apt to warp, like poplar or birch, and its shrinking unit is less than that of any other wood, excepting redwood. There is practically no shrinkage in redwood.

THE MOST DIFFICULT WOODS.--Ash is by far the most difficult wood to work. While not as hard as oak, it has the disadvantage that the entire board is seamed with growth ribs which are extremely hard, while the intervening layers between these ribs are soft, and have open pores, so that, for instance, in making a mortise, the chisel is liable to follow the hard ribs, if the grain runs at an angle to the course of the mortise.

THE HARD-RIBBED GRAIN IN WOOD.--This peculiarity of the grain in ash makes it a beautiful wood when finished. Of the light-colored woods, oak only excels it, because in this latter wood each year"s growth shows a wider band, and the interstices between the ribs have stronger contrasting colors than ash; so that in filling the surface, before finishing it, the grain of the wood is brought out with most effective clearness and with a beautifully blended contrast.

THE EASIEST WORKING WOODS.--The same thing may be said, relatively, concerning cherry and walnut. While cherry has a beautiful finishing surface, the blending contrasts of colors are not so effective as in walnut.

Oregon pine is extremely hard to work, owing to the same difficulties experienced in handling ash; but the finished Oregon pine surface makes it a most desirable material for certain articles of furniture.

Do not attempt to employ this nor ash until you have mastered the trade.

Confine yourself to pine, poplar, cherry and walnut. These woods are all easily obtainable everywhere, and from them you can make a most creditable variety of useful articles.

Sugar and maple are two hard woods which may be added to the list.

Sugar, particularly, is a good-working wood, but maple is more difficult. Spruce, on the other hand, is the strongest and toughest wood, considering its weight, which is but a little more than that of pine.

DIFFERENCES IN THE WORKING OF WOODS.--Different woods are not worked with equal facility by all the tools. Oak is an easy wood to handle with a saw, but is, probably, aside from ash, the most difficult wood known to plane.

Ash is hard for the saw or the plane. On the other hand, there is no wood so easy to manipulate with the saw or plane as cherry. Pine is easily worked with a plane, but difficult to saw; not on account of hardness, but because it is so soft that the saw is liable to tear it.

FORCING SAWS IN WOOD.--One of the reasons why the forcing of saws is such a bad practice will be observed in cutting white or yellow pine.

For cross-cutting, the saw should have fine teeth, not heavily set, and evenly filed. To do a good job of cross-cutting, the saw must be held at a greater angle, or should lay down flatter than in ripping, as by so doing the lower side of the board will not break away as much as if the saw should be held more nearly vertical.

These general observations are made in the hope that they will serve as a guide to enable you to select your lumber with some degree of intelligence before you commence work.

CHAPTER XIV

WOOD TURNING

ADVANTAGES OF WOOD TURNING.--This is not, strictly, in the carpenter"s domain; but a knowledge of its use will be of great service in the trade, and particularly in cabinet making. I urge the ingenious youth to rig up a wood-turning lathe, for the reason that it is a tool easily made and one which may be readily turned by foot, if other power is not available.

SIMPLE TURNING LATHE.--A very simple turning lathe may be made by following these instructions:

THE RAILS.--Procure two straight 2" 4" scantling (A), four feet long, and planed on all sides. Bore four 3/8-inch holes at each end, as shown, and 10 inches from one end four more holes. A plan of these holes is shown in B, where the exact s.p.a.cing is indicated. Then prepare two pieces 2" 4" scantling (C), planed, 42 inches long, one end of each being chamfered off, as at 2, and provided with four bolt holes. Ten inches down, and on the same side, with the chamfer (2) is a cross gain (3), the same angle as the chamfer. Midway between the cross gain (3) and the lower end of the leg is a gain (4) in the edge, at right angles to the cross gain (3).

THE LEGS.--Now prepare two legs (D) for the tail end of the frame, each 32 inches long, with a chamfer (5) at one end, and provided with four bolt holes. At the lower end bore a bolt hole for the cross base piece.

This piece (E) is 4" 4", 21 inches long, and has a bolt hole at each end and one near the middle. The next piece (F) is 2" 4", 14-1/2 inches long, provided with a rebate (6) at each end, to fit the cross gains (4) of the legs (C). Near the middle is a journal block (7).

[Ill.u.s.tration: _Fig. 241. Frame details._]