This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
In shrinking, the distance between rings remains onstant, and it is for this reason that the finest floors are made from quartered stuff; for (besides their greater beauty), the rings being all on end, no horizontal shrinkage will take
place; the width of boards remaining constant, and the shrinkage being only in their thick" ness; neither will tim ber shrink on end or in its length. Figures 132 and 133 show how timber will shrink. The first from a quartered log, the other from one with parallel cuts. The dotted part shows the shrinkage. The side-pieces G in Fig. 133 will curl, as shown, besides shrinking. By observing the directions of the annular rings, therefore, the future behavior of the timber can be readily predicted. Of course, the figures are greatly exaggerated to show the effect more clearly.
If the heart is not straight its entire length, the piece will twist lengthwise. Shrinkage is a serious danger, but the chief danger in the use of timber lies in its decay. All timber will decay in time, but if it is properly dried, before being built in, and all sap-wood discarded, and then so placed that no moisture can get to the timber, while fresh air has access to all parts of it, it will last for a very long time; some woods even for many centuries. In proportion as we neglect the above rules, will its life be short-lived. There are two kinds of decay, wet and dry rot. The wet rot is caused by alternating exposures to dampness and dryness; or by exposure to moisture and heat; the dry-rot, by confining the timber in an air-tight place. In wet rot there is "an excess of evaporation; " in dry rot there is an "imperfect evaporation." Beams with ends built solidly into walls are apt to rot; also beams surrounded solidly with fire-proof materials; beams in damp, close, and imperfectly ventilated cellars; sleepers bedded solidly in damp mortar or concrete, and covered with impervious papers or other materials; also timbers exposed only at intervals to water or dampness, or timbers in " solid " timbered floors.
Dry rot is like a contagious disease, and will gradually not only eat up the entire timber, but will attack all adjoining sound woodwork. Where rotted woodwork is removed, all adjoining woodwork.
Decay of Timber.
masonry, etc., should be thoroughly scraped and washed with strong acids.
Where wood has, of necessity, to be surrounded with fireproof materials, a system of pipes or other arrangements, should be made to force air to same through holes, either in the floors or ceilings, but in no case connecting two floors; the holes can then be made small enough not to allow the passage of fire. Where the air is forced in under pressure it would be advisable at times to force in disinfectants, such as steam containing evaporated carbolic acid, fumes of sulphur, etc.
Coating woodwork with paint or other preparations will only rot the wood, unless it has been first thoroughly dried and every particle of sap removed.
Timber must not be used too thin, or it will be apt to twist. For this reason floor-beams should not be used thinner than three inches. To avoid twisting and curling, cross-bridging is resorted to. That is, strips usually 2" X 3" are cut between the beams, from the bottom of one to the top of the next one, the ends being cut (in a mitre-box), so as to fit accurately against the sides of beams, and each end nailed with at least two strong nails. The strips are always placed in double courses, across the beams, the courses crossing each other like the letter x between each pair of beams.
This is known as "herring-bone" cross-bridging. Care should be taken that all the parallel pieces in each course are in the same line or plane. The lines of cross-bridging can be placed as frequently as desired, for the more there are, the stiffer will be the floor. About six feet between the lines is a good average. Sometimes solid blocks are used between the beams, in place of the herring-bone bridging. Cross-bridging is also of great help to a floor by relieving an individual beam from any great weight accidentally placed on it (such as one leg of a safe, or one end of a book-case), and distributing the weight to the adjoining beams Unequal settlements of the individual beams are thus avoided. Where a floor shows signs of weakness, or lacks stiffness, or where it is desirable to force old beams, that cannot be well removed, to do more work, two lines of slightly wedge-shaped blocks are driven tightly between the beams, in place of the cross-bridging. The beams are then bored, and an iron rod is run between the lines of wedges, from the outer beam at one end to the outer beam at the other, and, of course, at right angles to all. At one end the rod has a thread and nut, and by screwing up the latter the beams are all forced upwards, "cambered," and the entire floor arched. It will be found much stronger and stiffer; but, of course, will need levelling for both floor and ceiling. Under the head and nut at ends of rod, there must be ample washers, or the sides of end beams will be crushed in, and the effect of the rod destroyed.
In using wooden beams and girders, much framing has to be resorted to. The used joints between timbers are numerous, but only a very few need special mention here. Beams should not rest on girders, if it can be avoided, on account of the additional dropping caused by the sum of the shrinkage of both, where one is over the other. If framing is too expensive, bolt a wide piece to the under side of the girder, sufficiently wider than the girder to allow the beams to rest on it, each side. If this is not practicable bolt pieces onto each side of the girder, at the bottom, and notch out the beams to rest against and over these pieces. The bearing of a beam should always be as near its bottom as possible. If a beam is notched so as to bear near its centre, it will split longitudinally. Where a notch of more than one-third the height of beam, from the bottom, is necessary, a wrought-iron strap or belt should be secured around the end of beam, to keep it from splitting lengthwise.
If framing can be used, the best method is the "tusk and tenon" joint, as shown in Figs. 134 and 135. In the onecase the tenon goes through the girder and is secured by a wooden wedge on the other side; in the other it goes in only about a length equal to twice its depth, and is spiked from the top of girder. The latter is the most used. By both methods the girder is weakened but very little, the principal cut being near its neutral axis, while the beam gets bearing near its bottom, and its tenon is thoroughly strengthened to prevent its shearing off. The dimensions given in the figures are all in parts of the height of beams. Headers and trimmers at fire-places and other openings are frequently framed together, though it would be more advisable to use "stirrup-irons." The short tail-beams, however, can be safely tenoned into the header.
In calculating the strength of framed timber, the point where the mortise, etc., are cut, should be carefully calculated by itself, as the cutting frequently renders it dangerously weak, at this point, if not allowed for. For the same reason plumbers should not be allowed to cut timbers. As a rule, however, cuts near the wall are not dangerous, as the beam being of uniform size throughout, there is usually an excess of strength near the wall.
Stirrup-irons arc made of wrought-iron; they are secured to one timber in order to provide a resting-place for another timber, usually at right angles to and carried by the former. They should always lap over the farther side of the carrying timber, to prevent slipping, as shown in Fig. 13G.
The iron should be sufficiently wide not to crush the beam, where resting on if; the section of iron must be sufficient not to shear off each side of beams. The twist must not be too sudden, or it will straighten out and let the carried timber down. To put the above in formulae we should have: for the width of stirrup-iron (x) x = s/b. (c/f) (69)
Where x = the width of stirrup-iron, in inches. Where s = the shearing strain, in lbs., on end of beam, being carried.
Where b = the width of beam being carried, in inches.
For the thickness of stirrup-iron we should have: y = s/2.x (g/f) (70)
Which for wrought-iron (Table IV.) becomes,
y = 16000. x (71)
Where y=the thickness of stirrup-iron, in inches. Where s=the shearing strain on end of beam, in lbs. Where x=is found by formula (60).
Thickness of Stirrup-iron.
Providing, however, that y should never be less than one-quarter inch thick.
A girder carries the end of a beam, on which there is a uniform load of two thousand pounds. The beam is four inches thick, and of Georgia pine. What size must the stirrup-iron be?
The shearing strain at each end of the beam will, of course, be one thousand pounds, which will be the load on stirrup-irons. (See Table VII). From Table IV we find for Georgia pine, across the fibres, (c/f) =200, we have, therefore, for the width of stirrup-iron from Formula (69) x = 1000/ 4.200 1 1/2" Therefore the thickness of iron from Formula (71) should be y = 1000/16000.1 1/4 = 1/20" we must make the iron however at least 1/4" thick and therefore use a section of 1 1/4 x 1/4".
In calculating ordinary floor-beams the shearing strain can be overlooked, as a rule; for, in calculating transverse strength we allow only the safe stress on the fibres of the upper and lower edges, while the intermediate fibres are less and less strained, those at the neutral axis not at all. The reserve strength of these only partially used fibres will generally be found quite ample to take up the shearing strain.
The formulae for transverse strength are quite complicated, but for rectangular sections (wooden beams) they can be very much simplified provided we are calculating for strength only and not taking deflection into account.
Remembering that the moment of resistance of a rectangular section is (Table I) b.d2/6 and inserting into Formula (18) the value for m according to the manner of loading and taken from (Table VII), we should have:
For uniform load on beam.
u=b.d2/9.L. (k/f) (72)
For centre load on beam.
= b.d2/18.L. (k/f) (73)
Example stirrup-ironsRectangular beams.
Transverse strength of rectangular beams.