This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
In building-up beams to obtain increased strength, the most usual method is to lay 2 beams together sideways for short spans, as in the lintels over doors and windows; or to cut one down the middle, and reverse the halves, inserting a wrought-iron plate between, as shown in the flitch girder, Fig. 467. The reversal of the halves gives no additional strength, as many workmen suppose, but it enables one to see if the timber is sound throughout at the heart, and also allows the pieces to season better. A beam uncut may be decayed in the centre, and hence the advantage of cutting and reversing, even if no flitch-plate is to be inserted, defective pieces being then discarded. When very long and strong beams are required, a simple method is to bolt several together so as to break joint with each other, as shown in Fig. 468, taking care that on the tension side the middle of one piece comes in the centre of the span with the 2 nearest joints equidistant. It is not necessary in a built beam to carry the full depth as far as the supports; the strain is, of course, greatest in the centre, and provided there is sufficient depth given at that point, the beam may be reduced towards the ends, allowance being made for the loss of strength at the joints on tension side.
A single piece of timber secured to the under side of a beam at the centre, as in Fig. 469, is a simple and effective mode of increasing its strength. It will be observed that the straps are bedded into the sides of the beams; they thus form keys to prevent the pieces from slipping on each other. This weakens the timber much less than cutting out the top or bottom, as the strength of a beam varies only in direct proportion to the breadth but as the square of the depth. The addition of a second piece of timber in the middle is a method frequently adopted for strengthening shear legs and derrick poles temporarily for lifting heavy weights.