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.
Vertical load on slanting beam.
Nature of com pression large blocks.
Where, therefore, we are proportioning the sizes of struts - or later the sizes of washers - for their bearing area, across the wood, we will bear the above in mind, and if the safe values for crushing across the grain, as given in Table IV, require unusual dimensions, we will increase the amount of the safe value per square inch, as our judgment may dictate.
= 13700 pounds, which is near enough to be safe.
In calculating the other strut we should find that as a column 6 inches x 6 inches and 6 feet 3 inches long it can safely carry 14400 pounds, the actual strain being 12000 pounds. For pressure across the grain against the rafter and main tie-beam we have an area of about 6 inches x 8 inches against each or 48 square inches, therefore actual compression per square inch across the grain = 250 pounds, which they can safely stand, as these parts are of Georgia pine and the crushing area quite large.
We next proportion the sizes of tie-rods, which will be of wrought-iron. The safe tensional stress of wrought-iron being 12000 pounds per square inch we require areas as follows:
For the principal rod,
=18000/12000 = 1 1/2 square inches.
For the side rods,
5600/12000 = 7/15 or say 1/2 square inch.
The rods being circular the principal rod would need to be 1 3/8 inch diameter and the side rods 13/16 inch diameter or say 3/4 inch. We also place a small 5/8 inch diameter rod in each end panel to keep the tie-beam from sagging. The rods will all have to have their ends upset, or else they will not have enough sectional area between the threads. In practice, however, it would be cheaper, where the rods are so small and short, to enlarge their diameter and pay for the extra material, rather than to save this and have to pay for expensive blacksmith's work.
Screw ends should only be upset where the material saved fully counterbalances the labor.
We next proportion the washers, they bear against Georgia pine and across its grain and should, therefore, be proportioned at about 200 pounds per square inch. For the principal rod we will need, therefore, a bearing area for each washer of
14000/200= 70 square inches, or say 8 inches x 8 inches.
For the side rods we will need
12000/200= 60 square inches, or about the same, we will therefore to save expense make them all alike.
The lower washers are made to bear horizontally against both head (or nut) and tie-beam, but the upper washers have to be modelled to bear against the slanting side of rafter, and horizontally against nut. It will also be noticed that the lower end of the washer is "toed-in" to the rafter to keep the washer from sliding.
The wooden blocks which are screwed onto the rafter at each washer were made to allow for cutting away, to provide horizontal surfaces on which to rest the bridle irons which carried the purlins. It will be noticed that the principal tie-beam is pieced at the centre. The cut or halving was made slanting, so ;is to force each half to bear on and pull against the other half; and all sharp edges, where sudden increase in strains would take place wire avoided by rounding off, shown. Wrought-iron plates were placed over and below the cut and sufficient bolts placed each side of the cut, not to crush the wood.
Halving long timbers.
We now must still design the foot-joints.
The bearing area required must first be found; we can safely put 200 pounds per square inch on Georgia pine, across the grain, and our load at each reaction being 27000 pounds, we need
27000/200 135 square inches of bearing area, and as the timber is 10 inches wide, it would have to rest on its bearing for a length of 13 1/2 inches.
In this case, however, we made it 24 inches, because the principal rafter bearing on the tie-beam so far from the support, it would have been apt to bend it. The left end rested on an iron column and was bolted to it as shown in Figure 258. In Figure 259 is shown the right end, which rested on and was bolted to a wall. The latter was corbelled out under the truss and capped with blue stone.
We next provide at each foot an inclined bolt, to hold the rafter down to the tie-beam and keep it from jumping out of the strap, which it might do, if subjected to heavy transverse strains. We next "toe-in" the rafter to the tie-beam as shown, taking care to get enough bearing area and area ahead of the toe to keep it from pushing or shearing its way out through the tie-beam, or from being sheared off itself. In our case, however, the toe is made small, the only reliance we place on it being to steady the rafter sideways. The rafter transfers all the load vertically across the tie-beam, we shall therefore need the same length of bearing area on the tie-beam, 13 1/2 inches, as the latter needs on the wall.
We more than get this by means of the hardwood block, driven in tightly with its edge grain against the rafter and tie-beam and held in position by screwed on blocks.
We next calculate the strap. The strains coming on it are 54000 pounds tension and resisting this 58400 compression. The lesser will, of course, be the one it must resist. The width of strap required not to crush the rafter must first be determined. The strap bears almost along the rain of the rafter, but not quite, we will therefore reduce slightly the safe allowance for compression along the grain as given in Table IV, and allow, say, 670 pounds per square inch, we then require 54000/670= 80 square inches, or the rafter being 10 inches wide, a strap 8 inches wide. The thickness of strap should be sufficient not to shear off, we have two shearing areas one at each angle or