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.
Basis of Tables XIV and XV.
1 The rule for calculating the exact thickness will be found later, Formula (78).
The use of Table XV, is very similar to that of Table XIII, but that the safe uniform load is given (in the first column) in tons of 2000 pounds each. The continuation of the two 20" beams up to 42 feet span is given in the separate table, in the lower right-hand corner. To illustrate the Table: if we have a span of say 21 feet we pass down its vertical line; the first curve we strike is the 10 1/2"-90 pounds beam, which is three-quarter space beyond the horizontal line 5 (tons); therefore a 10 1/2"-90 pounds beam at 21 feet span will carry safely 5 3/4 tons uniform load, and will not deflect sufficiently to crack plaster. (Each full horizontal space represents one ton). The next beam at 21 feet span is 10 1/2"-105 pounds, which will safely carry 6 1/2 tons. Then comes the 12" -96 pounds beam, which will safely carry 7 tons, and so on down to the 20"-272 pounds beam, which will safely carry 33 3/4 tons.
If we know the span (say 17 feet) and uniform load (say 7 1/2 tons) to be carried, we pass down the span line 17' 0" and then horizontally along the load line 7 1/2 till they meet, which in our case is at the 9"-125 pounds beam; we can use this beam or any cheaper beam, whose curve is under it. We pass over the different curves under it, and find the cheapest to be the 12"-96 pounds beam, which we, of course, use.
Iron beams must be scraped clean of rust and be well painted. They should not be exposed to dampness, nor to salt air, or they will deteriorate and lose strength rapidly.
Steel beams are coming into use quite largely. They are cheaper to manufacture than iron beams, as they are made directly from the pig and practically in one process; while with iron beams the ore is first converted into cast iron, then puddled into the muck-bar, re-heated, and then rolled. Steel beams, however, are not apt to be of uniform quality. Some may be even very brittle; they are, however, very much stronger than iron (fully 25 per cent stronger), but as their deflection is only about 7, 3 per cent less than that of iron beams, there is but very little economy of material possible in their use. If steel beams are used they can be spaced one quarter distance (between centres) farther apart than given in Table XIV for iron beams; or they will safely carry one quarter more load than given in Table XV; but in no case, where full load is allowed, must the span in feet, (of steel beams), exceed twice the depth in inches. With full safe loads the deflection of steel beams will always be greater than that of iron beams (about 1/8 larger). Where, therefore, it is desirable not to have a greater deflection than with iron beams, add only 7 1/2 per cent to the distances between centres or "safe loads" as given in Tables for iron beams, instead of 25 per cent.
Steel beams will undoubtedly supersede iron beams before many years have passed; but in the present state of their manufacture their use is hardly to be recommended. Their strength and consistency is very variable. It has been found in some cases that steel beams broke suddenly when jarred, (that is, were very brittle,) though test pieces off the ends of these same beams gave very satisfactory results. If steel is used, not only should samples of each piece be carefully tested, for tenacity, ductility, elasticity, elongation, etc., but the whole beam itself should be tested by actual loading. It will be readily seen that the expense of such tests would bar the use of steel, but no architect can afford to take any chances in such an important part of his building.
Many writers even claim, that, "within the elastic limit," the additional stiffness of steel over iron does not appear; and that it is only beyond this limit that steel is somewhat stiffer than iron.
In using iron and steel beams it is very important that they be supported sideways, so as not to yield to lateral flexure. Where the beams are isolated and unsupported sideways, the safe load must be diminished. Just how much to diminish this load is the question. The practice amongst iron workers is to consider the top flange as a column of the full length of the span, obliged to yield sideways, and with a load equal to the greatest strain on the flange. Modifying, therefore, Formula (3) to meet this view, we should have: w1 = w/1+y.L2/b2 ______ (78)
Where to w= the safe load, in pounds, on a beam, lintel or straight arch supported sideways.
Lateral Flexure in beams.
Beams not braced sideways.