sizes recommended by the various sources, together with the sizes which modern regulations demand must be used, are tabulated later. For simplicity I have used the definitions used by James Newland. In a "single joisted floor", the "bridging" (common) joists span from wall to wall with no intermediate support. In a double floor the span of che "bridging" joists is broken by "binding joists" which span from wall to wall and which today would no doubt merely be called "beams". A "framed floor" has "bridging" joists again supported by "binding" joists (nowadays often called secondary beams) which are in turn carried by "girders" which we might well now term main or principal beams. Newland says that the rules which he gives "apply to ordinary cases of floors such as chose in dwellinghouses". and notes that for warehouse floors the loads are much greater. JAMES NEWLAND: 19TH CENTURY To my relief my next source, The Carpenter's Assistant by James Newland and published at some time in the 19th century, uses modern spelling and I no longer feel that I speak with a lifp (sorry, lisp). Newland is rather more mathematical. The depth of a joist for example is calculated thus: "Divide the square of the length in feet by the breadth in inches, and the cube root of the quotient, multiplied by 2.2 for fir, and 2.3 for oak, will give the depth in inches". Similar formulae are given for other members. RIVINGTON: 1901 AND 1925 Rivington's Notes on Building Construction of 1901 (which was edited by the wonderfully named W Noble Twelvetrees) gives exactly the same formulae as did Newland but attributes the rules to Tredgold. Interestingly all of these formulae result in oak members being some 3-5% deeper than fir. By 1925, Rivington seems to have concluded that finding the cube root of a number was a rather too complicated and Table 1:Comparison of recommendationsfrom 1726to 1991 Source Date Bridging Bindingjoists, Girders, Notes joists, l 5' 18' span, 24' span, span 6' centres 10' centres Neve 1726 8 X 3t 13 X 9 16 X 12 t 12' span Newland 19th 9.5 X 3 11 X 10 15 X 12 Fir century 10.5 X 2 Rivington 1901 As Newland above Mitchell 191 I 7.65 X 3 13 X 9 18 X 14 Northern pine Rivington, 1925 9.5 X? Baltic pine rule of thumb Rivington,table 1925 9.5 X 3 11 X 10 15 X 13 Baltic pine Mitchell 1930 11 X 3 not discussed Northern pine Mitchell, 1959 5.75 X 3 suggestssteel Group 1 timber calculation Mitchell, table 1959 9x2 suggestssteel Group I timber BS5268 1991 8x2 12 X 10 17 X 14 SC3 26 A typicalfloor but with a trussedbeam. has concluded that "The rule of thumb for the depth of common joists is to cake half span in feet; to chis number add 2 for che depth of the joist in inches". This edition of Rivington also gives the 'scantlings' or sizes of members in Baltic pine in tabular form and again acknowledges his source to be Thomas T redgold. MITCHELCS: 1911, 1930 AND 1959 The well known building construction book 'Mitchell's', in its 1911 edition, produces some empirical calculations. For example for beams under distributed load, W (the breaking weight in ewes) equals 2k X che breadth in inches X the square of the depth in inches divided by the length in feet. The constant k (the central breaking weight for samples of specified sizes) varies for different types of timber. This time oak is noted as being stronger than fir. The floor loading in "ordinary dwelling houses" is given as 1.25 cwts per square foot (we now use 30lbs per square foot). Although this is a very high value the sizes work out about right so there seems to be a built-in factor of safety. The 1930 edition of Mitchell has different empirical formulae. The 1959 CONTEXT 43
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