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Useful Charts and Tables.—No. 7.

28th March 1907, Page 6
28th March 1907
Page 6
Page 6, 28th March 1907 — Useful Charts and Tables.—No. 7.
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Which of the following most accurately describes the problem?

By George Watson, A.M.I.Mech.E.

Modulus of Section or " Z " for Channels and Girders.

The two charts reproduced in this number form a handy and rapid means of ascertaining the size of section required for a given bending moment on a frame or other part of a motor chassis, which may be made of channel or girder section steel.

Chart No. i gives the values of " Z " for all sizes up to 3 inches by 3 inches, whilst, for sizes above that but below 7;ii inches by 3.i inches, reference must be made to chart No. 2. All the sizes might have been plotted out on one chart, but the accuracy of the readings would have been seriously impaired. The charts are constructed from the formula IP. B. h.

Z = _ x '

where Z =The modulus of section, or moment of resistance. D-= The total depth of the section.

B=The total width of the section, d=The total depth, minus the thickness of the flanges. b=The total width, minus the thickness of the web. Whilst absolute accuracy is not claimed for these charts, the results obtained from them are sufficiently accurate for most practical purposes, either in designing, or comparing,

the relative strengths of two or more sections. Suppose a new vehicle is being designed, and it is proposed to make the side members of the frame of channel section, pressed out of steel plate, we must first find, by calculation or g-raphically, what will be the greatest bending moment at any section of the frame. Let this equal B1il and, if f equals the stress beyond which it is not proposed to subject the material, then the modulus of section at the point of maxi

BM mum bend should equal —, or BM=4Z. Having found the required value of Z for the section, we must now turn to the charts and find what section will give such a modulus as the one required.

These charts are necessarily more difficult to read than any of the others of this series, because there are so many factors to be taken into consideration in plotting them. For that reason, chain-dotted lines, as examples, have been added on the charts, and these lines are barbed at intervals, in order to make it quite clear in which direction the tracing point should travel. Two examples are given on chart No, and chart No. 2 respectively, which will make their use quite clear to the designer who may use them.


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