642 MODERN GASWORKS PRACTICE
is frequently the custom to allow for all struts to be of similar section, although in the case of the larger holders some reduction is admissible and conducive to economy. This particularly applies to holders having built up struts such as the lattice or open-box type. It must be borne in mind, however, that there is some tendency to deformation due to torsion, and this torsional force is a maximum at the top of the framing. Hence, too liberal a reduction in the section of the upper struts should not be permitted.
THE DESIGN OF STRUTS
Struts are best designed by trial and efi'ect methods. That is to say, an engineer acquainted with the practical construction of holders will decide upon what he considers to be a reasonable section, and will then prove the stability of his section by ordinary theoretical methods. For this purpose the safe load which the strut is capable of withstanding may be calculated from the following modification of the “ straight line ” formula which the author suggests as being the most adaptable for the purpose :—
Safe load =6^1 —0053— ) X area of section.
Where L= length of strut in indies, K= least radius of gyration.
Then, the section must be such that the safe load given is not less than the compressive stress in the mernber calculated as shown above.
To return to the example in question, the maximum stress in the bottom strut is 5-29 tons. For a holder of this size the struts would be most economically com-posed of ordinary R.S.J. The probable section required would be 10" by 5" by 30 Ib. per foot. Applying the test formula to this, we get—
Safe load = 6^1 — -0053^) X area
= ßfl - -0053 X 16 X 12) X 8-82
\ 1-05 7
= 6 (1 — -9699) X 8-82
= 52-92 X -0301
= 6-76 tons.
The section is therefore amply strong for the purpose.
(Note.—The valne of K for above may be readily obtained from one of the many sections tables published by the steel manufacturers, or it may be calculated from
the principle that K= \/__, where I is the least moment of inertia of the section.) A
THE DESIGN OF TIES
The design of the tie-bars which, with few exceptions, consist of plain round mild steel rods is simple. If the load on the tie is known. then the following formula gives the required diameter :—
Diameter in inch.es — \/ ljOa‘l ni 1 ons _
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