426. The iron used in straps and bolts should be of a quality tough and fibrous, so as not to be liable to snap with any sudden application of a strain which would otherwise be within its strength.

Mr. Kirkahly* has shown that the mere weight which destroys the cohesive force of iron does not indicate its quality. A high breaking strain may be due to the material being of superior quality, dense, fine, and moderately soft; or simply to its being very hard and unyielding. A low breaking strain, on the other hand, may be due to looseness and coarseness in the texture; or to extreme softness, although very close and fine in quality.

427. Fibrous iron elongates considerably under heavy strains; and when it breaks, the fractured part is much reduced in section, frequently in good iron to the extent of 30 or 40 per cent. In iron of inferior quality the area of the section at the point of fracture is scarcely reduced at all.

By careless forging, the best iron may be seriously injured, and give results little superior to those of the worst. Owing to uncertainty in the strength of inferior iron, it is more economical to use only such as may be of good quality, as it can be submitted with safety to a much greater load.

In proving iron (and all iron should be submitted to proof before it is used in important structures) it should not be subjected to a greater strain than one-half of that which would produce fracture, as a greater strain would injure the elasticity of the iron, and it would no longer be capable of resisting the sudden application of a heavy load. The greatest safe load to which iron should be permanently submitted is one-fourth of its nominal breaking weight.

428. The cohesive strength of different kinds of iron and steel is shown in the following Table, which will be of service in finding the dimensions of straps, etc.

* ' Experiments on Wrought Iron and Steel.'

429. The resistance of wrought iron to a shearing force equals 5/6ths of the cohesive strength of the section sheared.

430. "A skilful carpenter," says Professor Robison, " never employs many straps, considering them as auxiliaries foreign to his art;" * straps, however, may be made extremely useful in the practice of carpentry, particularly in such cases as the suspension of the tie-beam to the king post, and to secure the feet of the principal rafters to the tie-beam of a roof.

Strap for King or Queen Post - In Fig. 81, Art. 250, s shows a strap or stirrup for suspending the tie-beam to a king or queen post; its hold of the post may be improved by turning the ends, as at d d, in the section E; these, when well fitted, will, with the addition of bolts, give the strap a firm hold; or staples, as in Fig. 149, may be used for a slight roof. The strengths of straps for different bearings are stated below. When the longest unsupported part of the tie-beam is

* ' Encyclopaedia Britannica,' art. Carpentry.

These dimensions are quite sufficient for common purposes, but where the machinery of a theatre, or other heavy loads, are to be borne by the tie-beams, the straps must be made stronger in proportion to the load.

Where suspending pieces are used instead of queen posts, the same kind of strap will apply, as shown in Fig. 151.