8 tons per square inch.
An allowance of 30 per cent should be made to cover defects, such as air-holes, etc., in the castings.
Cast iron is not well adapted for structures intended to carry a live load, but if used for such, the working stresses would be reduced in the proportion shown by the factors of safety for the different cases given in the Table, p. 326.
The working stresses practically applied to wrought iron are as follows : -
5 tons per square inch.
1 Compression ....
4 to 4 1/2 „
These working stresses are in practice applied to girders with dead loads, and also to those carrying moderate live loads. This, of course, is not theoretically correct. When the load is all dead the working stresses may safely be higher - equal to 1/4 the breaking stress of the material; and when the live load becomes large in proportion to the weight of the girder (not a common case in girders connected with buildings), the working stresses must be reduced by a method which will be explained in Part IV.
For rolled girders the stresses may be taken slightly higher, i.e. at about 6 tons in tension and 5 tons in compression or shearing.
Where part of the load is live it is converted into an equivalent amount of dead load as described at page 468.
1 In calculating the area of the sections to which these stresses are applied, the rivet holes are deducted in the tension flange, but are not generally deducted in the compression flange. Some engineers deduct them in both flanges.
2 The shearing stress might be taken as high as the tensile stress, but that the former generally acts upon a group of rivets, some of which often get a larger share of the stress than the others, so that a lower limit is taken in order to be on the safe side.
Thus, with good bar iron (such as see Table, p. 318) a factor of safety
of 4 for a dead load would give a working stress of 2778/4 , or nearly 7 tons per square inch of section.
However, taking into consideration the sudden shocks caused by the wind, a working stress of 6 tons is high enough; and where the iron is of an unknown quality, it is better to allow only 5 tons per square inch.
Though the construction of bridges is a subject entirely beyond the limits of any part of these Notes, it may be as well to mention here the Board of Trade rule as to the working stress for bridges, because this rule has governed the practice with regard to bridges, and has to a great extent influenced it in other structures.
"In a wrought-iron bridge the greatest load which can be brought upon it, added to the weight of the superstructure, should not produce a greater strain on any part of the material than 5 tons per square inch."
Practically this rule is modified by taking the working stresses, as given above, all of them except the tensile stress being lower than the limits laid down by the rule.