On Some Common Errors in Iron Bridge Design

 24 weight of the girder. The remarkable economy due to replacing the clumsy end vertical by a sloping compression piece or ‘‘batter brace ” of theoretical section is now somewhat tardily being recognised by English Engineers. It has been, universal practice in America since the time of Whipple. This is an error to be avoided in future designs. In existing bridges it simply means waste of money, but does no further harm. 15. Unduly numerous systems of triangulation—This is a very common fault of the lattice girders of 30 or 40 years ago. A multiplicity of small bars, many of which for practical reasons- have to be much larger than, calculation, requires, which involve a large amount of complicated workmanship and often are not placed in proper relation to the points of attachment of the cross girdei’s, constitute a usual characteristic of early practice. Modern work, however, usually avoids this fault, and as it in most cases where it exists means waste and not weakness, it is not a matter needing any present action. 16. Inefficient forms of compression member.—In consequence- of the high compressive resistance of iron or steel the transverse dimensions of compression parts of girders have to be compara- tively small relatively to their length, consequently the tendency is to fail by buckling or long column action, rather than by direct crushing, and the strength attained depends largely on the success with which this tendency is combatted. When the column is so short as to fail by true crushing, the form of section is immaterial, provided only the required area is present, but when it is longer the form o£ section has a most profound influence on the resistance. The best form is that which presents the greatest resistance to bending laterally, either of the column as a whole or of any constituent part. Hence a good compression member must approach in form to an efficient beam, but as it may bend in any plane and not in one only, it must be an efficient beam in every direction. The ideal long column is a hollow cylinder which is a fairly and equally good beam in every direc- tion. The greater the diameter the greater the resistance to bending as a whole, but there is a limit to desirable increase of diameter and consequent reduction of thickness owing to the tendency of very thin tubes to give way by wrinkling or