Principle XXIX.

Rivets should not be used in direct tension.

In the days of iron rivets this was an important requirement, for the reason that the shanks were often so overstrained in cooling that the heads would fly off; but this does not occur with steel rivets. Nevertheless it is advisable to adhere to the rule, except for very unimportant members where there is a great excess in the number of rivets above the theoretical requirements.

Principle XXX.

For members of any importance two rivets do not make an adequate connection.

For such details as lattice bars, of course two rivets or even one rivet at each end will suffice; but where a direct calculable stress comes on the piece, and only two rivets at each end are used, it will be found that they will work loose, while, if three are used, they will not, unless they be overstrained by the calculated stress on the piece.

Principle XXXI.

Designs must invariably be made so that all metal-work after erection shall be accessible to the paint-brush, except, of course, those surfaces which are in close contact either with each other or with the masonry.

This clause very properly cuts out the use of closed columns, which are a fruitful source of condemnation of old bridges.

Principle XXXII.

In multiple-track structures, if any bracing-frames be used between panel points to connect the longitudinal girders of adjoining tracks, they must be designed without diagonals, in order to prevent the transference of any appreciable portion of the live load from one pair of girders to any other pair of same.