Class No. 6 represents a very good arrangement which can be modified to suit nearly any conditions of combined traffic. A good example of this type is the author's bridge over the Missouri River at Kansas City, Mo., owned by the Union Depot, Bridge, and Terminal Railroad Company, and known as the Fratt Bridge.

In designing combined bridges of all classes except No. 1, a considerable economy of metal may be effected legitimately by keeping the total live load for trusses as low as is proper with reference to the theory of probabilities. For instance, in Class No. 2 or Class No. 5 the live load for trusses may be determined by adding to the equivalent uniform live load for the steam-railway tracks, given by the diagram in Fig. 6e of "Bridge Engineering," a much lighter highway floor load per lineal foot of span than that prescribed in the specifications; because when the greatest train load is on the bridge, the chance of having simultaneously a heavy highway live load is very small. The longer the span the smaller may the live load per square foot of floor betaken when finding the total live load for the trusses. Again, in Classes No. 3 and No. 4 it would be legitimate to take the truss live load per lineal foot for the railway equal to twice the car load per lineal foot, and add thereto a small highway live load as in the last case. Finally, in Class No. 6 in case of a four-track bridge with cantilevered highways and foot-walks, it would be proper to assume the live load for the trusses equal to the sum of the car loads per lineal foot on the four tracks and ignore entirely the vehicular and pedestrian loadings; for the greatest probable live load from all classes of loading would never exceed the said four car loads.

This reduction of live load, however, can readily be carried to extremes, as was the case in the first accepted design of the great cantilever bridge over the St. Lawrence River near Quebec, and as is likely to be the case whenever the preparation of the specifications for a bridge is left either directly or indirectly to the contractor who is to build the structure. Good judgment, uninfluenced in any way by considerations of personal gain or by motives of false economy, should rule in the establishment of the live loads for the trusses of "combined" bridges.

It is very seldom that the designing engineer of a bridge has an opportunity to economize by the manipulation of unit stresses, because ordinarily he employes standard bridge specifications; and truly it is better for him to do so, for the reason that this question has already been very thoroughly threshed out; but, as will be explained presently, in certain structures, such as trestles, he may have an opportunity to employ his judgment in the determination of unit stresses for certain unusual but possible combinations of loads.

In modern specifications the aim is to have a fixed unit stress for each kind of material for all but extraordinary combinations of loads. In metals it is generally proper to place the ordinary intensity of working stress at one-half of the elastic limit of the material; but with high-alloy steels it