62 Stability of Structures in regard to Wind-Pressure. 



between the columns is not so great as to involve a possible 

 bending of the strutt. 



The most satisfactory method would be two strutts 

 crossing, and fastened in the centre where tliey pass one 

 another. As the stresses in all the braces of the pier will 

 be the same, the proper dimensions for the diagonal braces 

 can readily be determined. 



Besides collapsing from the weakness of the bracing, a 

 pier may fail through weakness in the columns themselves; 

 for suppose that the force of the wind, without being strong 

 enough to overturn the bridge, is just sufficient to remove 

 the whole weight from the windward columns, the burden 

 must be borne by the leeward ones. In the case of a pier 

 with two columns only, the leeward column would have to 

 bear the whole weight of one span of the bridge. It would 

 appear to be necessary, then, to design columns so as to meet 

 the compressive strains and tendency to buckle, due to the 

 imposition of such a load. 



3. Instability of Superstructure. — In this case the girders 

 would fail by overturning upon their bases. The over- 

 turning moment will be the wind's force upon the surface of 

 the girder exposed, multiplied by half the height of the 

 girder. The resisting power is in the stability of each 

 girder independently, and the strength of the strutts and 

 ties fasteniug the two together. This case is very analogous 

 to that of the instability of piers, only as the overturning 

 moment is very much smaller, the means of resisting it 

 present no difficulty. The mode of attaching the girders 

 together differs according to the position of the road and the 

 girders. If the road runs through the girders and the 

 headway is limited, arched braces usually connect the tops; 

 if not, the cross-pieces are straight. When the road passes 

 over the girders they are usually fastened in the same 

 way as the columns of a pier — viz., by horizontal and 

 diagonal braces. In this case, even more than in the 

 other, diagonal strutts would appear preferable to ties, 

 giving a greater rigidity to the superstructure under a 

 passing load. 



4. Lateral Weakness of the Superstructure. — Although 

 there is not much danger of girders being actually 

 broken b}^ the wind's force, it is quite conceivable 

 that in a long span they might be put into such a state of 

 lateral vibration as would unduly strain, and even inflict 

 damage upon, the structure. 



