DAMAGE TO BRIDGES DURING EARTHQUAKES 647 



the general rule. The bridge lies across the rift line of California at 

 a very acute angle and it is probable that the large shiftings on this line 

 have here played an important role in producing the damage.^ The 

 pulling of the girder out of its seat on the south abutment apparently 

 indicates a widening of the distance between banks during the earth- 

 quake, and this would appear to show that the shear on the rift 

 plane is here in a contrary sense to that which has generally obtained 

 at other places. 



Through inquiry of the engineering department of the Southern 

 Pacific Railway Company it was found that in addition to the Pajaro 

 bridge above mentioned sHght damage was sustained in other sections 

 of the road. Some few trestles were thrown out of line and "two 

 small drawbridges were affected slightly by the movement of the 

 landing piers toward the center piers — just enough in each case to 

 bind the bridge."^ On the Atchison, Topeka and Santa Fe Railway 

 there was a Bascule bridge in which the two leaves approached 

 each other so closely, as a result of the disturbance, that the bridge was 

 rendered inoperative until changes were made. This gradual ap- 

 proach of the two abutments has continued in the absence of sensible 

 shocks during the succeeding two years. ^ 



In the report of the subcommittee on railway structures to the 

 General Committee of the American Society of Civil Engineers'^ 

 it is stated that embankments crossing marshy ground generally 

 sank (sometimes as much as eleven feet), and that trestles in soft 

 material moved or were thrown down. Railway drawbridges across 

 little creeks and inlets about the Bay of San Francisco "were affected 

 by a slight movement of their piers, in many cases resulting in the 

 bridge binding so that it could not be opened until some repairs were 

 made." On the CaHfornia and Northwestern Railway the bridge at 

 Bohemia and the one over the Russian River at Healdsburg were 



» Loc. cit., p. 711. 



' Letter from J. H. Wallace, Assistant Chief Engineer, Southern Pacific Company. 



3 Letter from H. C. Phillips, Chief Engineer, A. T. and S. F. Ry. Co. 



4 "The Effects of the San Francisco Earthquake of April 18, 1906, on Engineer- 

 ing Constructions," Report of a general committee and six special committees of the 

 San Francisco Association of Members of the American Society of Civil Engineers 

 (with discussions by twenty-three others, Ed.), Trans. Am. Soc. Civ. Eng., Vol. LIX, 

 (December, 1907), pp. 208-329. 



