648 • WM. HERBERT HOBBS 



both slightly shifted on their piers at one end. At Duncan's Mills 

 on the North Shore Railroad a combination span a hundred and 

 twenty feet long "had the eye-bars in the lower chord buckled by 

 movement of the abutments toward each other" (see Fig. 6, -B).' 



The sub-committee appointed to investigate highway structures 

 in lieu of a report offered the statement that such structures had been 

 singularly immune from damage. This meager statement is curi- 

 ously contradicted by the evidence furnished in the printed discussions. 

 The highway bridge across a creek tributary to Tomales Bay near 

 Port Reyes Station, which had eight panels or sections, had its north 

 abutment sink two to three feet and approach the south abutment 

 so much that in reconstruction the north end panel was npt used but 

 was in part replaced by an apron (see Fig. 6, C) .^ At Watsonville was 

 a bridge which according to Derleth was distorted in the same manner 

 as the Salinas bridge above described and with which it is compared, ^ 

 In each case the deformation was ascribed to "differential surface 

 movement distinct from elastic vibration." 



Two additional instances have been mentioned in the report with 

 illustrations by Galloway,^ to wit: the Alder Creek bridge north of 

 Point Arena in Mendocino County, the other the Gualala bridge 

 south of the same point. The first mentioned (Fig. 6, D) now lies 

 on the bed of the stream across the rift line, the other, a steel structure, 

 had one end of the girder dropped twenty-eight feet. These bridges 

 were not examined by the engineers, and the views afford the only 

 evidence at hand. 



KINGSTON EARTHQUAKE OF JANUARY 1 4, I907 



A very recent example which illustrates the usual deformation of 

 a bridge at the time of a destructive earthquake, has been furnished 



1 J. H. Wallace, H. C. Phillips, R. M. Drake, and E. M. Boggs, sub-committee, 

 loc. cit. p. 259, PL 53, Fig. I. 



2 H. H. Wadsworth, M. Am. Soc, loc. cit., p. 270. ; 



3 Charles Derleth, 7oc. cit., pp. 311-15. 



4 John D. Galloway, M. Am. Soc. C. E., loc. cit.. Pis. 57, 58. From a personal 

 letter from Mr. Galloway it is learned that Alder Creek enters the ocean some twenty 

 miles north of Point Arena. The Gualala River is a mountain stream which enters the 

 ocean about the same distance south of Point Arena. The Gualala bridge, which was 

 built some ten years ago, gave trouble from the fact that the ground beneath the piers 

 showed a tendency to slide so that it was necessary to put the bridge up upon a false 

 work and realign the piers. 



