APPENDIX II. 



REVIEW OF THE FOREGOING PAPER BY MR. G. LINDENTHAL, * 

 CHIEF ENGINEER OF THE NORTH RIVER BRIDGE COMPANY. 



The designing of trestle structures has only in recent years been 

 undertaken by engineers; formerly it was left entirely to the practical 

 judgment of bridge carpenters, aided by a few general rules as to 

 strength and section of timbers issued to them by railroad managers. 

 Usually the sizes of timber most readily handled were employed, and 

 variation of section was avoided, as likely to lead to delay in getting 

 the material from the sawmills. Timber being cheap and labor dear 

 have sometimes led to constructions far from economical in an engineer- 

 ing sense. 



Trestle bridges on almost all railroads are regarded as temporary 

 structures, to be replaced with stone or iron structures and with filling. 

 If the financial condition of the railroad does not permit the permanent 

 work to be done before the timber structure becomes unsafe from wear 

 or rottenness, it must be replaced with a new timber structure, as being 

 the cheaper; yet it never loses its temporary character. It is true, 

 however, that much money can be saved in the correct designing of 

 trestles. 



The proposition to classify timber structures according to the mois- 

 ture they contain and to use different values for dimensioning for 

 different moisture classifications would, I think, unnecessarily compli- 

 cate the designing. It is better and simpler, as well as safer, to assume 

 that all timber going into trestles is green and that only the unit 

 stresses for green timber should be used in dimensioning it. f Accord- 

 ing to location and use, the timber will either remain green or will get 

 seasoned; in the latter case there is no harm done— the timber simply 

 gets stronger. Simplicity will always remain a valuable rule in design- 

 ing such structures. 



Insistence on selecting good timber, free of knots and wind shakes, 

 is justified, and the inspection of the timber in that regard should be 

 very rigid. 



*As explained in the letter of transmittal, the paper by Mr. A. L. Johnson, C. E., 

 was submitted to two leading bridge engineers for review, and extracts from the 

 expression of their views thus elicited are appended, believing that they will add to 

 the value of Mr. Johnson's paper and to its appreciation by the public— B. E. F. 



tThis is practically what the author of the paper has assumed. The moisture for 

 all timber in trestles has been taken at 18 percent, which is called "half-dry." The 

 increase in strength from the green to this condition is so slight as to be immaterial. 



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