ECONOMICAL DESIGNING OF TIMBER TRESTLE BRIDGES. 



INTRODUCTION. 



There are in the United States at least 2,000 miles of timber trestle, 

 representing - an expenditure of more than $60,000,000. These have 

 to be entirely replaced every nine years, on the average, making- an 

 annual expenditure of about $7,000,000, which, capitalized at <f per 

 cent, gives an invested capital of $175,000,000 necessary to maintain 

 these structures, consuming annually about 260,000,000 feet, B. M., of 

 timber, nearly all of it being in large sizes, very valuable for other 

 purposes. 



Any economy effected, upon however small a margin it is based, is 

 therefore, when multiplied so enormously, of great benefit not only to 

 the railroad companies but to our national forestry interests. 



The capital invested in timber structures is greatly in excess (proba- 

 bly twice as much) of that invested in structures of iron and steel. 

 Every piece in these latter structures is thoroughl} T inspected, both 

 chemically and physically, and carefulty designed to carry the imposed 

 load. Timber structures, on the other hand, have been designed 

 according to the general principal that the Lord takes care of His own, 

 as the great number of fatalities resulting from failures of these struc- 

 tures will attest. Experience is not only a dear teacher, hut requires 

 endless time to tell what he knows. That this time has not yet expired 

 is evidenced by the anachronisms of design existing in the present 

 practice. 



It is a well-known principle that a chain is no stronger than its 

 weakest link, and therefore, having one weak link, it is unnecessary 

 that the others should be any stronger. Yet there are thousands of 

 timber structures throughout this country to-day that have, in some 

 parts, barely sufficient strength to carry their working load (with a 

 factor of safety of only 1 — see table on pages 9 and 10), while other por- 

 tions of the same structure have from twenty to thirty times this strength. 

 The engineer who would design an iron structure in such a manner would 

 be the laughing stock of the most ignorant man in the profession. 



There are now about 800 miles of timber trestle bridges in this coun- 

 try so designed. What is the cause of this almost universal disregard 

 of the laws of economy!! One reason is that timber has been in the 

 past very cheap, and it was not necessary to consider sizes so carefully 

 as is done in iron and steel structures. The principal reason, probably, 



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