39 



retical calculations. Space is here too brief to enumerate all possible 

 reasons for this apparently wanton use of 12 by 12 inch lumber, but 

 the mention of a few will suffice to indicate the general trend of the 

 practical questions which the writer has in mind. A railroad corpora- 

 tion has to keep a large stock of lumber on hand for emergencies, sud- 

 den calls for new work not allowing time to obtain lumber from the 

 mills, and for other reasons. The uncertainty as to where the timber 

 is to be eventually used makes it impossible to have the timber cut to 

 specific sizes, and, again, to attempt to run a large lot of sizes would 

 be more wasteful in the end than to maintain a few stock sizes only. 

 Lumber can be bought more cheaphy by giving a general order for "the 

 run of the mill for the season " or " a cargo lot," specifying approximate 

 percentages of standard stringer size, of 12 by 12 inch stuff, 10 b}^ 10 

 inch stuff, etc., and a liberal proportion of 3 or 1 inch plank, all lengths 

 thrown in. The 12 b} r 12 inch stuff, etc., is ordered all lengths, from a 

 certain specified length up. In case of a wreck, washout, burn out, or 

 sudden call for a trestle to be completed in a stated time, it is much 

 more economical and practical to order a certain number of carloads of 

 " trestle stuff" to the ground and there to select piece after piece as fast 

 as needed, dependent only upon the length of stick required. When 

 there is time to make the necessaiy surveys of the ground and calcula- 

 tions of strength and to wait for a special bill of timber to be cut and 

 delivered, the use of different sizes for posts in a structure would be 

 warranted to a certain extent. 



The reasons presented indicate, therefore, that while it is not true 

 economy as far as the general timber consumption of the country is 

 concerned to use 12 by 12 inch sticks indiscriminately for posts, caps, 

 and sills of trestles, still, in the majority of cases, it is true practical 

 economy as far as the interest of the party who is paying for the work 

 is concerned. 



Another very important feature to consider in the designing of 

 trestles is not only to give a proper factor of safety as established from 

 timber tests, but to design those parts the failure of which would prove 

 absolutely disastrous in such a way as to give an additional guaranty 

 of safety against the racking of the structure from the unknown strains 

 and peculiar conditions resulting from the impact of fast-moving trains 

 with heavy concentrated wheel loads passing over the trestle, the cen- 

 trifugal force caused by high-speed trains upon curved trestles, the 

 lateral swaying induced by wind or the wobbling of the train back and 

 forth within the play of the gauge even on a straight track, and other 

 similar features.* 



There are also other practical points to consider, such as, for instance, 

 the possible shifting of the bearing caused by the undermining of some 

 mudsills, or the loosening or breaking of one of the posts, in which 



*The factor of safety of 5 was an average value reported by the engineers of the 

 railroad companies and is by the author considered sufficient only if applied to the 

 "minima" values of strength. — A. L. J. 



