18, 5 Valencia: Commercial Philippine Timbers 507 
so to proportion the speed of the moving head of the testing 
machine to the dimensions of the specimen that the resultant 
fiber strain will be a certain specified amount. The usual prac- 
tice is to adhere to the standards established by the Forest Serv- 
ice for the use of engineers of timber tests. These standards 
are as follows :'* 
Rate of 
Character of test. Specimen, fiber strain 
per minute. 
Inches. 
i ecy nce geek ES Me em Timber of structural size________. 0. 0007 
PO viento sunt -« Gem ieee, . sch see Small test specimen ______________ 0.0015 
‘ Compression parallel tograin__............_. Timber of structural size________. 0.0015 
WO io wnnlasnehe oii a hte SS pate Gri er on Sau Small test specimen _____________. 0. 0080 
Compression perpendicular to grain.__......______ Timber of structural size_._______ 0.0070 
WO. Siabined in deetek ca pe ee Small test specimen ______________ 0.0150 
Shearing parallel to grain.__.._..............._._.|___. bos fs de Si ok ONS a ar OEY Cee 0.0150 
When constant loads, amounting to a large fraction of the 
ultimate strength of timber, are sustained for very long periods, 
the deformation may continue to increase until rupture occurs, 
even though the stress encountered is far below the ultimate 
Strength of the timber as originally determined. 
Johnson * says that the strength of timber under any kind 
of permanent load is only about one-half its strength as found 
by actual, short-time tests. 
Factors of safety.—The factors of safety used in the design 
of timber structures in the United States, as given by different 
authorities, are as follows: Merriman * gives 8 for steady stress, 
10 for varying stress, and 15 for shocks; Rankine *’ gives 4 to 
5 for dead load and 5 to 10 for live load; the Cambria Steel 
Co. Handbook gives 10 for tension, 6 for extreme fiber stress 
in bending, 5 for compression along the grain, and 4 for com- 
Pression across the grain and for shear; and the (1909) Com-, 
mittee on Wooden Bridges and Trestles of the American 
Railway Engineering Association used the following factors of 
“Instructions to Engineers of Timber Tests, Circular (revised) U. S. 
Forest Service 38 (1909). 
“Johnson, J. B., Materials of Construction. New York, John Wiley and 
Son ( 1912). : 
* Merriman’s Mechanics of Materials, 468. 
“Rankine’s Handbook of Civil Engineering. 
