is, 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: 14 
Character of test. 
Specimen. 
Rate of 
fiber strain 
per minute. 
Bending tests 
Timber of structural size 
Inches. 
0. 0007 
Do 
0.0016 
0. 0015 
Compression parallel to grain 
Timber of structural size 
Do 
0. 0030 
Compression perpendicular to grain 
Timber of structural size __ 
0. 0070 
Do 
Small test specimen 
0. 0150 
Shearing parallel to grain 
do 
0. 0160 
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 15 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 16 gives 8 for steady stress, 
10 for varying stress, and 15 for shocks ; Rankine 17 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 
14 Instructions to Engineers of Timber Tests, Circular (revised) U. S. 
Forest Service 38 (1909). 
16 Johnson, J. B., Materials of Construction. New York, John Wiley and 
Son (1912). 
16 Merriman’s Mechanics of Materials, 468. 
1T Rankine’s Handbook of Civil Engineering. 
