13 



Table IV is a table of safe unit stresses * of the various kinds for the 

 materials employed in the construction of timber trestles. The "safe 

 unit stress' 1 is equal to the ultimate strength, as determined from the 

 test, divided by a quantity which is called the factor of safety. 



SAFE UNIT STRESSES. 



Table IV. — Safe unit stresses at 18 per cent moisture. 



[The values marked "D" were obtained from experiments made by the Forestry Division. Theother 

 values were obtained from various sources, chiefly the Tenth Census report, but so modified as to give 

 results comparable with Forestry Division values. To arrive at true average values of strength, 

 multiply safe loads by factor of safety given in each column. The values for resilience and tensile 

 strength are the ultimate values. The former is practically never used in designing. The latter is 

 a factor impossible to develop in practice, since the piece will always fail in some other way, usually 

 by shearing. (See descriptive text.)] 



In designing add one-half inch to each dimension obtained by use of 

 above table to allow for weathering. 



The values marked "D" in the table were obtained from tests made 

 by the Forestry Division and are considered quite reliable, especially 

 those for longleaf and shortleaf pine. These are, as indicated above, 

 for a moisture of 18 per cent, representing a half dry condition, and 

 were taken from a minimum moisture curve \ which represented the 

 average strength of the lowest 10 per cent of the nondefective pieces 

 tested. This curve gives values from 15 to 20 per cent less than the 

 mean values obtained for the species, and material of this strength can 

 readily be obtained even for full-sized beams and columns by an inspector 

 of average intelligence. 



The other values were obtained from various sources, chief of which 

 was the Tenth Census report. The} T were not taken as there given, 

 however, but modified in the following manner to make them compara- 

 ble with the Forestry Division values: The mean of the values given in 

 the census report for the " D" timbers in Table IV were compared with 

 the mean of these same values there given; the ratio of these two was 



* For a description of these stresses see page 29. 

 fSee Bulletin 8. 



