gJQ&j. 



FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



The great variation in strength which is noticeable in timber of the same species makes it necessary to accept 

 with caution the result of a limited number of tests as representing the average for the species, for it may have 

 happened that only all superior or all inferior material has been used in the tests. Hence we would not be entitled 

 to conclude, for instance, that pignut hickory is 14 per cent stronger than shagbark, as it would appear in the table, 

 for the 30 test pieces of the former may easily have been superior material. Only a detailed examination of the test 

 pieces or a fuller series of tests would enlighten us as to the comparative value of the results. 



The following data, therefore, are not to be considered as in any sense final values for the species, except where 

 the number of trees and tests is very large : 



Bes nits of tests in compression endwise, 

 [Pounds per square inch.] 



No. 



5 

 6 



7 



8 



9 



10 



11 



13 

 14 

 15 

 16 

 17 

 18 

 19 

 20 

 21 

 22 



24 

 25 

 26 

 27 

 28 

 29 

 30 

 31 

 32 



Species. 



JReduced to 1J pei cent moisture 



Longleaf pine 



Cuban pine 



Shortleaf pme 



Loblolly pine 



Reduced to 12 per cent moisture 



"White pine 



Bed pine 



Spruce pine 



Bald cypress 



White cedar 



Douglas spruce a 



White oak 



Overcup oak 



Post oak 



Co w oak 



Bed oak 



Texan oak 



Yellow oak 



"Water oak 



Willow oak 



Spanish oak 



S hagbark bi< kory 



Mo( kernut hickory 



Water hickory 



Bitternut hickory 



Nutmeg hickory 



Pecan hickory..". 



Pignut hickory 



White elm 



Cedar elm 



White asli , 



Green ash 



Sweet ffum 



Number 



oi tests. 



1,230 

 410 

 330 

 660 



130 



100 



170 



655 



87 



41 



218 



216 



49 



256 



57 



117 



40 



31 



153 



251 



1J7 



75 



14 



25 



72 



37 



30 



18 



44 



87 



10 



118 



Highest Lowest 

 single test, single test. 



11,900 



10, 600 



8,500 



11, 200 



8,500 



8, 200 



10, 000 



9,900 



6, 200 



8, 900 



12, 500 



9,100 



8,200 



11, 500 



9,700 



11, 300 



8, 600 



9 200 



11,000 



10, 600 



13,700 



12,200 



10,000 



11, 500 



12, 300 



10, 500 



13, 000 



8,800 



10, 600 



9,000 



0, 800 



8,900 



3,400 

 2,800 

 4,500 

 3,900 



3, 200 

 4, 300 

 4,400 

 2, 900 

 3, 200 

 4,100 

 5, 100 

 3, 700 

 5,900 

 4,600 

 5,400 

 5 800 



5, 500 



6, 200 

 4,200 

 3,700 

 5, 800 

 6, 200 

 6, 700 



7, 300 

 6,400 



5, 800 

 8, 700 

 4,900 

 6,200 

 5, 000 



6, 600 

 4,000 



Average 

 hi» best 10 

 pci cent 

 of tests. 



8,600 

 9,500 

 7,600 

 8,700 



6,800 

 8,100 

 8, 800 

 8, 500 

 6,000 

 8,100 

 11, 300 



8, 600 

 8, 100 

 9, 800 

 9,200 

 9,800 

 8,300 

 9, 000 

 8,700 



9, 500 

 10, 900 

 11, 600 



9, 600 



11, 200 

 11,000 

 10, 400 



12, 700 

 8,800 



10, 100 

 8, 700 

 9, 800 

 8, 500 



Average 

 lowest 10 

 per cent 

 ol tests. 



5,700 

 6, 500 

 4, 800 

 5,400 



4,000 

 4,900 



5, 600 

 4,200 

 4,400 



4, 200 



6, 300 

 6, 000 

 6,000 

 5, 600 

 5,500 

 6,900 

 5, 800 

 6,300 

 5, 500 

 5, 100 



7, 500 

 8,000 

 7,000 

 7,800 

 7,100 

 7, 300 

 8, 900 

 5, 000 

 6,500 

 5, 700 

 6, 000 



5, 600 



Average 

 of all tests. 



6,900 

 7,900 

 5, 900 

 6,500 



5,400 

 6, 700 

 7, 300 



6, 000 

 5, 200 

 5,700 

 8, 500 

 7,300 

 7,100 



7, 400 

 7,200 

 8,100 

 7,300 

 7,800 



7, 200 

 7,700 

 9,500 



10, 100 



8, 400 



9, 600 

 8, 800 

 9, 100 



10, 900 

 6, 500 

 8,000 



7. 200 



8, 000 

 7,100 



Proportion 



of tests 

 wit hm 10 

 per cent of 



average. 



Per cent. 

 53 

 61 



47 

 49 



49 

 54 

 GG 

 31 



79 

 28 

 40 

 70 

 58 

 51 

 36 

 62 

 58 

 75 

 51 

 61 

 79 

 G> 

 71 

 00 

 79 

 51 

 72 

 28 

 60 

 48 

 29 

 60 



Proportion 



of tests 



within 25 



per cent of 



average. 



Per cent 



90 

 93 

 90 

 84 



93 

 96 

 95 

 74 

 99 

 65 

 81 

 95 

 100 

 89 

 94 

 98 



100 

 88 

 94 

 97 

 99 



100 



100 

 97 

 95 



100 

 88 

 95 

 96 



100 

 97 



a Actual tests on "dry" material not reduced lor moisture. 



The variation in strength in wood of the virgin forest, as will be seen from the tables, is in some species so 

 great that by proper inspection and selection values differing by 25 to 50 per cent may be obtained from different 

 parts of the same tree, and values differing 100 to 200 per cent within the same species. These different es have all 

 their definite recognizable causes, to find and formulate which is the final aim of these investigations. 



The tests are intentionally not made on selected material (except to discard absolutely defective pieces), but on 

 material as it comes from the trees, so as to arrive at an average statement for the species, when a sufficient number 

 of trees has been tested. How urgent is the need for data of inspection as above indicated will appear from the 

 wide range of results lecorded. 



To enable any engineer to use the data here given with due caution and judgment, not only the ranges of values 

 and the average of all values obtained, but also the proportion of tests which came near the average values, have 

 been stated, as well as the average results of the highest and lowest values of 10 per cent of the tests. With this 

 information and a statement of the actual number of tests involved, the comparative merit of the stated values can 

 be judged. With a large number of tests, to be sure, it is more likely that an average value of the species has been 

 found. The actual test results have been rounded off to even hundreds in the tables. 



FACTORS OF SAFE1Y. 



With such lowest standard values, also lowest factors of safety could be employed. As to factors of safety, it 

 may be proper to state that the final aims of the present investigations may be summed up in one proposition, 

 namely, to establish rational factors of safety. It will be admitted by all engineers that the factors of safety as used 

 at present can hardly be claimed to be more than guesswork. There is not an engineer who could give account as 

 to the basis upon which numerically the factors of safety for wood have been established as "8 for steady stress; 

 10 for varying stress; 15 for shocks" (see Merriman's Testbook on the Mechanics of Materials); or as 4 to 5 for 

 "dead" load and 5 to 10 for "live" load (see Rankine's Handbook of Civil Engineering). 



