The following tables bring together in condensed form the average strength values resulting from a large number of tests made by the Forest Service on the principal structural timbers of the United States. These results are more completely discussed in other publications of the Service, a list of which is given on pages 157-159.

The tests were made at the laboratories of the U.S. Forest Service, in cooperation with the following institutions: Yale Forest School, Purdue University, University of California, University of Oregon, University of Washington, University of Colorado, and University of Wisconsin.

Tables XVIII and XIX give the average results obtained from tests on green material, while Tables XX and XXI give average results from tests on air-seasoned material. The small specimens, which were invariably 2" × 2" in cross section, were free from defects such as knots, checks, and cross grain; all other specimens were representative of material secured in the open market. The relation of stresses developed in different structural forms to those developed in the small clear specimens is shown for each factor in the column headed "Ratio to 2" × 2"." Tests to determine the mechanical properties of different species are often confined to small, clear specimens. The ratios included in the tables may be applied to such results in order to approximate the strength of the species in structural sizes, and containing the defects usually encountered, when tests on such forms are not available.

A comparison of the results of tests on seasoned material with those from tests on green material shows that, without exception, the strength of the 2" × 2" specimens is increased by lowering the moisture content, but that increase in strength of other sizes is much more erratic. Some specimens, in fact, show an apparent loss in strength due to seasoning. If structural timbers are seasoned slowly, in order to avoid excessive checking, there should be an increase in their strength. In the light of these facts it is not safe to base working stresses on results secured from any but green material. For a discussion of factors of safety and safe working stresses for structural timbers see the Manual of the American Railway Engineering Association, Chicago, 1911. A table from that publication, giving working unit stresses for structural timber, is reproduced in this book, see Table XXII.

TABLE XVIII
BENDING TESTS ON GREEN MATERIAL
Species Sizes Number of tests Per cent of moisture Rings per inch F.S. at E.L. M. of R. M. of E. Calculated shear
Cross Section Span Average per sq. inch Ratio to 2" by 2" Average per sq. inchRatio to 2" by 2" Average per sq. inch Ratio to 2" by 2" Average per sq. inch Ratio to 2" by 2"
InchesIns.


Lbs.
Lbs.
1,000 lbs.
Lbs.
Longleaf pine 12 by 12 138 4 28.6 9.7 4,029 0.83 6,710 0.74 1,523 0.99 261 0.86

10 by 16 168 4 26.8 16.7 6,453 .85 6,453 .71 1,626 1.05 306 1.01

8 by 16 156 7 28.4 14.6 3,147 .64 5,439 .60 1,368 .89 390 1.29

6 by 16 132 1 40.3 21.8 4,120 .83 6,460 .71 1,190 .77 378 1.25

6 by 10 180 1 31.0 6.2 3,580 .72 6,500 .72 1,412 .92 175 .58

6 by 8 180 2 27.0 8.2 3,735 .75 5,745 .63 1,282 .83 121 .40

2 by 2 30 15 33.9 14.1 4,950 1.00 9,070 1.00 1,540 1:00 303 1.00
Douglas fir 8 by 16 180 191 31.5 11.0 3,968 .76 5,983 .72 1,517 .95 269 .81

5 by 8 180 84 30.1 10.8 3,693 .71 5,178 .63 1,533 .96 172 .52

2 by 12 180 27 35.7 20.3 3,721 .71 5,276 .64 1,642 1.03 256 .77

2 by 10 180 26 32.9 21.6 3,160 .60 4,699 .57 1,593 1.00 189 .57

2 by 8 180 29 33.6 17.6 3,593 .69 5,352 .65 1,607 1.01 171 .51

2 by 2 24 568 30.4 11.6 5,227 1.00 9,070 1.00 1,540 1.00 303 1.00
Douglas fir (fire-killed) 8 by 16 180 30 36.8 10.9 3,503 .80 4,994 .64 1,531 .94 330 1.19

2 by 12 180 32 34.2 17.7 3,489 .80 5,085 .66 1,624 .99 247 .89

2 by 10 180 32 38.9 18.1 3,851 .88 5,359 .69 1,716 1.05 216 .78

2 by 8 180 31 37.0 15.7 3,403 .78 5,305 .68 1,676 1.02 169 .61

2 by 2 30 290 33.2 17.2 4,360 1.00 7,752 1.00 1,636 1.00 277 1.00
Shortleaf pine 8 by 16 180 12 39.5 12.1 3,185 .73 5,407 .70 1,438 1.03 362 1.40

8 by 14 180 12 45.8 12.7 3,234 .74 5,781 .75 1,494 1.07 338 1.31

8 by 12 180 24 52.2 11.8 3,265 .75 5,503 .71 1,480 1.06 277 1.07

5 by 8 180 24 47.8 11.5 3,519 .81 5,732 .74 1,485 1.06 185 .72

2 by 2 30 254 51.7 13.6 4,350 1.00 7,710 1.00 1,395 1.00 258 1.00
Western larch 8 by 16 180 32 51.0 25.3 3,276 .77 4,632 .64 1,272 .97 298 1.11

8 by 12 180 30 50.3 23.2 3,376 .79 5,286 .73 1,331 1.02 254 .94

5 by 8 180 14 56.0 25.6 3,528 .83 5,331 .74 1,432 1.09 169 .63

2 by 2 28 189 46.2 26.2 4,274 1.00 7,251 1.00 1,310 1.00 269 1.00
Loblolly pine 8 by 16 180 17 15.8 6.1 3,094 .75 5,394 .69 1,406 .98 383 1.44

5 by 12 180 94 60.9 5.9 3,030 .74 5,028 .64 1,383 .96 221 .83

2 by 2 30 44 70.9 5.4 4,100 1.00 7,870 1.00 1,440 1.00 265 1.00
Tamarack 6 by 12 162 15 57.6 16.6 2,914 .75 4,500 .66 1,202 1.05 255 1.11

4 by 10 162 15 43.5 11.4 2,712 .70 4,611 .68 1,238 1.08 209 .91

2 by 2 30 82 38.8 14.0 3,875 1.00 6,820 1.00 1,141 1.00 229 1.00
Western hemlock 8 by 16 180 39 42.5 15.6 3,516 .80 5,296 .73 1,445 1.01 261 .92

2 by 2 28 52 51.8 12.1 4.406 1.00 7,294 1.00 1,428 1.00 284 1.00
Redwood 8 by 16 180 14 86.5 19.9 3,734 .79 4,492 .64 1,016 .96 300 1.21

6 by 12 180 14 87.3 17.8 3,787 .80 4,451 .64 1,068 1.00 224 .90

7 by 9 180 14 79.8 16.7 4,412 .93 5,279 .76 1,324 1.25 199 .80

3 by 14 180 13 86.1 23.7 3,506 .74 4,364 .62 947 .89 255 1.03

2 by 12 180 12 70.9 18.6 3,100 .65 3,753 .54 1,052 .99 187 .75

2 by 10 180 13 55.8 20.0 3,285 .69 4,079 .58 1,107 1.04 169 .68

2 by 8 180 13 63.8 21.5 2,989 .63 4,063 .58 1,141 1.08 134 .54

2 by 2 28 157 75.5 19.1 4,750 1.00 6,980 1.00 1,061 1.00 248 1.00
Norway pine 6 by 12 162 15 50.3 12.5 2,305 .82 3,572 .69 987 1.03 201 1.17

4 by 12 162 18 47.9 14.7 2,648 .94 4,107 .79 1,255 1.31 238 1.38

4 by 10 162 16 45.7 13.3 2,674 .95 4,205 .81 1,306 1.36 198 1.15

2 by 2 30 133 32.3 11.4 2,808 1.00 5,173 1.00 960 1.00 172 1.00
Red spruce 2 by 10 144 14 32.5 21.9 2,394 .66 3,566 .60 1,180 1.02 181 .80

2 by 2 26 60 37.3 21.3 3,627 1.00 5,900 1.00 1,157 1.00 227 1.00
White spruce 2 by 10 144 16 40.7 9.3 2,239 .72 3,288 .63 1,081 1.08 166 .83

2 by 2 26 83 58.3 10.2 3.090 1.00 5,185 1.00 998 1.00 199 1.00
Note. - Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.
Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.
TABLE XIX
COMPRESSION AND SHEAR TESTS ON GREEN MATERIAL
Species Compression parallel to grain Compression perpendicular to grain Shear
Size of specimen No. of tests Per cent of moisture Cr. str. at E. L. per square inch M. of E. per square inch Cr. str. at max. ld., per square inch Stress area Height No. of tests Per cent of moisture Cr. str. at max. ld., per square inch No. of tests Per cent of moisture Shear strength per square inch
Inches

Lbs.1,000 lbs.Lbs.InchesInches

Lbs.

Lbs.
Longleaf pine 4 by 4 46 26.3 3,480
4,800 4 by 4 4 22 25.3 568 44 21.8 973

2 by 2 14 34.7

4,400







Douglas fir 6 by 6 515 30.7 2,780 1,181 3,500 4 by 8 16 259 30.3 570 531 29.7 765

5 by 6 170 30.9 2,720 2,123 3,490








2 by 2 902 29.8 3,500 1,925 4,030







Douglas fir (fire-killed) 6 by 6 108 34.8 2,620 1,801 3,290 6 by 8 16 24 33.7 368 77 35.8 631

2 by 2 204 37.9

3,430







Shortleaf pine 6 by 6 95 41.2 2,514 1,565 3,436 5 by 8 16 12 37.7 361 179 47.0 704

5 by 8 23 43.5 2,241 1,529 3,423 5 by 8 14 12 42.8 366



2 by 2 281 51.4

3,570 5 by 8 12 24 53.0 325









5 by 5 8 24 47.0 344









2 by 2 2 277 48.5 400


Western larch 6 by 6 107 49.1 2,675 1,575 3,510 6 by 8 16 22 43.6 417 179 40.7 700

2 by 2 491 50.6 3,026 1,545 3,696 6 by 8 12 20 40.2 416









4 by 6 6 53 52.8 478









4 by 4 4 30 50.4 472


Loblolly pine 8 by 8 14 63.4 1,560 365 2,140 8 by 4 8 16 67.2 392 121 83.2 630

4 by 8 18 60.0 2,430 691 3,560 4 by 4 8 38 44.6 546



2 by 2 53 74.0

3,240







Tamarack 6 by 7 4 49.9 2,332 1,432 3,032




24 39.2 668

4 by 7 6 27.7 2,444 1,334 3,360








2 by 2 165 36.8

3,190







Western hemlock 6 by 6 82 46.6 2,905 1,617 3,355 6 by 4 6 30 48.7 434 54 65.7 630

2 by 2 131 55.6 2,938 1,737 3,392







Redwood 6 by 6 34 83.6 3,194 1,240 3,882 6 by 8 16 13 86.7 473 148 84.2 742

2 by 2 143 36.8 3,490 1,222 3,980 6 by 6 12 14 83.0 424









6 by 7 9 13 74.7 477









6 by 3 14 13 75.6 411









6 by 2 12 12 66.5 430









6 by 2 10 11 55.0 423









6 by 2 8 12 56.7 396









2 by 2 2 186 75.5 569


Norway pine 6 by 7 5 29.0 1,928 905 2,404




20 26.7 589

4 by 7 8 28.4 2,154 1,063 2,652








2 by 2 178 26.8

2,504







Red spruce 2 by 2 58 35.4

2,750 2 by 2 2 43 31.8 310 30 32.0 758
White spruce 2 by 2 84 61.0

2,370 2 by 2 2 46 50.4 270 40 58.0 651
Note. - Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.
Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.
TABLE XX
BENDING TESTS ON AIR-SEASONED MATERIAL
Species Sizes Number of tests Per cent of moisture Rings per inch F.S. at E.L. M. of R. M. of E. Calculated shear
Cross Section Span Average per sq. inch Ratio to 2" by 2" Average per sq. inch Ratio to 2" by 2" Average per sq. inch Ratio to 2" by 2" Average per sq. inch Ratio to 2" by 2"
InchesIns.


Lbs.
Lbs.
 1,000 lbs.
Lbs.
Longleaf pine 8 by 16 180 5 22.2 16.0 3,390 0.50 4,274 0.37 1,747 1.00 288 0.75

6 by 16 132 1 23.4 17.1 3,470 .51 6,610 .57 1,501 .86 388 1.01

6 by 10 177 2 19.0 8.8 4,560 .68 7,880 .68 1,722 .99 214 .56

4 by 11 180 1 18.4 23.9 3,078 .46 8,000 .69 1,660 .95 251 .66

6 by 8 177 6 20.0 13.7 4,227 .63 8,196 .71 1,634 .94 177 .46

2 by 2 30 17 15.9 13.9 6,750 1.00 11,520 1.00 1,740 1.00 383 1.00
Douglas fir 8 by 16 180 91 20.8 13.1 4,563 .68 6,372 .61 1,549 .91 269 .64

5 by 8 180 30 14.9 12.2 5,065 .76 6,777 .65 1,853 1.09 218 .52

2 by 2 24 211 19.0 16.4 6,686 1.00 10,378 1.00 1,695 1.00 419 1.00
Shortleaf pine 8 by 16 180 3 17.0 12.3 4,220 .54 6,030 .50 1,517 .85 398 .98

8 by 14 180 3 16.0 12.3 4,253 .55 5,347 .44 1,757 .98 307 .76

8 by 12 180 7 16.0 12.4 5,051 .65 7,331 .60 1,803 1.01 361 .89

5 by 8 180 6 12.2 22.5 7,123 .92 9,373 .77 1,985 1.11 301 .74

2 by 2 30 67 14.2 13.7 7,780 1.00 12,120 1.00 1,792 1.00 404 1.00
Western larch 8 by 16 180 23 18.3 21.9 3,343 .57 5,440 .53 1,409 .90 349 .96

8 by 12 180 29 17.8 23.4 3,631 .62 6,186 .60 1,549 .99 295 .81

5 by 8 180 10 13.6 27.6 4,730 .80 7,258 .71 1,620 1.04 221 .61

2 by 2 30 240 16.1 26.8 5,880 1.00 10,254 1.00 1,564 1.00 364 1.00
Loblolly pine 8 by 16 180 14 20.5 7.4 4,195 .81 6,734 .72 1,619 1.10 462 1.45

6 by 16 126 4 20.2 5.0 2,432 .47 4,295 .46 1,324 .90 266 .84

6 by 10 174 3 21.3 4.7 3,100 .60 6,167 .66 1,449 .99 173 .54

4 by 12 174 4 19.8 4.7 2,713 .52 5,745 .61 1,249 .85 185 .58

8 by 8 180 9 22.9 4.9 2,903 .56 4,557 .48 1,136 .77 93 .29

6 by 7 144 2 21.1 5.0 2,990 .58 4,968 .53 1,286 .88 116 .36

4 by 8 132 8 19.5 9.1 3,384 .65 6,194 .66 1,200 .82 196 .62

2 by 2 30 123 17.6 6.6 5,170 1.00 9,400 1.00 1,467 1.00 318 1.00
Tamarack 6 by 12 162 5 23.0 15.1 3,434 .45 5,640 .43 1,330 .82 318 .75

4 by 10 162 4 14.4 9.7 4,100 .54 5,320 .41 1,386 .84 252 .59

2 by 2 30 47 11.3 16.2 7,630 1.00 13,080 1.00 1,620 1.00 425 1.00
Western hemlock 8 by 16 180 44 17.7 17.8 4,398 .69 6,420 .62 1,737 1.04 406 1.06

2 by 2 28 311 17.9 19.4 6,333 1.00 10,369 1.00 1,666 1.00 382 1.00
Redwood 8 by 16 180 6 26.3 22.4 3,797 .79 4,428 .57 1,107 .96 294 1.05

6 by 12 180 6 16.1 17.7 3,175 .66 3,353 .43 728 .64 167 .60

7 by 9 180 6 15.9 15.2 3,280 .69 4,002 .51 1,104 .96 147 .53

3 by 14 180 6 13.1 24.4

5,033 .64

291 1.04

2 by 12 180 5 13.8 14.4 3,928 .82 5,336 .68 1,249 1.09 260 .93

2 by 10 180 5 13.8 24.8 3,757 .79 4,606 .59 1,198 1.05 186 .67

2 by 8 180 6 13.7 20.7 4,314 .90 5,050 .65 1,313 1.15 166 .60

2 by 2 28 122 15.2 18.8 4,777 1.00 7,798 1.00 1,146 1.00 279 1.00
Norway pine 6 by 12 162 5 16.7 8.1 2,968 .56 5,204 .61 1,123 .97 286 1.02

4 by 10 162 5 13.7 12.0 5,170 .98 6,904 .82 1,712 1.48 317 1.13

2 by 2 30 60 14.9 11.2 5,280 1.00 8,470 1.00 1,158 1.00 281 1.00
Note. - Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.
Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.
TABLE XXI
COMPRESSION AND SHEAR TESTS ON AIR-SEASONED MATERIAL
Species Compression parallel to grain Compression perpendicular to grain Shear
Size of specimen No. of tests Per cent of moisture Cr. str. at E. L. per square inch M. of E. per square inch Cr. str. at max. ld., per square inch Stress area Height No. of tests Per cent of moisture Cr. str. at max. ld., per square inch No. of tests Per cent of moisture Shear strength
Inches

Lbs. 1,000 lbs.Lbs.InchesInches

Lbs.

Lbs.
Longleaf pine 4 by 5 46 26.3 3,480
4,800 4 by 5 4 22 25.1 572 52 20.2 984
Douglas fir 6 by 6 259 20.3 3,271 1,038 4,258 4 by 8 16 44 20.8 732 465 22.1 822

2 by 2 247 18.7 3,842 1,084 5,002 4 by 8 10 32 18.1 584









4 by 4 8 51 20.2 638









4 by 4 6 49 24.0 613









4 by 4 4 29 24.8 603


Shortleaf pine 6 by 6 29 15.7 4,070 1,951 6,030 8 by 5 16 4 17.8 725 85
1,135

2 by 2 57 14.2

6,380 8 by 5 14 3 16.3 757









8 by 5 12 5 15.1 730









5 by 5 8 6 13.0 918









2 by 2 2 57 13.9 926


Western larch 6 by 6 112 16.0

5,445 8 by 6 16 17 18.8 491 193 15.0 905

4 by 4 81 14.7

6,161 8 by 6 12 18 17.6 526



2 by 2 270 14.8

5,934 5 by 4 8 22 13.3 735


Loblolly pine 6 by 6 23
3,357 1,693 5,005 8 by 5 16 12 19.8 602 156 11.3 1,115

5 by 5 10 22.4 2,217 545 2,950 8 by 5 8 7 22.9 679



4 by 8 8 19.4 3,010 633 3,920 4 by 5 8 8 19.5 715



2 by 2 69


5,547







Tamarack 6 by 7 3 15.7 2,257 1,042 3,323 2 by 2 2 57 16.2 697 60 14.0 879

4 by 7 3 13.6 3,780 1,301 4,823








4 by 4 57 14.9 3,386 1,353 4,346








2 by 2 66 14.6

4,790







Western hemlock 6 by 6 102 18.6 4,840 2,140 5,814 7 by 6 15 25 18.2 514 131 17.7 924

2 by 2 463 17.0 4,560 1,923 5,403 6 by 6 6 26 16.8 431









4 by 4 4 6 15.9 488


Redwood 6 by 6 18 16.9

4,276 8 by 6 16 5 25.4 548 95 12.4 671

2 by 2 115 14.6

5,119 6 by 6 12 6 14.7 610









7 by 6 9 5 14.8 500









3 by 6 14 2 12.6 470









2 by 6 12 2 16.2 498









2 by 6 10 4 14.3 511









2 by 6 8 2 13.2 429









2 by 2 2 145 13.8 564


Norway pine 6 by 7 4 15.2 2,670 1,182 4,212 2 by 2 2 36 10.0 924 44 11.9 1,145

4 by 7 2 22.2 3,275 1,724 4,575








4 by 4 55 16.6 3,048 1,367 4,217








2 by 2 44 11.2

7,550







Note. - Following is an explanation of the abbreviations used in the foregoing tables:
F.S. at E.L. = Fiber stress at elastic limit.
M. of E. = Modulus of elasticity.
M. of R. = Modulus of rupture.
Cr. str. at E.L. = Crushing strength at elastic limit.
Cr. str. at max. ld. = Crushing strength at maximum load.
TABLE XXII
[b]WORKING UNIT-STRESSES FOR STRUCTURAL TIMBER[c]
EXPRESSED IN POUNDS PER SQUARE INCH
(From Manual of the American Railway Engineering Assn., 1911, p. 153)
NOTE. - The working unit-stresses given in the table are intended for railroad bridges and trestles. For highway bridges and trestles the unit-stresses may be increased twenty-five (25) per cent. For buildings and similar structures, in which the timber is protected from the weather and practically free from impact, the unit-stresses may be increased fifty (50) per cent. To compute the deflection of a beam under long-continued loading instead of that when the load is first applied, only fifty (50) per cent of the corresponding modulus of elasticity given in the table is to be employed.
KIND OF TIMBER BENDING SHEARING COMPRESSION Ratio of length of stringer to depth
Extreme fibre stress Modulus of elasticity Parallel to the grain Longitudinal shear in beams Perpendicular to the grain Parallel to the grain For columns under 15 diameters working stress Formulæ for working stress in long columns over 15 diameters
Average ultimate Working stress Average Average ultimate Working stress Average ultimate Working stress Elastic limit Working stress Average ultimate Working stress
Douglas fir 6100 1200 1,510,000 690 170 270 110 630 310 3600 1200 900 1200 (1 - l / 60 d) 10
Longleaf pine 6500 1300 1,610,000 720 180 300 120 520 260 3800 1300 980 1300 (1 - l / 60 d) 10
Shortleaf pine 5600 1100 1,480,000 710 170 330 130 340 170 3400 1100 830 1100 (1 - l / 60 d) 10
White pine 4400 900 1,130,000 400 100 180 70 290 150 3000 1000 750 1000 (1 - l / 60 d) 10
Spruce 4800 1000 1,310,000 600 150 170 70 370 180 3200 1100 830 1100 (1 - l / 60 d)
Norway pine 4200 800 1,190,000 590[d] 130 250 100
150 2600[d] 800 600 800 (1 - l / 60 d)
Tamarack 4600 900 1,220,000 670 170 260 100
220 3200[d] 1000 750 1000 (1 - l / 60 d)
Western hemlock 5800 1100 1,480,000 630 160 270[d] 100 440 220 3500 1200 900 1200 (1 - l / 60 d)
Redwood 5000 900 800,000 300 80

400 150 3300 900 680 900 (1 - l / 60 d )
Bald cypress 4800 900 1,150,000 500 120

340 170 3900 1100 830 1100 (1 - l / 60 d)
Red cedar 4200 800 800,000



470 230 2800 900 680 900 (1 - l / 60 d)
White oak 5700 1100 1,150,000 840 210 270 110 920 450 3500 1300 980 1300 (1 - l / 60 d) 12
These unit-stresses are for a green condition of timber and are to be used without increasing the live load stresses for impact. l = Length in inches.
d = Least side in inches.
[Footnote b: Adopted, Vol. 1909, pp. 537, 564, 609-611.]
[Footnote c: Green timber in exposed work.]
[Footnote d: Partially air-dry]