STIFFNESS PHYSICAL PROPERTIES OF WOOD Stiffness is the resistance which a stick offers to a force that tends to change its shape. The stiffness of a stick of wood varies directly with the cube of its thickness, and inversely with the cube of its length. In other words, doubling the length of a stick makes it only one-eighth as stiff as previously; doubling the thickness makes it eight times as stiff as before.
Timber testing engineers express the stiffness of wood by what is called the "modulus of elasticity," which is stated in 1,000 pounds per square inch. The modulus of elasticity for the principal woods tested in a green condition is as indicated in Table 6.
The softwoods are nearly as stiff as the hardwoods, and, in comparison with their weights, much stiffer than the hardwoods. For example, Western red cedar, with a specific gravity Modulus of Elasticity of Various Woods (Wood Tested in Green Condition; Modulus Given in Thousands of Pounds per Square Inch) Softwoods Cedar, Northern White 643 Fir, White 1,131 Cedar, Incense 754 Spruce, Red 1,179 Spruce, Englemann ... 832 Tamarack 1,236 Fir, Alpine 861 Douglas Fir 1,242 Cedar, Western Red... 886 Larch, Western 1,310 Hemlock, Black 936 Fir, Grand 1,311 Pine, Sugar 966 Fir, Amabilis 1,323 Spruce, White 968 Pine, Table Mountain.. 1,329 Pine, Western Yellow 977 Cypress 1,378 Pine, Lodgepole 993 Pine, Norway 1,384 Redwood 1,062 Pine, Shortleaf 1,395 Pine, White 1,073 Hemlock, Western .... 1,428 Hemlock, Eastern 1,123 Pine, Longleaf 1,662 Average Modulus of Elasticity 1,130 Hardwoods Willow, Black 489 Beech 1,242 Sumac 809 Elm, Slippery 1,264 Oak, Bur 877 Hickory, Nutmeg .... 1,289 Oak, Post 913 Cherry, Black 1,308 Laurel, Mountain .... 924 Osage Orange 1,329 Maple, Silver 943 Hickory, Big Shellbark 1,330 Sycamore 964 Oak, Red 1,330 Butternut 969 Hickory, Bitternut ... 1,399 Buckeye, Yellow 981 Maple, Red 1,420 Basswood 995 Ash, White 1,457 Ash, Black 1,033 Maple, Hard 1,474 Hackberry 1,040 Ash, Green 1,480 Elm, White 1,040 Birch, Sweet 1,490 Cherry, Red 1,042 Hickory, Shagbark.... 1,532 Ash, Pumpkin 1,043 Birch, Yellow 1,543 Tupelo 1,045 Hickory, Water 1,563 Witch Hazel 1,112 Cucumber 1,565 Gum, Red 1,138 Oak, Swamp White ... 1,593 Oak, Yellow 1,170 Hickory, Pignut 1,648 Dogwood 1,175 Hickory, Mockernut.. 1,672 Aspen, Largetooth 1,185 Oak, Tanbark 1,678 Oak, White 1,214 Locust, Honey 1,732 Elm, Rock 1,222 Locust, Black 1,849 Ash, Blue 1,241 Average Modulus of Elasticity 1,250 of only .29, has a modulus of elasticity of 886,000 pounds per square inch; while bur oak, which is twice as heavy, is not quite so stiff as western red cedar. A study of the tables affords many interesting comparisons of this sort.