£1<?4'-M 
 
 MECRmNICaL PROPERTIES 0? ;CE 
 
 By 
 
 JOHN T. DROA', Engineer 
 
 F prest Products Laboratory",- Forest Service 
 U, S. Department of Agriculture 
 
 
 Engelmann spruce is one of the lightest of the important commercial 
 woods grown in the United States. The specific gravity or density 
 of wood is a measure of the amount of wood substance per unit volume, 
 rts might be expected, therefore, the strength properties of wood are 
 related to its density, and a lightweight wood such as Engelinann 
 spruce is not so strong as the heavier woods. In general, this 
 results in lightweight woods being used to the greatest extent for 
 purposes where strength is not the main consideration and where the 
 light color, uniform texture, and lower weight are an advantage. 
 
 The bar graphs in figure 1 s:iow the relative weight, shrinkage, and 
 strength of the clear wood of Engelmann spruce and a number of other 
 commonly used species with which it is associated. The relations 
 shown are based on average values of species and are indicative only 
 of general trends. The properties of individual pieces or small 
 groups of material in all species may vary considerably above or 
 below the average. As a consequence, some overlapping between species 
 will occur, and differences of only a few percent in property rela- 
 tions should not be considered too important. 
 
 The importance of strength differences depends upon the use to which 
 a wood is put. The fact that Engelmann spruce is lighter and somewhat 
 weaker than many of the commonly used construction woods does not mean 
 that it cannot be utilized for purposes where strength is of some 
 importance. In housing, for example, 2- by U-inch studs of Engelmann 
 spruce at the usual 16-inch spacing would be entirely satisfactory, 
 since the size of these members is dictated more by custom t. ' an 
 engineering requirement for a piece of that size. Similarly, the 
 strength of Engelmann spruce is fully adequate to permit its use in 
 ordinary 1-inch lumber for wall and roof sheathing, subflooring, and 
 similar uses. For other housing members, such as floor and ceiling 
 joists or rafters, the lower strength properties can be compensated 
 for by using shorter spans, by increasing the size of the member to 
 
 ■^Maintained at Madison, Wis., in cooperation with the University of 
 
 Wisconsin. 
 
 Rept. Mo. 19UU-U -1- Agriculture-Ladison 
 
provide adequate strength for a desired span, or by the use of a 
 better grade. These factors are taken into account in setting up 
 maximum allowable spans. The required spans for Engelmann spruce 
 floor joists, flat-roof joists, ceiling joists, and rafters for 
 residential construction are listed in the Federal Housing administra- 
 tion publication, "Tables of Maximum Allowable Spans," FHA Bulletin 
 No. 2550, 
 
 For some other purposes the strength and physical characteristics of 
 Engelmann spruce are adequate to permit its use interchangeably with 
 comparable grades of other species having generally similar charac- 
 teristics. For example, spruce (which includes Engelmann spruce) is 
 listed among more than 20 other species that fall in group I of four 
 groups of woods commonly used for box construction. All of the other 
 species shown on the bar charts fall in group I also, and all woods in 
 that group can be used interchangeably as far as the thickness of 
 material and the size and spacing of nails are concerned. For such 
 use, the light weight of Engelmann spruce is a distinct asset, since 
 one of the objectives in construction of boxes and crates naturally 
 is to reduce the j weight of the shipping container to save shipping 
 costs and facilitate handling* 
 
 Engelmann spruce, like lodgepole pine, ordinarily has smaller knots 
 than ponderosa pine, but tney are present in larger numbers- These 
 wcods are in general use for containers requiring wider and thicker 
 slats, such as those used for shipping meat products, or for casket 
 boxes and the like. Engelmann spruce, because of its light weight, 
 would also be especially suitable for smaller containers in which the 
 component parts, for the sake of convenience of manufacture, are made 
 larger and thicker than actually required for the load to be carried. 
 
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 Rept. No. 19UU-U -2- October 1953