Basic Lumber Engineering for Builders
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Introduction | Table of Contents | Back Cover

by Max Schwartz

Chapter 1

Wood Basics

Wood has always been the most important construction material used in this country. Blessed with great forests, early American pioneers depended on wood for their frontier forts, cabins, and towns.

There are still more wood-frame buildings built in America than any other type. That means just about every contractor in America needs a working knowledge of practical wood engineering and construction. And you can't just rely on what you've learned through years of experience in the construction business. The country has suffered catastrophic hurricanes, tornadoes and earthquakes in the last decade. They've led to changes in the building codes and in local construction methods in areas where these disasters are likely to occur.

The introduction of composite and prefabricated structural members has led to additional revisions in the code, as well as new methods of design and construction.

When building codes were developed in the late 1920s, the writers adapted minimum standards for the "average" type of building and occupancy. They incorporated tables that listed minimum sizes of rafters, girders and joists for various spans. Most builders didn't worry about calculating stresses or deflections. They just went "by the code," or more accurately, by minimum requirements of the code. There was little need for engineering in wood construction except for beam spans greater than 25 feet.

The purpose of this book is to fill this gap and provide basic engineering rules for typical and nontypical conditions. It explains why wood is a complex structural material that reacts to the environment. It covers both the natural wood products and the newer composite and prefabricated wood structural members. Let's start with a look at how wood grows, is made into construction lumber, and how it changes with varying environmental conditions.

Figure 1-1 ( left )
Wood growth layers and grain direction

Figure 1-2 ( right )
Heartwood and Sapwood

Lumber Characteristics

A tree grows by adding new layers of cells from the center (pith) outward. Every year, a new annual growth ring made up of a light layer and a dark layer is added (shown in Figures 1-1 and 1-2). The thicker light layer is called springwood, which is the fast growth added during spring. The thinner dark layer is called summerwood. It's the slow growth added during summer. Since summerwood is denser and stronger than springwood, the proportion of summerwood to springwood determines the quality and strength of the wood.

A tree trunk is made up of sapwood and heartwood. Sapwood is located under the bark and allows water and nourishment to pass from the root system to the leaves. It's not a good idea to use lumber cut from sapwood for construction since it contains sap. Heartwood is inside the sapwood and gives structural strength to the tree. Heartwood is denser than sapwood and more resistant to shrinkage, warping, and decay.

Hardwood and Softwood

Hardwood such as oak or maple comes from deciduous (broadleaf) trees while softwood such as pine or fir comes from evergreen (cone bearing) trees. Softwood is used mainly for framing lumber and plywood. Hardwood is generally used for wood furniture, decorative interior paneling, trim, and strip flooring. Most common softwoods resist splitting and have excellent nail-holding qualities.

Wood Groups

Hardwoods and softwoods are classified into four groups according to their density (specific gravity). Group I is the densest and Group IV is the least dense. Lumber groups are shown in Figure 1-3.

Figure 1-4 shows some of the physical properties of common hardwoods. The densities shown in the figure are the approximate shipping weights of air-dried lumber in pounds per cubic foot. Specific gravity is based on a 12 percent moisture content.

Figure 1-3
Wood Groups

Redwood

Redwood is a special type of wood that's more resistant to the elements and infestation than other popular species used in construction. Although it's relatively expensive, redwood is widely used because:

• It shrinks very little and is resistant to warping, cupping, and nail-popping.

• The heartwood is naturally resistant to decay and insects.

• Redwood has fewer volatile resins, so it resists burning. If it ignites, it burns slowly and forms a protective char layer over the wood underneath.

• Redwood shrinks and swells very little during varying moisture conditions, and its unseasoned moisture content is low, ranging from 6 to 7 percent.

• Redwood holds paint and stains better than other softwoods.

• Redwood is graded in descending order of quality and cost as:

• Clear all-heart vertical grain (VG.)

• Clear all-heart

• Select heart

• Construction heart

• Utility

• Merchantable

Clear all-heart vertical grain is the highest redwood grade because it's all heartwood, free of knots. It's used for fine siding, paneling, cabinets, and finish carpentry. Select heart is also all heartwood, free from shakes and splits. Clear all-heart is similar to clear all-heart V.G. except that it contains some cream-colored sapwood. This grade is normally kiln-dried and used as a less expensive alternative to clear all-heart V.G. redwood.

Construction heart resists weathering and is economical. It's sold surfaced or rough, and it's normally unseasoned. It contains knots 2-1/2 to 4 inches in diameter and is normally used for posts, fencing, decking, and outdoor structures.

Utility grade redwood contains sapwood and is less resistant to decay and insects. It’s sold rough or surfaced and is usually unseasoned. This grade is used for subfloors, fencing, decking, and outdoor structures.

Merchantable grade is the most economical. It contains larger knots and more serious defects than are permitted in the higher grades. Both heartwood and sapwood pieces are included in this grade, as well as shakes, stains, splits, and knotholes. It's used for decking, subfloors, temporary low-cost construction, and interior work where a knot-free surface isn’t required. It's sold surfaced or rough, and is usually unseasoned.

Lumber Grading

Lumber is graded according to the number, size and location of grains, knots, and checks in it. Higher grade lumber has the least amount of these defects. Lumber graders classify softwood lumber for construction purposes into three basic groups:

• Lumber for primary construction purposes

• Lumber for secondary construction, such as wales and studs used in
  building concrete forms

• Lumber for architectural purposes

Hardwood mills grade their lumber by the percentage of board that's free of defects. They also grade lumber by the amount of usable lumber in each piece, or the number of clear face cuttings that can be made between knots and cracks. They classify hardwood as firsts, seconds, select, No. 1 common, and No. 2 common.

Firsts is the highest grade of hardwood. Each piece must be at least 4 inches wide and 5 feet long or 3 inches wide and 7 feet long. Six-inch widths are allowed if 92-2/3 percent of each piece is clear of knots and defects.

Seconds are usually mixed with Firsts and called FAS (Firsts and Seconds). At least 83 percent of each board must be clear. Each piece of Seconds must also be at least 4 inches wide and 5 feet long or 3 inches wide and 7 feet long.

Select is another rating of hardwood. Each piece of Select must also be at least 4 inches wide and 5 feet long or 3 inches wide and 7 feet long. Each piece must be 91-2/3 percent clear.

No. 1 common is the fourth class of hardwood. Each piece of No. 1 common must be at least 4 inches wide and 2 feet long or 3 inches wide and 3 feet long.

The lowest grades include No. 2 common, sound wormy, No. 3A common, and No. 3B common. Each piece must be at least 3 inches wide and 2 feet long.

Figure 1-5
Plain- or flat-sawn lumber

Lumber Shapes

The way timber is cut into lumber affects its strength, its degree of shrinkage, and its appearance. Plane- or flat-sawn lumber is cut in slices parallel to one side of the log, as shown in Figure 1-5. Quarter-sawn lumber is made by sawing lumber perpendicular to the exterior of the log, as shown in Figure 1-6. Quarter-sawn lumber is more expensive than flat-sawn lumber because there’s more waste and labor in cutting it. But quarter-sawn wood twists and cups less, holds paint better, wears more evenly, and swells and shrinks less than flat-sawn lumber. The amount that a piece of lumber will shrink and warp depends on where the piece was cut from the tree. Wood will shrink and warp perpendicular to the grain as it loses or gains moisture, but it won’t shrink or swell much parallel to the grain. See Figure 1-7.

Figure 1-6
Quarter-sawn lumber

Mills saw, edge, trim, and plane trees into lumber. The ultimate shape of lumber depends on its size and how it's used in construction. Some of these shapes are:

1. Boards: Lumber less than 2 inches thick and 2 or more inches wide. Boards less than 6 inches wide are called strips.

2. Dimension lumber: The National Grading Rule for dimension lumber classifies dimensions into width and use categories. These are:

• Dimensions up to 4 inches wide are classified as structural light framing, light framing, and studs

• Dimensions 5 inches and wider are classified as structural joists and planks

• Appearance framing grade is 2 inches and wider and used for high strength and appearance

3. Beams and stringers: These carry loads on the narrow face of a member. Members are 5 inches or more thick, and at least 2 inches wider than they are thick.

4. Joists and planks: These members are similar to beams and stringers except they're 2 to 4 inches thick and at least 6 inches wide. Joists are installed to carry loads on the narrow face, while planks used for decking or flooring carry loads on the wide face.

5. Posts and timbers: This shape includes lumber with a square or almost square cross section used for posts or columns. These members are 5 inches or more thick and less than 2 inches wider than they are thick.

Figure 1-8 shows more categories of various types of lumber.

Round Sections

Lumber mills also manufacture timber with round cross sections. Round shapes are used for pilings, trestles, wharves, falsework, utility poles, and gin poles. Douglas fir and southern yellow pine are the most common species used for pilings in North America. Telephone and power companies normally use Douglas fir, southern pine, and lodgepole pine for their poles.

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