LUMBER USED IN CIVIL ENGINEERING CONSTRUCTION PROJECTS

Design values for lumber are contained in grading rules established by the National Lumber Grades Authority (Canadian), Northeastern Lumber Manufacturers Association, Northern Softwood Lumber Bureau, Redwood Inspection Service, Southern Pine Inspection Bureau, West Coast Lumber Inspection Bureau, and Western Wood Products Association.

The rules and the design values in them have been approved by the Board of Review of the American Lumber Standards Committee. They also have been certified for conformance with U.S. Department of Commerce Voluntary Product Standard PS 20-94 (American Softwood Lumber Standard).

In addition, design values for visually graded lumber may be established in accordance with ASTM D1990, ‘‘Standard Practice for Establishing Allowable Properties for Visually-Graded Dimensional Lumber from In-Grade Tests of Full- Size Specimens.’’

Design values for visually graded timbers, decking, and some species and grades of dimension lumber are based on provisions of ‘‘Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber,’’ ASTM D245.

ASTM D245 also specifies adjustments to be made in the strength properties of small clear specimens of wood, as determined in accordance with ‘‘Establishing Clear Wood Strength Values,’’ ASTM D2555, to obtain design values applicable to normal conditions of service.

The adjustments account for the effects of knots, slope of grain, splits, checks, size, duration of load, moisture content, and other influencing factors. Lumber structures designed with working stresses derived from D245 procedures and standard design criteria have a long history of satisfactory performance.

Design values for machine stress-rated (MSR) lumber and machine-evaluated lumber (MEL) are based on nondestructive tests of individual wood pieces. Certain visual-grade requirements also apply to such lumber.

The stress rating system used for MSR lumber and MEL is checked regularly by the responsible grading agency for conformance with established certification and quality-control procedures.

ALLOWABLE STRESSES FOR LUMBER BASIC INFORMATION AND TUTORIALS

The National Design Specification for Wood Construction (NDS) (AF&PA, 1997) makes comprehensive recommendations for engineered uses of stress-graded lumber. Stress values for all commercially available species groups and grades of lumber produced in the U.S. are tabulated in the NDS.

The moduli of elasticity for all species groups and grades are also included in these tables. These tabulated values of stresses and moduli of elasticity are called base design values. They are modified by applying adjustment factors to give allowable stresses for the graded lumber.

The adjustment factors reduce (or in some cases increase) the base design stress values to account for specific conditions of use that affect the behavior of the lumber. A list of these adjustment factors and a discussion of their use follows.

Load Duration — CD

The stress level that wood will safely sustain is inversely proportional to the duration that the stress is applied. That is, stress applied for a very short time (e.g., an impact load) can have a higher value than stress applied for a longer duration and still be safely carried by a wood member. This characteristic of wood is accounted for in determining allowable stresses by using a load duration factor, CD.

The load duration factor varies from 20 for an impact load (duration equal to one second) to 0.9 for a permanent load (duration longer than 10 years). ACI Committee 347 recommends that for concrete formwork, a load duration factor appropriate for a load of 7 days should be used. This corresponds to a value for CD of 1.25.

ACI Committee 347 says this load duration factor should only be applied to concrete forms intended for limited reuse. No precise definition of limited reuse is given by the ACI committee, but the no increase for duration of load should be used for concrete forms designed to be reused a high number of cycles.

Moisture — CM

Wood is affected by moisture content higher than about 19%. Higher moisture content significantly softens the wood fibers and makes it less stiff and less able to carry stresses. The reduction in allowable strength depends on the type of stress (e.g., shear stress is affected less than perpendicular to grain compressive stress) and the grade of the lumber.

Size — CF

Research on lumber allowable stresses has shown that as cross-sectional size increases, allowable stresses are reduced. A size factor, CF, is used to increase base design values for different sizes of lumber.

Repetitive Members — Cr

The NDS allows bending stresses to be increased for beams that share their loads with other beams. The increased allowable stress is referred to as a repetitive member stress. For a beam to qualify as a repetitive member, it must be one of at least three members spaced no further apart than two feet and joined by a load-distributing element such as plywood sheathing.

When these three requirements are met, the allowable bending stress can be increased by 15%. This corresponds to a value for Cr of 1.15. Repetitive member stresses may be appropriate for some formwork components. Because the intent of allowing increased stress for repetitive flexural members is to take advantage of the load sharing provided by continuity, gang panels assembled securely by bolting or nailing and intended for multiple reuse would seem to qualify for this increase.

ACI Committee 347 specifies that they should not be used where the bending stresses have already been increased by 25% for short duration loads.

Perpendicular to Grain Compression — Cb

Allowable perpendicular to grain bearing stress at the ends of a beam may be adjusted for length of bearing according to: lb is the length of bearing parallel to grain.

Horizontal Shear Constant — CH

Shear stress in lumber beams used as components of concrete forms is usually highest at the ends of the members. For beams having limited end defects (e.g., splits, checks, cracks), the values of allowable shear stress can be increased. This is done by using a shear constant CH that depends on the size of end defects and varies from 1 to 2.

Temperature — CT

Sustained high temperatures adversely affect some properties of wood. It is unusual for concrete forms to be exposed to temperatures high enough to require the use of a temperature adjustment factor. For temperatures in excess of 100°F, the stresses and moduli should be adjusted using CT.

Stablity — CP

Like all columns, wood shores will safely carry axial loads in inverse proportion to their effective slenderness. The more slender a wood shore is, the less load it will support because of the increased influence of buckling. Prior to the 1997 edition of the NDS, wood columns were divided into three categories (short, intermediate, and long) according to their slenderness.

Allowable stresses and loads were then found using three different formulas — one for each category. Beginning with the 1997 NDS, allowable loads for all wood columns are found using a stability adjustment factor, CP, that reduces the base stress to account for the buckling tendency of the column. It is no longer necessary to divide wood shores into three categories to find allowable loads.

GLUED-LAMINATED STRUCTURAL MEMBERS BASICS AND TUTORIALS

GLUED-LAMINATED STRUCTURAL MEMBERS BASIC INFORMATION
What Are Glued-Laminated Structural Members?

The gluing together of multiple laminations of standard 2-in.-nominal-thickness lumber has been used for many years to produce large beams and girders. This is really the only option for using sawn wood for large members that are beyond the feasible range of size for single sawn pieces.

However, there are other reasons for using the laminated beam that include the following:

Higher Strength. Lumber used for laminating consists of a moisture content described as kiln dried. This is the opposite end of the quality range from the green wood condition ordinarily assumed for solid-sawn members.

This, plus the minimizing effect of flaws due to lamination, permits use of stresses for flexure and shear that are much higher than those allowed for single-piece members. The result is that much smaller sections can often be used, which helps to offset the usually higher cost of the laminated products.

Better Dimensional Stability. This refers to the tendency for wood to warp, split, shrink, and so on. Both the use of the kiln-dried materials and the laminating process itself tend to create a very stable product. This is often a major consideration where shape change can adversely affect the building construction.

Shape Variability. Lamination permits the production of curved, tapered, and other special profile forms for beams. Cambering as compensation for service load deflection, sloping for roof drainage, and other useful custom profiling can be done with relative ease. This is otherwise possible only with a truss or a built-up section.

Laminated beams have seen wide use for many years and industry wide standards are well established. Cross-sectional sizes are derived from the number of laminations and the size of the individual pieces used. Thus depths are multiples of 1.5 in. and widths are slightly less than the lumber size as a result of the finishing of the product.

Minor misalignments and the unavoidable sloppiness of the gluing process result in an unattractive surface. Finishing of the sides of beams may simply consist of smoothing them off, although various special surface textures can also be created.

Investigation and design of glued-laminated timber members are done primarily with the procedures explained for solid-sawn beams as described in Section 4.4. Criteria for design is provided in most building codes, in the National Design Specification (NDS) (Ref. 3), and in the literature provided by manufacturers and suppliers of the products.

These elements are manufactured products and are mostly not able to be transported great distances, so information about them should be obtained from local suppliers. Individual elements of glued-laminated timber can be custom profiled to produce a wide variety of shapes for structures.

For very large elements this is not a problem, but for smaller structures the curvature limits of 2-in.-nominal-thickness lumber may be critical. Manufacturers of laminated products usually produce the arch and gabled elements as standard forms.

Structural design of the products is usually done by the manufacturer’s engineers. Form limits, size range, connection details, and other considerations for these products should be investigated with individual manufacturers.

Custom shapes can be produced, such as those with double curvature. Many imaginative structures have been designed using the form variation potential of this process.

Columns may be produced with 1.5-in. laminations, presenting the same advantages as those described for beams: higher strength and dimensional stability being most critical. It is also possible to produce glued-laminated columns of greater length than that obtainable with solid-sawn members.

In general, laminated columns are used less frequently than beams or girders and are mostly chosen only when a special shape is desired or when some of the inherent limitations of other options are restrictive.

ALLOWABLE STRESS FOR LUMBER USED IN CONSTRUCTION BASICS AND TUTORIALS

ALLOWABLE STRESS FOR LUMBER USED IN CONSTRUCTION BASIC INFORMATION
What Is The Allowable Stress For Lumber Used In Construction?


The National Design Specification for Wood Construction (NDS) (AF&PA, 1997) makes comprehensive recommendations for engineered uses of stress-graded lumber. Stress values for all commercially available species groups and grades of lumber produced in the U.S. are tabulated in the NDS.

The moduli of elasticity for all species groups and grades are also included in these tables. These tabulated values of stresses and moduli of elasticity are called base design values. They are modified by applying adjustment factors to give allowable stresses for the graded lumber.

The adjustment factors reduce (or in some cases increase) the base design stress values to account for specific conditions of use that affect the behavior of the lumber. A list of these adjustment factors and a discussion of their use follows.

Load Duration — CD
The stress level that wood will safely sustain is inversely proportional to the duration that the stress is applied. That is, stress applied for a very short time (e.g., an impact load) can have a higher value than stress applied for a longer duration and still be safely carried by a wood member.

This characteristic of wood is accounted for in determining allowable stresses by using a load duration factor, Cduration factor varies from 20 for an impact load (duration equal to one second) to 0.9 for a permanent load (duration longer than 10 years).

ACI Committee 347 recommends that for concrete formwork, a load duration factor appropriate for a load of 7 days should be used. This corresponds to a value for CD of 1.25. ACI Committee 347 says this load duration factor should only be applied to concrete forms intended for limited reuse.

No precise definition of limited reuse is given by the ACI committee, but the no increase for duration of load should be used for concrete forms designed to be reused a high number of cycles.

Moisture — CM
Wood is affected by moisture content higher than about 19%. Higher moisture content significantly softens the wood fibers and makes it less stiff and less able to carry stresses. The reduction in allowable strength depends on the type of stress (e.g., shear stress is affected less than perpendicular to grain compressive stress) and the grade of the lumber.

Size — CF
Research on lumber allowable stresses has shown that as cross-sectional size increases, allowable stresses are reduced. A size factor, CF, is used to increase base design values for different sizes of lumber.

Repetitive Members — Cr
The NDS allows bending stresses to be increased for beams that share their loads with other beams. The increased allowable stress is referred to as a repetitive member stress.

For a beam to qualify as a repetitive member, it must be one of at least three members spaced no further apart than two feet and joined by a load-distributing element such as plywood sheathing.

When these three requirements are met, the allowable bending stress can be increased by 15%. This corresponds to a value for Cr of 1.15. Repetitive member stresses may be appropriate for some formwork components.

Because the intent of allowing increased stress for repetitive flexural members is to take advantage of the load sharing provided by continuity, gang panels assembled securely by bolting or nailing and intended for multiple reuse would seem to qualify for this increase.

ACI Committee 347 specifies that they should not be used where the bending stresses have already been increased by 25% for short duration loads.

Perpendicular to Grain Compression — Cb
Allowable perpendicular to grain bearing stress at the ends of a beam may be adjusted for length of bearing according to: lb is the length of bearing parallel to grain.

Horizontal Shear Constant — CH
Shear stress in lumber beams used as components of concrete forms is usually highest at the ends of the members. For beams having limited end defects (e.g., splits, checks, cracks), the values of allowable shear stress can be increased. This is done by using a shear constant CH that depends on the size of end defects and varies from 1 to 2.

Temperature — CT
Sustained high temperatures adversely affect some properties of wood. It is unusual for concrete forms to be exposed to temperatures high enough to require the use of a temperature adjustment factor. For temperatures in excess of 100°F, the stresses and moduli should be adjusted using CT.

Stablity — CP
Like all columns, wood shores will safely carry axial loads in inverse proportion to their effective slenderness. The more slender a wood shore is, the less load it will support because of the increased influence of buckling. Prior to the 1997 edition of the NDS, wood columns were divided into three categories Cb = (1b + .375) lb (short, intermediate, and long) according to their slenderness.

Allowable stresses and loads were then found using three different formulas — one for each category. Beginning with the 1997 NDS, allowable loads for all wood columns are found using a stability adjustment factor, CP, that reduces the base stress to account for the buckling tendency of the column.

It is no longer necessary to divide wood shores into three categories to find allowable loads.