Determining Allowable Design Values for Wood

Richard E. Nolan, PE, SECB

Course Outline

This is a three hour course. The course begins with introducing the student to the most popular wood products but the course focuses on sawn lumber. The difference between a reference and allowable design values is explained. The six most commonly needed types of wood design values are introduced and explained. An illustration is then given which shows how the design value types relate to the direction of the wood grain. Next, the ten most significant adjustment factors are introduced. This is followed by how to determine the numerical value of the adjustment factors. How to apply the adjustment factors to the reference design values is then explained. The course is then completed by showing realistic examples that use the methodology that was presented in the course. A multiple-choice quiz is administered at the end which is designed to enhance the understanding of the course material.

• Introduction
1. Sawn Lumber
2. Engineered Lumber
• Reference Vs. Allowed Design Values
1. Explain difference between reference and allowed design values
• Types of Design Values
1. Bending
2. Tension
3. Shear Parallel to Grain
4. Compression Perpendicular to Grain
5. Compression Parallel to Grain
6. Modulus of Elasticity
• Wood Grain
• Factors that Affect Reference Design Values
2. Moisture Factor
3. Size Factor
4. Flat use Factor
5. Temperature Factor
6. Beam Stability Factor
7. Repetitive Factor
8. Column Stability Factor
9. Buckling Stiffness Factor
10. Bearing Area Factor
11. Incising Factor
• Adjusting the Reference Design Values
2. Adjusting Tension Parallel to Grain
3. Adjusting Compression Parallel to Grain
4. Adjusting Compression Perpendicular to Grain
5. Adjusting Shear Parallel to Grain
• Examples
• Determination of Allowable Bending Design Value for a Beam
• Determining Number of Header Supports

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of course materials.

Learning Objective

At the conclusion of this course, the student will:

• Know the different between sawn and engineered wood products;
• Understand the difference between reference and allowable design values;
• Know the six most needed types of design values for wood;
• Know the ten most significant adjustment factors;
• Know how to determine the value of the adjustment factors;
• Know how to convert reference design values to allowable design values;
• Understand the concepts of perpendicular and parallel to the grain;
• Know how to size a header using all the critical design values (bending, shear, bearing, deflection); and
• Learn how to determine the proper number of header supports.

Intended Audience

Engineers, Architects or builders who need to determine the allowable design values for wood.

Benefit for Attendee

Be certain that wood components can safely carry a load because the correct allowable design value was used.

Course Introduction

Wood is the most popular building material for residential homes and other smaller buildings. There are many types of wood products on the market used in the construction industry today. Some of the engineered wood products include I-joists, laminated veneer and glued laminated timber. However, the most familiar and well used is still sawn lumber. This course will provide the methodology for determining the allowable design values to use when designing sawn lumber components, such as beams, headers, trusses, floor joists, decking, etc. However, much of the same methodology and adjustment factors are also applicable to the other types of engineered lumber.

This course teaches the methodology used by the National Design Specification (NDS) for Wood Construction published by the American Forest & Paper Association and the American Wood Council. It is recommended but not essential that the student obtain a copy of this nationally recognized standard. Owning a copy of the NDS will not only increase the understanding of the course material but also be an invaluable source of information for anyone designing wood structures. The NDS contains reference design values for all types of sawn lumber and other engineered wood products. The copy of the NDS can be purchased from www.amc.com.

Course Content

The course content is contained in the following PDF file:

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.

Course Summary

The necessary types of design values needed to design structural wood components are presented (e.g., Fb, Fc, Fc┴, Fv, Ft, E). The significant adjustment factors needed to adjust the reference design values are presented (e.g., CD, CM, Ct, CC, CF, Cr, Ci, CP, CT, Cb). How to determine the value of the adjustment factors will be presented except those for CT, CP, and CL which were beyond the scope of the course. Circumstances when CT, CP, and CL are equal to unity are explained.  How to determine CT, CP, and CL when they do not equal 1.0 can be found in the reference. Reference design values need to be adjusted by all the applicable adjustment factors in order to obtain the correct allowable design values. Example problems are provided at the end of the course which utilized the information learned.