Wood Frame Construction Manual—a Valuable Structural Design Guide

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All Things Wood
Issue #10222 - January 2018 | Page #73
By Frank Woeste and John "Buddy" Showalter

While the International Residential Code (IRC) gives the structural requirements and prescriptive design data for residential framing, additional help for the non-engineer is available through the Wood Frame Construction Manual for One- and Two-Family Dwellings (WFCM) published by the American Wood Council (AWC). The WFCM offers a pathway to accomplishing a code-conforming framing design for everyone from a layman to the most experienced residential designer, engineer, or architect. Building code officials will also find the document valuable for plan review.

The WFCM includes design and construction provisions for connections, wall systems, floor systems, and roof systems. A range of structural elements are covered, including sawn lumber, structural glued laminated timber, wood structural sheathing, I-joists, and trusses.

Demonstrating its functionality as a design resource, in 2014, Virginia Tech offered a two-day program on how to use the WFCM to design the framing system of the two-story home depicted in Figure 1. Using the manual, the instructors led the participants through the design process assuming all of the load conditions contained within the IRC. Covering an intense two days (which some participants suggested should have been at least three days), the 91-page workbook for the course can still be downloaded and printed.

How to Get Started with the WFCM

The objective of this article is to introduce the reader to the scope of WFCM and provide guidance for starting to use the manual on a limited basis (such as the design of a single girder or header). Step 1 is to be aware of the scope and limitations of the manual, and the applicability of the manual for your jurisdiction. Step 2 is to select the section and table that applies to your specific problem and carefully ensure the design assumptions and results tabulated align precisely with your situation.

It is important to understand that each edition of the WFCM will be recognized by the respective building code governing your jurisdiction. The 2018 WFCM has been recognized by the 2018 IBC and IRC, so it will eventually be the version that governs your locality. At this time, however, the 2015 WFCM is the version that is in effect (or pending) in the majority of jurisdictions. Therefore, this article will focus on the 2015 WFCM.

Step 1: 2015 WFCM Scope and Limitations

Developed by AWC, the 2015 WFCM is referenced directly in the 2015 International Building Code and 2015 IRC.

  • Under the 2015 IRC (R301.1.1 Alternative provisions), the 2015 WFCM is “…permitted subject to the limitations of this code and the limitations therein.”

The practical significance of IRC R301.1.1 is that it provides a basis for the Building Official to accept designs based on the WFCM, provided the subject design falls within the limitations of the IRC and WFCM. A free “view only” copy of the manual is online.

WFCM Scope

“Engineered” and “prescriptive” design provisions in WFCM Chapters 2 and 3, respectively, are based on the following loads from ASCE 7-10 Minimum Design Loads for Buildings and Other Structures:

  • 0-70 psf ground snow loads
  • 110-195 mph (700-year return period 3-second gust basic wind speeds)
  • Seismic Design Categories A-D

If you’re not sure about the design loading requirements per the applicable code for your location, the local Building Code Official should be consulted for this information.

WFCM Limitations

The applicability limitations of the WFCM are given in the top section of Table 1 (p. 2). The building dimensions are limited to:

  1. Mean Roof Height, 33 ft.
  2. Number of Stories, 3
  3. Building Length and Width, 80 ft.

In the lower section of the WFCM Table 1, the load types and ranges of gravity load levels and wind speeds, and seismic categories covered by the manual, are tabulated:

Further, within each chapter of the WFCM, an additional “limitations” table is provided. Table 3 (p.114), for example, provides additional “prescriptive design limitations” that help the designer or code official determine whether their particular situation can be solved by the prescriptive limits, or might require an engineered solution per Chapter 2. An example of a limit to Chapter 3 prescriptive provisions is a 10’ loadbearing wall height. Tables in WFCM Chapter 3 only allow for 10’ loadbearing walls similar to prescriptive IRC provisions. However, Chapter 2 would allow an engineered solution up to 20’ for loadbearing walls.

Step 2: Tips to Solve your Design Problem

After establishing that your design problem fits within the scope and limitations of the WFCM, the next step is to align your specific design problem with the applicable table. For demonstration purposes, the following is a simple wood-framing design example.

Floor Girder Design Requirements
  • One floor only, center bearing
  • Building width, 30-2 (ft-ins)
  • 16-ft. solid-sawn joists bearing on top of girder
  • Girder span 8-ft
  • Floor Live Load, 40 psf, L/360 live load deflection
  • Dead load, 10 psf (wood sheathing subfloor, carpet or wood flooring)
Ensure Design Requirements and Table Assumptions Align

The WFCM contains two tables (pp. 276–277) that apply to “one floor only” and 50 psf total load, but only Table 3.24A2 on p. 277 matches the lateral support condition stated above: Joists connected to top of girder. The table caption refers to a “Laterally Supported (Raised) Header…” and the required load and deflection criteria, thus Table 3.24A2 applies to the example problem.

An excerpt from Table 3.24A2 and footnotes follow: [See PDF or View in Full Issue]

Entering Table with the Required Data

Use of the tables will generally require interpolation between the tabulated values. This process is tricky and a formula may help. For example, for the problem at hand, the Building Width is 30-2. Assuming a 3-2x12 (No.2 Southern Pine) girder may be adequate, the following formula using ft-in format can be used to obtain the allowable girder span for a 30-2 Building Width.

Span for 3-2x12 girder = 9-6 - (9-6 minus7-9) x (30-2 minus 24-0)/(36-0 minus 24-0)

                             = 9-6 - (1-9) x (6-2)/(12-0)

Converting to inches = 114” – 21” x 74”/144” = 103.2” or 8-7

Being that the maximum calculated girder span is 8-7 and thus greater than 8-0, a 3-ply 2x12 Southern Pine girder is adequate when the joists are toe-nailed to a nail-laminated girder. (Toe-nailing of joists to the girder provides required lateral support for the girder.)

Importance of Table Footnotes

“Look up” design tables are valid when all conditions stated in the caption and table footnotes are satisfied or met. For example, in the example girder selection, we assumed No.2 Southern Pine. According to Footnote 1:

  • “Tabulated spans are based on the lowest Fb, Fv, and E for #2 Grade Douglas Fir-Larch, Hem-Fir, Southern Pine, and Spruce-Pine-Fir. For #3 Grade lumber, spans shall be multiplied by 0.75.”

Specie groups other than the four listed may not be adequate for the tabulated spans.

Having determined that a 3-2x12 Southern Pine girder is adequate for the example 30-2 Building Width, is it also adequate for the case wherein the two 16-ft joists would be replaced by 30-2 engineered wood product (EWP) continuous span joists or floor trusses? This question can be answered by invoking footnote 3 of Table 3.24A2:

  • Tabulated spans assume headers supporting single span floor joists. For headers supporting continuous two span floor joists, tabulated spans shall be multiplied by 0.89

Multiplying the allowable girder span by 0.89 yields an allowable span of 92”, or 7-8, less than the required span of 8-0. The calculated girder span must be reduced by the factor of 0.89 because the use of a two-span continuous 30-2 EWP joist or truss puts 25% more load on the girder than for the two single-span joist case.

The joist design advantage of using a two-span continuous joist or truss is the fact that a lower stiffness value is required for satisfying the Span/360 live load deflection limit. This example points to the need for framing designers to be specific in their design results as it is likely that changes may be made in the construction process (such as two single-span solid-sawn joists being replaced by one continuous span 30-2 EWP or floor truss).

Learning by Practice

The WFCM is a fabulous residential framing design resource. To make the best use of the manual, we recommend continuing education on using the manual, either on-line or in-person. Recorded online education, complete with continuing education credits, is available on the 2015 WFCM at: http://www.awc.org/education/wfcm-2015. AWC has several free webinars scheduled or available on-demand to provide an overview of changes to the 2018 AWC standards and 2018 building codes. In addition, while not the focus of our next Virginia Tech short course, “Structural Design Topics in Wood Construction,” we will provide and use the WFCM in four of the twelve subjects to be covered.

Frank Woeste, P. E., Professor Emeritus, Virginia Tech and a wood construction consultant. He can be reached by e-mail at: fwoeste@vt.edu.

John “Buddy” Showalter, P.E., joined the American Wood Council (AWC) in 1992, and serves as vice president of technology transfer. He can be reached at: bshowalter@awc.org.

You're reading an article from the January 2018 issue.

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