Log structures

Where can I find information on log structures?

1. International Codes Council
ICC Consensus Committee on Log Structures (IS-LOG): Information on the development of the ICC Standard on Design, Construction and Performance of Log Structures (ICC-400).

ICC Headquarters
5203 Leesburg Pike
Suite 600
Falls Church, VA 22041
Phone: (888) ICC-SAFE (422-7233)
Fax: (703) 379-1546
Website: http://www.iccsafe.org/

2. Log Homes Council
National Association of Home Builders
1201 15th Street
Washington, D.C 20005
Phone: (800) 368-5242, ext. 8576
Fax: (202) 266-8141
Email: efulton@nahb.com
Website: http://www.loghomes.org

All Log Home Council members must participate in a monitored Log Grading Program.

Their technical consultant is:
Mr. Rob Pickett
Rob Pickett & Associates
PO Box 490
Hartland, VT 05048-0490
Phone: (802) 436-1325
Fax: (803) 436-1325
E-mail: robpickett@mindspring.com
Website: http://robpickettandassoc.com/

3. Visual Stress Grading of Wall Logs and Sawn Round Timbers Used in Log Structures, by Edwin J. Burke, Ph.D., Univ. of Montana, Wood Design Focus, Vol. 14, No. 1. Abstract: This article explains the need for and the history and process of stress grading logs used in construction of log structures and offers practical information for engineers, architects, and code officials working with this type of construction system. For ordering information, visit http://www.forestprod.org/wdfindex.html

Dr. Edwin J. Burke
Professor of Wood Science
University of Montana
School of Forestry; Wood Science Associates
2617 Garland Drive
Missoula, MT 59803
Phone: (406) 251-4325
Fax: (406) 251-6189
E-mail: eburke@bigsky.net

4. ASTM Standard D3957-90(1996)e1 Standard Practices for Establishing Stress Grades for Structural Members Used in Log Buildings distinguishes between two types of sawn or machined timbers with different grading rules for each. Sawn Round Timbers and Wall Logs are defined. For more information, visit http://www.astm.org/

5. International Log Builders Association
PO Box 775
Lumby, British Columbia V0E 2G0 Canada
Phone: (250) 547-8776
Fax: (250) 547-8775
Toll-free: (800) 532-2900
E-mail: info@logassociation.org
Website: http://www.logassociation.org

ILBA has a document on their website that is primarily a construction guide.

6. Timber Products Inspection
1641 Sigman Road
Conyers, GA 30012
Phone: (770) 922-8000
Fax: (770) 922-1290
E-mail: jrobison@tpinspection.com
Website: http://www.tpinspection.com/LogHomeServices.html

TP developed and now provides a nationwide grading and gradestamping program to log home manufacturers.

7. Forest Products Society
Wood Design Focus
Volume 14, No. 1 - Log Structures
2801 Marshall Court
Madison, WI 53705-2295
Phone: (608) 231-1361
Fax: (608) 231-2152
Website: http://www.forestprod.org/wdf.html

8. Investigation of Old Faithful Inn Log Shearwalls
Barry Snyder
November 25, 2003
Montana State University
Professional Paper
Advisor: Dan VanLuchene
E-mail: DanV@ce.montana.edu

9. How Log Buildings Resist Lateral Loads, by Tom Hahney, P.E.
International Log Builders Association
Log Building News #32
PO Box 775
Lumby, British Columbia V0E 2G0 Canada
Phone: (250) 547-8776
Fax: (250) 547-8775
Toll-free: 1-800-532-2900
E-mail: info@logassociation.org
Website: http://www.logassociation.org

10. Testing of Lateral Resistance of Handcrafted Log Walls - Phase I and II
Forintek Canada Corp. by Dr. Marjan Popovski
Project No. 3512/3512A
2665 East Mall
Vancouver, B.C.
Canada V6T 1W5
Phone: (604) 224-3221
Fax: (604) 222-5690
Website: http://www.forintek.ca/

Load duration factor

What is the load duration factor (C_D)?

In section 2.3.2 of the 2005 NDS , it's stated that:

2.3.2.1 Wood has the property of carrying substantially greater maximum loads for short durations than for long durations of loading. NDS Table 2.3.2 gives frequently used load duration factors. The 1997 NDS Commentary section 2.3.2 notes the following:

The characteristic behavior of wood structural members to carry a greater maximum load for short durations than for long load durations was reported as early as 1841. Load duration tests conducted on in-grade lumber in bending, tension, and compression and reported in 1986 and 1988 indicate the standard load duration curve is a generally adequate representation of the effect of load duration on strength. The long record of satisfactory performance with wood structures designed using load duration adjustment factors, and the results of load duration tests on full-size members, substantiate the general applicability
of the standard strength-load duration relationship (NDS Appendix B). As more information is developed on the frequency of occurrence and the duration of maximum design loads specified in building regulations,
use of different adjustment factors than those currently specified may be appropriate for individual load cases. Such changes would be associated with improved definition and recognition of the duration of the loads
involved rather than a revision of the underlying relation between strength and time under load.

Structural lumber grades vs. appearance characteristics

Where can I find information about structural lumber grades vs. appearance characteristics?

See the following links for information:

1. Marley's PDF
2. FPL's Wood Handbook

Full versus reduced diameter lag screws

What is the difference between full diameter and reduced diameter body lag screws (also known as lag bolts)?

Full diameter screws have a larger unthreaded portion than the root diameter. Reduced diameter body screws' shank portion is the same as the root diameter of the screw. See the figure in Table L2 of the 2005 NDS for more clarification.


The reason the root diameter was used in the 2005 NDS was to better address the use of "reduced body diameter" lag screws (vs. "full body diameter") - and to better address the condition where the full length of the fastener is threaded.

Because "reduced body diameter" lag screws have a shank diameter approximately equal to the root diameter of "full body diameter" lag screws, design values for these fasteners are smaller than those provided in the 1997 NDS edition for "full body diameter" lag screws.


Root diameter (Dr), rather than the shank diameter, is used to calculate the tabulated lag screw design values, such as the ones shown in Table 11J.

Please refer to Section 11.3.6 Dowel Diameter in the 2005 NDS where it states:

"11.3.6.1 When used in Tables 11.3.1A and 11.3.1.B, the fastener diameter shall be taken as D for unthreaded full-body fasteners and Dr for reduced body diameter fasteners or threaded fasteners except as provided in 11.3.6.2..."

where 11.3.6.2 states:

" 11.3.6.2 For threaded full body fasteners (see Appendix L), D shall be permitted to be used in lieu of Dr when the bearing length of the threads does not exceed 1/4 of the full bearing length in the member holding the threads...Alternatively, a more detailed analysis accounting for the moment and bearing resistance of the threaded portion of the fastener shall be permitted (see Appendix I)."

Self-tapping lag screws

Are self-tapping lag screws covered by provisions in the NDS?

Design provisions and design values in the National Design Specification® for Wood Construction (NDS®) are applicable to lag screws conforming to ANSI/ASME Standard B18.2.1-1981. Tabulated design values are based on lag screws conforming to ANSI/ASME Standard B18.2.1-1981 and having assumed bending yield strengths provided in the table footnotes. Note that self-tapping lag screws are not addressed in ANSI/ASME B18.2.1 and are not specifically covered by provisions of the NDS. Specifically, the NDS does not address fabrication and assembly requirements, withdrawal design values, or lateral design values for self-tapping lag screws.

For self-tapping lags screws with dimensions similar to those provided in ANSI/ASME B18.2.1, the general form of the yield equations should apply for determining lateral design values. Accordingly, tabulated design values would also apply provided that the self-tapping lag screw dimensions meet or exceed the dimensions ASME B18.2.1 and the bending yield strength equals or exceeds the assumed bending yield strength in the table footnotes. In order to use lateral design provisions of the NDS, it must be assumed that fabrication and assembly of connections using self-tapping screws permits the development of the full bearing strength of the wood beneath the lag screw or permits yielding of the lag screw (i.e. installation does not damage the wood member or connection).

Finally, it should be noted that NDS Section 7.1.1.4 indicates that connections, other than those covered in the provisions, are not precluded from use where it is demonstrated by analysis, tests, or extensive experience that the connections will perform satisfactorily in their intended end use.

What is the IRC?

What is the IRC?

The IRC is the International Residential Code published by the ICC.

In-Grade Testing

Where can I find background on In-Grade Testing for design values shown in the NDS Supplement?

Changes in the 1991 NDS to dimension lumber design values are based on a comprehensive testing program conducted by the North American forest products industry called In-Grade Testing. Here's an excerpt from section 4.2.3.2 of the NDS Commentary:

"The testing program conducted over an eight year period, involved the destructive testing of 70,000 pieces of lumber from 33 species groups. A new test method standard, ASTM D4761, was developed to cover the mechanical test methods used in the program. A new standard practice, ASTM D1990, was developed to codify procedures for establishing design values for visually graded dimension lumber from test results obtained from in-grade test programs."

There are also a couple of 5-6 page articles on the subject:

"Lumber Design Values from In-Grade Test Results," Wood Design Focus, Volume 2, No. 2, 1991, Forest Products Society.
"In-Grade: What it means," Western Wood Products Association, Rev. 12-94.

In addition to these references, the Wood Handbook (Chapter 6), published by the Forest Products Lab, deals with lumber stress grades and derivation of design properties. It gives a good overview of the development of "small-clear" design values and "in-grade." It also provides some additional references for further study. Here's a link to the pdf file on the FPL website for Chapter 6:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch06.pdf

Note that concurrent with development of new design values in the 1991 NDS, behavioral equations for column, beam, and beam-column design also changed as a result of the In-Grade Testing program. Therefore, an advisory was issued with the 1991 NDS indicating that new design values were to be used simultaneously with new design equations and pre-1991 design values be used with pre-1991 design equations.

I-joists

What are I-joists and where can I find them?

Chapter 7 of the 2005 NDS says:

7.1.2 The term "prefabricated wood I-joist" refers to a structural member manufactured using sawn or structural composite lumber flanges and wood structural panel webs bonded together with exterior exposure adhesives, forming an "I" cross-sectional shape.

Design procedures and other information provided in the NDS apply only to prefabricated wood I-joists conforming to all pertinent provisions of ASTM D 5055.

For more information on I-joists, contact a manufacturer.

BOCA, ICBO, and SBCCI

What is BOCA, ICBO, and SBCCI?
And what are the UBC, NBC, and SBC?

The International Conference of Building Officials (ICBO), the Building Officials Code Administrators (BOCA), and the Southern Building Code Congress International, Inc. (SBCCI) have been consolidated into the International Code Council (ICC). Visit http://www.iccsafe.org/news/nr/2002/021001consolidation.html for the ICC news release.

The National Building Codes (NBC) was developed by BOCA, the Standard Building Code (SBC) was developed by SBCCI, and the Uniform Building Code (UBC) was developed by ICBO. The NBC, SBC, and UBC were consolidated into the International Building Code (IBC) when ICC was formed.

Past NDS editions

Where can I find more information about past NDS editions?

The first edition of the National Design Specification for Wood Construction was printed in 1944 by the National Lumber Manufacturers Association (NLMA). Revisions of the 1944 edition of the Specification were issued in 1948, 1950, 1951, 1952 and 1953. New editions were published in 1957, 1960, 1962, 1968, 1971, 1973, 1977, 1982, 1986 and 1991. The 1968 edition was the first published under the Association's new name, the National Forest Products Association (NFPA). In 1993, NFPA merged with the American Paper Institute to form the American Forest & Paper Association (AFPA).

The scope of the Specification has remained essentially unchanged since the first edition was issued in 1944. Information on machine stress rated lumber and timber piles was introduced into the 1971 edition and expanded in the 1973 edition. The name of the Specification was changed to the "National Design Specification for Wood Construction" in the 1977 edition.

The oldest design values on record with AWC are from 1922. Click here to see them.

We have archival copies of the NDS beginning with the 1944 edition. Contact the Help Desk.

Wood guard rails or hand rails

What are the design criteria such as deflection limits and strength for wood guard rails or hand rails?

The International Building Code (available from ICC) requires handrails and guards be designed to resist a load of 50 plf applied in any direction at the top of the handrail or guard, a single concentrated load of 200 lbs applied in any direction at the top, and a 50 lb. load applied horizontally over a one square foot area of the plane of the intermediate rails or balusters (applied normally), transferring these loads through supports to the structural elements of the building. These loads need not be assumed to act concurrently. In addition, there is a provision that when using allowable stress design (working stress design), the allowable stress for members and their attachments are permitted to be increased by one-third.

Most rail systems are hardwoods and need a different set of design values (Note that most of the hardwoods used typically do not have a grade stamp, they would have to be graded in some way to determine design values, assuming they are in the NDS Supplement ). Furthermore, no criteria for deflection exists for this application. The test criteria are commonly interpreted to mean ultimate load at failure.

Wood guardrails used in highway/bridge construction

Where do I find information on wood guardrails used in highway/bridge construction?

Hand rails and guard rails used in highway/bridge construction, have criteria available from the sources below.

AITC's Glued Laminated Timber Bridge Systems Manual. (303) 792-9559. http://www.aitc-glulam.org/

Wood Transportation Structures Research Website: http://www.fpl.fs.fed.us/wit/index.html (304) 285-1591. na-wit@fs.fed.us

See timber bridges

Group action factor (Cg)

How does the group action factor apply to connections? And does the group action factor, C_g, apply to sill plates and ledgers?

It does not apply to sill plates and ledgers. See the slides for more details.

Nominal lateral design values for split ring connectors, shear plate connectors, bolts with D less than or equal to 1", or lag screws in a row are multiplied by a group action factor per section 10.3.6 of the 2005 NDS.

An obvious application is the bottom chord tension splice of a heavy timber truss. It can be shown through shear-lag analysis that fasteners arrayed in this manner do not load up equally. The group action factor accounts for load redistribution that occurs in this type of joint.

The group action factor does not apply to sill plates or ledgers because such unit loads along the length of the member are not axial. See the slides for a more detailed explanation.

Where to find information on green building design?

Where to find information on green building design?

National Association of Homebuilders Research Center developed NAHB Model Green Home Building Guidelines . It is available from the NAHB-RC website.

Also try Green Building Guidelines©: Meeting the Demand for Low-Energy, Resource-Efficient Homes© available at http://www.psic.org/store/index.php#item20.

Green Building Initiative - Green Globes - http://www.thegbi.org/home/

Impact of LEED on Wood Markets - http://www.awc.org/pdf/TheImpactofLEED.pdf

Green Wood: Building Green with Wood - http://www/awc/org/pdf/StructureMag-Green-Wood-7-05.pdf

Understanding Green Building Ratings - http://www.structuremag.org/archives/2005/August%202005/Understanding-Green-Building-Ratings.pdf

The Unseen Connection: Building Materials and Climate Change (by Bruce Lippke, Ph.D.) - http://www.awc.org/pdf/LippkeArticleCAForestsWinter2006.pdf