HomeMy WebLinkAboutWood foundation AFBPA�
/� AMERICAN FOREST £� PAPER ASSOCIATION
� American Wood Council
°' Engineered and Traditional Wood Products
Design Requirements for
the Permanent Wood Foundation System
Prepared for the State of Minnesota
by the
American Forest& Paper Association
Introduction
The Permanent W ood Foundation(PWF)system uses pressure preservative-treated dimension lumber
and plywood,constructed similarly to wood frame walls,as a building foundation. Foundation walls
transfer the weight of the structure,via concrete,gravel or crushed stone footings,to suitable bearing
conditions. In addition to transferring vertical loads,the walls of the foundation also resist horizontal
forces resulting from backfill. Therefore, the structural elements of a foundation wall are under
compression and bending loads, simultaneously. Dimension lumber(e.g. 2x6, 2x8), when sheathed
with plywood,provides an ideal structural system for resisting these forces. When properly designed
for the appropriate loads and detailed to prohibit water intrusion, the PWF provides a structurally
sound foundation system.
PWF Acceptance
The PWF system is recognized by all U.S.model building codes and the U.S.Department of Housing
and Urban Development. The American Forest & Paper Association (AF&PA) is the publisher of
TechnicalReport#7,BasicRequirementsfor thePermanent WoodFoundation System-1987(TR7)
with 1992 revisions, the building-code-recognized reference for the design and construction
requirements of the PWF. This document provides detailed information on design load criteria,
treated lumber and plywood requirements, fastener durability, waterproofing materials, and footing
design. The publication does not provide prescriptive solutions for the appropriate size, grade, or
species of lumber or plywood. The loads induced on foundation walls are unique and may result in
a required analysis of the resisting system. With one exception presented later in this paper, it is the
position of AF&PA that foundations must be designed for each unique building configuration and soil
condition.
The State ofMinnesota's buildingcode has adopted the 1997 Uniform Building Code(UBC). Chapter
18,Division II of the 1997 UBC is based on TR7. Therefore,any jurisdiction enacting the Minnesota
State Building Code must follow the provisions of this section in the design, construction, and
fabrication of wood foundations. Further, any jurisdiction enacting the Minnesota State Building
Code has the option of referencing Appendix Chapter 3, Division III, of the UBC. This appendix
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America's Forest �r Paper People S"^—Improving Tomorrow's Environment Today'n
chapter is further amended by the Minnesota State Building Code to reference the 1992 CABO One
and Two-Family Dwelling Code (OTFDC). The OTFDC requires that "far most soils," wood
foundation walls "shall be designed assuming a lateral soil pressure of 30 pounds per cubic foot
equivalent fluid weight."Since lateral soil pressures vary depending on the type of soil,higher design
pressures are often warranted. It is the recommendation of AF&PA that PWF system design be in
accordance with the most current edition of the National Design SpecificationOO for Wood
Construction (NDSOO) and Supplement and TR7. AF&PA defers to Minnesota laws governing
"design" for the purpose of enforcement of this requirement.
Notchin�and Borin�
AF&PA believes it is not the intent of the code to permit the notching or boring of wood foundation
studs as permitted by sections 2320.11.9 and 2320.11.10 of the 1997 UBC for wall studs. The forces
acting on a foundation wall stud may resist sizeable permanent forces,which are uncommon in above
grade,exterior wall studs. Conversely,however, there is nothing in the code to prohibit notches and
holes when designed in accordance with good engineering practice. As notches and holes remove
fiber that is otherwise considered to be present when an analysis is performed, as long as a designer
considers the loss of cross-section in an analysis, there is no reason to prohibit notches or holes.
Other Sources of Information
The International Residential Code(IRC)published by the International Code Council, provides a
simple,prescriptive solution for the PWF. It is the position of AF&PA that foundations may comply
with this prescriptive solution in lieu of design. Wood foundations must strictly comply with the
prescriptive requirements in addition to the provisions of TR7. Under the IRC backfill is limited to
4' in height. Further, there must be equal height of fill around all sides of the foundation, and the
distance between points oflateral suppart(the point at which an interior wall is perpendicular to and
abuts the exterior foundation wall)cannot exceed the width of the building(cannot exceed the smaller
of either the building length or width). Minimum dimensions and strength characteristics are
specified in the IRC for wall studs, along with minimum thicknesses for plywood sheathing. When
these limitations cannot be met, the prescriptive provisions of the code do not apply. Therefore, any
structure or portion thereofoutside the prescriptive requirements must be competently designed.The
requirement for design of the P WF is no different than it is for other structural elements which are not
prescribed.
ThePerrnanent Wood Foundation System,Design,Fabrication,and Installation Manual(DFI)is no
longer published by the American Forest&Paper Association.Last published in 1987,the prescriptive
design solutions are no longer in compliance with the model building codes or the National Design
SpecificationOO for Wood Construction (NDSOO).Although the DFI has never been referenced in the
model building codes, it has been popular as a guidance document for foundation designers, and the
design details contained therein are still used by them.
However,a Permanent WoodFoundation Design and Construction Guide,is now published by the
Southern Forest Products Association on behalf of the Southern Pine Council. It provides the most
current design and construction information prepared by the industry.The Guide is not referenced in
the building codes, nor was it developed for that purpose. Rather, the Guide is intended to provide
a homeowner, builder, designer, or building official with best practices reference materiaL It should
not be considered an alternative to the requirement for structural engineering,unless recognized far
that purpose by the State.
American Forest&Paper Association Page 2
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Fipu�e 2
Ftoor/Ceiling Assembly-One Hour
(See page 12 of this report for footnotes)
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Ff�ure 3
Well Assembty-pns Hour(Umtted Load eearing)
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FROM : Extreme Panels/INSULSPAN PHONE N0. : 5074235531 Jan. 23 2003 05:27PM P12
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Re ort Ilio.NER-520
Footnotes fo� Figure 2 - Floor/Ceiling and RooflCeiling Assembly - One Hour ,
1. Insulspan Panels-48 inches (1220 mm)wide(minimum), 5-1/2 inch t88.9 mm)thick(maximum)EPS core having 7/16 inch
(1 S.l mm�thick OS8 skins. Panels having 7'/a, 9'/�and 1 1�/,inch thick cores are also permitted in roof/ceiling essemblies_
2. Splines- 2x wood spline studs. When the tables indicate that no spline studs are required for structural capac�ty, 3 inch
(76.2 mm) wide, 5/8 inCh (15.9 mm) thick OSB Surface splines are permitted under both skins instead of spline siuds.
3. Fasteners-OSB skins are fastened to spline studs using 6d nails @ 6 inches (152 mm)an center. When using OSB surfaoe
splines, the surface spfines shall be installed using a gap f;lling expanding foam sealant and 1 inch (Z5.4 mml fong, No, 6
drywall screws at 6 inches (152 mm) on �enter on each side of the joint.
4. Gypsum Wallboerd•a Ceiling surface consisting of two layers of 5/8 inch(15.9 mm1 Lhitk Type X gypsu►n wallboard. The
gypsum panels are attached to the Insulspan panels using 2 inch (51 mm) fong, A-ppint, bugle head drywall screws at 6
inches 1 i 52 mm)on center along the sheet perimeters and on a 12 inCh (30a mm) x 12 inch (304 mm) spacing in the field
of the sheets. A11 seams shall be staggered. Exposed seams shall be treated with an application of tape, followed by three
coats of US Gypsum Corporation Durabond 90 joint compound.
5. Roof Covering- (on r0of/cCiling assemblies only) -a cocle complying roof eovering.
Footnotes fo� Figure 3 - Wa11 Assembly• One Hour (Lim7ted Load Bearing)
7. Insutspan Panels-3-1/2 inch (88.9 mm1 or 5-1/2 inch (140 mm� ihick �PS core havin� 3/8 inch (9.62 mm) or 7/16 inch
I1 1.1 mm) thick OSB skins. Structural load shall noL exceed 27.4 °i6 of allowable load nor 1250 Ib/ft (18.2kN/m).
Z. Spli�es- 2x wood studs @ 48 inches (1220 mm) on center.
3. Top and 6otiom Plates- 2x wood top and bottom plates
4. Neils-OSB skins are fastened with 6d commvn nails at 6 inChes (152 mm) on center ai panel edges (vertical Splines)and
at top and bottom ptates.
5. Gypsum Wallboard-Ywo layers of %: inch (12_7 mm1 �hick USG FireGode 'C " installetl on 2he each side of the assembiy_
Installation of wellboard on Ihe interior side only of exterior walls is permiLTed in jurisdiciions using the Standard Building
Code or the BOC1r/National Building Code when such walls are located greater Lhan 5 feet 1524 mm) from a properry line
or assumed proper[y line.
The frst layer of gypsum wallboard is installed horizontally over the Insulspan panels using a continuous 3/8 inch diameter
bead of construction adhesive (Miracle DSA 20 drywall sdhesive)at 24 inches (609 mm)on eenter across the width of the
panels and 1 inch (25.4 mm) fong No. 6 bugle head drywal!screws at 8 inches 203 mm)on center along th�e perimeterand
12 inches (304 mm) on center at the two adhesive lines.
The second layer of gypsum wallboa�d is installed vertically in the same manner as the first layer using 1-5/8 inch(41.3 mm)
long bugle head drywall screws.