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Section AS106 Strawbale Walls—Structural

California Residential Code 2022 > AS Strawbale Construction > AS106 Strawbale Walls—Structural
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AS106.1 General
Plastered strawbale walls shall be permitted to be used as structural walls in accordance with the prescriptive provisions of this section.
AS106.2 Building Limitations and Requirements for Use of Strawbale Structural Walls
Buildings using strawbale structural walls shall be subject to the following limitations and requirements:
  1. Number of stories: Not more than one, except that two stories shall be allowed with an approved engineered design.
  2. Building height: Not more than 25 feet (7620 mm), except that greater heights shall be allowed with an approved engineered design.
  3. Wall height: In accordance with Table AS105.4, AS106.13(2) or AS106.13(3) as applicable, whichever is most restrictive.
  4. Braced wall panel lengths: The greater of the values determined in accordance with Tables AS106.13(2) and AS106.13(3) for buildings using strawbale braced wall panels, or in accordance with Item 4 of Section AS105.2 for buildings with load-bearing strawbale walls that do not use strawbale braced wall panels.
AS106.3 Loads and Other Limitations
Live and dead loads and other limitations shall be in accordance with Section R301. Strawbale wall dead loads shall not exceed 60 psf (2872 N/m2) per face area of wall.
AS106.4 Foundations
Foundations for plastered strawbale walls shall be in accordance with Chapter 4, Figure AS105.1(1), Figure AS105.1(2) or an approved engineered design.
AS106.5 Orientation and Configuration of Bales
Bales in strawbale structural walls shall be laid flat or on-edge and in a running bond or stack bond, except that bales in structural walls with unreinforced plasters shall be laid in a running bond only.
AS106.6 Plaster on Structural Walls
Plaster on load-bearing walls shall be in accordance with Table AS106.12. Plaster on shear walls shall be in accordance with Table AS106.13(1).
AS106.6.1 Compressive Strength
For plaster on strawbale structural walls, the building official is authorized to require a 2-inch (51mm) cube test conforming to ASTM C109 to demonstrate a minimum compressive strength in accordance with Table AS106.6.1.
TABLE AS106.6.1
MINIMUM COMPRESSIVE STRENGTH FOR PLASTERS ON STRUCTURAL WALLS
PLASTER TYPE MINIMUM COMPRESSIVE
STRENGTH (psi)
Clay 100
Soil-cement 1,000
Lime 600
Cement-lime 1,000
Cement 1,400
For SI: 1 pound per square inch = 6894.76 N/m2.
AS106.7 Straightness of Plaster
Plaster on strawbale structural walls shall be straight, as a function of the bale wall surfaces they are applied to, in accordance with all of the following:
  1. As measured across the face of a bale, straw bulges shall not protrude more than 3/4 inch (19.1 mm) across 2 feet (610 mm) of its height or length.
  2. As measured across the face of a bale wall, straw bulges shall not protrude from the vertical plane of a bale wall more than 2 inches (51 mm) over 8 feet (2438 mm).
  3. The vertical faces of adjacent bales shall not be offset more than 3/8 inch (9.5 mm).
AS106.8 Plaster and Membranes on Structural Walls
Strawbale structural walls shall not have a membrane between straw and plaster, or shall have attachment through the bale wall from one plaster skin to the other in accordance with an approved engineered design.
AS106.9 Mesh
Mesh in plasters on strawbale structural walls, and where required by Table AS105.4, and where used to resist wind uplift in accordance with Section AS106.14, shall be installed in accordance with Sections AS106.9.1 through AS106.9.4.
AS106.9.1 Mesh Laps
Mesh required by Table AS105.4 or AS106.12 shall be installed with not less than 4-inch (102 mm) laps. Mesh required by Table AS106.13(1) or in walls designed to resist wind uplift of more than 100 plf (1459 N/m) in accordance with Section AS106.14, shall run continuous vertically from sill plate to the top plate or roof-bearing element, or shall lap not less than 8 inches (203 mm). Horizontal laps in such mesh shall be not less than 4 inches (102 mm).
AS106.9.2 Mesh Attachment
Mesh shall be attached with staples to top plates or roof-bearing elements and to sill plates in accordance with all of the following:
  1. Staples. Staples shall be pneumatically driven, stainless steel or electro-galvanized, 16 gage with 11/2-inch (38 mm) legs, 7/16-inch (11.1 mm) crown; or manually driven, galvanized, 15 gage with 1-inch (25 mm) legs. Other staples shall be as designed by a registered design professional. Staples into preservative-treated wood shall be stainless steel.
  2. Staple orientation. Staples shall be firmly driven diagonally across mesh intersections at the required spacing.
  3. Staple spacing. Staples shall be spaced not more than 4 inches (102 mm) on center, except where a lesser spacing is required by Table AS106.13(1) or Section AS106.14, as applicable.
AS106.9.3 Steel Mesh
Steel mesh shall be galvanized, and shall be separated from preservative-treated wood by Grade D paper, No. 15 roofing felt or other approved barrier.
AS106.9.4 Mesh in Plaster
Required mesh shall be embedded in the plaster except where staples fasten the mesh to horizontal boundary elements.
AS106.10 Support of Plaster Skins
Plaster skins on strawbale structural walls shall be continuously supported along their bottom edge. Acceptable supports include: a concrete or masonry stem wall, a concrete slab-on-grade, a wood-framed floor in accordance with Figure AS105.1(2) and an approved engineered design or a steel angle anchored with an approved engineered design. A weep screed as described in Section R703.7.2.1 is not an acceptable support.
AS106.11 Transfer of Loads to and From Plaster Skins
Where plastered strawbale walls are used to support superimposed vertical loads, such loads shall be transferred to the plaster skins by continuous direct bearing in accordance with Figure AS105.1(3) or by an approved engineered design. Where plastered strawbale walls are used to resist in-plane lateral loads, such loads shall be transferred to the reinforcing mesh from the structural member or assembly above in accordance with Figure AS105.1(3) or AS105.1(4) and to the sill plate in accordance with Figure AS105.1(1) or AS105.1(2) and with Table AS106.13(1).
AS106.12 Load-Bearing Walls
Bearing capacities for plastered strawbale walls used as load-bearing walls in one-story buildings to support vertical loads imposed in accordance with Section R301 shall be in accordance with Table AS106.12.
TABLE AS106.12
LLOWABLE SUPERIMPOSED VERTICAL LOADS (LBS/FOOT) FOR PLASTERED LOAD-BEARING STRAWBALE WALLS
WALL
DESIGNATION
 PLASTERa
(both sides) Minimum thickness in inches each side
 MESHb STAPLESc ALLOWABLE
BEARING CAPACITYd
(plf)
A Clay 11/2 None required None required 400
B Soil-cement 1 Required Required 800
C Limee 7/8 Required Required 500
D Cement-lime 7/8 Required Required 800
E Cement 7/8 Required Required 800
For SI: 1 inch = 25.4mm, 1 pound per foot = 14.5939 N/m.
  1. Plasters shall conform to Sections AS104.4.3 through AS104.4.9, AS106.7 and AS106.10.
  2. Any metal mesh allowed by this appendix and installed in accordance with Section AS106.9.
  3. In accordance with Section AS106.9.2, except as required to transfer roof loads to the plaster skins in accordance with Section AS106.11.
  4. For walls with a different plaster on each side, the lower value shall be used. For walls with plaster on only one side, half of the tabular value shall be used.
  5. Shall use hydraulic or natural hydraulic lime.
AS106.12.1 Precompression of Load-Bearing Strawbale Walls
Prior to application of plaster, walls designed to be load-bearing shall be precompressed by a uniform load of not less than 100 plf (1459 N/m).
AS106.12.2 Concentrated Loads
Concentrated loads shall be distributed by structural elements capable of distributing the loads to the bearing wall within the allowable bearing capacity listed in Table AS106.12 for the plaster type used.
AS106.12.3 Roof-Bearing Assembly
Roof-bearing assemblies shall be of nominal 2-inch by 6-inch (51 mm by 152 mm) lumber with 15/32-inch (12 mm) plywood or OSB panels fastened with 8d nails at 6 inches (152 mm) on center in accordance with Figure AS105.1(3) and Items 1 through 6, or be of an approved engineered design.
  1. Assembly shall be a box assembly on the top course of bales, with the panels horizontal.
  2. Assembly shall be the width of the strawbale wall and shall comply with Section AS106.11.
  3. Discontinuous lumber shall be spliced with a metal strap with not less than a 500-pound (2224 N) allowable wind or seismic load tension capacity. Where the wall line includes a braced wall panel the strap shall have not less than a 2,000-pound (8896 N) capacity.
  4. Panel joints shall be blocked.
  5. Roof and ceiling framing shall be attached to the roof-bearing assembly in accordance with Table R602.3(1), Items 2 and 6.
  6. Where the roof-bearing assembly spans wall openings, it shall comply with Section AS106.12.3.1
AS106.12.3.1 Roof-Bearing Assembly Spanning Openings
Roof-bearing assemblies that span openings in strawbale walls shall comply with the following at each opening:
  1. Lumber on each side of the assembly shall be of the dimensions and quantity required to span each opening in accordance with Table R602.7(1).
  2. The required lumber in the assembly shall be supported at each side of the opening by the number of jack studs required by Table R602.7(1), or shall extend beyond the opening on both sides a distance, D, using the following equation:
    (Equation AS-1)
    where:
    D = Minimum distance (in feet) for required spanning lumber to extend beyond the opening
    S = Span in feet
    R = BL/BC
    BL = Design load on the wall (in pounds per lineal foot) in accordance with Sections R301.4 and R301.6
    BC = Allowable bearing capacity of the wall in accordance with Table AS106.12
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