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|California Straw Bale Code|
|State of California Guidelines for Strawbale Structures
AMENDED IN SENATE SEPTEMBER 1, 1995
CHAPTER 4.5 GUIDELINES FOR STRAW-BALE STRUCTURES
Article 1. General Provisions and Definitions
a) The Legislature finds and declares all of the following:
1) There is an urgent need for low-cost, energy-efficient housing in California.
2) The cost of conventional lumber-framed housing has risen due to a shortage of construction-grade lumber.
3) Rice straw is an annually renewable source of cellulose that can be used as an energy-efficient substitute for stud-framed wall construction.
4) The state has mandated that the burning of rice straw be prohibited as specified in statute by the year 2000 in an annual phased reduction.
5) As a result of the mandated burning reduction, growers are experimenting with alternative straw management practices. Various methods of straw incorporation into the soil are the most widely used alternatives. The two most common methods are nonflood incorporation and winter flood incorporation. Economically viable off-farm uses for rice straw are not yet available.
6) Winter flooding of rice fields encourages the natural decomposition of rice straw and provides valuable waterfowl habitat. According to the Central Valley Habitat Joint Venture component of the North American Waterfowl Management Plan, in California's Central Valley, over 400,000 acres of enhanced agricultural lands are needed to restore the depleted migratory waterfowl populations of the pacific flyway. Flooded rice fields are a key and integral part of the successful restoration of historic waterfowl and shorebird populations.
7) Winter flooding of rice fields provides significant waterfowl habitat benefits and should be especially encouraged in areas where there is minimal potential to impact salmon as a result of surface water diversions.
8) An economically viable market for rice straw bales could result from the use of rice straw bales in housing construction.
9) The present requirement for costly design engineering in the Uniform Building Code is severely restricted. Existing regulatory requirements are costly and severely restrict the development of straw-bale housing.
10) Statutory guidelines for the use of straw-bale housing would significantly benefit low-cost housing, agriculture, and fisheries in California.
b) It is therefore the intent of the Legislature to adopt safety guidelines for the construction of structures including, but not limited to, single-family dwellings that use baled rice straw as a structural or nonstructural loadbearing or nonloadbearing material, provided that these guidelines shall not be effective within any city or county unless and until the legislative body of the city or county makes and express finding that the application of these guidelines within the city or county is reasonably necessary because of local conditions. 18944.31.
a) Notwithstanding any other provision of law, the guidelines established by this chapter shall not become operative within any city or county unless and until the legislative body of the city or county makes an express finding that the application of these guidelines within the city or county is reasonably necessary because of local conditions and the city or county files a copy of that finding with the department.
b) In adopting ordinances or regulations, a city or county may make any changes or modifications in the guidelines contained in this chapter as it determines are reasonably necessary because of local conditions, provided the city or county files a copy of the changes or modifications and the express finding for the changes or modifications with the department. No change or modification of that type shall become effective or operative for any purpose until the finding and the change or modification has been filed with the department. 18944.32. Nothing in this chapter shall be construed as an exemption from Chapter 3 (commencing with Section 5500) of, or Chapter 7 (commencing with Section 6700) of, Division 3 of the Business and Professions Code relative to preparation of plans, drawings, specifications, or calculations under the direct supervision of a licensed architect or civil engineer, for the construction of structures that deviate from the conventional framing requirements for wood-frame construction.
For the purposes of this chapter, the following terms are defined as follows:
a) "Bales" means rectangular compressed blocks of rice straw, bound by strings or wire.
b) "Department" means the Department of Housing and Community Development.
c) "Flakes" means slabs of straw removed from an untied bale. Flakes are used to fill small gaps between the ends of stacked bales.
d) "Laid flat" refers to stacking bales so that the sides with the largest cross-sectional area are horizontal and the longest dimension of this area is parallel with the wall plane.
e) "Laid on-edge" refers to stacking bales so that the sides with the largest cross-sectional are vertical and the longest dimension of this area is horizontal and parallel with the wall plane.
f) "Straw" means the dry stems of cereal grains left after the seed heads have been removed.
a) Subject to the availability of funds, on or before January 1, 2002, the California Building Standards Commission shall transmit, to the department and to the Legislature, a report regarding the implementation of this chapter.
b) The implementation report shall describe which cities and counties have utilized this chapter, and the number and type of structures that have been built pursuant to local ordinances. The implementation report may include recommendations to amend the guidelines established by this chapter, or any other related matters.
c) The California Building Standards Commission may accept and use any funds provided or donated for the purposed of this section.
ARTICLE 2. GUIDELINES FOR MATERIALS
a) Bales shall be rectangular in shape.
b) Bales used within a continuous wall shall be of consistent height and width to ensure even distribution of loads within wall systems.
c) Bales shall be bound with ties of either polypropylene string or baling wire. Bales with broken or loose ties shall not be used unless the broken or loose ties are replaced with ties which restore the original degree of compaction of the bale.
d) The moisture content of bales, at the time of installation, shall not exceed 20 percent of the total weight of the bale. Moisture content of bales shall be determined through the use of a suitable moisture meter, designed for use with baled rice straw or hay, equipped with a probe of sufficient length to reach the center of the bale, and used to determine the average moisture content of five bales randomly selected from the bales to be used.
e) Bales in loadbearing walls shall have a minimum calculated density of 7.5 dry density of 7.0 pounds per cubic foot. The calculated dry density shall be determined after reducing the actual bale weight by the weight of the moisture content.
f) Where custom-made partial bales are used, they shall be of the same density, same string or wire tension, and where possible, use the same number of ties as the standard size bales.
g) Bales of various types of straw, including wheat, rice, rye, barley, oats, and similar plants, as determined by the building official, shall be acceptable if they meet the minimum requirements of this chapter for density, shape, moisture content, and ties.
ARTICLE 3. CONSTRUCTION GUIDELINES
a) Straw-bale walls, when covered with plaster, drywall or stucco, shall be deemed to have the equivalent fire resistive rating as wood-frame construction with the same wall-finishing system.
b) Minimum bale wall thickness shall be 13 inches.
c) Buildings with structural loadbearing bale walls shall not exceed one story in height, and the unloaded bale portion of the wall shall not have a width/to/height ratio less than 0.18 loadbearing walls shall not exceed a height-to-width ratio of 5.6:1 (for example, the maximum height for a wall that is 28 inches in width 23 inches thick would be 10 feet minus l8 inches), unless the structure is designed by an engineer or architect licensed by the state to practice, and approved by the building official.
d) The ratio of unsupported wall thickness to wall length shall not be less than 0.064 to thickness, for loadbearing walls, shall not exceed 15.7:1 (for example for a wall that is 28 inches in width 23 inches thick, the maximum unsupported length allowed is 80 30 feet), unless the structure is designed by an engineer or architect licensed by the state to practice, and approved by the building official.
e) The allowable vertical load (live and dead load)on top of loadbearing bale walls shall not exceed 400 pounds per square foot, and the resultant load shall act at the center of the wall. Straw-bale structures shall be designed to withstand all vertical and horizontal loads as specified in the latest edition of the Uniform Building Code.
f) Foundations shall be sized to accommodate the thickness of the bale wall and the load created by the wall and roof live and dead loads. Foundation or stem walls which support bale walls shall extend to an elevation of not less than six inches above adjacent ground at all points. The minimum width of the footing shall be the width of the bale it supports, except that the bales may overhand the exterior edge of the foundation by not more than four three inches to accommodate rigid perimeter insulation. Footings shall extend a minimum of 12 inches below natural, undisturbed soil, or to the frost line, whichever is lower.
(1) Vertical reinforcing bars with a minimum diameter of one-half inch shall be embedded in the foundation to a minimum depth of seven inches, and shall extend above the foundation by a minimum of 12 inches. These vertical bars shall be located along the center line of the bale wall, spaced not more than two feet apart. A vertical bar shall also be located within one foot of any opening or corner, except at locations occupied by anchor bolts.
(2) Nonbale walls abutting bale walls shall be attached by means of one or more of the following methods or by means of an acceptable equivalent:
A. Wooden dowels of five-eighths inch minimum diameter and of sufficient length to provide 12 inches of penetration into the bale, driven through holes bored in the abutting wall plate stud, and spaced to provide one dowel connection per bale.
B. Pointed wooden stakes, a minimum of 12 inches in length and one and one-half inches by three and one-half inches at the exposed end, fully driven into each course of bales, as anchorage points.
C. Bolted or threaded rod connection of the abutting wall, through the bale wall, to a steel nut and steel or plywood plate washer, a minimum of six inches square and a minimum thickness of three-sixteenths of an inch for steel and one-half inch for plywood, in a minimum of three locations.
A. Structural Loadbearing bale walls shall be anchored to the foundation at intervals of six feet or less. There shall be embedded in the foundation a minimum of two one-half inch diameter steel anchor bolts per wall, with one bolt located within 36 inches of each end of each wall. Sections of one-half inch diameter threaded rod shall be connected to the anchor bolts, and to each other, by means of threaded coupling nuts, and shall extend through the roof plate bearing assembly and be fastened with a steel washer and nut.
B. Straw/bale walls and roof plates Bale walls and roof bearing assemblies may be anchored to the foundation by means of other methods which are adequate to resist uplift forces resulting from the design wind load. There shall be a minimum of two points of anchorage per wall, spaced not more than six feet apart, with one located within 36 inches of each end of each wall.
C. With structural loadbearing bale walls, the dead load of the roof and ceiling systems shall will produce vertical compression of the walls. Regardless of the anchoring system used to attach the roof plate bearing assembly to the foundation, prior to installation of wall finish materials, the finish materials nuts, straps, or cables shall be retightened to compensate for this compression.
(1) A moisture barrier shall be used between the top of the foundation and the bottom of the bale wall to prevent moisture from migrating through the foundation so as to come into contact with the bottom course of bales. This barrier shall consist of one of the following:
A. Cementitious waterproof coating.
B. Type 30 asphalt felt over an asphalt emulsion.
C. Sheet metal flashing, sealed at joints.
D. Another building moisture barrier, as approved by the building official.
(2) All penetrations through the moisture barrier, as well as all joints in the barrier, shall be sealed with asphalt, caulking, or an approved sealant.
(1) For nonstructural nonloadbearing walls, bales may be laid either flat or on-edge.
Bales in structural loadbearing bale walls shall be laid flat and be stacked in a running bond, where possible, with each bale overlapping the two bales beneath it. Overlaps shall be a minimum of 12 inches. Gaps between the ends of bales which are less than six inches in width may be filled by an untied flake inserted snugly into the gap.
(2) the first course of bales shall be laid by impaling the bales on the rebar verticals and threaded rods, if any, extending from the foundation. When the fourth course has been laid, vertical #4 rebar pins, or an acceptable equivalent, long enough to extend through all four courses, shall be driven down through the bales, two in each bale, located so that they do not pass through the space between the ends of any two bales. The layout of these rebar pins shall approximate the layout of the rebar pins extending from the foundation. As each subsequent course is laid, two pins, long enough to extend through that course and the three courses immediately below it, shall be driven down through each bale. This pinning method shall be continued to the top of the wall. In walls seven or eight courses high, pinning at the fifth course may be eliminated.
(3) When Alternative pinning method: when the third course has been laid, vertical #4 rebar pins, or an acceptable equivalent, long enough to extend through all three courses, shall be driven down through the bales, two in each bale, located so that they do not pass thorough the space between the ends of any two bales. The layout of these rebar pins shall approximate the layout of the rebar pins extending from the foundation. As each subsequent course is laid, two pins, long enough to extend through that course and the two courses immediately below it, shall be driven down through each bale. This pinning method shall be continued to the top of the wall.
(4) Only full-length bales shall be used at corners of structural walls, unless exceptions are approved by an engineer or architect licensed by the state to practice, and approved by the building official. loadbearing bale-walls.
(5) Vertical #4 rebar pins, or an acceptable alternative, shall be located within one foot of all corners or door openings.
(6) Staples, made of #3 or larger rebar formed into a "U" shape, a minimum of 18 inches long with two six-inch legs, shall be used at all corners of every course, driven with one leg into the top of each abutting corner bale.
(1) All structural loadbearing bale walls shall have a roof plate bearing assembly at the top of the walls to bear the roof load and to provide a the means of connecting the roof structure to the foundation. The roof plate bearing assembly shall be continuous along the tops of structural loadbearing bale walls.
(2) An acceptable roof plate bearing assembly option consists of two double two-inch by six-inch, or larger, horizontal top plates, one located at the inner edge of the wall and the other at the outer edge. Connecting the two doubled top plates, and located horizontally and perpendicular to the length of the wall, shall be two-inch by six-inch cross members, spaced no more than 72 inches center to center, and as required to align with the threaded rods extending from the anchor bolts in the foundation. The double two-inch by six-inch top plates shall be face-nailed with 16d nails staggered at 16-inch o.c., with laps and intersections face-nailed with four 16d nails. The cross members shall be face-nailed to the top plates with four 16d nails at each end. Corner connections shall include overlaps nailed as above or an acceptable equivalent, such as plywood gussets or metal plates. Alternatives to this roof plate bearing assembly option shall provide equal or greater horizontal and vertical rigidity vertical rigidity and provide horizontal rigidity equivalent to a continuous double 2-by-4 top plate.
(3) The connection of roof framing members to the roof plate shall comply with the appropriate sections of the California Building Code.
All openings in structural loadbearing bale walls shall be a minimum of one full bale length from any outside corner, unless exceptions are approved by an engineer or architect licensed by the state to practice. Wall or roof load present above any opening shall be carried, or transferred, to the bales below by one of the following:
(1) A frame, such as a structural window or door frame.
(2) A lintel, such as an angle-iron cradle, wooden beam, or wooden box beam. Lintels shall be at least twice as long as the opening is wide and extend a minimum of 24 inches beyond either side of the opening. Lintels shall be centered over openings.
(3) A roof plate bearing assembly designed to act as a rigid beam over the opening.
(1) All weather-exposed bale walls shall be protected from water damage. However, nonbreathing moisture barriers shall not be used on the upper two-thirds of vertical exterior surfaces of bale walls in order to allow natural transpiration of moisture from the bales.
(2) Bale walls shall have special moisture protection provided at all window sills. Unless protected by a roof, the tops of walls shall also be protected. This moisture protection shall consist of a waterproof membrane, such as asphalt-impregnated felt paper, polyethylene sheeting, or other moisture barrier, as approved by the building official, installed in a manner that will prevent water from entering the wall system at windowsills or at the tops of walls.
(1) Interior and exterior surfaces of bale walls shall be protected from mechanical damage, flame, animals, and prolonged exposure to water. Bale walls adjacent to bath and shower enclosures shall be protected by a moisture barrier.
(2) Cement stucco shall be reinforced with galvanized woven wire stucco netting or an equivalent, as approved by the building official. The reinforcement shall be secured by attachment through the wall at a maximum spacing of 24 inches horizontally and 16 inches vertically.
(3) Where bales abut other materials, the plaster or stucco shall be reinforced with galvanized expanded metal lath, or an acceptable equivalent, extending a minimum of six inches onto the bales.
(4) Earthen and lime-based plasters may be applied directly onto bale walls without reinforcement, except where applied over materials other than straw.
(1) All wiring within or on bale walls shall meet all provisions of the California Electrical Code. Type "NM" or "UF" cable may be used, or wiring may be run in metallic or nonmetallic conduit systems.
(2) Electrical boxes shall be securely attached to wooden stakes driven a minimum of 12 inches into the bales, or an acceptable equivalent.
Water or gas pipes within bale walls shall be encased in a continuous pipe sleeve to prevent leakage within the wall. Where pipes are mounted on bale walls, they shall be isolated from the bales by a moisture barrier.
GEIGER RESEARCH INSTITUTE OF SUSTAINABLE BUILDING