STRAWBALE DRAFT CODE
By Ed Taylor
This was put together at the request of a Shire Building Surveyor. He had so many questions he wanted answered that in the end he asked me to write a code. Where to start!?! If this looks a little familiar, that is no suprise. It is based on the Code adopted by Austin, Texas for strawbale construction. The main reason for that was to give me a layout guide so that I could include all relevant details. The content itself is all-Australian, and mostly based on experience.
This document is supplied on an all-care-no-responsibility basis. There is no copyright or any other strings attached to it. It is presented here as a guide -useful I hope- to anyone interested in convincing their local council that their application for a strawbale building has merit. It fills in the gaps that the Building Code of Australia does not yet cover. Hopefully in time strawbale building will have its own niche in that code.
If anyone out there has any practical ideas for additions or modifications to this code, feel free to contact me. It can be be revised at a moments notice.
SECTION 9901 - PURPOSE
The purpose of this document is to establish minimum prescriptive standards of safety for the construction of structures which use baled straw as a load bearing or non-load bearing material.
SECTION 9902 - SCOPE
The provisions of this document shall apply to all structures utilizing straw-bales in the construction of wall systems. Load bearing structures shall not be limited to any particular class of building or structure.
SECTION 9903 - DEFINITIONS
For the purpose of this document, certain terms are defined as follows:
STRAW is the dry stems of cereal grains left after the seed heads have been removed.
BALES are rectangular compressed blocks of straw, bound by strings or wire.
FLAKES are slabs of straw removed from an untied bale. Flakes are used to fill small gaps between the ends of stacked bales.
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.
LAID ON-EDGE refers to stacking bales so that the sides with the largest cross-sectional area are vertical and the longest dimension of this area is horizontal and parallel with the wall plane.
SECTION 9904 - MATERIALS
9904.1 Specifications for Bales.
9904.1.1 Type of Straw. Bales of various types of straw, including, but not limited to, wheat, rice, rye, barley, oats and similar plants, shall be acceptable if they meet the minimum requirements for density, shape, moisture content, and ties.
9904.1.2 Shape. Bales shall be rectangular in shape.
9904.1.3 Dimensions. Bales used within a continuous wall shall be of consistent height and width to ensure even distribution of loads within wall systems. A standard bale is generally 900mmx450mmx360mm.
9904.1.4 Ties. 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.
9904.1.5 Moisture Content. Moisture content of bales, at time of installation, shall not exceed 20% of the total weight of the bale. Moisture content of bales shall be determined by one of the following:
9904.1.5.1 Field Method. A suitable moisture meter, designed for use with baled straw or hay, and equipped with a probe of sufficient length to reach the center of the bale, shall be used to determine the average moisture content of 5 bales randomly selected from the bales to be used.
9904.1.5.2 Laboratory Method. A total of 5 samples, taken from the center of each of 5 bales randomly selected from the bales to be used, shall be tested for moisture content by a recognized testing lab.
9904.1.5.3 Common Sense Method. If the bales have been cut from the paddock and delivered direct to the building site during the summer period, then it can be safely assumed that the moisture content is less than 20%. This can be confirmed by the supplier.
9904.1.6 Density. Bales in load-bearing structures shall have a minimum calculated dry density of 16kgs. per bale. The calculated dry density shall be determined after reducing the actual bale weight by the weight of the moisture content, as determined in section 9904.1.5. The calculated dry density shall be determined by dividing the calculated dry weight of the bale by the volume of the bale, or a general approach to strawbale consistency may be exercised as follows.
9904.1.6.1 Consistency. If the bale supplier is made aware of the end use of the bales, then by appropriate harvesting techniques he will be able to supply consistent density bales.
9904.1.7 Custom Size Bales. 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. Custom bales of various sizes can be supplied by the bale supplier if sufficient notice is given.
SECTION 9905 - CONSTRUCTION AND GENERAL REQUIREMENTS
9905.1 General. Bale walls, when covered with plaster, drywall or render shall be deemed to have the equivalent fire resistive rating as wood frame construction with the same wall-finishing system. This is based on tests done in 1993 in New Mexico, USA, in accordance with ASTM E-119.
9905.2 Wall Thickness. Nominal minimum bale wall thickness shall be 450mm.
9905.3 Wall Height. Bale walls shall not exceed one and a half story in height and the bale portion shall not exceed a height to width ratio of 6.5 : 1 (for example, the maximum height for the bale portion of a 450mm thick wall would be 2925mm), with the ground story floor and half wall above being tensioned independently of each other.
Exception: In non-load bearing exterior end walls of structures with gable or shed roofs, an approved continuous assembly may be required at the roof bearing assembly level.
9905.4 Unsupported Wall Length. The ratio of unsupported wall length to thickness for load bearing bale walls shall not exceed 16 : 1 (for a 450mm thick wall, the maximum unsupported length allowed is 7200mm). A timber box section structure or other equivalent structure must be securely attached to the footing in a self supporting mode to enable a run of wall to exceed that distance.
9905.5 Allowable Loads. The allowable vertical load (live and dead load) on the top of load-bearing bale walls shall not exceed 2000 kilograms per square metre (psm) and the resultant load shall act at the center of the wall. Bale structures shall be designed to withstand all vertical and horizontal loads as specified in Load Bearing Tests carried out in accordance with the BCA.
9905.6 Footings.
9905.6.1 General. Footings shall be sized to accommodate the thickness of the bale wall and the load created by the wall and roof live and dead loads. Footing (stem) walls which support bale walls shall extend to an elevation of not less than 100mm above adjacent ground at all points. The minimum width of the footing shall be the width of the bale it supports, plus the thichness of the render.
9905.6.2 Concrete strip footings shall be at least 500mm wide, extend no less than 100mm above ground level, and extend 300mm below ground level. Reinforcing will be in accordance with the B.C.A.
9905.6.3 Alternative strip footings, such as beam and pylon or "T" shaped shall be designed and approved by a structural engineer.
9905.6.4 A stumped system of footing must allow for the live and dead loads of the wall and roof systems. It will provide 500mm of width for the bale wall, with appropriate dampcoursing provisions. This shall be designed and approved by a structural engineer.
9905.6.5 Concrete raft slabs shall be laid and attached in accordance with the B.C.A, taking into consideration the minimum requirements for the height and width of the stem wall as per 9905.6.2.
9905.6.6 Suspended floors shall be constructed in accordance with the B.C.A.
9905.6.7 Termite protection of concrete slabs and stumped/suspended floor systems shall comply with the provisions of the B.C.A.
9905.7 Wall and Roof Bearing Assembly Anchorage.
9905.7.1 General. Vertical reinforcing bars with a minimum diameter of 12mm, embedded in the foundation a minimum depth of 150mm, and extending above the footing a minimum of 300mm may be used at wall ends or at corners if necessary. Two per bale per minimum one bale from end of wall or corner.
9905.7.2 Intersecting Walls. Walls of other materials intersecting bale walls shall be attached to the bale wall by means of one or more of the following methods or an acceptable equivalent:
1. Wooden dowels at least 25mm in diameter of sufficient length to provide 300mm of penetration into the bale, driven through holes bored in the abutting stud, and spaced to provide one dowel connection per bale.
2. Pointed wooden stakes, at least 300mm in length and 100mm by 50mm at the exposed end, fully driven into each course of bales, as anchorage points.
3. 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 150mm square and a minimum thickness of 6mm for steel and 12mm for plywood, in at least three locations.
4. Brick ties laid in mortar joints and attached to the bale wall as per methods 2 or 3.
9905.7.3 Wall Anchor System.
9905.7.3.1 General. Bale walls and roof bearing assemblies shall be anchored to the footings by 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 1200mm apart, with one located within 450mm of each corner or end of wall. The dead load of the roof and ceiling systems will produce vertical compression of the bales. Regardless of the anchoring system used to attach the roof bearing assembly to the footing, prior to installation of wall finish materials bolts or straps shall be re-tightened to compensate for this compression.
9905.7.3.2 Precompression is a system used whereby the natural settling processes of the strawbale wall are speeded up artificially and thus time is saved. This can be accomplished by the mechanical methods described below.
9905.7.3.3 Gripple system. This system uses a tie down method with gripples or other similar inline fence strainers. Galvanized fencing wire of 2.5mm diameter or equivalent multistrand cable is located at intervals of 900mm or less, with a minimum of two anchor points per wall. An anchor point shall be located no more than 500mm from a wall end or corner. The wire or cable can either run a complete loop through the footing and over the wall, or the ends are attached to appropriate hardpoints set in the footing. Two gripples are used per anchor point, one each side of the wall. This allows even loading when they are tensioned.
9905.7.3.4 Gripple system with air compression. The same hold down system and specifications as described above applies. However, compression of the bale wall is achieved by using an inflatable sausage that is laid along the top of the wall and attached to the footing with chains. The sausage is inflated, the bale wall compresses, the gripples tensioned to take up the slack, and the bag and chains removed from the wall. The bag will compress up to six metres of wall in a single operation.
9905.7.3.5 Allthread or bookerthread system. Load bearing bale walls shall be anchored to the footing by 12mm diameter steel allthread or booker thread embedded at least 150mm in the footing at intervals of 1200mm or less. A minimum of two anchor rods per wall shall be provided with one rod located within 500mm of each end of each wall. Sections of 12mm 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 bearing assembly and be fastened with a steel washer and nut.
9905.7.4 Moisture Control.
9905.7.4.1 A moisture barrier shall be used along the top of the foundation to prevent moisture from migrating through the footing into the bottom course of bales. This barrier shall consist of one of the following:
1. bituminous waterproof coating;
2. rubberized aluminium over an asphalt emulsion;
3. sheet metal flashing, sealed at joints;
4. other approved building moisture barrier. All penetrations through the moisture barrier, as well as all joints in the barrier, must be sealed with asphalt, caulking or an approved sealant.
9905.7.4.2 The bale wall shall sit on a drainage system of blue-metal or other approved aggregate contained by brickwork one course high or a timber equivelant. The overall width of the system shall be the width of the footing.
9905.7.5 Stacking and Pinning. Bales in load-bearing walls shall be laid flat and stacked in running bond where possible, with each bale overlapping the two bales beneath it. Bales in non load-bearing walls may be laid either flat or on-edge and stacked in running bond where possible. For non-load bearing walls, bales may be laid either flat or on-edge. Bales in load bearing walls shall be laid flat and stacked in a running bond, where possible, with each bale overlapping the two bales beneath it. Overlaps shall be a minimum of 300mm. Gaps between the ends of bales which are less than 150mm in width can be filled by an untied flake inserted snugly into the gap.
The first course of bales shall be laid by impaling the bales on the vertical bars or threaded rods, if any, extending from the footing. When the fourth course has been laid, bamboo or other stakes at least 15mm in diameter and long enough to extend through all four courses(1400mm), shall be driven down through the bales, one to two in each bale, as many or as few as required to maintain the stability of the wall before tensioning. 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.
Only full-length bales shall be used at corners of load bearing walls. Staples, made of 12mm or larger rebar formed into a "U" shape, at least 450mm long with two 250mm legs, shall be used at all corners of every course, driven with one leg into the top of each abutting corner bale. In lieu of staples, corner bales may be tied or staked together, by a method approved by a structural engineer.
9905.7.5.1 Alternative pinning method. When the third course has been laid, vertical bamboo pins, or an acceptable equivalent, long enough to extend through all three courses(1000mm), shall be driven down through the bales, two in each bale, located so that they do not pass within 150mm of, or through the space between the ends of any two bales. The layout of these pins shall approximate the layout of the rebar pins extending from the foundation if used. As each subsequent course is laid, two such 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.
9905.7.6 Roof Bearing Assembly. Load bearing bale walls shall have a roof bearing assembly at the top of the wall to bear the roof load and to provide a means of connecting the roof structure to the foundation. The roof bearing assembly shall be continuous along the tops of structural walls. Acceptable roof bearing assembly options consist of the following.
9905.7.7 Two 100mm X 50mm hardwood timbers on edge, one located at the inner edge of the wall and the other at the outer edge. Connecting the two top plates and located horizontally and perpendicular to the length of the wall shall be 100mm X 50mm cross members spaced no more than 1800mm center to center, and as required to align with the threaded rods extending from the anchor bolts in the footing if used. The 100mm X 50mm cross members shall be attached to the top of the top plate timbers with Tek screws or equivalent. Corner connections shall include overlaps attached as above or an acceptable equivalent such as plywood gussets or metal plates.
9905.7.8 A hardwood timber plank of at least 200mm x 38mm with a central spine of hardwood timber 100mm x 50mm on edge, screwed to the top and laid on the centre line of the bale wall as an inverted "T". This shall be continuous, with corner connections as above.
The connection of roof framing members to the roof bearing assembly shall comply with the appropriate sections of the B.C.A.
9905.7.9 Openings and Lintels.
9905.7.9.1 General. All openings in load bearing bale walls shall be a minimum of one full bale length from any outside corner.
.9905.7.9.2 Openings. Openings in exterior bale walls shall not exceed 50 percent of the total wall area based on interior dimensions, and where the wall is providing resistance to lateral loads.
9905.7.9.3 Lintels. Wall and/or roof load present above any opening shall be carried, or transferred to the bales below by one of the following:
1. a structural frame, (eg. 200mmx100mm timber or equivalent)
2. a lintel (such as an angle-iron cradle, wooden beam, wooden box beam). Lintels shall be at least twice as long as the opening is wide and extend at least 600mm beyond either side of the opening. Lintels shall be centered over openings, and shall not exceed the load limitations of section 9905.5 by more than 25 percent.
9905.7.9.4 Attachment of framework. All door and window box frames shall be attached to the bale walls with wooden dowels or equivalent no smaller than 25mm in diameter. These shall be driven through the sides and top of the frames no less than 300mm into the bales, and spaced no more than 900mm apart, with a minimum of two per section of frame.
9905.7.10 Moisture Protection. All weather-exposed bale walls shall be protected from water damage. An approved building moisture barrier may be used to protect at least the bottom course of bales, but not more than the lower one-third of the vertical exterior wall surface, in order to allow natural transpiration of moisture from the bales. The moisture barrier shall have its upper edge inserted at least 150mm into the horizontal joint between two courses of bales, and shall extend at least 75mm inches below the top of the foundation. Bale walls shall have special moisture protection provided at all window sills, unless full cover is supplied by a verandah or equivalent. This shall consist of an extended sill with approriate drip drainage in accordance with the B.C.A. All external door and window openings will be flashed in accordance with the B.C.A. 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 craft paper, polyethylene sheeting, galvanized metal flashing or other acceptable moisture barrier, installed in such manner as to prevent water from entering the wall system at window sills, openings or at the tops of walls.
9905.7.11 Wet areas. Strawbale walls in wet areas shall be made moisture proof by the following methods.
1. Applying a moisture barrier to the interior face of the bale wall before final rendering.
2. Sealing the interior face of the rendered bale wall with an approved sealant.
3. Applying an approved physical barrier, such as tiles or an equivalent, to the interior face of the rendered bale wall.
4. Attaching a timber or other framework to the interior face of the rendered bale wall with a minimum air gap of 75mm, to which approved wet area sheeting is attached. This shall be finished off as per methods 2 or 3.
At all times due attention shall be paid to joints and corners, with appropriate flashing in accordance with the B.C.A.
9905.7.12 Wall Finishes. 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.
The joint between the bottom of the wall render and the top of the stem wall will be reinforced to ensure that it is water proof and not liable to cracking.
9905.7.13 Earthen and lime/cement-based plasters may be applied directly onto the exterior and interior surface of bale walls without reinforcement, except where applied over materials other than straw, or where the bales have been laid on edge. Where bales abut other materials the plaster/render shall be reinforced with galvanized expanded metal lath, or an acceptable equivalent, extending a minimum of 300mm onto the bales. Weather-exposed earthen plasters shall be stabilized using a method approved by the building code.
9905.7.14 Cement render would consist of a 7-9:1:2 mix of sand, cement and lime putty.
Lime render would consist of a 2-4:1 mix of sand and lime putty.
Plaster would be as per manufacturer's instructions.
Earthen renders would consist of a suitable sand/clay mix, and stabilized with an appropriate medium where necessary.
9905.7.15 Electrical. All wiring within or on bale walls shall meet all provisions of the Building Code of Australia. Wiring shall be run in approved conduit systems, mounted on the bale wall with wire ties and rendered over. Outlets shall be securely attached to wooden stakes driven a minimum of 300mm into the bales, or an acceptable equivalent.
9905.7.16 Plumbing. 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. Where a sleeve is to run through an external wall, its outer end shall sit lower than its inner end, thus allowing any leakage to drain to the outside.
9906 - PRIVACY/LANDSCAPE WALLS
9906.1 General. This section covers free-standing or attached bale privacy or landscape walls, not exceeding 2m in height, from final grade to top of wall. Bales may be stacked either flat or on-edge. Alternate methods, other than those listed in this section, may be approved by the building surveyor.
9906.2 Footings. The minimum footing shall consist of an 150mm thick reinforced concrete stem wall, over an approved footing. Minimum width of the stem wall shall be equal to the width of the bottom bale.
9906.2.1 Reinforcement. Bales shall be pinned as described in 9905.7.5.1. The bale wall shall be anchored as per 9905.7.3.
9906.2.2 Maximum run of unsupported wall is 7.2m. A wall return, pier or freestanding box section or equivalent shall be installed if the run of wall is to exceed this distance.
9906.2.3 Moisture Barrier. A moisture barrier shall be used between the top of the stem wall and the first course of bales. A moisture barrier shall also be used to protect the tops of bales at the top of walls, and shall extend 300mm down on either side of the wall.
9906.2.4 Wall Finish. Walls shall be finished with cement or lime render, or stabilized mud plaster, with a minimum thickness of 20mm.