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Performance Solution for earth rammed tyre walls

The following Performance Solution is for presentation to your local council inspector and/or building surveyor to show that the foundation, footings and structural integrity of the rammed-earth-tyre wall out-performs the structural integrity of tranditional timber-framed and/or concrete construction. The evidence presented in this Performance Solution meets and exceeds the Performance Requirements defined in the National Construction Code, Section B (Structure).

Process for Performance Solution

  1. Performance Based Design Brief: (a) stakeholders submitting evidence, (b) history of construction method and assumptions, (c) building type, function, size (height/floor space) and plausible location for earthen wall buildings using this construction method.
  2. Nominated applicable Performance Requirements as per the NCC (and state legislation).
  3. Agreed analytical assessment process for each agreed Performance Requirement including agreed acceptance criteria, e.g. evidence of suitability, verification method, expert judgement, comparison to 'deemed to satisfy'. Testing methods could include: comparative/absolute, qualitatitive/quantitative, deterministic/probabalistic, empirical calculations, in-situ/lab testing, computer aidied modelling/simulation.
  4. Required scope of supporting evidence to achieve a Performance Solution.
  5. Final report, including: (a) overview of the PBDB (as per above), (b) 'performance requirement' analysis methods used, (c) results from analysis, (d) conclusions
  6. Acknowledgement of experts, participants and references.

Terminology:

  • ABCB = Australian Building Code Board, who are commisioned via by the Government as a standards body responsible for the NCC/BCA, Code Mark & WaterMark certification schemes.
  • NCC aka BCA - National Construction Code / Building Code of Australia
  • PBDB - a 'Performance Based Design Brief' is a collaborative process whereby stakeholders agree crtieria (aka performance requirements) for expert evaluation of an engineered 'performance solution'.
  • Performance Solution - is a set of 'Performance Requirements' which are agreed by a group of stakeholders to measure the safety of a proposed 'performance solution'.
  • Performance Requirement(s) - are the measures by which an agreed set of expert stakeholders agrees and evaluates to assure a 'performance solution' meets the requirements setout by the NCC for recognition as a 'performance solution'.
  • Expert Judgement - the judgement of an expert who has the qualifications and experience to determine whether a Performance Solution or Deemed-to-Satisfy Solution complies with the Performance Requirements.
  • TESE - Tyre-Encased-Soil Elements aka "Rammed Earth Tyres' which are engineereed to perform as building blocks in a load-bearing external wall.
  • φ = load capacity reduction faction, e.g. the lognormal distribution reliability index (structural actions, resistance and responses), equivalence to ISO2394 . NB These are represented as random variables in probalistic modelling.

Performance Based Design Brief

Stakeholders

"A performance solution may be aided by the early and significant involvement from regulatory authorities, peer reviewer(s) and/or a technical panel" (ABCB):

Proposal summary

History of rammed earthen wall construction for structurally habitable buildings:

  • Evidence of rammed earth wall construction for habitable building walls has been archeologically documented in: (a) Mesopotamia Fertile Crescent, 7-9th century BCE (b) Yangshao & Longshan Yellow River settlement 5th century BCE, and (c) by the 2nd cetury BCE the use of rammed earth was documented by the Chinese Imperial court as it own construction practice "夯土 Hāng tǔ" meaning "rammed earth" Subequently, every indigenous culture with settlement based dwellings have used rammed earth to achieve structural endurance.
  • Widespread use of rammed earth to build large community settlements with multiple stories is documented in: (a) Bam Iran aka Arg-e Bam, Persian from 5th cetury BCE, (b) Pueblo Bonito in Chaco Canyon, built by the Indigenous Ancestral Puebloans in 3rd century BCE; and (c) when the Spanish conquistadors encountered Native American settlements in the 11th century, rammed earthen dwellings were widespread with over 600 dwellings (single occupancy units) which are still standing in the Anasazi cliff top of the Four Corners area, Colorado (USA); and (d) France (based on Roman occupation and construction) widely uses "pisé walls" (from the Latin origin "pisé de terre"). First documented and used in Lyons France 1562.
  • 1925 US Department of Agriculture published a report for homesteading farmers showing evidence that "rammed earth structures endure indefinitely and can be constructed for less than two-thirds of the cost of standard frame houses".
  • 1970s 'Earthship Biotecture' pioneered the use of tyres for earth rammed structures: the first schematic in the 1970s Taos New Mexico by architect Michael Reynolds. Earthship rammed earth walls have been built on every content, other than Antartica.
  • Currently, there are approximatley 3 billion people living in earthen buildings worldwide who could benefit from the structural integrity which rammed earth tyres provide over earthen structures which are not laterally encased with steel tyre walls.
  • In 2020 Tyre Stewardship Australia invested $100k to have rammed earth tyre walls tested by the University of South Australia, in response to the excessive stockpile of tyres available as recycled building material.

Rammed earth disadvantages:

  • soil granularity of internal construction aggregate materials varies widely;
  • susceptable to cracking in earthquakes (if not laterally encased);
  • susceptable to erosion (if not encased and protected from environments with excess water/wind); and
  • history of rammed earth buildings being associated with ad hoc indigenous cultures makes investment by bankers/insurance institutions difficult.

Analytical assesment process and acceptance criteria

The below assessment methods are outlined in the ABCB Structural Reliability Verification Method Handbook

Method of assesment

As per the NCC, a combination of Assessment Methods A2.2(2) will be utlised:

  • NCC Vol.1 Verification Method BV1 and V2.1.1. which meet the compliance requirements for BP1.1 and BP1.2 in NCC Vol.1 and NCC Vol.2 Part 2.1.1(a)(b)(c)
  • Performance Requirements (absolute) > A2.2(2)(c) expert judgement > qualitative;
  • Partial supporting evidence AS.2(1)(a) Compliance with Performance Requirements (absolute) > A2.2(2)(b)(i)(ii) Verification method > Quantitative > Deterministic/Probabilistic;

Assessment criteria

Performance solution assessment criteria (in scope):

  • Tyre wall structural reliability performance (variation in dimensions as a result of the fabrication/construction process. A measure for the allowable tolerances/measurements of the dimensions of the rammed earth tyre wall in comparison to precast tilt-up concrete panels (AS3850-2003), e.g. axial and lateral loads such as side impact/collapse of tyre wall.
  • Tyre wall robustness performance (uncertainties in the structural modelling of the component). A measure for the uncertainties of construction of the structural rammed earth tyre wall will be decided by the expert stakeholder panel.
  • Tyre wall 'serviceability performance' (variability in the mechanical properties of the materials). A measure for ‘quality control’ of the material manufacturing process will be agreed by the expert stakeholder panel, e.g.

Deemed out of scope (for this assessment):

  • expansion of load bearing external wall utilising butresses and/or lateral support
  • multiple storey construction height and vertically transferrable loads
  • fire resistence level and thermal insulation gradients

Scope of rammed earth tyre wall buildings to be asssessed

Building classes:

  • Class 1 - Residential single storey
    • Function: detached house, townhouse, boarding house, guest house:
    • Size: up to three storey, less than 300m2 and less than 12 simultaneous occupant use.
    • Location: Zones 1-6, N3+ wind regions
  • Class 2 - Residential
    • Function: apartment with single occupancy units (SOUs)
    • Size: floor area >300m2 and 9m in height
    • Location: Zones 1-6, N3+ wind regions
  • Class 3 - Residential single storey
    • Function: boarding house, guest house, motel and hotel
    • Size: floor area >300m2 and 9m in height
    • Location: Zones 1-6, N3+ wind regions
  • Class 4 - Residential within a commerical building
    • Function: caretakers and single occupancy units within single storey commercial buildings.
    • Size: >300m2
    • Location: Zones 1-6, N3+ wind regions
  • Class 5 - Commerical single storey
    • Function: office, professional consulting SOUs, etc.
    • Size: floor area >300m2 and 9m in height
    • Location: Zones 1-6, N3+ wind regions
  • Class 6 - Commercial single storey
    • Function: shops, restaurants, cafe, shop, laundry, hair-dresser, showroom, marketplace, etc.
    • Size: floor area >300m2 and 9m in height
    • Location: Zones 1-6, N3+ wind regions
  • Class 10 - Residential single storey
    • Function: shed
    • Size: >10m2 and less than 3m in height
    • Location: Zones 1-6, N3+ wind regions

NB Building Class 7, 8 & 9 were out of scope for this performance solution due to further tests required for fire safety compliance for external load bearing fire walls with a Fire Rating Level of 120/120/120.

Client brief

The buttresses on Earth#1 were used for a number of reasons (structural, health/amenity, aesthetic and accessibility)

  • The buttress feature was started by an American engineer which required them for structural integrity to lengthen and increase the overall size of the building (ISO6707-1), thereby ensuring the structural integrity of the earth/tyre wall from Earthquakes and High Wind Regions (N3+). Subsequent engineers have determined that these buttresses are not required for areas in Australia with wind and/or Earthquake area.  Nonetheless, the buttresses provide 'peace of mind' for those who might experience hurricane force winds on the coast or in alpire regions. See attached engineers report. Most importantly, the Performancee Solution provided by the buttresse(s) enable earthen tyre walls to be expanded to the legal maximum size of a building in Australia.
  • The Buttresses were used in Earth #1 primarily to solve health and amenity issues.  For example, they are used as the basis for mounting wall framework and solid mounting for hanging doors (which could also be achieved via other framing standards). The buttresses are over-engineered with internal steel reinforced rebar, which results in an internal wall system which has structural stability for a continous wall of 3x the current length.
  • Design criteria: buttresses have become a core design feature of Earthships, alongside the bottle walls.  Buttresses, other than their ancient historical use in architecture, are also a feature of the Earth1, helping create division of space and a natural line of sight to the beautiful ceiling of Earth 1.  In short, the buttresses in Earth1 are a key aesthetic feature. 
  • Livability / accessibility: In addition to providing division between room, the concrete buttresses contrasted against the earthen adobe walls provide a perfect location for the wood heater as a thermal radiating mass.  While the exemplar energy efficiency (5 star) does not need the additional heat, it is still comforting to have a fire on the SouthCoast of Australia when Antarctic winds are blowing.

Supporting evidence

The supporting evidence for the 3x assessment criteria (structural reliability performance, robustness performance and serviceability performance) are provided below. As per the ABCB handbook on Structural Reliability Verification Methods, there are three categories which must be evaluated for structural probabilities to compare the load capacity reduction factor:

  • Kf = Strength performance (variation in dimensions as a result of the fabrication/construction process. A measure for the allowable tolerances/measurements of the dimensions of the rammed earth tyre wall in comparison to precast tilt-up concrete panels (AS3850-2003).
  • Ks = Robustness performance (uncertainties in the structural modelling of the component). A measure for the uncertainties of construction of the structural rammed earth tyre wall will be decided by the expert stakeholder panel.
  • Km = Serviceability performance (variability in the mechanical properties of the materials). A measure for ‘quality control’ of the material manufacturing process will be agreed by the expert stakeholder panel.

For each of the above, the suggested assessment acceptance criteria is:

  • Strength performance (Kf) will be evaluated via Quantitative Verification Methods as per A2.2(2)(b)(i)(ii) whereby quantitative (lateral/vertical tests) comparison to the structural dimensions of pop-up concrete walls
  • Robustness performance (Ks) will be evaluated via A2.2(2)(d) comparison with Deemed-to-Satisfy provisions
  • Serviceability performance (Km) will be evaluated via Qualitative Expert Evaluation as per A2.2(2)(c) and will require a framework for assuring construction quality build control of the laterally encased rammed earth tyres.

Evidence for structural reliability performance

(Kf) structural engineers will compare the lateral/vertical testing done by the University of South Australia to the load tolerances for concrete and timber.

Evidence for robustness performance

(Ks) robustness performance will be comparatively measured against Deemed-to-Satisfy provision for wind, snow and earthquake action over 100 years in each climate zone.

Evidence for serviceability performance

The evidence for serviceability performance (Ks) will be evaluated via A2.2(2)(d) comparison with Deemed-to-Satisfy provisions, i.e. practicing builders and structural engineers will evaluate the construction process of rammed earth tyres and their probability for defective quality control manufacturing.

Final Report

The final report should clearly demonstrate that compliance with the NCC Performance Requirements agreed in the PBDB has been achieved.

  • An overview of the PBDB, including:
    • Scope of the project
    • Relevant stakeholders
    • Applicable NCC Performance Requirements
    • Approaches and methods of analysis
    • Any assumptions that were made
    • Acceptance criteria and safety factors agreed to by stakeholders
  • Overview and outline of the analysis, modelling and/or testing carried out:
    • Method of analysis used
    • Calculations and outcomes
    • Limitations of NCC performance solutions: structural componets and connections are not calculated.
    • The sensitivities, redundancies and uncertainty studies carried out
    • The results obtained and relevance to the PBDB
  • Evaluation of results including:
    • Comparison of results with acceptance criteria
    • Any further sensitivity studies undertaken
    • Any expert judgement applied and its justification
  • Conclusions:
    • Specifications of the final design that are deemed to be acceptable
    • The NCC Performance Requirements that were met
    • All limitations to the design and any conditions of use

The above template for this report was provided via: https://www.abcb.gov.au/news/2021/ncc-provision-developing-performance-solutions

References