This research project was initiated by a group of interested design practitioners interested in providing some data to inform the debate on how to approach the refurbishment and/or replacement of the ‘Housing Commission towers’ located in inner Melbourne suburbs.
Designed and built in a time of post-war modernist vision and social aims, Melbourne’s 44 housing towers were constructed between 1960 and 1975 and comprised 7,834 apartments - ranging from 27m2 bedsits to 96m2 three-bedroom units. Located on 21 estates, the towers provide homes for some of our most vulnerable populations (low-income, elderly, sick) and account for 10% of the city’s social housing stock.
We have used the Atherton Gardens tower as a case study due to spatial data being available. Whilst acknowledging that every case would need to be evaluated individually, there are ten towers with this same plan, and at 20 storeys with 180 dwellings the building is broadly representative of the average scale of the towers currently slated for demolition.
The intention of this research is not to advocate for one scenario or another, but rather to provide some realistic comparative approaches that could be applied to any of the 44 towers.
Context
International Context
Our public housing towers form part of an international modernist movement to provide high rise public housing throughout the 1950s & 1960s, with many countries now looking at how to best to update this aging building stock. Needing to be updated to current building codes and living standards, European approaches have included the successful retrofitting of existing towers. With lower costs and environmental impacts, European case studies such as Lacaton Vassal’s Bordeaux housing have even managed to avoid social upheaval of needing decant residents.
With both architecture and engineering professions and the State Government having policies to transition to zero emission buildings over the next 20 years, the operational energy and carbon embodied in the towers have been considered as an important part of this research. The embodied carbon in the concrete of the towers has been quantified and resultant emission impacts of rebuild and refurbishment options compared. In comparison to a new building meeting the energy efficiency requirements of our current building codes, the operational energy savings of retrofitting options has also been identified.
While the significant social impacts and costs associated with the relocation of residents from the towers is beyond the scope of this research, strategies that can minimise the need and time to relocate residents are seen to hold great public value - both in avoiding breaking up established communities and avoiding the significant decanting costs involved. The demolition of all the towers and resulting long-term decanting of approximately 10,000 residents into the Melbourne rental market will add further pressure to the housing crisis we are currently facing.
Sustainability
Social Considerations
Methodology
Methodology
Whilst there have been many architectural schemes proposed for these towers with various potential improvements and modifications, this study excludes design as a variable and simply looks at the comparative case of refurbishing versus demolition and rebuild.
The study is not a proposal for a real design or building outcome, but an ‘apples for apples’ comparison that could inform future designs for the towers and/or their replacements.
We considered 3 scenarios:
Case 1 - Demolish & Rebuild.
Case 2 - Retrofit: Minimal Intervention for Building Code compliance.
Case 3 - Retrofit: Upgraded Intervention to meet requirements of social housing standards.
Demolish and rebuild an equivalent building to current standards, minimum compliance (equivalent to current new social housing construction approach).
Externally: provide seismic strengthening, provide new insulated thermal layer to existing precast facade. Internally: refurbish building and bring up to current regulatory standards, while maximising environmental initiatives that would reduce operational costs for residents. This scenario would meet the requirements of current market housing.
As per Case 2, but adding fully new external bracing facades with balconies for each apartment. This scenario would meet the requirements of current social housing.
Case 1
Case 2
Case 3
Methodology
For each of these cases, we evaluated three key figures: capital cost of construction, operational cost (per annum) and embodied carbon of building works. In addition, we considered the time taken for construction and also possible staging, as this would affect the length of time residents would have to move out for and be housed elsewhere.
There are many other factors that could be considered, including the direct costs of decanting residents and rental of alternative housing during construction plus indirect costs of relocation including impact on education, health and wellbeing – however these have been excluded from this study and can be referred to elsewhere.
To determine the scope of works required we assessed the existing building spatially and consulted with an experienced team for advice on works required. This advice is high level and would require verification if works were to proceed – however onsite verification of actual building conditions was not feasible within the scope of this project.
The key areas of technical consideration informing the cost plan were:
Building Surveyor: access and egress, requirements for fire protection/ fire separation between dwellings
Structural engineer: reinforcement of existing structure to meet current requirements for seismic (earthquake) resistance. Overall performance of building and structural principals. Likelihood of concrete cancer and potential rectification.
Acoustic engineer: requirements for acoustic separation between dwellings
Environmental engineer: requirements for thermal performance of envelope and options for services including heating, cooling and ventilation.
A cost plan was developed for each scenario taking into account the scope of works and analysing the likely construction approach that would be taken by a large-scale commercial builder. A detailed explanation of assumptions for each of the above is included below.
Existing Conditions & Design Assumptions
Existing Building
The existing building was prefabricated and constructed as a total precast system, with precast slabs supported on precast walls throughout. The walls combine to provide lateral load resistance using predominantly the weight of the building to clamp the walls down. In some of the later towers, the walls are post-tensioned down at the lower floors.
The primary internal and external walls are only 100mm thick in the upper floors, thickening to 150 for levels 7-12 and 175 below this. Secondary internal walls are thinner throughout. Floors are 125mm thick throughout.
The structure transfers above ground floor to reinforced concrete frames: these“legs” providing the open public spaces predominantly on the wings of the building. Some walls run to ground around the stair and lift cores, plus surrounding the ancillary spaces central to the building. The buildings are piled under these legs and core structures.
The buildings are exposed aggregate and/or painted: it is assumed that the panels have remained dry and hence carbonation of the concrete is not yet critical. The panels should be treated with a specialist cementitious coating to assist with extending their life further against carbonation. This warrants further investigation in the buildings, and it may vary according to the accuracy of production, concrete quality and potential degradation in use.
Generally it has been assumed that carbonation (or ‘concrete cancer’) is more likely in the external facade due to weather exposure. Hence our approach has been to clad over the facade and encapsulate the panels in Case 2, and to replace the external facade completely in Case 3.
Floor Plan
Building Section
Structural Approach
The buildings have an inherent seismic resistance from their height: the nature of earthquakes possible in Australia captured by the standard AS1170.4 illustrates how much less energy is imparted into buildings with long periods (those that sway slowly). Site specific hazard analysis (PSHA)_using the latest research is likely to show these “code” demands are conservative.
The precast nature of the floors means they have less tying capability as more conventional in situ structures. However the distributed seismic resistance means the demands on the floors to do this tying are small, with the exception of first floor.
The existing building has significant irregularity at the open ground floor, and the change in seismic resistance is particularly critical for the small tying capability of the precast floors. This can be retrofit with supplementary seismic resistance fattening the “legs” and providing tying across the structure in key locations.
This work, carried out at the low level, has little disruption to any refurbishment project above.
Plan Level 0 (Ground Floor)
Showing extent of new strengthening works
Seismic Strengthening Zones
Isometric view
Design Assumptions
Fire Separation, Access & Egress
The existing building floor-to-floor levels are very tight at 2590mm (compared to say 3000 for contemporary construction). The existing concrete slabs are 125mm thick, leaving 2465mm internally from floor level to underside of slab. This poses some difficulties for compliance as internal minimum ceiling heights of 2400 leave only 65mm for finishes. In addition, the 125mm slabs would not achieve the required 90 minute fire-rating between dwellings in their current state. The consultant team reviewed this condition and noted that a number of the existing buildings were upgraded with fire sprinklers in recent decades (and fire sprinklers are assumed in all 3 cases). By adding 2 layers of 16mm fire-rated plasterboard on 28mm resilient mounts, the distance between fire source and existing concrete reinforcement is increased and the overall FRL between dwellings is increased. Further review by a fire engineer would be required for a performance solution specific to the actual conditions of each building.
Acoustics
In tandem with fire separation, the current construction of single-layer concrete walls and slabs is unlikely to meet current requirements for acoustic separation between dwellings. The above ceiling linings on resilient mounts would improve transmission between floors to achieve compliance, assuming new carpets are installed to all units. An additional acoustic layer has been assumed for party walls between adjacent dwellings on the same floor; this discontinuous layer being offset to one side of the concrete walls to avoid symmetrical acoustic reverberation. Both the new wall and ceiling linings are able to be absorbed into the refurbished apartment interior layouts.
Internal Architectural Fit-out
For costing purposes, the interior of each apartment has been assumed as fully refurbished, with all new services, finishes, fittings and joinery within the existing structural shell. In the refurbished cases 2+3, it is assumed that 80% of internal walls will be retained, as these have a structural role for lateral racing of the egg crate concrete structure. The existing walls provide a number of limitations on internal apartment layout, and room functions are assumed to remain as per existing in both case 2 and 3, with additional amenity to living rooms for Case 3 through new balconies.
Upgrades to Public Areas
Allowance has been made for new lifts and full replacement of service pipe runs for the refurbished cases. Existing fire stairs are spatially compliant, however new stair nosing’s, handrails and indicators would be required and are included. The cost plan also assumes a fully new roof to the existing structure.
Typical Floor Plan
Typical Section
Environmental
Understanding the environmental requirements
Case 1 was considered as a brand new building of the same design and scale as the existing building, but aligned with current building code minimum compliance. Energy consumption was estimated based on dynamic 3D thermal simulation of a year’s operation using industry standard software Integrated Environmental Solutions: Virtual Environment (IESVE). Water consumption was estimated in line with the Green Building Council of Australia’s Potable Water Calculator Guide.
Waste generation was calculated based on Sustainability Victoria waste generation rates for multi unit developments. Embodied carbon was estimated based on the Race to Net Zero Carbon report by the Low Carbon Living CRC, Low Carbon Institute and UNSW.
Cases 2 and 3 considered an environmental retrofit targeting maximum performance using financially viable initiatives. Initiatives considered included upgrading the facade performance, adding heat recovery ventilation, installing domestic hot water heat pumps, adding a rooftop solar PV system, installing efficient water fixtures and fittings, installing rainwater collection for laundry and toilet flushing, and greywater diversion for subsoil irrigation. Operating costs and carbon were calculated using the same methodology as in Case 1, but including the cumulative impact of all of these initiatives.
Embodied carbon was calculated in Cases 2 and 3 by estimating the equivalent embodied carbon for a new build (in line with the methodology for Case 1), and subtracting materials being retained on site based on the quantity surveyor cost analysis. Some reductions in embodied carbon of new materials were also considered to allow for low carbon material selection where possible.
Inputs used in the calculation of environmental impact and operating costs can be found in appendix.
Produces more energy than it consumes
$770,000
$250,000
$250,000
Embodied Carbon
Embodied Carbon Summary tCO2
Embodied Carbon
Breakdown
47,000 tCO2
30,000 tCO2
31,000 tCO2
Results Summary
These results demonstrate that retrofitting options can achieve substantial savings across both capital and operational costs in comparison to a new build of the same floor area and configuration. There are also significant reductions in embodied carbon. The overall time taken for construction of retrofit options is marginally less than new build, however the real benefits are in the ability to stage construction level by level (or in groups of levels) which would allow existing residents to be relocated for a much shorter period of time. The reduced impact on existing communities, and the ability for residents to stay connected to existing networks of support and maintain coherence of social bonds should be a major consideration for any project on these sites.
The costing results for Case 2 and 3 are quite close, which suggests that the benefits of a new braced façade with additional amenity in the form of balconies for all apartments and larger windows (Case 3) would potentially be the best value-for- money outcome.
Community Impact
Understanding the impact on residents across cases
All tenants are vacated and relocated for 25 months.
Staged relocation possible, floor by floor in sync with proposed work schedule.
Discussions
Key Findings
Summary of key findings for Atherton Gardens
~$35-41 million reduction in capital cost (-25-30%)
~$500,000 per annum reduction in operating costs (-70%)
Opportunity to retain residents through construction programming
Includes earthquake upgrade
34-36% reduction in embodied carbon
Typologies
Identifying the opportunity of how it impacts all 44 buildings
Identifying the impacts across all 44 buildings (based on Case 3)
~$1.5 Billion reduction in capital cost (-25%)
~$22 Million per annum reduction in operating costs (-70%)
62,920 tonnes per annum reduction in operational carbon (-84%)
~27,000 residents homes are restored
748,000 tCO2 reduction in embodied carbon (-36%)
Recommendations for Further Work
Retrofitting options for the towers should be properly investigated and solutions that could be applied across multiple sites properly designed and costed. Individual sites should then be assessed to see if retrofitting options, along with additional medium density infill buildings, may be able to provide less disruptive and lower cost solutions to the redevelopment of our public housing estates.
This study is not intended as a stand-alone solution to the future of Melbourne’s Housing Commission estates; however it provides evidence to assist with retrofit of the existing towers as one of a number of strategies for increasing housing numbers and urban design amenity on these important public sites. A range of medium-rise infill buildings and substantial landscape design specific to each site would be required to augment refurbished existing towers, in consultation with existing residents and communities.
Cost Parameters
Comparative Retrofit Scenarios for 20-Storey Tower at
