ConstructioNews
Vol 29 No 6 (07-2020)
Recovery Work for Block 4 at the Central Police Station Compound
Background
The Central Police Station compound (CPS), with a history of more than 170 years, has been conserved and revitalized as Tai Kwun – Centre for Heritage and Arts by the Hong Kong Jockey Club (HKJC) and the Government of the HKSAR since 2008. The HKJC has been responsible for the revitalization. Of the sixteen historic buildings, fifteen have been restored and the CPS Compound was opened to the public in May 2018. The remaining building, the Married Inspectors’ Quarters (Block 4), sustained a partial collapse during the restoration work in May 2016. The recovery work is now planning to be conducted.
Introduction of Block 4
Block 4 is one of the sixteen historic buildings in the CPS Compound and locates at a prominent location overlooking the Pottinger Ramp to the north and the Parade Ground to the west. It is a three-storey masonry building with mostly timber floors supported by brick walls. Block 4 is formed by two adjoining parts, the north wing and the east wing. It was built as a domestic building in the 1860s and used as police officers’ quarters.
Renovation before Collapse
When Block 4 was decommissioned in 2005 to 2006, it was in a dilapidated state. Pre-work surveys conducted by conservation architects and structural engineers from 2009 to 2011 noted an array of problems in Block 4, including cracks in the brick walls and evidence of termite infestation in some of the floor timber. After works started in 2012, more defects were disposed upon removal of internal and external finishes. The brickwork was determined as in a poor to very poor condition in some areas, especially those of the roof eaves and some brick piers could be clearly noticed.
The recovery work aimed at repairing the defects and making changes to fulfill statutory requirements, including the installation of a lift and ramps, lavatories, and associated internal alterations to walls and floors. All original joinery components were to be repaired or replaced on a like-for-like basis if found beyond repair. Pottery balusters with granite handrails on the verandahs were to be repaired and strengthened. For hand rails on the upper levels, supplementary rails were to be installed. The clay roof covering was to be replaced on a like-for-like basis with the addition of thermal insulation and a vapour barrier. The main staircase was built of cantilever granite stone steps from the ground floor to the first floor, and timber from the first floor to the second floor. Since all stairs did not meet the current safety standards, plus the original plan for Block 4 limited its use, therefore the stone steps and most of the timber stairs were planned to be retained after modifications. The upper section of timber steps were planned to be replaced because they were very steep, and in order to comply with fire safety requirements, steel was to be used.
When Block 4 partially collapsed on 29 May 2016, three fourth of the planned recovery work had been finished and strengthening work was in progress in the north wing. The incident resulted in the loss of the northwest corner of that wing. The area of the collapsed part was about eight metres by ten metres, or approximately 15% of the Block 4 footprint. Vertically, the loss extended from roof level down to the top of the granite revetment walls. The associated floor structures on the first and second floors were also lost.
Measures were taken to ensure the stability of Block 4 after the partial collapse. These included removing the damaged parts, making safe the exposed roof structure, erecting extensive propping, horizontal ties and scaffolding, and installing sensors to monitor the stability of the building. Existing building materials including granite stonework, joinery and metal parts of balconies were salvaged and stored for reuse where possible.
Renovation Scheme
The partial collapse directed a new thought on the renovation scheme for Block 4. The HKJC suggested a new proposal in 2018 and insisted on the three key principles, namely safety, authenticity and integrating old and new architecture.
Safety
Safety is the most important principle that underpins all work at CPS. While adhering to the principle of “changing only as much as necessary and as little as possible”, HKJC suggested to conserve and revitalize the heritage site to meet modern standards so that it can be opened up safely for sustainable public benefits.
Authenticity
The recovery plan will preserve key architectural traits of Block 4, like the brick facade, which is a significant heritage element of the building. The recovery team will also reinstate or reuse salvaged materials to reduce the loss of heritage and their values.
Integrating old and new architecture
The 2018 proposal aimed at revitalizing Block 4 as a medium scale venue for public activities, befitting the ambience of the CPS Compound and benefitting a broad audience. It was envisaged that the ground floor will be reserved for a visitor centre and heritage galleries, whilst the upper floors will be used for public activities and events.
The extant part will largely maintain the current internal configuration but require extensive structural strengthening with steel and reinforced concrete. Eight percent of the extant part adjacent to the collapsed part will be removed to facilitate safe construction. The plan also called for improving public circulation by opening up the verandahs of the extant part. To maintain the stability of the existing main staircase, the recovery plan proposed rebuilding the stair with concrete.
The rebuilding of the collapsed part will be linked to the extant part to form an architecturally and functionally connected building. With a double layered Chinese style pan-and-roll tile roof, light colour brick walls and a glazed balcony on the second floor, the new annex will be a modest and contemporary expression of Block 4.
Update on Recovery Plan
Structural frame
In order to safely recover Block 4, priority must be given to retaining the exterior walls of the extant part as they are the façade of the building. The creation of a structural frame inside the building is needed to be done before strengthening the exterior walls so as to take the load away from the external walls. The existing brick walls cannot be relied upon for strengthening because they, too, are very weak and supported with props. The bonding quality of the masonry was low as acknowledged from the previous collapse. There were hidden cracks and voids in the brickwork and some bricks were seemingly unfired. The specialists concluded that the building is considered to have a very low margin of safety. Reinforced concrete frame was selected as the optimal solution as it is easier to adapt to the uneven profile of the building and offers the best protection with the lowest risk. The new concrete frame will include new concrete floors, columns and walls. The concrete walls will also act as stiff shear walls to reduce wind loads transferred to the weak brickwork. The concrete frame will be tied to the external brick walls with stainless steel bolts. Vertical sliding joints will be added between the concrete frame and the brickwork to take away vertical loads from the brickwork. In addition, brick arches will be supported by stainless steel arches underneath, which can prevent the bricks from falling and thereby enable it to function.
Roof
To strengthen the roof, a new system of steel roof trusses will be added to the existing timber truss system to take over the weight of the roof from the timber members. The steel trusses will be connected to the concrete frame, hence taking away the load from the brickwork. Most of the existing timber trusses will be retained and repaired but will not bear structural load.
Retaining wall
Adding new strengthening materials will increase weights on Block 4. It is therefore important to ensure that the weight will not create undue load to the granite retaining wall adjacent to Arbuthnot Road, on which Block 4 is sitting. It was concluded to install a cast mass concrete behind the existing retaining walls to reduce most disruption. This will help stabilize and maintain the appearance of the retaining wall, another crucial heritage characteristic of the CPS Compound.
Foundation
A cellular raft system will be built for the extant part as the new foundation as it could even out the load across the footprint of the building and sit well with the cast mass concrete behind the retaining wall. Furthermore, the system can provide a uniform load platform for the whole building because the new annex will also be built on a cellular raft foundation. This method will also require removing the existing interior walls and floors during work and reinstating them after the construction of the foundation system.
Heritage Consideration
The partial collapse of Block 4 has already substantially damaged the historic fabric or the character defining elements of the building. The updated plan will undoubtedly result in a further loss, especially the timber floor joists, plaster ceilings, brick internal walls and plaster finishes, and the staircases. However, these elements will be salvaged and reinstated where possible, for example, all joinery items such as floor boards, paneled doors, architraves and skirting, the stones and some of the timber on the stairs. The reinstatement will largely follow the original spatial configuration of Block 4 except the new stairs, lift and lavatories, the opening up of the verandahs and the new annex.
Conclusion
The partial collapse of Block 4 resulted in a profound loss of historic fabric but highlighting the challenges facing conservation and revitalization projects at the same time. Heritage revitalization is not merely about preserving a historic building but revitalizing it for modern use sustainably. Public safety should be placed in the first priority. Hoping that Block 4 can be recovered safely as soon as possible, the building will then join the other buildings and facilities in the CPS Compound to provide excellent value to Hong Kong.
Reference: Tai Kwun Website
The Central Police Station compound (CPS), with a history of more than 170 years, has been conserved and revitalized as Tai Kwun – Centre for Heritage and Arts by the Hong Kong Jockey Club (HKJC) and the Government of the HKSAR since 2008. The HKJC has been responsible for the revitalization. Of the sixteen historic buildings, fifteen have been restored and the CPS Compound was opened to the public in May 2018. The remaining building, the Married Inspectors’ Quarters (Block 4), sustained a partial collapse during the restoration work in May 2016. The recovery work is now planning to be conducted.
Introduction of Block 4
Block 4 is one of the sixteen historic buildings in the CPS Compound and locates at a prominent location overlooking the Pottinger Ramp to the north and the Parade Ground to the west. It is a three-storey masonry building with mostly timber floors supported by brick walls. Block 4 is formed by two adjoining parts, the north wing and the east wing. It was built as a domestic building in the 1860s and used as police officers’ quarters.
Renovation before Collapse
When Block 4 was decommissioned in 2005 to 2006, it was in a dilapidated state. Pre-work surveys conducted by conservation architects and structural engineers from 2009 to 2011 noted an array of problems in Block 4, including cracks in the brick walls and evidence of termite infestation in some of the floor timber. After works started in 2012, more defects were disposed upon removal of internal and external finishes. The brickwork was determined as in a poor to very poor condition in some areas, especially those of the roof eaves and some brick piers could be clearly noticed.
The recovery work aimed at repairing the defects and making changes to fulfill statutory requirements, including the installation of a lift and ramps, lavatories, and associated internal alterations to walls and floors. All original joinery components were to be repaired or replaced on a like-for-like basis if found beyond repair. Pottery balusters with granite handrails on the verandahs were to be repaired and strengthened. For hand rails on the upper levels, supplementary rails were to be installed. The clay roof covering was to be replaced on a like-for-like basis with the addition of thermal insulation and a vapour barrier. The main staircase was built of cantilever granite stone steps from the ground floor to the first floor, and timber from the first floor to the second floor. Since all stairs did not meet the current safety standards, plus the original plan for Block 4 limited its use, therefore the stone steps and most of the timber stairs were planned to be retained after modifications. The upper section of timber steps were planned to be replaced because they were very steep, and in order to comply with fire safety requirements, steel was to be used.
When Block 4 partially collapsed on 29 May 2016, three fourth of the planned recovery work had been finished and strengthening work was in progress in the north wing. The incident resulted in the loss of the northwest corner of that wing. The area of the collapsed part was about eight metres by ten metres, or approximately 15% of the Block 4 footprint. Vertically, the loss extended from roof level down to the top of the granite revetment walls. The associated floor structures on the first and second floors were also lost.
Measures were taken to ensure the stability of Block 4 after the partial collapse. These included removing the damaged parts, making safe the exposed roof structure, erecting extensive propping, horizontal ties and scaffolding, and installing sensors to monitor the stability of the building. Existing building materials including granite stonework, joinery and metal parts of balconies were salvaged and stored for reuse where possible.
Renovation Scheme
The partial collapse directed a new thought on the renovation scheme for Block 4. The HKJC suggested a new proposal in 2018 and insisted on the three key principles, namely safety, authenticity and integrating old and new architecture.
Safety
Safety is the most important principle that underpins all work at CPS. While adhering to the principle of “changing only as much as necessary and as little as possible”, HKJC suggested to conserve and revitalize the heritage site to meet modern standards so that it can be opened up safely for sustainable public benefits.
Authenticity
The recovery plan will preserve key architectural traits of Block 4, like the brick facade, which is a significant heritage element of the building. The recovery team will also reinstate or reuse salvaged materials to reduce the loss of heritage and their values.
Integrating old and new architecture
The 2018 proposal aimed at revitalizing Block 4 as a medium scale venue for public activities, befitting the ambience of the CPS Compound and benefitting a broad audience. It was envisaged that the ground floor will be reserved for a visitor centre and heritage galleries, whilst the upper floors will be used for public activities and events.
The extant part will largely maintain the current internal configuration but require extensive structural strengthening with steel and reinforced concrete. Eight percent of the extant part adjacent to the collapsed part will be removed to facilitate safe construction. The plan also called for improving public circulation by opening up the verandahs of the extant part. To maintain the stability of the existing main staircase, the recovery plan proposed rebuilding the stair with concrete.
The rebuilding of the collapsed part will be linked to the extant part to form an architecturally and functionally connected building. With a double layered Chinese style pan-and-roll tile roof, light colour brick walls and a glazed balcony on the second floor, the new annex will be a modest and contemporary expression of Block 4.
Update on Recovery Plan
Structural frame
In order to safely recover Block 4, priority must be given to retaining the exterior walls of the extant part as they are the façade of the building. The creation of a structural frame inside the building is needed to be done before strengthening the exterior walls so as to take the load away from the external walls. The existing brick walls cannot be relied upon for strengthening because they, too, are very weak and supported with props. The bonding quality of the masonry was low as acknowledged from the previous collapse. There were hidden cracks and voids in the brickwork and some bricks were seemingly unfired. The specialists concluded that the building is considered to have a very low margin of safety. Reinforced concrete frame was selected as the optimal solution as it is easier to adapt to the uneven profile of the building and offers the best protection with the lowest risk. The new concrete frame will include new concrete floors, columns and walls. The concrete walls will also act as stiff shear walls to reduce wind loads transferred to the weak brickwork. The concrete frame will be tied to the external brick walls with stainless steel bolts. Vertical sliding joints will be added between the concrete frame and the brickwork to take away vertical loads from the brickwork. In addition, brick arches will be supported by stainless steel arches underneath, which can prevent the bricks from falling and thereby enable it to function.
Roof
To strengthen the roof, a new system of steel roof trusses will be added to the existing timber truss system to take over the weight of the roof from the timber members. The steel trusses will be connected to the concrete frame, hence taking away the load from the brickwork. Most of the existing timber trusses will be retained and repaired but will not bear structural load.
Retaining wall
Adding new strengthening materials will increase weights on Block 4. It is therefore important to ensure that the weight will not create undue load to the granite retaining wall adjacent to Arbuthnot Road, on which Block 4 is sitting. It was concluded to install a cast mass concrete behind the existing retaining walls to reduce most disruption. This will help stabilize and maintain the appearance of the retaining wall, another crucial heritage characteristic of the CPS Compound.
Foundation
A cellular raft system will be built for the extant part as the new foundation as it could even out the load across the footprint of the building and sit well with the cast mass concrete behind the retaining wall. Furthermore, the system can provide a uniform load platform for the whole building because the new annex will also be built on a cellular raft foundation. This method will also require removing the existing interior walls and floors during work and reinstating them after the construction of the foundation system.
Heritage Consideration
The partial collapse of Block 4 has already substantially damaged the historic fabric or the character defining elements of the building. The updated plan will undoubtedly result in a further loss, especially the timber floor joists, plaster ceilings, brick internal walls and plaster finishes, and the staircases. However, these elements will be salvaged and reinstated where possible, for example, all joinery items such as floor boards, paneled doors, architraves and skirting, the stones and some of the timber on the stairs. The reinstatement will largely follow the original spatial configuration of Block 4 except the new stairs, lift and lavatories, the opening up of the verandahs and the new annex.
Conclusion
The partial collapse of Block 4 resulted in a profound loss of historic fabric but highlighting the challenges facing conservation and revitalization projects at the same time. Heritage revitalization is not merely about preserving a historic building but revitalizing it for modern use sustainably. Public safety should be placed in the first priority. Hoping that Block 4 can be recovered safely as soon as possible, the building will then join the other buildings and facilities in the CPS Compound to provide excellent value to Hong Kong.
Reference: Tai Kwun Website