7ChapterAuthors:

Er. Lau Joo Ming

Mr Andrew Yoong Yaw Yuan

Mr Wang Chien Looi Design forDesign forBuilding Performance

Design forBuilding Performance

Design for

Introduction

Design for Strength and Durability

Design for Tropical Environment

Maintenance, Monitoring and Repair

New Performance Standards with Assure 3

Conclusion

1. Introduction

••2

Thousands of people were made homeless when a fire broke out at Bukit Ho Swee Estate in 1961. The

people who survived the fire became destitute and had to find shelter among friends and relatives. The Housing and Development Board (HDB) quickly acted to house 6,000 people in rental flats to alleviate the situation. As there were insufficient rental flats, many people were still without homes. The government then ordered the construction of more flats to cater for those made homeless. The HDB responded by building 5 blocks of flats within 9 months.

When HDB was set up in 1960, HDB had a mission to provide low cost housing for a growing population. HDB inherited a huge backlog of demand for housing due to the increase in population in Singapore in the 1950s. Many people were staying in squatters and slums with poor sanitary facilities. Based on a survey, there was a need to build over 100,000 flats in the next 10 years.

Between 1960 and 1965, HDB built 53,000 small flats such as the standard 1-room, 2-room and 3-room flats. These flats were built to accommodate low income families who could only afford to pay low rent. In 1964, the government initiated a bold plan and announced the Home ownership scheme to encourage Singaporeans to buy homes. During the next 5 years, between 1966 to 1970, HDB changed focus and started to build better quality flats with larger floor areas.

Figure 1: A new HDB building at Kallang Basin built in 1970, contrast against the old dilapidated building in the foreground (Source: HDB annual report 1971).

Between 1971 and 1980, HDB provided a wider variety of flats to cater to an increasing discerning population. The flat types included 3-room flats and 4-room flats. These flats range from 70 to 135 sq.m in floor area. In the following decade between 1981 and 1990, the demand for housing increased. In 1981, HDB had introduced a new range of Executive flats and Executive Maisonnette flats for those who can afford to pay for better housing. A record 50,600 new applications for public housing was received by HDB that year. HDB accelerated the building programme and built 327,000 residential flats between 1981 and 1990. This was achieved in part with the use of prefabricated housing.

Figure 2: Vibrant bands of colour are created in HDB building at Serangoon North. The building was constructed using façade with tiles during the 1980s.

Figure 3: Precast facades with colourful motifs decorate a HDB building at Jurong East built during 1990s.

In the next decade between 1991 and 2000, HDB town planning moved in line with the new URA revised concept plan which was introduced in 1991. The new concept plan encouraged a mix of housing between private and public housing. The new town planning concept was implemented in HDB new towns at Sengkang, Punggol and Sembawang.

HDB has a wealth of experience in providing a unique system of public housing, having built about 970,000 flats. This chapter provides an insight to the engineering design process, the reasons why certain changes were made, the improvements carried out to improve the performance of the building and maintenance issues. Important design considerations such as robustness, resistance to lateral loads, durability, insulation and waterproofing are highlighted.

••3

The development of an HDB estate is part of a long process involving planning, design, construction and maintenance. HDB employs

a team of architects and engineers to provide in-house consultancy for HDB projects. During the planning and design stage, the architects have determine the size of the development, the number of dwelling units to build, the number of carparks and types of amenities. The architects prepare the architectural design drawings while the engineers produce the detailed structural drawings which show how the buildings are built.

HDB buildings are designed as reinforced concrete frame structures. The design of the buildings must meet the requirements of quality, stability and durability and has to be carried out in compliance with the prevailing Codes of Practice, the Building Control Act and Regulations and Statutory requirements.

2. Design for Strength and Durability

Table 1: List of codes of practice.

2.1 Quality assurance in structural design

The design work carried out by the former Structural Engineering Department had evolved into a system, where the design process were documented and shared among engineers within the Department. The documented procedures were then refined into a Quality Assurance system.

The advantage of the Quality Assurance (QA) system is that it provides a systematic and multi-level checking system for the structural design. The former Structural Engineering Department had adopted a total Quality Management System which consisted of QA in design, QA in construction and QA in maintenance. These 3 systems are linked to each other and complement one another.

HDB buildings are

designed as reinforced

concrete frame structures.

••4

Code of practice Title

SS CP65 Singapore Standard on Code of Practice for Structural Use of Concrete

BS 5950 British Standard on Structural Use of Steel work

SS CP4 Singapore Standard on Code of Practice for Foundation

BS 6399 Design Loading for buildings

BS 8007 Design of concrete structure for retaining aqueous liquids

5••

Figure 4: View of Geoinfosys website.

••5

Level 1

The design engineer proposes the structural system, prepares design calculations, and checks the structural drawings. The Professional Engineer who is a Senior engineer approves the structural system, checks the design and drawings. The critical areas of quality control and other comments from the Professional engineer and the design engineer are recorded, and sent to the construction engineer on site.

Level 2

The second level of checks is carried out by the Design Review panel, which comprise a team experienced engineers. Two Design Reviews are conducted. In the first Design Review, the structural system, concept, floor plan and the loading plan is reviewed. In the second Design Review, the panel reviews the drawings and focus on the specifications, durability, cost-effectiveness and ease of construction.

Improvements in design and detailing are recorded formally in Design review records. The Design review records are circulated to other design engineers so that the design improvements can be carried out for other design projects.

2.1.2 The structural design of HDB buildings includes foundation design and superstructure design. The design work for foundation start once the basic structural system has been developed and preliminary sizes of the structural members

2.1.1 Design checks

Before foundation design can proceed, the engineers have to understand the ground condition where the buildings will be built. HDB engages Soil investigation (SI) term contractors to carry out boring to collect soil samples for testing at the laboratory. The location and depth at which the samples are taken are determined by the engineer. The soil samples are sent to the geotechnical laboratory for testing to obtain the properties of the soil.

During the last 45 years, HDB had carried out thousands of soil investigation work and the geotechnical information from the boreholes had been recorded and archived. In 2004, the geotechnical information from the past records were saved in a computer database to facilitate retrieval of data. This was carried out in an R&D project, “Development of a geotechnical information system for singapore.” The R&D project for digitizing, storing, database development and setting up a website was a joint collaboration with NUS and partly funded by BCA. The information has been made available to the public through the websitewww.eptc.com.sg.

2.2 Foundation works for HDB residential buildings

have been determined. The foundation would then be designed to support the building weight.

Major geological formations in Singapore are broadly sedimentary Jurong Formation in the south and the west, residual soil of Old Alluvium to the east, granitic Bukit Timah Formation ranges from central to the north and clayey Kallang Formation distributed along the coast line island wide, especially at river mouths.

These varied soils has different impacts and influences on the types of foundations to be used for a building. Some soils are inherently better suited for driven piles while other types of soil condition require bored piles. The different types of soil pose challenges to engineers designing pile foundation systems for high rise HDB residential blocks island wide.

2.2.1 Geology of Singapore

Figure 5: Drilling rods are removed to change over to sampler when the correct depth is achieved.

••62.2.2 Deep foundation system

The foundation is a critical part of the building structure as the foundation has to support the entire building self weight, live load and lateral loads. The 2 main types of piles used in HDB’s foundation systems are:

(a) Steel piled foundation

Steel H-Piles adopted by HDB varies from 200X200mm X12mm (denoted as Am pile) to 500X500mm X 20mm (denoted as Gm pile) with nominal working loads from 40 tonnes to 215 tonnes. Steel H-piles are normally designed for areas in which the subterranean soils are poor and/or have high water table.

Close attention was paid to steel pile driving and installation process. Hammers of correct weights and types would be selected for driving different sizes of piles. Type of soil strata for a specific site could also have significant influence on the types of hammers to be deployed as well as the sizes of pile used. In Jurong West where sedimentary soils were found, large sized piles with over 100T working loads would have great difficulty

penetrating hard clays such as shales and mudstones. This could result in piles unable to develop adequate skin friction (as evidenced by gradually increasing settlement of pile under maintained load test). The solution was to use small piles and correspondingly smaller hammer to achieve a deeper pile penetration. Smaller piles were able to reduce fracturing of the hard clay as the soil displacement would be small especially at the pile toe.

Many piling sites in Jurong West required very hard driving. In order to achieve a faster rate of pile driving contractor many use oversized hammers or excessive hammer drops during driving. This could damage the pile heads during hard driving and cause buckling of the smaller piles. The problem of damaged pile heads was

Sizes of bored piles adopted by HDB varies from 600 mm diameter to 1500 mm diameter with nominal working load of 150 tonnes to 900 tonnes respectively. They are normally designed for areas in which the subterranean soils are dense with low ground water table.

For the eastern part of Singapore, bored pile concreting was normally carried out due to the soil conditions. The subsoil types normally fall under the Old Alluvium geological classification. Commonly of cemented sands with higher unit skin friction, the length of the pile formed (typically 15 m to 20 m) was much shorter than those in the northern and western regions (usually exceeding 20 m). As a result, the number of piles that could be formed per day per boring machine was much higher. Typically between 6 to10 numbers of 700 mm diameter bored piles of about 18 m length could be achieved in eastern worksites as compared to about 2 to 6 numbers for other locations.

••7

(b) Bored pile foundationeven more pronounced in Bukit Panjang and Chua Chu Kang Towns which lie in the Bukit Timah granite formation. Contractors had to be careful to avoid damage to the pile when hard soil stratum such as granite bedrock or boulders were suddenly encountered during pile driving (also known as “sudden set”).

In areas where poor soft soils exceeded 30 m deep, pile capacity for steel piles were reduced to avoid excessive settlement of the pile head from elastic compression of long piles. Where pile penetration were shallow say less than 8 m deep, downgrading and bracing of individual piles within pile group had to be carried out to prevent disturbance to the pile by pile cutting and excavation. Any downgrading and bracing of shorter piles were checked against possible excessive differential settlement of piles, especially if the adjacent piles were much longer.

Tests were carried out to evaluate the performance of the piles. Either 2 or 3 piles representing different pile sizes and/or depths would be randomly selected for the Standard Load test. The kent ledge method of loading were done at early stages of piling to verify carrying capacities of piles. This was done early to detect and arrest problems so that remedial action could be taken before the bulk remedial action could be taken before the bulk of the piles were installed. To prevent cracks of the piles were installed. To prevent cracks in building structure due to excessive in building structure due to excessive settlement, the maximum allowable settlement settlement, the maximum allowable settlement of steel piles loaded to twice their nominal of steel piles loaded to twice their nominal working load would be 25mm.working load would be 25mm.

Regular checks were also carried out on Regular checks were also carried out on the grade and quality of the steel pile sections the grade and quality of the steel pile sections such as pile deformities, welding and splicing such as pile deformities, welding and splicing of the pile sections.. of the pile sections..

Figure 6: Welding of joints between steel piles.Figure 6: Welding of joints between steel piles.

In Yishun Town, there were many long steel casings up to 22m long that were used to support the wall of the bore holes and to keep out the ground water, Water bailors had to be used to remove water that was found either on the granite bearing stratum or had seeped down the casing walls. In certain cases, poor “seating” of the steel casing on the uneven granite bearing stratum resulted in the entry of fine sand or silt slurries. When this occurred, the steel casing would have to be rotated and reseated repeatedly to contain entry of slurries before concrete could be poured. The use of long casings required repeated jointing and cutting (during casing insertion/ withdrawal) which prolongs the overall construction time. Concrete used for casting of bored piles were normally added with super plasticizers and retarders to extend the setting time ( normally up to 3 hours or more).

In Bukit Batok, tremie concreting method was employed to overcome high ground water table not uncommon in an area with Bukit Timah granite formation. Long tremie pipes and higher grade concrete (from Grade 35) were important requisites for underwater concreting of piles. Very often, bentonite slurry or other similar stabilizing mediums were added into the boreholes to support the borehole walls and thus facilitate the concreting works. Higher slump concrete (about 100 mm to 150 mm) was necessary to allow the continuous discharge of the concrete out of the tremie pipes at the bottom of boreholes. Water in the borehole would often be displaced progressively as concreting continued.

Tests were also conducted to evaluate the performance of the bored piles. The bored piles were tested up to twice their nominal working load using the kent ledge method. Allowable settlement was fixed at 3 mm per tonne subjected to a maximum of 25 mm.

2.3.1 The structural system

The structural system consists of columns, walls, beams and slabs with precast façade walls or brickwall cladding.

The loads from the slabs are carried by the beams and transferred to the columns. The forces acting on the beams and columns can be modeled using sub-frame. Factors of safety for the dead loads and live loads can be easily applied when using software programmes.

HDB carried out standardization of the sizes of the beams, columns and walls. The dimensions of columns were fixed at 300mm wide and the length varies from 600mm with increments of 100mm. As far as possible, the column sizes were kept uniform from ground

2.3 Superstructure design

Figure 7: Preparation of steel cage before

casting of bored pile.

••8

floor to the top floor in order to improve buildability. The width of the beams was consistent with the columns and was fixed at 300mm wide. The depth of the beam was standarised at 500mm or 600mm.

The slabs were either reinforced concrete slabs or composite slab with precast prestress plank forming the bottom half and concrete topping. The Structural floor slab thickness inclusive of the plank was standardised at 150, 175 or 200mm.

All public housing residential floor heights were fixed at 2800mm. By fixing the floor to floor height, the height of precast façade and other vertical precast components can be standardized. This standardized height resulted in higher productivity and better utilization of mould. The savings were critical in ensuring that the precast implementation for public housing remained viable.

One major spin off from the standardization of beam and column sizes was the implementation of prefabricated reinforcement cages. HDB worked closely with the steel suppliers to introduce a range of standard beam and column reinforcement cages, which could be fabricated at the factory. HDB engineers would design the beams and columns and select prefabricated steel cages using a newly developed code. The use of prefabricated cage code facilitated the identification and subsequent automation of bar meshing and bending processes.

2.3.2 Design for robustness

Buildings are designed to support the self weight of the building, live load and lateral loads acting on the building such as wind loads.

The loads from self weight of the building, live loads and lateral loads can be calculated with some degree of certainty. In accordance with CP65 and BS8110, factors of safety were also applied to the building loads to increase the overall building design strength.

The method of calculating the building design strength is adequate for most normal loading cases. However, there is a remote possibility that there are abnormal loads or accidental loads acting on the building. This can be caused by some unforeseen accident eg gas explosion within a flat. When such loads occur, engineers are concerned whether the loads can cause part of the building to fail.

Designing buildings for robustness means designing the building to cater for accidental loads, and to ensure the buildings are not subjected to the effects of progressive collapse.

Figure 9: Prefabricated beam cage are placed in the beam formwork before casting.

Figure 8: Prestress planks form the bottom half of the slab and are used as permanent formwork.

••9

All HDB buildings are designed to British standard BS8110 requirement for robustness. This include the provision of peripheral ties, internal ties, external column/wall ties and vertical ties.

The load bearing structural elements such as columns and walls are the critical members supporting the building. For each structural frame, a three column system was used to provide redundancy and increase the robustness. When the two column structural frame was adopted, additional catenary bars were added to the slab reinforcement. This design allowed the slabs to transfer loads to adjacent columns when a column failed.

The building structure should not have any inherent weakness. All columns loads are transferred directly floor to floor down to the foundation. No transfer beams are used for HDB high rise residential buildings.

HDB buildings are designed to resist lateral loads caused by wind. For the purpose of design, the wind loads which are dynamic in nature are assumed as static loads acting on the building at each floor level. The design wind load is calculated using the Code of Practice CP3.

Besides meeting stability requirements, the building structure itself have to be strong enough to resist wind loads. The wind loads are transferred from the floor plate to the shear walls through diaphragm action. As the walls are different in size, the distribution of the loads are dependent on the stiffness of the walls. Walls which are stiffer will attract more loads.

The wind loads cause the building to deflect and increase in the loads of the beams, walls and columns. The axial forces, shear forces and bending moments caused by the wind loads can be determined by running software programmes such as SECAD. The programme can also calculate deformation of the building due to wind load. The total deformation of the building has to be less than 1 /500 of the building height. This limit is set to avoid cracks in the building.

2.3.3 Design for wind load

Figure 10: Models of HDB buildings at Duxton Plain are placed on a rotating table and subjected to wind load testing at NUS.

••10

2.4 Design for durability

Careful selection of materials used in the construction of buildings help to ensure that buildings last. Concrete and steel are often used in building construction because the materials have high strength and are durable.

The concept of durability was important in HDB design even before the British Standard BS8110 was introduced. However the BS8110 highlighted various aspects of durability that made it clear how concrete quality was critical to the durability of building structures.

Durability can be defined as the ability of the building to perform its function over a period of time or service life. As the building consists of many components and is constructed using different building materials, the durability of the building is often related to the durability of the building components.

It is important to understand that some of the building components are replaceable while some

are not. For example, windows and doors can be replaced easily when the windows and doors have deteriorated. However parts of the building structure cannot be replaced easily especially if the components are load bearing structure such as columns and walls.

The durability of the building structure is therefore critical to the durability of the building, and is often used by engineers to predict the service life of the building.

Some of the oldest HDB buildings are located at Tiong Bahru Estate. The 3 to 4 storey high residential blocks were built by the Singapore Improvement Trust (SIT) between 1937 to 1949. The buildings are still occupied and show no serious signs of deterioration. The buildings are generally in good condition even though the buildings are about 60 years old.

Figure 11: Residential buildings at Tiong Bahru Estate built in 1940s are in good condition.

••11

The durability of the building can be improved by reviewing the entire process from design to construction. In terms of design, HDB specified better grades of concrete for construction of HDB buildings. During the 1980s, HDB developed a comprehensive material quality control system to ensure that the concrete quality used was consistently good. These quality control systems are still implemented today.

Concrete is the basic material used for both reinforced concrete structures and precast concrete components. Over the years, HDB has taken steps to ensure that the quality of the concrete meets the latest prevailing Codes of practice.

HDB design is based on the British Codes of practice. Prior to 1974, HDB engineers used the British Code of Practice CP 114 to decide on the concrete and design method. After 1974, HDB proceeded to use British Code of Practice CP 110 which replaced the CP114.

2.4.1 Improvement in concrete grade

With the introduction of British Standard BS8110 in 1986, HDB adopted the new standard in the design of HDB buildings. The BS8110 highlighted the importance of the concrete material in the durability of concrete structures. Durability of concrete is dependent on the performance of concrete in terms of permeability to water and oxygen. The exposure of the concrete would therefore govern the minimum

Table 2: Implementation of better concrete grade.

grade of concrete. In accordance with the BS8110, it was therefore necessary to specify a higher concrete grade in HDB buildings. In 1987, HDB specified that the minimum concrete grade was Grade 30. This improve-ment in concrete grade was also made possible with advances in use of admixtures in the production of concrete.

••12

Code Concrete grade Year of implementation

CP114/ CP110 20 Before 1986

BS 811025 1986 (transition)

30 1987

CP65 40 1996

13••

Steel reinforcement bars are provided in concrete structure to improve the performance of the building structure. Together with the concrete, the steel reinforcement bars behave in composite action and lend strength to the structure. In reinforced concrete columns for example, the steel reinforcement increases the load carrying capacity of the columns.

As buildings get older, reinforced concrete structure start to deteriorate. For buildings more than 10 years old, concrete spalling have been known to occur. Concrete spalling occurs on reinforced concrete beams, columns and slabs when the steel reinforcement starts to corrode.

According to the British Standard BS 8110, concrete cover affects the durability of the reinforced concrete structures. In new reinforced concrete structure, the concrete surrounding the steel reinforcement protects the steel against rusting. However over time, the presence of carbon dioxide in the air can react with the concrete and alter this protective behaviour. The duration of protection provided by the concrete to the steel reinforcement

2.4.2 Improvement in concrete cover

depends on the thickness of the concrete cover.

With the introduction of British Standard BS8110 and SS CP65, the minimum concrete cover was increased to 25mm for HDB buildings.

During the 1980s, HDB implemented major improvements in quality control on site. As durability of concrete depended on the concrete cover, HDB introduced special precast concrete spacers and plastic spacers. The spacers are designed to be placed between the steel reinforcement and the formwork so that the steel reinforcement would not shift during casting. The spacers are produced by HDB approved suppliers to ensure that the dimensions are accurate. These spacers were so effective, the use was also widely accepted by precasters in production of precast concrete components.

In mid 1980s, HDB received feedback on spalling concrete occurring on the ceilings of toilets and kitchens in HDB flats. As a further improvement, HDB specified galvanized reinforcement mesh in toilets and kitchens to prevent rusting of steel reinforcement. The zinc coating on the surface of the steel reinforce-ment acts to protect the steel from corroding. This would effectively stop rusting of steel reinforcement.

The provision of galvanized reinforcement mesh was implemented for toilets in 1985, and for kitchen in 1987. After a study was conducted in 1992, the provision of galvanized reinforce-ment mesh for kitchen was discontinued as the use of higher concrete strength was found to be adequate in preventing spalling concrete.

2.4.3 Changes in detailing

••13

Tests are also conducted on the materials used in production on concrete. Samples of cement, sand and coarse aggregate are taken on site and are tested at the laboratory at HDB’s Prefabrication Technology Centre. The materials normally come from a single source or manufacturer. In the event the materials come from different source, care is taken to ensure that the materials are not mixed.

••14

The quality of building works is dependent on the workmanship of the contractor. On each building site, HDB has engineers and clerk-of-works who will check the quality of the building works to ensure that the buildings are constructed properly using good quality materials as specified in the drawings.

2.4.4 Material quality control

The concrete and steel supplied to HDB construction sites have to meet a strict testing regime, which is specified and controlled by HDB.

For every 40 cubic meters of concrete produced, the building contractors have to prepare concrete cube samples. The samples are tested at 7 days and 28 days at the laboratory at HDB’s Prefabrication Technology Centre. The tests done on concrete cube samples on the 7th day is for monitoring purpose while the tests done on 28th day is used to determine if the concrete strength meets the passing criteria set by HDB.

The concrete strength of cubes tested on 28th day is evaluated based on rolling average of 4 samples and individual sample. For example, to meet the characteristic strength of 40N/mm2, the average of 4 samples must

2.4.4.1 Quality control for concrete and steel

be more than 43 N/mm2 and the individual sample must be more than 37N/mm2. In the event of concrete cube failures, HDB would ask the contractor to take corrective action to improve the concrete strength and to carry out non-destructive test on the structure. Administrative charges are also imposed on the building contractor in the event of concrete cube failures.

Figure 12: Centralised testing of concrete cube at the laboratory at Prefabrication Technology Centre.

15••

For steel reinforcement, samples are taken from the site and sent to the laboratory at HDB Prefab-rication Technology Centre for testing. For sampling, 3 pieces of steel bars are cut from each size of steel reinforcement used on site.

S/N TYPE OF MATERIAL TYPE OF TEST

1 Cement

a) Consistency

b) Soundness

c) Setting time

2 Sand

a) Sieve analysis

b) Silt content by decantation method

c) Bulk density

d) Relative density

3 Aggregate

a) Sieve analysis

b) Flakiness index

c) 10% fines value

c) Bulk density

d) Relative density

e) Angularity number

f) Elongation index

Table 3: List of tests conducted on cement, sand and aggregate conducted at PTC.Table 3: List of tests conducted on cement, sand and aggregate conducted at PTC.

2.4.4.2 Control of ready mixed concrete suppliers

During the 1990s, concrete supplied to HDB construction sites were gradually changed from site batching to ready mixed concrete. When the concrete is produced off site at the production plant of the ready mixed concrete supplier, there would be a system of Quality control to check the quality of concrete. Another advantage was the land space would be available on site if there

was no concrete batching plant. This frees up the space to allow storage space for storage of precast components.

To ensure consistent good quality concrete, HDB introduced a registration system for ready mixed concrete suppliers. Only registered suppliers that meet HDB’s criteria are allowed to be listed in the Approved list of suppliers to supply concrete to the HDB building contractors.

The supplier is required to have a set of The supplier is required to have a set of established Quality Control procedures in the concrete batching system, cube sampling at batching plant and site auditing during concreting. The supplier is required to provide all data and test results for verification by HDB. The materials such as cement, sand, coarse aggregate have to be tested according to the latest standard.

••15

The ready mixed concrete suppliers have to prepare concrete cubes for testing at HDB’s Prefabrication Technology Centre. When there are concrete cube failures or disruption of concrete supply reported, demerit points are given and recorded. The demerit system allowed HDB to monitor the performance of the supplier.

There are currently 22 ready mixed concrete plants which are on the HDB’s list of approved products and suppliers.

HDB buildings have been designed to protect the occupants in the building from the environment outside. The two

main important design considerations are heat insulation and waterproofing. The design is applied consistently to all HDB buildings through the use of the standard details provided in the tender drawings and standard specifications. The standard details and specifications are reviewed periodically to take into account the latest improvement and introduction of new materials.

3. Design for Tropical Environment

The weather in Singapore is warm and humid throughout the year. During the day, the ambient temperature is between 31 deg and 34 deg Celcius.

The HDB buildings are designed to be naturally ventilated. Windows are provided to improve circulation of air and allow the flats to be cooled down naturally.

To prevent heat build up within the flat, HDB provides the following features:

a) Ferrocement secondary roof panels

Ferrocement secondary roof panels are placed on top of the main roof to create an air gap between the secondary roof and main roof. The air gap is a natural insulation layer. Even though the roof panels are heated up but the main roof remains cool.

The thermal transmittance value (U-value) for the system is 0.563 W/ m2 oK.

The ferrocement secondary roof panel system represent an improvement over the previous roofing panel system, which consists of a layer of cement sand screed, an insulation layer and 3-layer bituminous felt waterproofing membrane. The ferrocement secondary roof panel system is shown in Figure 13. In the event of any leak, the ferrocement panels could be removed to assess the condition of the roof and carry out the repairs.

3.1 Insulation from heat

••16

17••

Figure 13: Isometric view of secondary roofing system.

b) Precast gable end wall

The precast gable end walls are precast panels with an insulation layer within the precast component. The precast gable end walls are provided as an external façade facing the western side of the building to insulate the building from heat from afternoon sun.

Figure 14: Precast gable ends walls at Serangoon North.

c) Sun breakers

Sun breakers are sun shade devices fixed permanently on the building. The sun breakers are placed over the window to cover the windows facing the afternoon sun. HDB has implemented two types of sun breakers - the ferrocement panel

and the aluminium panel. Both types of sun breakers were used successfully to reduce the heat from direct sunlight and reduce sunlight coming into the flat to a more comfortable level.

Figure 15: Sunbreakers are provided to shade windows at at Ang Mo Kio.

••17

3.2 Waterproofing

Singapore experiences tropical climate with rain throughout the year. The highest rainfall occurs during the months of December and April. HDB has taken measures to improve the performance of the building in terms of waterproofing. Studies have been carried out to review the roofing system and the waterproofing system.

a) Main roof

The main structural roof is cast with a 1 in 50 slope to allow rain water to drain off at the edge of the building. Before the concrete sets, the main roof is power float to create a dense concrete surface which will act as water barrier.

During the 1980s and early 1990s, the roofing system consisted of a 7-layer of waterproofing membrane and an elevated secondary roofing slab over the main roof. The water proofing membrane installation was critical to overall performance of the waterproofing system. The application of this membrane was time consuming and laborious

as the waterproofing layer which consisted of 7 layers had to be applied layer by layer.

In 1998, HDB implemented the new ferrocement secondary roof panels for new building projects. The new ferrocement roof panels are specially designed with raised edges to allow the panels to interlock with each other. Similar to roof tiles, the ferrocement roof panels are installed at a steeper slope to drain the rain water effectively. Since the rain water is drained off at the secondary roof level, there is no need to provide another layer of waterproofing membrane.

Figure 16: New ferrocement roofing system.

Figure 17: Precast facade walls create a

new skyline at Tampines.

••18

19••

b) External walls

The external walls can be constructed from fair faced bricks or precast walls. Both brick wall and precast wall are subjected to thermal stress and building movement due to self weight and wind load. The thermal stress and building movement can cause cracks to occur at the joints or on the walls itself, and can lead to water seepage.

Brick wall

Brick walls are less commonly used in HDB buildings today but were commonly used for

constructing walls back in the 1960s and 1970s. The construction of brick walls is labour intensive. Skilled workers are required to brick laying and preparation of mortar between each layer of brick. If mortar was not prepared properly, gaps can occur and allow water to flow through.

Water seepage can occur for a variety of reasons. This include:

i) mortar bedding is too dry or too wet;

ii) poor pointing works;

iii) poor compaction of mortar;

iv) bricks not wetted before laying; and

v) cracks formed in bricks.

The water seepage problems have led to HDB to consider other alternative systems. Precast walls was one method which was found to be more effective.

Precast wall

Precast walls were implemented in HDB building contracts in the 1980s to improve the site productivity and improve quality of building construction. The most common type of precast walls are the precast façade walls and the precast gable end walls.

The use of precast walls offer better protection against water seepage because concrete is less porous than bricks. Another advantage is that the production of precast panels is carried out at the factory where there is better quality control to ensure the quality of precast walls is consistent. To control the problem of water seepage through the precast joints, HDB uses a multiple layer of defense against water seepage through the joints.

Figure 18: Bricks were used to form external brickwall cladding.

••19

There are 2 types of joints, based on the location of the joint:

i) Vertical joint

For the vertical joints between precast components, sealants are provided to seal the gaps between precast components. The sealants is normally 13mm deep and the width of the sealant is normally about 14mm, depending on the actual gap between the precast components. Behind the sealant there is a decompression chamber to trap any moisture and causes the water to flow down. Behind the decompression chamber, there is a cast-in-situ concrete joint that seals the gap between the precast component.

ii) Horizontal joint

The horizontal joint is designed as an open drain concept. There is a 30mm kerb on top of the lower precast component to prevent rain water from flowing into the internal unit. A compressible waterproofing strip is placed on top of the precast component and becomes compressed when the next level of precast component is installed. Non- shrink grout is used to patch up the kerb on the internal surface of the precast component. When the grout is dried, a layer of flexible waterproofing membrane is used to coat over the joint area.

Figure 20: Horizontal waterproofing detail.

Figure 19: Vertical waterproofing detail.

c) Window

Windows consists of two parts, the window frame and the glazing. The window frames are fabricated in the factory and brought to site for installation during construction stage. The window frame dimension has to be smaller than the window opening to allow the window frame to fit. The gap between the window frames and the window opening are filled with cement mortar using a hand pump. Sometimes, the gaps are not filled properly because the cement mortar mix is the either too wet or too dry. This means that there is a possibility of leaks occurring through the gap between the window frame and the window opening.

••20

21••

Recently, HDB carried out a study to fix the window frame in the precast façade before delivery to site. The study showed that by casting-in the window frame the problem of water seepage through the window frames could be eliminated.

HDB takes a pro-active approach to maintenance of the HDB properties. Prior to the formation of the Town

Councils in 1988, HDB had to carry out maintenance of the common areas outside the flats, including maintenance of lifts, water tanks and lighting. After the Town Councils were formed, HDB re-organised the area offices and focused on serving the HDB lessees.

As a responsible developer, HDB’s takes on a wider role even after the flats are sold to the residents on a 99-year lease. HDB has encouraged the residents to maintain their flats. This is done through a combination of public education, direct

4. Maintenance, Monitoring and Repair

communication to the lessees and legislation. For internal defects found within the units, HDB has offered to lessees to carry out goodwill repairs for spalling concrete and ceiling leakages.

4.1 Cyclical maintenance by Town Council

When the Town Councils were formed under the Town Councils Act in 1988, the responsibility for physical maintenance of the common properties was handed over to the Town Councils. The Town Council also became responsible for the financial upkeep of the estate and had to collect fees from the residents for the maintenance services rendered. The HDB as landlord and lessor, continued to carry out functions such as handling tenancy and lease matters, collection of sale installments, rent and carpark parking charges.

There are currently 16 Town Councils in Singapore who are maintaining the common properties. The common properties, as defined in the Town Councils Act include all parts of

There are currently 16 Town

Councils in Singapore

who are maintaining the

common properties.

••21

the building which are outside the flats. This includes the external walls, staircases, lifts, lift lobby, common corridors, roof, water tanks and linkways. The types of work and the frequency of repair is shown in the table below.

Item Frequency

1. Building repair and redecoration Every 6 - 7years

2. Re-roofing Every 10 years

3. Electrical re-wiring Every 15 -20 years

4. Lift servicing

Lift testing

Lift automatic rescue device

Every 2 weeksReplacement of parts every 5 years

Quarterly inspectionTest every yearFull load testing every 5 years

Quarterly

5.Water pumpset Pumpset replacement Every 10 years

6. CATV servicing Half yearly

The frequency of the repair is dependent on the durability of the building material. For example, in the case of building repair and redecoration, the frequency of the work is dependent on the quality of paint. HDB had worked with suppliers during the 1980s implemented the acrylic emulsion paint which can last at least 5 years without showing signs of deterioration. Now, with improvement in paint technology the paint manufacturers and the contractor were able to provide a 6-year warranty to cover defects such as discoloration, peeling and algae growth.

As part of the HDB maintenance programme, HDB had coordinated and ensured that building inspection for the HDB

properties were carried out to monitor the condition of the buildings. Where repairs have to be carried out at external areas or common areas, the Town councils are informed on the need to carry out the works. The planning work is currently coordinated by HDB Building Technology Department.

4.2 Monitoring and inspection

4.2.1 Building inspection programme

Under the Building Control Act, buildings which are for residential purpose have to be inspected at 10 years interval. Other buildings such as shops and industrial buildings have to be inspected at intervals of 5 years. The inspection have to be carried out by Civil/Structural professional engineers to ensure that the buildings inspected are safe and free from structural defects. HDB engages a Consultant to carry out the building inspections.

Currently, HDB has an existing stock of over 9,000 residential blocks of which about 7,400

Table 4: Cyclical maintenance work.

••22

23••

blocks are over 10 years. HDB engineers have to plan a programme to carry out visual inspection of the buildings in a timely and organized manner.

The works involved are:

a. Planning the programme for inspection of buildings.

b. Inspection of building, highlight defects and prepare building inspection report.

c. Recommending repair and rectification method by engineer and vetted by a Qualified Person (QP).

d. Sending Certificate endorsed by the QP to other departments

e. Submitting records of inspected buildings to HDB Building Control Unit.

After the inspections are carried out by the Consultant, the certificate and the building inspection reports are filed in the Visual inspection report database. The building data collected are critical for preventive maintenance as they relate to the long-term safety, durability and integrity of the HDB buildings. The information is also useful in upgrading and improvement consultancy services, Addition and Alteration works, and Repair & Redecoration works carried out by Town Councils.

4.2.2 Structural health monitoring

The existing methods carried out by HDB for evaluating the condition of the buildings such as periodic building inspection and carrying out non-destructive test provides useful data for assessing the condition of the buildings. These methods allow HDB engineers to assess the current condition of the buildings at the time of the survey but do not record the change in the condition of the building over time.

The idea of Structural Health Monitoring was considered as an alternative. This monitoring method involves the use of fibre-optic sensor to evaluate the condition of the building. Critical structures that hold the building such as the column can be monitored to obtain measurements on the behaviour of the columns eg shrinkage and creep.

HDB carried out a pilot implementation at a building under construction at Punggol. We investigated the use of fibre-optic sensor from SOFO system. Ground floor columns were selected as the columns could be monitored during construction and during the service life of the building. The installation of the sensors was done before casting of the column. The sensors were attached to the column reinforcement bars before casting and were embedded within the columns.

Figure 21: Monitoring of column loads carried out at a building under construction at Punggol.

••23

By measuring the total compressive strain in the column, the elastic strain could be calculated after deducting the creep and shrinkage components. The measurements of the column compressive strain were comparable to the theorectical elastic strain.

4.3 Maintenance repairs carried out by HDB

The building structure and building components will deteriorate over time if the building is not maintained regularly. While some of the deterioration may result in building cracks and spalling, some of the deterioration may be less obvious. The problems may manifest in the form of water seepage or quality of water delivered to the flats. The HDB has to work with the staff from Town Council to monitor the condition of the building and take appropriate action to solve the problem.

4.3.1 Water tank replacement programme

Clean water is provided for all HDB households through a network of pipes. Before water can be delivered to HDB flats, the water is stored in water tanks located at the roof of HDB buildings. The provision of water tanks on the roof ensures a constant supply of water to HDB residents.

During the 1970s to mid 1980s, water tanks were commonly made from mild steel. In mid 1980s, there were concerns that the mild steel water tanks would start to corrode. A major programme was initiated by HDB to change the mild steel water tanks to precast concrete water tank. Compared to mild steel water tanks, the precast water tank would be more durable. Another advantage was that the installation of precast concrete water tank would be faster and be less disruptive to the residents in terms of noise and dust.

Figure 22: Precast water tank are lifted to the roof during the water tank replacement programme.

••24

25••

Before the replacement could be carried out, HDB engineers had to carry out a feasibility study to check the condition of the HDB building and to re-calculate the water tank loading to the columns.

The water tank replacement work involved the following:

a) inspection and assessment of the condition of the building.

b) provision of overhead safety barriers at the perimeter of ground floor and exits and entry points.

c) provision of temporary water tanks to provide continuous water supply.

d) removal of existing water tank room roof

e) supply and installation of precast water tanks.

f) re-installation of booster pump and other M&E services.

Figure 23: Precast water tanks are tested for water tightness for 28 days.

The precast water tanks were tested to ensure there were no leaks before delivery to site. With better quality control and stringent test, the precast water tank comes a 10 year warranty provided by the contractor. Water stored in the water tanks are sent to Public Utilities Board (PUB) to check the quality of water. The water is fit for drinking after passing all the water quality tests set by PUB.

4.3.2 Goodwill repair assistance programme

Ceiling leaks can occur at occur in older residential blocks and affect both private apartments and HDB flats. Ceiling leaks can be caused by natural deterioration of the waterproofing membrane or poor renovation works. The ceiling leak problem normally occurs at wet areas such as bathrooms and kitchen area.

Under the lease agreement with the lessee, the lessee is responsible for maintenance of the internal area of his flat. For new flats, HDB provides a standard 1 year Defects Liability Period to cover defects caused by poor workmanship and materials.

Figure 24: Spalling concrete occurring on the ceiling of a toilet.

••25

For public housing flats, HDB has an on-going programme called the Goodwill Repair Assistance programme (GRAP). This programme is available to residents who have ceiling leak problems. HDB come in to meet with the upper floor lessee to obtain their agreement to carry out the repair. Apart from repairing ceiling leaks, HDB carries out repair of spalling concrete which occur as a result of the ceiling leak.

The objective of the GRAP is to promote neighbourliness and harmonious living among the lessees. The HDB act as mediator to resolve the ceiling leak issue. The programme is a cost sharing scheme where the government pays 50 percent of the cost of repair, the upper floor lessee pays 25 percent and the lower floor lessee pays the balance 25 percent. During the financial year ending on Mar 2005, there were 20,100 households who benefited from the one time goodwill repair exercise.

4.3.3 Goodwill rivet replacement programme

The incidents of falling external windows in HDB estates was a major safety concern in 2004 and 2005. The falling of windows could easily have caused serious injuries or even fatalities to the residents walking on the ground floor. HDB carried out investigations to determine the cause of the falling windows. It was found that many of the windows that have fallen were casement windows secured with aluminium rivets.

HDB buildings are designed according to building code and other relevant statutory requirement. Aluminium window frames which are used by HDB since 1985 met the requirements specified in Singapore Standard for Aluminium Alloy Windows. Although the windows complied with the prevailing industry standard when they were first fitted, the aluminium rivets used to secure the casement windows were prone to corrosion over time. These rivets were less durable than stainless steel rivets and require regular checking and maintenance to ensure serviceability.

••26

27••

Due to rising statistics of falling windows in HDB estates, HDB worked quickly with the Building Control Authority to formulate a legislative framework. The Building Control (Retrofitting of Casement Windows) Order 2004 was introduced by the Building Construction Authority and came into force on 1 Oct 2004. The order required the residents at both HDB flats and private apartments to be responsible for replacement of aluminium rivets with stainless steel rivets. The replacement was necessary for casement windows in residential units, which were prone to falling off if the aluminium rivets became corroded.

A grace period of one year was given before enforcement. To encourage residents to carry out the retrofitting of windows, a Goodwill Rivet Replacement Programme (GRRP) was implemented in Apr 2004. The GRRP was a goodwill exercise where the government and the flat owners share the cost of the repair work.

Figure 25: Falling windows were a major safety concern before GRRP.

HDB further embarked on a communication plan to create awareness among residents on the importance of carrying out the repair and maintenance of the windows. Besides letters, others such as newspaper advertorials, exhibitions and video clip were planned to achieve overall and timely impact.

To ensure that there were enough competent contractors for smooth implementation, HDB conducted courses to train all the registered window contractors before implementation. This was to equip them with the skills and knowledge on the minimum safety requirements for retrofitting of windows. The GRRP was carried out for 53,600 flats as at Nov 2005.

••27

HDB buildings are

designed according to

building code and other

relevant statutory

requirement.

S/No. Type of Major Defects Warranty Period

I Spalling concrete 10 years

Ii Water seepage from the external walls 5 years

Iii Ceiling Leak at toilet and kitchen 5 years

Table 5: Fixed period warranties for major defects.

The new Assure 3 warranty is a statement of quality of HDB buildings. Over the years HDB has taken various steps during design

Figure 26: UPVC pipes are placed at the correct locations in the toilet area before casting.

Figure 27: Precast facade with cast-in-window frames are stored in the factory yard, before delivery to site.

and construction stage to improve the quality of buildings. The improvement measures which lead to the introduction of the extended warranty system are as follows:

a) Cast-in UPVC pipes for toilet and kitchen

In the past, sanitary pipes are not cast together with the floor slab. Openings were provided at the toilet and kitchen locations, and the UPVC pipes would be fixed in place by the sanitary subcontractor. Gaps around the pipes would be filled by non-shrink grout. By fixing the UPVC pipe and casting the floor slab together, there are less joints which would help prevent leakage. Leakage caused by poor workmanship would also be eliminated.

b) Precast façade panels with cast-in window frames

HDB has been using precast façade panels to clad the building structure since the 1980s. In the past, the window frames were installed separately after all the precast façade panels have

••28

5. New Performance Standards with Assure 3

In late 2004, HDB announced a new warranty called Assure 3 for HDB flats launched under the BTO (Build-to-order)

and SERS (Selected-Enbloc-Redevelopment scheme) from 2005 onwards. Assure 3 is extended warranty scheme which would be offered to residents to assure them that during the period of warranty, their flats will be free from three types of defects, namely water seepage from exterior and leakage within flats occurring in the toilet. In the event that there are any defects, HDB would carry out the repairs free-of-charge to the residents. Table 5 shows the warranty period for the three defects.

been installed. By re-engineering the work process, the window frames are now installed at the precast factory during the production of precast façade panels. The window frames are fixed to the precast mould and cast together with the precast façade. This eliminates the gap between the window frame and the precast façade which would lead to leakage.

••29

Figure 28: Window frame fixed to the mould before casting of precast facade.

c) Higher grade concrete

HDB has introduced better and stronger concrete grade for the construction of HDB buildings. From 1996, HDB improved the concrete grade from grade 30 to grade 40 to improve the performance of concrete in terms of durability. Providing a higher concrete grade would mean the concrete is more dense and less susceptible to deterioration caused by carbonation, which would lead to spalling concrete.

The new extended warranty scheme, Assure3 would be offered to all new flat owners who apply for their HDB flats launched under the BTO and SERS from 1 Jan 2005.

A s a developer for public housing, HDB has provided a comprehensive Quality Assurance system to ensure that the

buildings are well designed and constructed to meet the performance requirements in terms of quality and durability.

HDB has played a major role in the local Construction industry by introducing and implementing the latest prefabrication technology in the construction of public housing flats. The use of better materials and precast components will be the key to improved durability for HDB buildings.

Over the past 45 years, HDB has re-invented itself to meet the changing needs of the public and has pushed the technology envelope in construction technology. The benefits of the pursuing excellence in design and construction have resulted in the delivery of good quality and affordable homes.

6. Conclusion

BluePrints for Successful Public Housing Development