Guidelines on Structural Survey and Appraisal of Historical Buildings Part I- Materials and Structural Forms

SEB GUIDELINES SEBGL – MT6

Guidelines on Structural Survey and Appraisal of Historical Buildings

Part I: Materials and Structural Forms

Structural Engineering Branch Architectural Services Department August 2012

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CONTENTS Content Page 1.

................................................................................ ........................................... ............... Introduction ....................................................

1

2.

Historical Buildings in Hong Kong .........................................................

2

3.

Record Drawings and Calculations of Historical Buildings …………..

4

4.

Construction Materials ........................................................ ............................................................................ ....................

5

5.

Structural Forms .................................................... ................................................................................ .................................. ......

27

6.

Specification and Corrosion Protection ..................................................

72

7.

List of References ................................................. ............................................................................. .................................... ........

72

Annex A – Examples of Structural Forms of Graded Historical Buildings

Acknowledgment

Structural Engineering Branch would like to record their thanks to Ir Eric P W CHAN and Ir K Y MA for their help in preparing the manuscripts.

Copyright and Disclaimer of Liability

This Guideline or any part of it shall not be reproduced, copied or transmitted in any  form or by any means, electronic or mechanical, including photocopying, recording, or or any information storage and retrieval system, without the written permission from  Architectural Services Department. Moreover, this t his Guideline is intended for the internal use of the staff in Architectural Services Department only, and should not be relied on by any third party. No liability is therefore undertaken to any third party. While every effort has been made to ensure the accuracy and completeness of the information contained in this Guideline at the time of publication, no guarantee is given nor responsibility taken by  Architectural Services Department for errors or omissions in it. The information is  provided solely on the basis that readers will be responsible for making their own assessment or interpretation of the information. Readers are advised to verify all relevant representation, statements and information with their own professional knowledge.  Architectural Services Department accepts no liability for any use of the said information and data or reliance placed on it (including the formulae and data). Compliance with this Guideline does not itself confer immunity from legal obligations.

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CONTENTS Content Page 1.

................................................................................ ........................................... ............... Introduction ....................................................

1

2.

Historical Buildings in Hong Kong .........................................................

2

3.

Record Drawings and Calculations of Historical Buildings …………..

4

4.

Construction Materials ........................................................ ............................................................................ ....................

5

5.

Structural Forms .................................................... ................................................................................ .................................. ......

27

6.

Specification and Corrosion Protection ..................................................

72

7.

List of References ................................................. ............................................................................. .................................... ........

72

Annex A – Examples of Structural Forms of Graded Historical Buildings

Acknowledgment

Structural Engineering Branch would like to record their thanks to Ir Eric P W CHAN and Ir K Y MA for their help in preparing the manuscripts.

Copyright and Disclaimer of Liability

This Guideline or any part of it shall not be reproduced, copied or transmitted in any  form or by any means, electronic or mechanical, including photocopying, recording, or or any information storage and retrieval system, without the written permission from  Architectural Services Department. Moreover, this t his Guideline is intended for the internal use of the staff in Architectural Services Department only, and should not be relied on by any third party. No liability is therefore undertaken to any third party. While every effort has been made to ensure the accuracy and completeness of the information contained in this Guideline at the time of publication, no guarantee is given nor responsibility taken by  Architectural Services Department for errors or omissions in it. The information is  provided solely on the basis that readers will be responsible for making their own assessment or interpretation of the information. Readers are advised to verify all relevant representation, statements and information with their own professional knowledge.  Architectural Services Department accepts no liability for any use of the said information and data or reliance placed on it (including the formulae and data). Compliance with this Guideline does not itself confer immunity from legal obligations.

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1.

Introduction

1.1

ArchSD has committed to enhance our our services services by developing and providing providing service on Government-wide Government-wide total asset and facilities management. Under this objective, SEB has committed to carry out detailed structural survey of all Government buildings aged 30 or above by the Fiscal Year 2017/18, in addition to providing routine and emergency maintenance services to these buildings. Concurrently,  Buildings (Amendment) Ordinance 2010  2010   was enacted by the Legislative Council in June 2011, which introduced the Mandatory Building Inspection Scheme (“MBIS”) into  Buildings Ordinance. Ordinance. Under the MBIS, owners of buildings aged 30 years or above (except domestic buildings not exceeding 3 storeys) are required to carry out inspections (and, if necessary, repair works) of the common parts, external walls and projections of the  buildings once every 10 years.

1.2

Besides the MBIS, the public have have increasingly awareness awareness of values of historical buildings, and would like to conserve or revitalise such buildings as far as possible. Well-known revitalisation projects projects completed by ArchSD in the  past decade include: Restoration and Preservation of King Law Ka Shuk completed in 2001 (UNESCO Asia-Pacific Heritage Awards for Cultural Heritage Conservation 2001-Merit Award), the Hong Kong Heritage Discovery Centre completed in 2005 (HKIA Annual Awards 2005-Special Architectural Award (Heritage)), Dr Sun Yat-sen Museum (the former Kom Tong Hall) completed in 2007 (Quality Building Award 2008-Merit Award (Heritage)), Stanley Blake Pier completed in 2007 (HKILA Award 2008-Silver Medal Award and HKIA Annual Awards 2008-Special Architectural Award (Urban Design)), and Conversion of Yau Ma Tei Theatre and Red Brick Building into a  Xiqu Activity Centre completed in 2011.

1.3

However, in carrying out out the detailed structural structural survey and appraisal appraisal of the the historical buildings, project officer may have noted that the design methods, loadings, materials and construction of these buildings are very much different from the current practice. practice. Addis (1997) commented that “ [m]any wonderful buildings have been demolished or irreparably damaged because the chosen engineers have had inadequate experience of old buildings or certain types of construction”. construction”. This is particularly the cases for historic buildings, some of them  being graded or declared. Clancy and Stagg (2004) list a number of essential requirements for structural engineers for carrying out structural survey and appraisal of historical buildings, including: knowledge of the type of building; in-depth but also broad understanding of structural theory; ability to recognise what is original and what are extensions and alterations to a building; good knowledge of behaviour of all major construction materials; knowledge of past as well as present codes of practice and design standards; etc.

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1.4

This set of guidelines consists of two parts, providing information and guidelines for project officer in carrying out detailed structural survey, appraisal and/or design for adaptive reuse of historical buildings. Part I of this set of guidelines (this “Guideline”) concentrates on the materials and structural forms of historical buildings, and provides project officer: i)

ii) iii)

a review of the construction materials, structural forms and the construction methods of Government buildings in Hong Kong since the th mid-19  century till the end 1970s; a summary of the strength of the construction materials in historical  buildings; and a list of references containing information on historical buildings that are useful in carrying out detailed structural survey, appraisal of historical  buildings and/or adaptive reuse.

Part II deals with the methods and procedures for carrying out detailed structural survey, appraisal and/or adaptive reuse of historical buildings, and  provides project officer: i) an overview of the evolution of building legislation and design codes in Hong Kong; ii) the common structural defects identified in historical buildings; and iii) guidelines on carrying out detailed structural survey and appraisal for existing or adaptive reuse of such buildings. 2.

Historical Buildings in Hong Kong

2.1

Protection of Historical Buildings in Hong Kong

2.1.1 In this Guideline, “historical buildings” are distinguished from “historic  buildings”. The term “historical building” is used to define “a building of traditional construction with age over 50”. “Historic building” is defined as “a  building of architectural or historic interest or significance. The interest or significance may be local or national, and may be a consequence of, for example, the building’s age, built form or location. It may result from its connection with a person or persons, or with local or national events or industry; or from a combination of these or other factors” (Urquhart 2007). 2.1.2 In Hong Kong, historical buildings are graded by Antiquities and Monuments Office (“AMO”). According to AMO (URL: www.amo.gov.hk/), 1,444 historical buildings including both Chinese and Western styles in Hong Kong have been assessed up till 14 June 2012. 929 historical buildings have been “graded”, and there are a total of 101 “declared monuments”. Many of them are still serving the public, such as study halls, art galleries, resources centre, museums and places for worship. ArchSD, as the maintenance agent of Government buildings, is responsible for the maintenance (including the  building structures) of a number of them, e.g. the old Supreme Court Building, the former Wan Chai Post Office, the former Kowloon British School, the Court of Final Appeal Building. Project officer is further advised to seek comments from AMO, should their new development projects with deep excavation or

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with foundation system generating signification level of vibration are close to declared monuments or graded historical buildings. 2.2

Declared Monuments Hong Kong has many monuments which need proper preservation. Under  Antiquities and Monuments Ordinance, places, buildings, site or structure “with historical, archaeological, or paleontological significance” may be declared as “declared monuments” by the Antiquities Authority, after the consultation with the Antiquities Advisory Board and with the approval of the Chief Executive. Once a building is declared, it receives legal protection for preservation, and AMO is empowered to prevent alterations, or to impose conditions upon any  proposed alterations of such buildings or places, in order to protect the monument. A particular point to be noted for project officer in carrying out detailed structural survey and/or alteration works in a declared monument is that  Antiquities and Monuments Ordinance prohibits any works (including building works, routine maintenance, repair, plant or fell trees, demolition) being carried out on such site without a permit (s.5).

2.3

Graded Historical Buildings Graded historical buildings (available: www.amo.gov.hk/form/historical.pdf ; accessed: 14 June 2012) are classified under a three-tier grading system ( Table 1). Although grading of historical building is for AMO’s internal reference and graded historical buildings do not enjoy statutory protection, project officer should note that AMO should be notified of and invited to comment on any  building proposals or demolition applications affecting such buildings. Table 1. Grading for Historical Building Grading Meaning Grade I Buildings of outstanding merit which should be preserved at all costs. Grade II Buildings of special merits, efforts should be made to selectively preserve. Grade III Buildings of some merits, but not yet qualified for consideration as possible monument. These are to be recorded and used as a pool for future selection.  Notes:

As at 14 June 2012, there are 160 historical buildings with Grade I status. As at 14 June 2012, there are 324 historical buildings wit h Grade II status. 3 As at 14 June 2012, there are 645 historical buildings with Grade III status. 2

(Source: AMO) 2.4

Principles of Conservation The aim of conservation is to retain and safeguard the cultural significance of a  place with unswerving respect of the existing fabric, which includes the aesthetic, historical and physical integrity of the cultural property. The emphasis in conservation is on saving what may seem an ordinary or even worthless part of the fabric, e.g. a joist, lime-washed wall. Minimal intervention is therefore the basic component underlying all conservation principles. That is, only change that is absolutely essential for the building’s own good is

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acceptable; and it should only be carried out after all other options have been exhausted (Allwinkle et al   1997). Lam (2003) therefore derived the level of intervention based on the level of cultural significance ( Table 2). His list of levels of intervention is: replica, relocate, partial retention, replace, restore, repair, consolidate, and do-nothing, whilst Ross (2002) advanced the ‘five Rs’retain, repair, reinforce, replicate and replace - for remedial works for a deteriorated structure, in the ascending level of intervention. Table 2. Levels of Intervention of Different Options in Conservation

(Source: Lam 2003) 3.

Record Drawings and Calculations of Historical Buildings

3.1

ArchSD has developed an information system (the “RDRS”) in storing the record drawings and calculations of Government buildings in electronic form. These record drawings and calculations can be retrieved from the following URLs: Record drawings: http://asdweb/rdrs/ Calculation: http://asdweb/rdrs/ SEB has also produced as-constructed structural layout of some of the historical  buildings with the full reports, and they can be retrieved in the following URL: http://asdiis/sebiis/2k/application/dssr/archives.aspx

3.2

In addition to the soft copy, SEB has also kept microfilms of some record drawings and hard copy of the calculation. Project officer can approach PTO/3 to retrieve the hard copy, though all calculations are being converted to soft copies and will be stored in the RDRS. However, project officer should note that most of the record drawings and calculations of buildings completed before the Second World War (the “WWII”) had been destroyed during the war. For those records, project officer may try the Public Records Office, which may contain as-built drawings (usually the architectural drawing) of these pre-WWII Government buildings. The University of Hong Kong also uploads a full-text image database in the following URL providing online access to pre-WWII Government publications such as reports on public works, proceedings of the

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Legislative Council, and reports of government departments and special committees, which contains valuable information on the progress of pre-WWII  public works projects: http://sunzi.lib.hku.hk/hkgro/index.jsp There are also in the market two books, namely  Measured Drawings Volume One: Hong Kong Historical Chinese Buildings   (Wong and Liu 1999a) and  Measured Drawings Volume Two: Hong Kong Historical Western Buildings (Wong and Liu 1999b), containing the as-surveyed drawings of historical  buildings in Hong Kong (including those of former Kom Tong Hall, Tsim Sha Tsui former KCRC Clock Tower, etc). However, these two books again show the as-surveyed architectural layouts of these buildings, though structural information can be deduced from the architectural layouts. 4.

Construction Materials Used in Historical Buildings

4.1

Materials always come first in the advancement of construction techniques. Technological development is usually preceded by the advancement of new materials, which would then be followed by research to understand their  behaviour and finally, design methods would be derived to carry out the design. Hence, an understanding of the major construction materials available (since the th mid-19  century) will first be described, which will be followed by a summary of the structural forms. Design methods and evolution of design codes will be described in Part II of this set of guidelines.

4.2

The major construction materials for historical buildings were: brick/masonry, structural steel/cast-iron/wrought iron, timber, and later reinforced concrete. The following provides a brief historical summary of the major construction materials together with their strengths at different ages. Knowledge on this area may help project officer to recognise roughly the construction materials and their strength, once the year of construction of a historical building is known. The following summary of the historical development of construction materials is based on the information and review provided in Addis and Bussell (2003), Bussell (1997, 1999, 2007), Sutherland (2001) and Ma (2007), and hence these sources will not be acknowledged in the text. Moreover, as there are only a few  publications on historical development of structural engineering practice in th th Hong Kong especially in the 19  century and early 20  century, reference has to be made to that in the UK, as general engineering practice in Hong Kong has  been very much based on and influenced by those in the UK by reasons of historical ties.

4.3

Brick, Masonry and Timber

th 4.3.1 Table 3  summarises the changes in construction materials since the mid-16 century. Brick, masonry and timber were used as the main construction th materials since earliest times of human civilisation. It was only in the mid-19 century that with experience of the load-bearing and spanning capabilities of cast iron and wrought iron, engineers started to investigate the two new alternative materials – steel and reinforced concrete - to replace brick, masonry

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or stone. Even then, brick, masonry or timber was continually used in Hong Kong as structural materials. Typical structural materials used for load bearing elements were Canton grey brick, red brick and Hong Kong granite bonded by lime-mortar or Roman cement (later by Portland cement) as vertical elements, whilst Manila hardwood or teak, American, China or Baltic fir were used for roofing or f1ooring. The Murray House (originally completed in 1846), the Flagstaff House (the former office and residence of the Commander of the British Forces in Hong Kong built in the 1840s), St John’s Cathedral (completed in 1849), and the former Central School at Hollywood Road (completed in 1889) ( Figure 1(a), Figure 1(b)  and Figure 1(c)) were typical examples during this period.

Figure 1(a). Layout of 1/F of the former Central School at Hollywood Road (completed in 1889 and destroyed during the WWII) (Source: AMO)

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Figure 1(b). South elevation of the former Central School at Hollywood Road (completed in 1889 and destroyed during the WWII) (Source: AMO)

Figure 1(c). Former Central School at Hollywood Road facing Staunton Street (completed in 1889 and destroyed during the WWII) (Source: AMO) Table 3. Chronology of timber, masonry and brick construction

(Source: Beckmann and Bowles 2004) Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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4.3.2 Timber 4.3.2.1 Timber is one of the oldest building materials. Timber may be classified according to the zone at which it is obtained from a tree trunk ( Figure 2). Those obtained near the centre are called “heartwood” ( ), and are usually darker than the portion adjacent to the bark. Those light coloured wood is called “sapwood” ( ). Sapwood and heartwood are about equally strength, and the main difference between them is that sapwood has lower natural decay resistance than heartwood (Australian National Association of Forest Industries 2004).

心材

邊材

(a) Terminology (b) Camphor 樟樹 Figure 2. Tree cross-section (Source: Merriam-Webster Online Dictionary) 4.3.2.2 Timber may also be classified into the two types according to its species, namely, softwood (or conifer) and hardwood (or non-conifer). Softwood comes from the coniferous (cone-bearing) species such as the pines, spruces and Douglas fir with seeds in a cone-like structure. Hardwood comes from the  broadleaved group of species such as the eucalypts, oaks, and meranti. The terms “softwood” and “hardwood” do not indicate softness or hardness of  particular timbers. Many hardwoods are even softer and lighter than some softwoods, e.g. willow, poplar, balsa, paulownia. 4.3.2.3 The strength properties of timber are determined by: timber species, degree of seasoning as measured by moisture content (dry timber being stronger than green) and the presence of defects (e.g. knot). The presence of moisture is also a contributing factor for fungal attack. In trees, moisture content may be as much as 200% of the weight of wood substance; but it will lose moisture after harvesting. Such initial drying of the wood after harvesting and milling will not cause a change in its dimension. However, as the wood is dried out to a moisture content below about 30%, further gain or loss of moisture will cause dimensional change. 4.3.2.4 There is a lack of study or laboratory testing results on the strength properties of the timber used in historical buildings, and indeed, it is doubted whether  proper structural design had been carried out for such timber structures at the time of construction. References to the UK practice at that time are not appropriate, as both timber species and the degree of seasoning differed. In Hong Kong, most timber spcies comes from the tropical forests in Southeast Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Asia, whilst those in the UK come from temperate forests. A good reference on the timber commonly available in the market in around the 1950s in Hong Kong is Timber Used in Hong Kong   (Tamworth 1952). In particular, Tamworth (1952) recorded that the timber species that were used at that time include Teak ( ), Douglas Fir ( , Seraya ( 抄 ), Keruing ( , Kapur ( 山 ), Balau ( 一號抄), Billian ( ), Selangan Batu ( ), Chengal ( ), 二號抄 ), Krabak ( Selangan Batu Merah, ( 紅 抄 ), and Oak ( 木). Table 5(a) summarises the mechanical properties (obtained from laboratory test at that time) of some of these timber species in Hong Kong. In the 1950s, grading of different timber species was still in infancy stage, which should have become mature in the 1970s. Table 5(b) gives two classes of timber used in Hong Kong in the 1970s, and lists the permissible stresses as given in  Building (Construction)  Regulations 1976 .

仔木, 英木) 樟

柚木

花旗松) 娑羅 木 冰片香木, 樟 巴勞, 杪木, 桂蘭 娑羅 木 橡

油 坤甸鐵 橡果木

4.3.2.5 Comparison between the tested flexural yield values in Table 5(a)  and the  permissible flexural values in Table 5(b)  shows that the tested values are much higher than the permissible values. These may be attributed to the fact that the values in Table 5(a)  had not included the effect of defects (e.g. the  presence of knots), creep effect, overloading, sampling size, etc (Tamworth 1952). A safety factor must therefore be applied to these values to get the corresponding permissible stresses. Tamworth (1952) suggested the use of a FOS of 5.5. Urquhart (2007) further reminded engineers that the strength and elastic modulus of old timbers are often greater than those specified in current codes of practice. Moreover, it was further stated that “not simply the timber strengths, sizes, and spans that should be assessed, because a major weakness in an old floor may be the joints and connections between the timber members, and between other structural elements” (Urquhart 2007).

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Table 5(a). Mechanical properties proper ties of timber used in Hong Kong in the 1950s

Type of Wood

Moisture Content (%) (as tested)

Flexural Yield Strength (MPa)

As a Beam Ultimate Flexural Strength (MPa)

Modulus of Elasticity (GPa)

Ultimate Shear Strength (MPa)

Flexural Yield Strength (MPa)

Ultimate Flexural Strength (MPa)

As a Post Modulus of Elasticity (GPa)

Compressive Yield Strength Perpendicular to Grain (MPa)

Teak (柚木) Douglas Flr (Coast) (海岸花旗松) Douglas Flr (Mountain) (山脈花旗松) Red Seraya 紅 ( 娑羅抄木) White Seraya (白娑羅抄木)

52

49

79

11.5

7.2

28

41

13.4

7.3

36

33

52 52

10.7

6.4

24

27

-

3.5

38

25

44

8.1

6.1

18

21

-

3.1

63

32

53

11.4

6.2

20

29

13.4

2.4

73

32

55

9.8

5.9

19

30

12.0

2.4

Keruing (油仔木)

65

43

71

15.0

7.6

26

39

17.2

4.2

Kapur (冰片香木,

山樟) Balau (巴勞,一號抄)

55

53

84

16.0

8.3

29

44

17.2

5.1

50

78

122

18.1

13.1

39

69

20.7

9.5

(坤甸鐵樟)

38

83

134

18.1

14.1

44

80

22.8

17.4

Oak (橡木) To convert the tested figures to values at 12% MC, multiply these figures by

90

31

57

9.2

8.3

20

28

11.4

4.5

1.8

1.59

1.31

1.45

2

2

2

2

2

Billian

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Table 5(b). Permissible Permissibl e stresses of timber used in Hong Kong in the 1970s Maximum permissible values Kind of stresses Class A Timber Class B Timber Flexural stress 7.0MPa 5.5MPa Shear stress 0.7MPa 0.7MPa Modulus of elasticity 11GPa 8.3GPa Compressive stress 2.4MPa 1.7MPa  perpendicular to grain Compressive stress in 6.6MPa to 1.1MPa 5.3MPa to 0.9MPa  posts or struts for l/b ratio from 6 to 58 for l/b ratio from 6 to 58 (Source: Source: Building (Construction) Regulations Regulations 1976 )

4.3.3 Masonry and Brick Precise strength data of masonry and brick are rarely required for appraisal, as wall and pier sections were typically sized by rules of thumb and were usually quite lightly stressed by comparison with their crushing strength (IStructE 2010).

Table 5(b). Permissible Permissibl e stresses of timber used in Hong Kong in the 1970s Maximum permissible values Kind of stresses Class A Timber Class B Timber Flexural stress 7.0MPa 5.5MPa Shear stress 0.7MPa 0.7MPa Modulus of elasticity 11GPa 8.3GPa Compressive stress 2.4MPa 1.7MPa  perpendicular to grain Compressive stress in 6.6MPa to 1.1MPa 5.3MPa to 0.9MPa  posts or struts for l/b ratio from 6 to 58 for l/b ratio from 6 to 58 (Source: Source: Building (Construction) Regulations Regulations 1976 )

4.3.3 Masonry and Brick Precise strength data of masonry and brick are rarely required for appraisal, as wall and pier sections were typically sized by rules of thumb and were usually quite lightly stressed by comparison with their crushing strength (IStructE 2010). In addition, the strength of masonry and brick is influenced by mortar strength rather than its crushing strength. Table 4 shows the strength of granite, brick and mortar mortar alone. The characteristic strength f k  k   of the masonry or brick wall can then be obtained using the following equation in  BS EN 1996 :  Design of  Masonry Structures: Structures: α β f k     = K×f  b ×f m k = where f k  k =   characteristic compressive strength of masonry; K = a constant obtained in  BS EN 1996-1-1  1996-1-1  (= 0.45 for dimensioned natural stone masonry); f  b = mean compressive strength strength of masonry or brick; f m = compressive strength of mortar; α for lime mortar = 0.65; and β for lime mortar = 0.25. Project officer should note that the lime mortar in  BS EN 1996  contains   contains more than 65% by mass of Portland cement clinker. However, pure non-hydraulic lime without cement was used in historical buildings, and no provisions are  provided in BS in  BS EN 1996  for   for such such material. The values of α and β quoted above should therefore be used with caution. Table 4. Compressive strength of granite, brick and mortar Material Compressive Strength (MPa) Granite 140-200 Brick 2-20 Mortar 0.5-1.0 (Source: Source: IStructE 2010)

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4.4.1

Cast Iron and Wrought Iron

In the 19th century, apart from masonry and timber, the main structural materials were cast iron (containing 2-5% carbon) and wrought iron (containing 0.1-0.5% carbon) ( Table 6). Cast iron, while molten, molten, is easily poured into sand moulds. It has a relatively high compressive stress, a natural resistance to rust;  but a very low tensile capacity. Moreover, cast iron is brittle and can fail suddenly. Wrought iron is very very malleable (and was therefore called “malleable iron”), and its main weakness is that it is stronger in tension than in compression. Both cast iron and wrought iron vary widely in physical properties and are vulnerable to flaws, and as such, a conservative FOS had been used. Table 6. Chronology of cast iron, wrought iron and steel construction

(Source: Source: Beckmann and Bowles 2004) 4.4.2 Structural Steel Commercially viable, large-scale production of steel through iron conversion did not take place until 1855, with the development of the Bessemer (or Bessemer–Kelly) process. This process enabled enabled a massive production of of steel for structural purposes. purposes. By 1900, steel had largely replaced wrought wrought iron for structural work in the UK, because steel has its advantage that it was more readily rolled into long and heavy sections such as joists and channels. However, most of the early use of structural steel was restricted to horizontal elements as floor beams. Cast iron columns (mainly circular hollow) and th  brick/masonry walls continued to be used until early 20  century; but from the 1880s they were progressively superseded by wrought iron columns (Bussell 1997).

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In these early days, different manufacturers produced their own sections to suit the orders of their respective clients.  Dorman Long and Company Section Book  was first published in 1887 while the  Redpath Brown & Company Handbook  was published in 1892. The standardization of sections was made in 1904 when the UK Engineering Standards Association standardised the section as well as the quality of steel. These sections were made in the forms of fat, L-, T-, U- and I-sections; but hollow sections (though were available in the mid-1930s) were not used in building works until the 1970s (Addis 1997). Project officer may consult  Historical Structural Steel Handbook (Bates 1991) to check the sectional properties of these early days steel sections.  BS 15: Structural Steel  for Bridges, etc., and General Building Construction appeared in 1906 (which was amended in 1912, 1941, 1948 and 1961) specifying the chemical composition and mechanical properties of structural steel. In Hong Kong, in around 1909, structural steel had been in use in Blake Pier  pavilion and old Supreme Court Building. The as-surveyed drawings of Blake Pier pavilion further showed that the original pavilion was built with cast iron  posts, similar to the practice in the UK (Wong et al  2007). 4.4.3 Differentiating Structural Steel, Wrought Iron and Cast Iron Table 7  shows the differences in mechanical properties among cast iron, wrought iron and structural steel. In historical buildings, project officer may sometimes be required to differentiate among cast iron, wrought iron and steel. Cast iron beams can easily be identified, as they usually had unequal flanges with large tension flange ( Figure 3 and Photo 1), since cast iron is weak in tension. Moreover, its span was usually no more than 4m (Rabun 2000). Cast iron columns could be made in circular in section ( Figure 4 and Photo 2), and structural steel hollow sections were only available in the 1960s. Table 7. Differences in mechanical properties among cast iron, wrought iron and structural steel Mechanical property

Tensile strength Compressive strength Ductility

Cast iron

Poor Good Poor, brittle tensile failure mode (Source: Bussell 1997)

Wrought iron Good Good Good

Steel

Good-excellent Good-excellent Good

Figure 3. Typical cross-section of cast iron beams (Source: Bussell 1997)

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Photo 1. Typical cross section of cast iron beam showing thicker and wider tension flange (Source: Beal 2011)

Photo 2. Cast iron columns at Flagstaff House Annex (built in the 1840s)

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Figure 4. Typical cast iron column with bolt connections (Source: Bates 1991 and CIRIA 1986)

It may, however, be difficult to differentiate wrought iron from structural steel, due to their similar way and the similar forms in which they were produced. th One particular way is to note that in the early 20  century rolled steel sections were often ‘engraved” with the name of the manufacturer and the particulars of the section ( Photo 3, Photo 4 and Photo 5). Table 8 lists the characteristics of such materials for visual identification. Samples, of course, may be taken for chemical analysis or metallographic inspection; but project officer should note that such sampling should not affect the historical values of the buildings. Moreover, project officer should note that flame cutting should not be used in obtaining samples from the structure, as the operation may damage or weaken cast iron, while with wrought iron or steel the resultant heat affected zone will give a misleading picture of the metal’s structure. Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Photo 3. Steel sections engraved with manufacturer (Cargo Fleet) in former Yau Ma Tei Theatre (completed in 1930)

Photo 4. Steel sections engraved with manufacturer (Port Talbot) in former Yau Ma Tei Theatre (completed in 1930)

Photo 5. Steel sections engraved with manufacturer (Glengarnock) in old Supreme Court Building (completed in 1912)

4.4.4 Connections Bolting and riveting were usually used to join structural steel sections together th th in the late 19  and early 20   centuries. Project officer should note that rivets were not (or should not be) used for cast iron, because the hammering forces forming rivets could facture the brittle cast iron. Instead, they were connected together by iron bolts. Hence, if project officer finds that riveting was used to  joining sections together, this serves as a quick way to differentiate it from cast iron. Welding by electric arc was introduced in the UK during the 1920s. However, it did not become an established practice even in British constructional steelwork until the 1940s, and as late as in the 1960s, both in the UK and Hong Kong, riveting was still chosen to connect some of the steelwork. Hence, if welding was used in joining sections together, it is likely that it was constructed after the WWII. Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Table 8. Differences among cast iron, wrought iron and steel

(Source: Bussell 1997)

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4.4.5 Strength 4.4.5.1 Pre-Second World War In the early days, manufacturers seldom measured and quoted the yield stress of their steel, as the yield stress was not used in the design, which was based on their permissible stress. Beal (2011) quoted that the average ultimate tensile strength (UTS) of cast iron and wrought iron to be 93MPa (6t/in²) and 280-370MPa (18-24t/in²) respectively. However, project officer should note high variability in the strength of early days production, and hence a FOS of 4 was adopted to calculate its permissible stress. Similarly, for structural steel, there was no specified yield stress. Beal (2011) quoted that in the early days of structural steel, the average UTS was about 432-494MPa (28-32t/in²), and again, there was considerable “variability of the strengths, even between  pieces in the same building” (Addis 1997). Similar to cast iron and wrought iron, a FOS of 4 was applied to obtain its permissible stress. The earlier paragraphs noted that the first edition of  BS 15  (specifying the chemical composition and mechanical properties of mild steel) was published in 1906, and  London County Council 1909 Act was the first official document specifying a permissible bending stress of 116MPa (7.5t/in²) for both tension and compression for mild steel structural steel. In 1927, the Institution of Structural Engineers recommended a permissible stress of 124MPa (8t/in²) for  both bending and axial compression in steel and the same value was adopted in the first edition of BS 449: 1932. High tensile structural steel (with maximum carbon content of 0.3%) was available in the early 1930s, and  BS 548: High Tensile Structural Steel for  Bridges etc, and General Building Construction  was published in 1934 to cover their mechanical properties (Bussell 1997). The allowable stresses of high tensile steel were increased by approximately 50% (except for column stresses where the increases depended on the slenderness ratio). Though high tensile structural steel could have higher allowable stresses, their relative higher carbon content led to the formation of brittle martensitic layer near the weld causing weld hardening and consequent danger of weld failure. There is no information on whether such high tensile structural steel had been used in Hong Kong, and it was usually engraved with the words “H.T.” to distinguish itself from mild steel structural steel (Bussell 1997). Table 9  shows the values of strengths for wrought iron and steel and the appropriate partial safety factors as stated in The Assessment of Highway  Bridges and Structures (2001) (“ BD 21/01”) and IStructE (2010).

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Material

Table 9. Strength of Cast Iron, Steel and Wrought Iron in Pre-WWII Historical Buildings Ultimate Yield Permissible Elastic Material Strength Stress Stress Modulus Factor (MPa) (MPa) (MPa) (GPa) Tension 90 – 250

Cast iron Compression 775 – 1100

Wrought iron1

Tension 324 – 371 Compression 278 – 593

Mild Steel (pre-1955)

Tension 433 – 494 5 3 330

 No defined yield point

220

3

232 – 386 5 230 3

Tension 32 – 39 (depending on DL/LL ratio) 3 4 33 Compression 154 3 124 4 Tension and compression 141 3 77 4 Tension and compression 147 3 4 116

80-150 3

90-138

170-220 1.20 2 200 3 190 - 210 205 3

1.05-1.15 3 1.25

High Tension Tension Tensile 262 – 317 6 6 7 510 – 593 165 Steel 1  Notes: IStructE (2010) notes that the strength of wrought iron is directional, with at right-angles of the line of rolling being about two-thirds to three-quarters of that in the line of rolling. 2 The load factors for DL and LL should be 1.1 and 1.5 respectively (IStructE 2010. 3 Values given in BD 21/01 4 Values given in London County Council 1909 Act 5 Values given in BS 15 6 Values given in BS 548 7 Values given in BS 449

(Source: BD 21/01 and IStructE 2010) 4.4.5.2 Post-Second World War During the WWII, in order to economise scarce materials, an amendment was made to BS 449: 1932 in 1939 to increase the permissible stress of mild steel to 154MPa (10t/in²), while the permissible stresses for axial tension and compression remained unchanged. After the WWII,  BS 15: 1948  increased minimum yield stresses for mild steel to 235MPa (15.25t/in²) for thin sections and 228MPa (14.75t/in²) for steel with thickness greater than 19.1mm thick. With the increase in minimum yield stresses,  BS 449:1948  retained the ‘wartime’ permissible bending stress of 154MPa (10t/in²) and increased the  permissible stress for direct tension to 139MPa (9t/in²). In 1959, the  permissible bending stress of mild steel was increased to 162MPa, then in 1969 to 165MPa and in 1989 was increased to 180N/mm².

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 BS 968: High Tensile Steel (Fusion Welding Quality)  for Bridges and General  Building Construction  was issued in 1941 (and was amended in 1943 and 1962) for high tensile structural steel in order to solve the problem of weld hardening of the earlier high tensile structural steel, and the permissible steel specified in  BS 968 were higher than for mild steel but less than those to  BS 548. In 1968, the first edition of  BS 4360: Weldable Structural Steel  was published to cover both mild steel and high tensile structural steel. Table 10(a) lists the chronology of the yield stress, and permissible bending and compressive stresses of mild steel structural steel since the WWII, and Table 10(b) lists the mechanical properties of high tensile structural steel. Table 10(a). Strength of post-WWII mild steel structural steel Permissible Permissible Material Yield Stress Design Year Bending Compressive Standard (MPa) Code Stress (MPa) Stress (MPa) 1948  BS 15 154 139 235 (≤ 19mm)  BS 449 228 (> 19mm) 1961  BS 15 162 147 247 (≤ 19mm)  BS 449 232 (> 19mm) 1969  BS 4360 165 155 255 (≤ 16mm)  BS 449 245 (> 16mm) 1986  BS 4360 180 170 275 (≤ 16mm)  BS 449 265 (> 16mm) (Source: Beal 2011) Table 10(b). Strength of high tensile structural steel in BS 968:1962 Ultimate Permissible Permissible Tensile Design Yield Stress (MPa) Bending Compressive Strength Code Stress (MPa) Stress (MPa) (MPa) 441 - 536  BS 449 200 186 317 (≤ 16mm) 310 (> 16mm but ≤32mm) 303 (> 32mm) (Source: Bates 1991)

4.4.5.3 Material Testing Adopting the above characteristic strengths is normally adequate for structural appraisal such that if this assessment is satisfactory, then the member is adequate. No material testing is therefore required. In order to cater for the variability of material and any inaccuracy of the typical value, IStructE (2010) recommends a conservative material factor of 1.25 to be adopted, instead of the 1.0 currently adopted in the design of structural steel and 1.05-1.15 suggested by BD 21/01.

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If it is decided necessary for sample materials for strength tests, project officer should note that adequate samples must be taken to get a 95% confidence limit on the characteristic value of the yield stress. The 95% confidence limit is k times the standard deviation below the sample mean. As the strength of the samples usually follows a “normal” distribution,  BS EN 1990:2002: Basis of Structural Design offers guidance on the value of k to be adopted for different number of samples. As only a small number of samples can be taken or available for testing, the characteristic value with 95% confidence level will be very low with respect to the sample mean. This characteristic value being low is not because the sample mean is low; but because the sample size is small. For example, if only three samples have been taken, it will be necessary to use a characteristic value 3.37 standard deviations below the sample mean. Bussell (1997) commented that testing for strength therefore is generally worthwhile “only when an initial assessment (using typical strength values) shows that the structure is neither significantly overstressed, nor understressed in its intended use.”  BD 21/01 also gives a cautionary advice on sampling for testing that “[i]t must  be appreciated that the yield stress of wrought iron determined from samples 2 varies over a wide range, typically from 180 to 340 N/mm  and this range is not necessarily much narrower when samples are taken from the same structure. It is, therefore, unlikely that a few test results will provide any more reliable information about the strength of the material in the structure as a whole than the value given in clause 4.9 of  BD 21 which is based on a large number of tests.” Therefore, this Guideline recommends to use the values quoted in Table 9 for pre-WWII historical buildings and those summarized by Beal (2011) ( Table 10) for post-WWII historical buildings, rather than testing unless in special circumstances. Alternatively, a proof load test may be required to check the adequacy of a structure. For details of load test, project officer may refer to Clause 16.2 of Code of Practice for the Structural Use of Steel 2011  and Clause 13 of Code of  Practice for the Structural Use of Concrete 2004  issued by Buildings Department for steel structures and r c structures respectively. 4.5

Reinforced Concrete

4.5.1 Although reinforced concrete (rc) is now the most common structural material in building works especially in Hong Kong, rc was only used in Europe and the th US for building works after the mid-19  century and only used in Hong Kong in th the early 20  century. The breakthrough in technology occurred in 1824, when an English inventor, Joseph Aspdin, invented and patented “Portland Cement”, a fast-curing hydraulic cement formed by burning ground limestone and clay together. The name “Portland” was used in order to liken it in people’s minds to the stone from Isle of Portland in Dorset, which had been used as building stones for famous buildings in the UK, e.g. St Paul's Cathedral, Buckingham Palace, Tower of London, London Bridge. By 1870, unreinforced concrete spanning between and enveloping iron beams had been used as the floors of  buildings, due to its good fire resistance.

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4.5.2 It was generally agreed that using rc as a structural material began in 1848, when Joseph Louis Lambot of France found that adding thin steel bars or steel fibres to concrete could greatly increase the strength of the concrete, making it  better for use in a variety of applications. He then built a rowboat by concrete reinforced with a grid of thin iron rods. Such composite was then used making reinforced garden tubs, road guardrails, and reinforced beams. The first rc  building works in the US was a house in New York completed in 1876 by William E Ward with concrete floor slabs reinforced with a two-way grid of iron rods and supported on concrete beams reinforced with wrought iron I beams. The first rc structure in the UK was Weaver’s Mill in Swansea completed in 1897 by François Hennebique, a French concrete specialist. Using rc as the construction materials then became increasingly common in the late th th 19  century and early 20  century in the US and Europe ( Table 11), due to the ability of concrete to resist fire, carry heavy loads, and dampen noise, which make it a good choice for factory and apartment buildings. In 1903, the first rc “skyscraper” - Ingalls Building - of 16 storeys together a single storey basement  built was completed in Cincinna, Ohio, the US by A O Elzner. The first rc framed building in the UK was the 11-storey Royal Liver Building in Liverpool completed in 1909. 4.5.3 However, the use of rc in buildings was later in Hong Kong than that in the US th and Europe. The majority of buildings in Hong Kong before the 20  century were built with brick/masonry columns or walls with timber floors on timber th  joists or steel joists. In the late 19  century, unreinforced concrete slabs on steel th  joists appeared. Since the early 20  century, floor system using rc slabs resting on steel joists had been used. The first Government building using rc slabs on rc beams in Hong Kong was the Public Works Department Store completed in 1912. The first rc framed building in Hong Kong was the Gaol Extension completed in 1914 ( Report of the Director of Public Works for the year 1915). Photo 6  shows the in-situ  rc work in 1932 for Gardens Services Reservoir (a civil engineering project) in Central as part of the Shing Mun Water Supply Scheme.

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Photo 6.  In-situ rc work in 1932 in Gardens Service Reservoir, Upper Albert Road underneath Zoological and Botanical Garden (Source: Ho 2001) Table 11. Chronology of concrete construction

(Source: Beckmann and Bowles 2004) 4.5.4 Concrete strength 4.5.4.1 Pre- Second World War In the UK, there is little information on the concrete strength before the WWI (IStructE 2010).  London County Council 1909 Act specified a permissible direct compressive stress of 2.5MPa, and hence the typical concrete cube strengths were in the range of 11–15MPa. By the 1930s, typical cube strengths had risen to 15–20MPa. The first edition of  Reinforced Concrete  Designers’ Handbook   by Charles Reynolds published in 1932 stated the cube strengths of Grade 1:2:4, Grade 1:1.5:3 and Grade 1:1:2 concrete to be 2100psi (14MPa), 2250psi (17MPa) and 2625psi (18MPa) respectively (Clarke 2009). For concrete in pre-WWII historical buildings,  BD 21/01 Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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therefore recommends that their cube strength may be taken conservatively as not exceeding 15MPa in back analysis. In Hong Kong, Exceptional Building Regulations 1931 was the first legislation specifying the permissible concrete stress according to the mix proportions. 1:2:4 mix was specified to have a permissible direct compressive stress of 600psi (4MPa), and 1:1:2 mix was specified to have a permissible direct compressive stress of 750psi (5MPa). As compared with later codes, the adopted permissible stresses were lower, as there was a lack of confidence on the quality of concrete. Besides using natural or crushed aggregate, crushed  brick might have been used as aggregate (IStructE 2010). In at least two such  pre-WWII buildings (the former Kom Tong Hall and the old Supreme Court Building) it was found that brick fragments were used as aggregate in the concrete of the roof slabs (Photo 7) and non-structural elements.

Photo 7. Concrete mix with brick fragments as aggregate in the roof slabs of former Kom Tong Hall (built in 1914)

4.5.4.2 Post- Second World War The three commonly used concrete mixes after the WWII were 1:1:2 (known as “Grade C”), l:1.5:3 (known as “Grade B”), and 1:2:4 (known as “Grade A”), with corresponding characteristic cube strength of 30MPa, 25MPa and 20MPa. Another common practice at these days was that the horizontal elements (slabs and beams) were cast with Grade 1:2:4 mix, and the vertical elements (columns and walls) were cast with higher grade, e.g. Grade 1:1:2 mix. 4.5.5 Strength of steel reinforcement 4.5.5.1 Pre- Second World War The earliest steel reinforcement was in the form of wire mesh supplied as a  patented pre-fabricated product, and there is little information on the strength of such steel reinforcement. A catalogue of the early wire mesh (American Steel & Wire Company 1908) showed that its ultimate tensile strength was in the range of 586MPa (85000psi). 12 nos. of samples had been cut from the wire mesh reinforcement in former Kom Tong Hall (built in 1914) which showed that the ultimate tensile strength ranged from 722MPa to 784MPa. Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Despite the relatively high ultimate tensile strength of such wire mesh, project officer should note that these wires were twi sted together ( Photo 7), and hence in order to mobilise its full tensile strength, larger deformation might have occurred. Moreover, project officer should note that manufacturers at that time could also supply patented steel wire with higher ultimate tensile strength to suit the span and loading of a particular project. For steel reinforcement, a RIBA report of 1911 recommended a minimum yield stress of 221MPa (32000psi) and a permissible tensile stress of 110MPa (16000psi), and this was also stated in  London County Council By-Laws 1915 . In 1934, the UK Department of Scientific and Industrial Research (“DSIR”)  published a code (“Code of Practice for the Use of Reinforced Concrete in  Buildings”), which increased the permissible tensile stress for mild steel to 125MPa (18000psi).  London County Council By-Laws 1938  then adopted this  permissible stress. In Hong Kong,  Exceptional Building Regulations 1931 was the first legislation specifying the permissible tensile stress for steel reinforcement as 110MPa (16000psi), which was generally in line with the UK  practice. 4.5.5.1 Post- Second World War After the WWII, ‘high tensile’ steel became available for use in reinforced concrete in the UK. However, in Hong Kong, high tensile steel reinforcement was only employed in Government building works in the 1960s. CP114:1948 maintained the pre-WWII permissible reinforcement stresses of 125MPa for mild steel, and for high yield steel reinforcement a permissible stress of 0.5fy (185MPa) was suggested. In CP 114: 1965, the permissible tensile stress for high tensile steel reinforcement was increased to 230MPa (for φ≤9.5mm) and 205MPa (for φ>9.5mm). Similar to structural steel, no characteristic yield stress was specified till 1969, when BS 4449: 1969 and BS 4461: 1969 specify the characteristic yield stress for mild steel and high yield bars to be 250MPa (which has then remained unchanged) and 410MPa respectively.  BS 4449: 1978 then increased the characteristic yield stress for mild steel and high yield  bars to be 460MPa (for φ≤16mm) and 425MPa (for φ>16mm). However, steel reinforcement to  BS 4449: 1978  was not used in the design of rc till the late 1980s. Moreover, even though the Register of Registered Structural Engineers was introduced in Hong Kong in 1974, Authorized Architects (changed to Authorized Persons in 1974 simultaneously with the introduction of Registered Structural Engineers) were allowed to carry out structural design for private sector buildings up till 1987. During this period, enhanced  permissible stresses in the steel reinforcement were allowed when the design was carried out under the supervision of Registered Structural Engineers. For example, the permissible stress in high yield steel reinforcement could be increased to 230MPa, whereas Authorized Architect could only use a  permissible stress of 185MPa. This Guideline, however, does not note that such distinction was made in the structural design of Government buildings, as structural design of all Government buildings should have been carried out by  professional structural engineers. Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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4.5.6 Table 12  lists the characteristic strength of concrete cube and steel reinforcement in chronological order in Hong Kong. Table 12. Chronology of characteristic characteristic strengths of concrete and and steel reinforcement till the mid-1980s Concrete Steel Reinforcement

Year

PreWWII

PostWWII

Grade

Cube strength (MPa)

1:1:2 1:1.5:3 1:2:4 1:1:2 1:1.5:3

20 18 16 30 25

Permissible direct Yield Stress compressive (MPa) stress (MPa) 5 Mild steel bar 4.5 only 221 4 7.6 Mild steel bar 250 6.5

Permissible working stress (MPa) 1

110 2 124 Mild steel bar 140

High yield bar High yield bar 410 230 1 and  Exceptional Building Regulations 1931 1931..  Notes: Value given in RIBA Report and Exceptional 2 Value given in DSIR Code 1:2:4

20

5.3

4.5.7 Material Factors 4.5.7.1 IStructE (2010) notes that the material factor of 1.5 in  BS 8110  8110  for concrete has been chosen largely because of uncertainties in the quality of materials a nd workmanship, compaction, curing, etc, and hence is of the view that if concrete strengths are ascertained by tests on cores from the actual structure supplemented by ultrasonic pulse velocity or rebound hammer measurements to assess the variability, it may be reasonable to reduce the overall value of the material factor. However, should back-analysis back-analysis of the structural integrity of of a historical building be required, this Guideline still recommends the use of a material factor of 1.5 for concrete because: a)  b)

there was a high variability in the materials in such building; and full extensive tests (especially with adequate number of samples) are usually not carried out.

4.5.7.2 For steel reinforcement, the material factor currently adopted in Hong Kong is 1.15, and IStructE (2010) is also of the view that if samples have been obtained from a number of representative members and tested and the consistency of the mechanical properties of the other bars has been checked using non-destructive means, there may be a case for reducing the factor to 1.05. However, given that it is usually not practical to extract a large number of bar samples for testing, this Guideline recommends that the current material factor of 1.15 needs to be modified in order to cater for the greater variability of material properties in early days, and a material factor of 1.25 for structural steel as discussed disc ussed in earlier paragraph may therefore be adopted.

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5.

Structural Forms and Structural Systems

5.1

Horizontal Flooring System

5.1.1 Timber Floors Timber flooring was the then practice before the invention of concrete. An example of such flooring system that can still be found today include that in Red Brick Building (formerly the Yau Ma Tei Pumping Station) ( Photo 8) and Mongkok Police Station (formerly temporary premises of Diocesan Boy’s School) (Photo 9).

Photo 8. Timber floor in Red Brick Building (built in 1895)

Photo 9. Timber floor in Mongkok Police Station (built in 1925)

5.1.2 Unreinforced Concrete Floors Floors The concept of brick jack-arch was adapted using concrete as flooring system in th the 18   century in Europe. Figure 5 and Figure 6  show a typical flooring system using brickwork with a concrete fill and topping, spanning between  parallel iron beams. This flooring system consists of wrought-iron tie-rods, to Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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tie the cast iron or wrought iron beams during concreting to carry the arch thrusts from the bricks. Photo 10  shows the brick jack arch floor in Albert Dock in the UK. Later the bricks were replaced replaced by concrete concrete ( Figure 7). In Hong Kong, Block 10 of Lei Yue Mun Park and Holiday Village ( Photo 11) and old Supreme Court Building ( Photo 12) had used such flooring system.

Figure 5. Isometric view of brick jack arch floor (Source: Source: modified from Beckmann and Bowles 2004)

Figure 6. Cross section of brick jack arch floor (Source: Source: modified from Beckmann and Bowles 2004)

Figure 7. Concrete jack jack arch floor Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Photo 10. Jack arch floor on cast-iron columns in Albert Dock in the UK (built between 1842-48) (Source: Curtin and Parkinson 1989)

Photo 11. Jack arch floor in Lei Yue Mun Park th th and Holiday Village Block 10 (built in the late 19  or early 20  century)

Photo 12. Jack arch floor in old Supreme Court Building (completed in 1912)

‘Joist-concrete’ (or later known as “filler-joist construction”) ( Figure 8) was th developed in the late 19   century, as an alternative to jack arch floor. In this floor system, steel or wrought iron beams were placed at 0.6-1.2m c/c (or 2-4 ft) and the gaps between were infilled with concrete to complete the floor. The Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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unreinforced concrete slabs span between joists as a series of shallow arches. This technique also provided a degree of fire-protection to the beams themselves. At that time, safe-load tables had been published by steel manufacturing firms so as to allow engineers to select suitable sizes of beams and slabs to suit spans and loadings, similar to the roof cladding system nowadays. CIRIA Report No. 111: Structural Renovation of Traditional  Buildings (CIRIA 1986) noted that beams deeper than 300mm were built up by riveting plate sections together ( Figure 9). Similar construction has also been found in Block 10 of Lei Yue Mun Park and Holiday Village ( Photo 13) and in old Supreme Court Building ( Photo 14).

Figure 8. Filler-joist floor

Figure 9. Elevation of built-up wrought iron girder (Source: Bussell 1997)

Photo 13. Filler joist floor in Lei Yue Mun Park and Holiday Village Block 10 th th (built in the late 19  or early 20  century)

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Photo 14. Built-up filler joist floor with steel plates on flanges for 8m span in old Supreme Court Building (completed in 1912)

5.1.3 Reinforced Concrete Floor th

As stated above, rc started to be used in the early 20 century. There were a number of different types of steel reinforcement at that time. Instead of using square grid of reinforcing bars as today, steel reinforcement was in the form of metal wire mesh ( Figure 10(a)) supplied in roll form ( Figure 10(b)), or Isection ribs. Such form of steel reinforcement enabled their production off-site (Figure 11), and could facilitate the transportation of the steel reinforcement. The mesh spans between the beams by means of catenary action (Stuart 2010). Hence, the concrete serves only as the wear surface and as the mechanism by which the imposed loads are transmitted to the mesh. Because the concrete is not structurally stressed in this type of system, the composition and quality of the concrete is not as important as in a true flexural slab. Such form of steel reinforcement was found in the former Kom Tong Hall ( Photo 15) and old Supreme Court Building.

(a) when laid (b) when supplied Figure 10. Wire mesh steel reinforcement (Source: American Steel & Wire Company 1908) Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Photo 15. Wire mesh steel reinforcement in the former Kom Tong Hall (built in 1914)

Figure 11. Fixing of wire mesh steel reinforcement on site (Source: American Steel & Wire Company 1908) th

The filler joist construction of the late 19  century was later replaced by rc slabs on steel beams. An example is the former Central Fire Station ( Photo 16) (commonly known as “Shui Che Kwun”) located at the corner of Queen’s Road Central and Wellington Street (now the site for Hang Seng Bank Headquarters Building), where in the rc details ( Figure 12) required steel mesh reinforcement was detailed to be bent up at support to take the hogging moment and to wrap around the steel beam at support.

Photo 16. Former Central Fire Station (completed in 1926 and demolished in 1982) (Source: AMO)

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Figure 12. Wire mesh steel reinforcement in slabs at former Central Fire Station (built in 1924)

5.1.4 Hollow Block Floor Hollow-block floors were commonly used from the 1950s to 1970s in Hong Kong. Such form of construction has been found in City Hall Annex at Central (now Hong Kong Planning and Infrastructure Exhibition Gallery) and Central Government Offices ( Photo 17). They were constructed by placing clay or  precast hollow cement sand blocks ( Figure 13) on formwork, and concrete was then cast to form ribs spanning in one direction ( Figure 14). Its advantages are its lightweight, the excellent sound insulation and thermal insulation. The clay or cement sand blocks were not usually included in the design, and hence the topping can be very thin (may be of 50mm). Similarly, the width of the ribs can  be as small as 50mm. ArchSD general specification (1968 edition) specified that the outer casing of the hollow block should be of 1in (25mm) thick and there should be a 1in× 83 in key along each side (though such keys are not noted in the hollow blocks in these two buildings). It further specified the ends of  blocks to be filled solid to a depth of 3 inches with concrete.

Figure 13. Clay or precast hollow cement sand blocks

Figure 14. Section of hollow-block floor

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Photo 17. Hollow block floor in old Supreme Court Building (additional works of the 1960s)

5.1.5 Roof Structure For pitched roof, either timber or structural steel has been used to form trusses. Figure 15  and Figure 16  show the typical structural forms of western-styled th th timber roofs in the late 19  and early 20  centuries. Similar timber trusses have  been found in former Clubhouse of Royal Yacht Club at Oil Street, North Point (Photo 18 and Figure 17), Mongkok Police Station ( Photo 19) and Hong Kong Museum of Medical Sciences Society (former Pathological Institute) (( Photo th 20). In the early 20   century, structural steel was used in pitched roof th construction. Examples of such early 20   century structural steel roof trusses are found in Blake Pier Pavilion ( Figure 18), former Yau Ma Tei Cinema (Photo 21) and old Supreme Court Building ( Photo 22).

Figure 15. King-post timber truss (Source: www.builderbill-diy-help.com/)

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Figure 16. Queen-post truss (Source: www.builderbill-diy-help.com/)

Figure 17. Western-styled timber truss in former Clubhouse of Royal Yacht Club at Oil Street, North Point (built in 1908)

Photo 18. Western-styled timber roof truss in former Clubhouse of Royal Yacht Club at Oil Street, North Point (built in 1908)

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Photo 19. Western-styled timber roof truss in Mongkok Police Station (built in 1925)

Photo 20. Western-styled timber roof truss in Hong Kong Museum of Medical Sciences Society in Mid-Levels (completed in 1906)

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Figure 18. Structural steel roof truss in former Blake Pier (completed in 1909)

Photo 21. Structural steel roof truss in former Yau Ma Tei Cinema (built in 1930)

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Photo 22. Joints at roof truss in old Supreme Court Building (completed in 1912)

5.2

Vertical Elements

5.2.1 Masonry and/or Brick Walls In the earlier paragraphs, it was noted rc framed buildings did not appear in Hong Kong till 1914, and hence the majority of vertical load-bearing elements were made up of masonry and bricks. In some cases, walls were made with masonry facing backed with bricks ( Figure 19(a)  and Photo 23), or masonry with rubble heart ( Figure 19(b)).

(a) (b) Figure 19. Masonry wall mixed with brick or rubble (Source: CIRIA 1986)

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Photo 23. Masonry wall backed with bricks in old Supreme Court Building (completed in 1912)

Moreover, even with the appearance of rc framed buildings, masonry and bricks were still used as vertical elements in ArchSD projects till the early 1950s. A rule of thumb had been specified in the UK  Metropolitan Building Act 1844 (Figure 20 and Figure 21), which was incorporated into  Buildings and  Nuisances Ordinance 1856 . Brick or masonry walls were specified to “be of the thickness of not less than 230mm at the upper storey, 340mm immediately  below the upper storey, and 450mm at the storey (if any) immediately, the said two stories.” Similarly,  Buildings Ordinance 1950  provided, inter alia, the following rule of thumb for the thickness of such walls: a) 230mm for height of wall not exceeding 3.66m;  b) 340mm for height of wall between 3.66m and 7.62m; c) 450mm for the lowermost storey and 340mm for other storey(s) for height of wall between 7.62m and 12.20m.  Building (Construction) Regulations 1976  still contained similar rules of thumb for masonry or brick construction. Most pre-WWII buildings, though with rc slabs and beams, were built with masonry and/or brick vertical walls and columns. One of the possible reasons was that the buildings in ArchSD projects at that time were relatively low-rise (most not exceeding three storeys), and rc frame action was not required in resisting wind. Another reason was that steel was relatively expensive and scarce resource at that time, and using masonry and brick could yield a more cost-effective design.

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Figure 20. Thickness of walls for dwelling houses under Metropolitan Building Act 1844

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Figure 21. Thickness of walls for warehouses under Metropolitan Building Act 1844

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Figure 22 shows the typical buildings using brick walls as vertical elements in Shaukeiwan Police Station (note that the framing plan of G/F shows the structural layout above that floor). Typical structural materials used for load  bearing elements were Canton grey brick, red brick and granite bonded by limemortar of lime and sand or by Portland cement (or its forerunner Roman cement - a natural hydraulic cement). They were built in single length, with no movement joints. Lintels may sometimes be non-existent and loads were applied to door or window frames.

Figure 22. Structural layout of rc slabs on brick walls in Shaukeiwan Police Station (built in 1949)

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5.2.2 Steel Framed Building th

In the 19   century, besides masonry and brick, cast iron was used as vertical elements, due to its good compressive strength. Wrought iron was not widely used for columns, because cast iron was better in compression and was cheaper than wrought iron. The first steel framed building in the UK was the Ritz Hotel completed in 1904. However, steel framed buildings were only permitted in the UK under London County Council Act 1909 , 5 years after the completion of the Ritz Hotel. Similar form of structures has been found in Hong Kong in former Kom Tong Hall ( Photo 24) (though not all columns are steel as some vertical elements being masonry or brick walls), the former Central Fire Station, Queen Mary Hospital Main Block ( Photo 25  and Figure 23) and the third generation Hong Kong and Shanghai Bank Building ( Photo 26). Steel framed structure then became increasingly common for commercial  buildings, hotels, and residential flat in the UK between 1909 and 1939 (Gibbs 2000). For such structures, they were usually clad in brick or stone ( Figure 24), for architectural reasons and for fire protection. Initially, it was further assumed that the cladding surrounding the steelwork would prevent moisture ingress and avoid corrosion problems (Gibbs 2000). Yet such assumption was not correct, and one of major problems of such structures was the corrosion of the embedded steelwork ( Photo 27). Project officer may refer to TAN 20: th Corrosion in Masonry Clad Early 20  Century Steel Framed Buildings (Gibbs 2000) on the causes of corrosion and the possible repair methods of such structures. The corrosion problem was subsequently recognised in the 1930s in the UK, when all steelwork was required before erection to be coated with one coat of boiled oil, tar or paint, and after erection by an additional coat of boiled oil, tar, paint or cement wash. Gibbs (2000) further noted that steelwork in some of such steel framed structures may be painted with red lead or coated with bituminous coating. In the record drawing of Queen Mary Hospital of 1934, it was noted that all structural steelwork had been specified to be painted with one coat of red lead paint following the prevailing UK practice at that time, though the design was done in Hong Kong by in-house staff (Fung 1997). Photo 28 shows the steel frames under trial fabrication in Dorman Long yard in Middlesbrough, the UK, which were then taken apart and shipped to Hong Kong. Photo 29 and Photo 30 show the erection of the steel frames on the site Queen Mary Hospital in 1935. Photo 31 shows the red paint on a steel beam in such building.

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Figure 23. Framing plan of Queen Mary Hospital Main Block (completed in 1937)

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Figure 24. Brick clad steel framed building (Source: Gibbs 2000)

Photo24. Brick clad steel column in former Kom Tong Hall (built in 1914) Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Photo 25. Queen Mary Hospital (built in 1937) (Source: Hospital Authority)

Photo 26. Corrosion of brick clad steel column in the former Kom Tong Hall (built in 1914)

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Photo 27. Steel frames of Queen Mary Hospital (completed in 1937) under trial fabrication in Middlesbrough, the UK (Source: Historical Photographs of China) (available: http://hpc.vcea.net/Collection/Introduction; accessed: 29 May 2012))

Photo 28. Steel frames of Queen Mary Hospital (completed in 1937) under erection on site at Pokfulam in 1935 (Source: Fung 1997)

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Photo 29. Steel frames of Queen Mary Hospital under erection on site at Pokfulam in 1935 (viewed from Pokfulam Road) (Source: Fung 1997)

Photo 30. Steel frames of old Hong Kong and Shanghai Bank Building under construction in 1934 (completed in 1935 and demolished in 1984) (Source: Historical Photographs of China) (available: http://hpc.vcea.net/Collection/Introduction; accessed: 29 May 2012))

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Photo 31. Red lead paint on steel beam in steel framed building (Source: Gibbs 2000)

5.3

Foundations

5.3.1 There is a lack of historical review and publication of the foundation systems in Hong Kong (and indeed in the world) (Przewłócki et al   2005), though foundation is vital to the structural stability of a building. Up till the 1950s, shallow foundation (pad or strip footing) was the predominant type of foundation, as most of the buildings were low-rise. The first building legislation,  Buildings and Nuisances Ordinance 1856   already contained a rule of thumb for the width and depth of the founding level, which remained unchanged till  Buildings Ordinance 1950.  Buildings Ordinance 1950 still specified that the footings should be founded on “sound stone, brick, concrete, or other equally hard substance, carried down to a depth of not less than twice the thickness of the wall in the lowest storey”. The same section also specified the footings to be stepped so that width of such foundation “shall diminish gradually towards the upper surface thereof in regular steps or offsets” ( Figure 25, Figure 26, Figure 27, and Photo 32).  Buildings Ordinance 1950 also  provided depth of the founding level and width of the foundation might be required to vary for soft ground. No special provision had yet been specified for  piled foundation. Layers of granitic stone ( Figure 28, Photo 33 and Photo 34) were usually laid underneath such footings, probably to form a rigid platform to lay the masonry or brick walls and spread the loading over soft ground. Reinforced concrete footings were not common, and an early e xample of the use of rc pad footings were in 1908 ( Figure 28(b)) (Bowen and Measor 1958).

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Figure 25. Unreinforced concrete footings for brick walls

Figure 26. Unreinforced concrete footings for masonry walls

Figure 27. Footings of former Married Quarters at Hollywood Road (built in 1950)

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Figure 28(a). As-surveyed footing of former Clubhouse of Royal Yacht Club (built in 1908)

th

Figure 28(b). Reinforced concrete pad footing in early 20  century (Source: Bowen and Measor 1958)

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Photo 32. Stepped footing in former Kom Tong Hall (built in 1914)

Photo 33. Layers of granitic stones underneath wall footing in the former Central School at Hollywood Road (completed in 1889 and destroyed during the WWII) (Source: AMO)

Photo 34. Layers of granitic stones underneath wall footing in the former Central School at Hollywood Road (completed in 1889 and destroyed during the WWII) (Source: AMO) Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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5.3.2 Besides brick or masonry walls on concrete (reinforced or unreinforced) footings, steel framed buildings usually founded on steel base plates resting on a steel grillage inside concrete footing. Figure 29(a) and Figure 29(b) show the th steel stanchion footings in early 20   century and in the 1930s, and Photo 35 shows the steel grillage underneath the footings in the former Kom Tong Hall (built in 1914). Reinforced concrete footings replaced such steel grillage concrete footing in the UK in around the 1940s in order to conserve structural steel during the WWII (Bowen and Measor 1958). As the pressure distribution underneath a raft foundation had not yet been ascertained till the 1950s, the use of raft foundation was not common til l that time (Bowen and Measor 1958). .

Figure 29(a). Footing of steel stanchion Ritz Hotel in London (the first steel framed building in the UK built in 1904) (Source: Chrimes 2001)

Figure 28(b). Footing of steel stanchion in the 1930s (Source: Bowen and Measor 1958)

Photo 35. Steel grillage underneath footing in former Kom Tong Hall (built in 1914) Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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5.3.3 The early use of piles was believed to be for dockyards and piers, and timber  piles were used. When applied to building structures, Przewłócki et al   (2005) noted that, the early scheme was to lay the foundation of the brick wall on a stone layer or on the timber platform supported by timber piles ( Figure 30). Timber piles were always placed below the lowest expected groundwater level,  because builders from the oldest times had known that timber did not decay if  permanently immersed under the ground water table. An early use of timber  piles in building works in Hong Kong could be traced to 1846, when timber  piles were used in the Exchange Building on No. 7 Queen’s Road (which housed the second generation Supreme Court) ( Photo 36). The timber piles there were Manila hardwood each of 225mm square in cross-section and of length 12ft (3.7m). Timber piles can still be found for the foundation of the third generation Supreme Court (1912-1985) (which was later converted into the former Legislative Council), where 1,447 nos. of timber piles ( Photo 37(a)) were installed. The piles were China fir with dimension of about 200mm either in square or circular shape and length of 5m. These piles were cut off at about 200mm clear of the face of the footings. Similarly, timber piles were used to found the old Alexandra House in Central ( Photo 37(b)).

Figure 30. Early timber piled foundation (Source: Przewłócki et al  2005)

Photo 36. Exchange Building on No. 7 Queen’s Road (Source: Hong Kong Museum of History) Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Photo 37(a). Timber pile in old Supreme Court Building (completed in 1912)

Photo 37(b). Timber piles in old Alexandra House at Central (completed in 1904 and demolished in the 1950s) (Source: Cameron 1979) th

5.3.4 In the late 19   century, driven precast concrete piles first appeared in Europe th (Photo 38(a)). In the early 20  century, steel H-piles and concrete (precast or cast-in-situ) piles appeared in the market. It was said that timber piles were faded out in the UK owing to the increasing scarcity of supplies of suitable long straight lengths, and to the cost of transport ( The Structural Engineer   1933).  Numerous pile driving formulae had been derived (e.g. Dutch formula, Engineering News formula) (Sandover 1933). Hiley formula first appeared in a  paper entitled “The Impact of Imperfectly-Elastic Bodies and the Effect of the Hammer Blow in Pile-Driving” published in the Transactions of the Society of  Engineers  in 1923. Diesel hammers, originally invented in Germany, only appeared in the market of the UK in the early 1960s (Bullen 1961).

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Photo 38(a). Earliest driven concrete piles in 1894 (Source: The Structural Engineer  1924)

5.3.5 In summary, types of piles in historical buildings in Hong Kong include: a) timber piles;  b) driven pre-cast concrete piles; c) driven cast-in-situ piles (e.g. Vibro and Frankie piles); and d) driven steel H-piles (though uncommon in building works and more common in civil engineering works (e.g. piers). Hand-dug caissons appeared later in the market in the early 1960s. Cylinder  piles in the form of large diameter bored piles and auger piles ( Photo 38(b) appeared in the UK only in the 1950s, and only appeared in Hong Kong in the late 1960s.

Photo 38(b). Early auger piles (Source: Bowen and Measor 1958)

5.3.6 Driven precast concrete piles were shod with a cast iron shoe, and could be up to 60 ft (18m) long). Driven cast-in-situ concrete piles (the “Simplex” system), invented by Frank Shuman of Philadelphia in 1903, appeared in the market around the WWI (Chrimes 2001). It was then followed by two other systems of driven cast-in-stiu piles, namely, Frankie piles (invented by Belgian Edgard Frankignoul in 1909) and Vibro piles (invented by A Hiley in 1920). Pressure  piles were invented in Germany in the early 1920s, and were first used in the UK in 1928 by J F Barr (Bullen 1961). By then, there were two common Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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methods of installing cast-in-situ concrete piles. The first method (commonly called “Vibro pile”) was to drive a steel tube fitted with a conical shoe to the required set, followed by pouring concrete into the hole. The tube was withdrawn, and at the same time vibrated so as to consolidate the concrete and force it into the surrounding ground. The shoe, of course, remained at the  bottom of the hole. Vibro (HK) Limited was established in Hong Kong in 1929, specialised in pile driving works. The second method (commonly called “Frankie pile”) was to pour a plug of dry concrete placed nearly dry in the  bottom end, followed by dropping an internal hammer onto the concrete plug which dragged the tube into the ground by internal friction. When the required depth was reached, additional energy (by greater drop height of the hammer) forced the plug out of the tube to form a bulb of dry concrete. Then, concrete was placed in the tube and consolidated by means of the hammer, the tube being withdrawn meanwhile. 5.3.7 An early application of using driven steel H-piles in Hong Kong was in Blake Pier (completed in 1900) at Central (Wong et al   2007). The Central Market was founded on 390 nos. of Vibro cast-in-situ concrete piles. However, piles were not common for Government buildings, as most of them were low rise. The then Central Government Offices, 7-storey buildings, were founded on pad footings with bearing pressure of 250kPa. The City Hall at Central, built in 1962, was one of Government buildings employing piled foundation, as the  buildings lie on the reclaimed land of the then new waterfront. 5.4

Summary of Structural Forms of Historical Buildings

5.4.1 Annex A gives examples of the structural forms and load-transfer mechanisms of pre-WWII graded historical buildings maintained by ArchSD in chronological order. For those pre-WWI buildings, timber floor and/or unreinforced concrete slabs were common. Timber trusses were also common as the pitched roof. Masonry or brick walls and columns were the norm. For those post-WWI buildings, rc slabs on rc beams and columns might be used, though brick walls were still widely used as vertical elements. The typical  buildings are generally 2 to 3 storeys in height. Table 13  summarises the th th structural forms from mid-19   century to mid-20   century for Government  buildings, and project officer should note that this is a broad-bush classification and actual construction may deviate from suc h classification.

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Vertical elements masonry or  brick walls

Year 1840s – 1900s

1910s – 1930s

masonry or  brick walls / steel frames cladded with  brick or masonry masonry walls /rc columns

1930s – 1950s

Horizontal floor systems timber floor /  jack arch floor / filler  joists floor filler joists floor / rc slabs

rc slabs with steel reinforcemen t on rc beams

Roof systems

Foundation

timber trusses / wrought iron trusses

footings or timber piles

timber trusses footings, timber / steel trusses  piles, driven  precast concrete  piles, driven steel H-piles steel trusses / rc slabs

footings, driven  precast or castin-situ piles, driven H-piles

5.4.2 Chinese-Styled Historical Buildings In Hong Kong, traditional Chinese houses existed long before it became a British colony, and such buildings can still be seen in the New Territories. There are a number of Chinese-styled declared monuments maintained by ArchSD, including the Old House in Shatin Wong Uk, Sheung Yiu Folk Museum, Sam Tung Uk Museum, etc. Figure 31, Photo 39 and Photo 40 show the typical layout of such houses, with clay tiles on timber roof battens supported by round timber purlins ( Photo 41) which rested on brick ( Photo 42) or masonry walls ( Photo 43). Typical Chinese-styled tile roof can be single layer or double layer ( Figure 32). Probably because of the lack of resources in early days, sometimes the walls might be of masonry or brick facing with rubble,  pebble, mud or sand heart ( Photo 44), or a combination of masonry and brick (Photo 45) without mortar or with mud, clay or lime/cement mortar, or clay  brick (Photo 46). For wealthy clan, the traditional Chinese house usually consisted of more than one hall ( ) and courtyard in plan with more than one bay ( ) in width ( Photo 47, Figure 33, Figure 34, and Figure 35). The entrance hall, the courtyards and the halls were located along the central axis usually in south-north direction. Side rooms were attached on each side of the entrance hall and the halls, while the kitchen and the bathroom were located on t he left and right sides of the front courtyard respectively. Such house was mainly constructed of Canton grey  bricks and granite blocks with its walls supporting the pitched roofs of wooden rafters, purlins and Chinese-style tiles ( Figure 36  and Figure 37). For larger span roof, rather than using timber trusses, the traditional Chinese construction used a series of timber joists (e.g. tailiangshi goujia ( ) in Figure 38).

進

間

抬樑式構架

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Photo 39. Typical Chinese-styled house (Chung Old House at Tsuen Wan) (Source: AMO)

Photo 40. Typical Chinese-styled house (Heung Yuen Wai at Sha Tau Kok) (built in 1928) (Source: AMO)

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(a) G/F Plan

(b) Section Figure 31. Typical Chinese-styled house

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Photo 41. Clay tiles on timber joists – Chung Old House at Tsuen Wan

Figure 32. Typical Chinese-styled clay tile roof (single and double layer) Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Photo 42. Chung Old House at Tsuen Wan (showing the brick walls)

Photo 43. Chung Old House at Tsuen Wan (showing the masonry walls)

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Photo 44. Wall with masonry facing and rubble heart (Source: www.world-housing.net)

Photo 45. Brick wall mixed with masonry with and without mortar in former Yau Ma Tei Cinema (built in 1930)

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th

Photo 47. Wong Uk, Shatin (built in the 19  century) (Source: AMO)

Figure 33. Front elevation of Wong-Uk at Shatin (Source: CM Wong & Associates Ltd)

Figure 34. Side elevation of Wong-Uk at Shatin Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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(Source: CM Wong & Associates Ltd)

Figure 35. Architectural layout of Wong Uk at Shatin (showing two-hall-and-two-courtyard in plan with three bays in width) (Source: AMO)

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Figure 36. Structural layout at cockloft of Wong Uk at Shatin (Source: CM Wong & Associates Ltd)

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Figure 37. Structural layout at roof of Wong Uk at Shatin (Source: CM Wong & Associates Ltd)

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(a) Isomeric view

(b) Section Figure 38. Traditional tailiangshi goujia for large span (Source: and  1991

王其钧 2006

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马炳坚

)

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唐樓

Tenement buildings (or more commonly called “ tong lau ( )) were then  built throughout the territory, which were mostly constructed with timber joists (later rc slabs and beams) on masonry or brick walls. Photo 48, Figure 39, Figure 40 and Figure 41 show typical layouts of such tenement buildings. The width of these buildings was restricted to 15 ft (about 5m) as the timber joists were supplied in such length.

Photo 48. Tong lau at Shanghai Street (Source: AMO)

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Figure 40. Plan of tong lau (Source: modified from Chadwick 1882)

Figure 41. Typical layout of tong lau in Hong Kong (Source: modified from Chadwick 1882)

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Foundations for such Chinese-styled historical buildings differed from that adopted for Western-styled buildings, which were either founded on pad or strip footings or piles. Probably due to the unavailability of concrete, the brick or masonry walls were enlarged at base, which were then rested on a strip or pad footing formed by granitic stone ( Figure 42). However, it should be noted that the brick or masonry walls, or internal posts might sometimes rest directly on soil without any stepped bricks, nor stone beneath.

Figure 42. Typical foundation for Chinese-styled historical buildings (Source: modified from )

北京土木建筑学会 2006

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6.

Specification and Corrosion Protection

6.1

Specification In the early days, there was no standardised general specification for building works in the then Public Works Department. At that time, the specification on materials and workmanship was stated in the tender drawings and particular specifications issued with the tender documents. The earliest edition of general specification appeared in 1962, when General Specification of Materials and Workmanship in Connection with the Construction of Buildings for the Hong  Kong Government in the Colony of Hong Kong   was issued by the then Architectural Office. The general specification has then been revised and updated in 1967, 1968, 1970 (metric edition), 1976, 1984, 1993 and 2003, until its current version of General Specification for Buildings  (2007 Edition). Copies of 2003 and 2007 editions are available in ArchSD web site at URL: www.archsd.gov.hk/. For editions of the general specification before 2003,  project officer may visit ArchSD library on 35/F or the libraries of local universities (which keep all editions of the general specification). SEB also hold soft copy of the section on concrete works of 1968 and 1970 editions, and soft copy of 1976, 1984, 1993 and 2003 editions. Project officer can approach CSE/1 for these soft copies.

6.2

Corrosion Protection Both structural steel and steel reinforcement is susceptible to corrosion resulting in the loss of cross-sectional area. The corrosion of steel reinforcement causes concrete spalling. In the early days, red lead paint was used for corrosion  protection of structural steel. This was evident in the notes in the structural drawings and the recent works. The record drawings of Queen Mary Hospital Main Block (built in 1937) specified that the steelwork was to be applied with red lead paint, and in the adaptive reuse of Blake Pier Pavilion in new Stanley Pier the steelwork built in 1909 was found to be protected with red lead paint. For rc, corrosion protection of steel reinforcement, same as current practice, relied on the concrete cover. However, the concrete cover for each structural element has been improved over the years with the advancement of codes. Project officer should therefore refer to the prevailing code at the time of construction for the adopted concrete covers.

7.

List of References

Project officer should note that the above paragraphs can neither serve a comprehensive review of all the construction materials, structural forms and construction methods of historical buildings in Hong Kong, nor contain all information required for structural survey and appraisal of historical buildings. Hence, the following list of references is provided:

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History of Structural Materials, Design and Construction Addis, B (1997), “Concrete and Steel in Twentieth Century Construction: from Experimentation to Mainstream usage,” in Stratton, M (1997) (ed), Structure and Style: Conserving Twentieth Century Building  (London: E&FN Spon) pp. 122-42. Addis, B and Bussell, M (2003), “Key Developments in the History of Concrete Construction and the Implications for Remediation and Repair,” in MacDonald, S (2003) (ed), Concrete Building Pathology (Oxford: Blackwell Science) pp. 15-106. American Steel & Wire Company (1908), Triangle Mesh Concrete  Reinforcement Engineers’ Handbook  (American Steel & Wire Company). Basil, S W (1929), “Some Historical Notes on the Applications of Iron and Steel to Building Construction,” The Structural Engineer , 7(1), pp. 4-12 (available: www.istructe.org/thestructuralengineer ; accessed: 11 May 2012). Bates, W (1991),  Historical Structural Steelwork Handbook   (London: British Constructional Steelwork Association Ltd) (available: www.steelconstruction.org/component/documents; accessed: 11 May 2012). Beal, A N (2001), “A History of the Safety Factors,” The Structural Engineer  89(20), pp. 20-6 (available: www.istructe.org/thestructuralengineer ; accessed: 11 May 2012). Bussell, M (1997), Appraisal of Existing Iron and Steel Structures (Ascot: SCI). Bussell, M (1999), “Problems and Possibilities – Cast Iron, Wrought Iron, Steel,” in Verhoef, L G W (ed) (1999),  Proceedings of the International Congress on Urban Heritage and Building Maintenance - Iron and Steel  (Faculty of Architecture, Delft University of Technology) Bussell, M (2007), “Use of iron and steel in buildings,” in Forsyth, M (ed) (2007), Structures & Construction in Historic Building Conservation (Oxford: Blackwell), pp. 173-91. Bussell, M (2008), “Concrete and Reinforced Concrete,” in Forsyth, M (2008) (ed),  Materials & Skills for Historic Building Conservation  (Oxford: Blackwell Publishing Ltd). Clarke, J L (2009), Technical Report No. 70:  Historical Approaches to the  Design of Concrete Buildings and Structures (Surrey: The Concrete Society). Cotta, R (2008), “Stone: Granite,” in Forsyth, M (2008) (ed),  Materials & Skills  for Historic Building Conservation (Oxford: Blackwell Publishing Ltd).

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Department of Transport (2001),  BD 21/01: The Assessment of Highway  Bridges and Structures  (London: Department of Transport) (available: www.dft.gov.uk/ha/standards/dmrb/vol3/section4/bd2101.pdf ; accessed: 8 May 2012). th

Gibbs, P (2000), TAN 20: Corrosion in Masonry Clad Early 20  Century Steel  Framed Buildings (Edinburgh: Historic Scotland). Ma, K Y (2007), The Development of Hong Kong Structural Engineering Standards after the Second World War and before 1997   (Hong Kong: The University of Hong Kong) (Unpublished MA Dissertation) (available: http://sunzi.lib.hku.hk/ER/detail/hkul/3862073; accessed: 11 May 2012).  Ng, H K (2011), “Evolution of the Hong Kong Wind Code,”  Hong Kong  Engineer , 130(4), pp. 17-8 (available: www.hkengineer.org.hk ; accessed: 11 May 2012). Tamworth, I P (1952), Timber Used in Hong Kong   (Hong Kong: Ye Olde Printerie).

Structural Appraisal and Surveys of Historical Buildings Beckmann, P (1995), Structural Aspects of Building Conservation  (London: McGraw-Hill). Beckmann, P and Bowles (2004), Structural Aspects of Building Conservation nd (Oxford: Elsevier Butterworth-Heinemann, 2 ). Buildings Department (2012),  Practice Guidebook on Compliance with  Building Safety and Health Requirements for Adaptive Re-use of and  Alteration and Addition Works to Heritage under the Buildings Ordinance (Hong Kong: Buildings Department) (available: www.bd.gov.hk/; accessed: 23 July 2012). CIRIA (1986), CIRIA Report No. 111: Structural Renovation of Traditional  Buildings (London: CIRIA). Clancy, B and Stagg, B (2004), “Are ‘Structural Surveys’ Proper Engineering?” The Structural Engineer , 82(1), pp. 27-32 (available: www.istructe.org/thestructuralengineer ; accessed: 11 May 2012). D’Ayala, D F and Forsyth, M (2007), “What is Conservation Engineering,” in Forsyth, M (ed) (2007), Structures & Construction in Historic Building Conservation (Oxford: Blackwell), pp. 1-11. Hume, I (2007), “The Philosophy of Conservation Engineering,” in Forsyth, M (ed) (2007), Structures & Construction in Historic Building Conservation (Oxford: Blackwell), pp. 12-18.

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rd

IStructE (2010), Appraisal of Existing Structures (London: IStructE, 3  ed). Rabun, J S (2000), Structural Analysis of Historic Buildings: Restoration,  Preservation, and Adaptive Reuse Applications for Architects and Engineers (New York: John Wiley & Sons). Ross, P (2002), “Appraisal and Repair of Timber Structures,” The Structural  Engineer , 80(17), pp. 26-9 (available: www.istructe.org/thestructuralengineer ; accessed: 11 May 2012). Koon, C M (2010), “Structural Appraisal of Reinforced Concrete Buildings with Historic Values,”  Presented at Seminar on Concrete Damage  Assessment, Concrete Repair and Concrete Mix Technology, Hong Kong, China, 2 February 2010. Urquhart, D (ed) (2007), Guide for Practitioners 6 - Conversion of Traditional  Buildings: Application of the Building Standards Part I - Principles and  Practice  (Edinburgh: Scottish Building Standards Agency) (available: www.historic-scotland.gov.uk ; accessed: 11 May 2012).

王其钧 (2006),《中国建筑图解词典》(北京: 机械工业出版社)。 北京土木建筑学会 (2006),《中国古建筑修缮与施工技术》(北京: 中国计划 出版社)。 马炳坚 (1991),《中国古建筑木作营造技术》(北京: 科学出版社)。 Historical Review of Foundation Bowen, F M and Measor, E O (1958), “Foundations and Sub-structures,” The Structural Engineer , 36(13), pp. 57-65 (available: www.istructe.org/thestructuralengineer ; accessed: 14 August 2012). Bullen, F R (1961), “Presidential Address Notes on the History of Foundation Engineering,” The Structural Engineer , 39(12), pp. 385-404 (available: www.istructe.org/thestructuralengineer ; accessed: 14 August 2012). Chrimes, M (2001), “Concrete Foundations and Substructures: a Historical Review,” in Sutherland, J, Humm, D and Chrimes, M (2001) (eds), Concrete:  Background to Appraisal (London: Thomas Telford Publishing). Przewłocki, J, Dardzinska I and Swinianski, J (2005), “Review of Historical Buildings’ Foundations,” Geotechnique, 55(5), pp. 363–72. Sandover, J A M (1933), “Foundation,” The Structural Engineer , 11(8), pp. 338-51 (available: www.istructe.org/thestructuralengineer ; accessed: 11 May 2012).

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Sutherland, J (2001), “Introduction,” in Sutherland, J et al (eds) (2001),  Historic Concrete: Background to Appraisal (London: Thomas Telford). Tomlinson, M J, Driscoll, R and Burland, J B (1982), “Discussion: Foundations for Low-rise Buildings,” The Structural Engineer , 60A(8), pp. 242-53 (available: www.istructe.org/thestructuralengineer ; accessed: 11 May 2012).

Other References Allwinkle, S et al   (1997), TAN 11: Fire Protection Measures in Scottish  Historic Buildings (Edinburgh: Historic Scotland). Australian National Association of Forest Industries (2004), Timber Manual  Datafile P1: Timber Species and Properties (Deakin: NAFI, revised edition) (available: www.nafi.com.au ; accessed: 3 August 2012). Buildings Department (2011),  Draft Code of Practice for Mandatory Building  Inspection Scheme and Mandatory Window Inspection Schemes  (Hong Kong: Buildings Department) (available: www.bd.gov.hk/; accessed: 13 October 2011). Cameron, N (1979), The Hongkong Land Company Ltd: a Brief History  (Hong Kong: Offset Printing). Chadwick, O (1882), Mr Chadwick's Reports on the Sanitary Condition of Hong  Kong (with Appendices and Plans) (London: Colonial Office). Fung C M (1997),  A History of Queen Mary Hospital 1937-1997   (Hong Kong: Queen Mary Hospital). Ho, P Y (2001), Water of a Barren Rock – 150 Years of Water Supply in Hong  Kong  (Hong Kong: The Commercial Press). Lam, S L (2003), Conservation of Historic Buildings in Hong Kong   (Hong Kong: Architectural Services Department) (available: www.archsd.gov.hk/english/reports/e3121.pdf ; accessed: 17 May 2012). Pang, P and Chan, W T (2010), “Fire Engineering for a Sustainable Future,”  Presented at Fire Division Symposium on Fire Engineering for a Sustainable  Future, Hong Kong, China, 15 March 2010. Wong, W S and Liu, A (1999a) (eds),  Measured Drawings Volume One: Hong  Kong Historical Chinese Buildings (Beijing: China Planning Press). Wong, W S and Liu, A (1999b) (eds),  Measured Drawings Volume Two: Hong  Kong Historical Western Buildings (Beijing: China Planning Press).

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Urban Redevelopment Authority, Singapore (2011), Conservation Guidelines (Singapore: URA) (available: www.ura.gov.sg/conservation/; accessed: 17 May 2012).

Case Studies Curtin, W and Parkinson, E (1989) “Structural Appraisal and Restoration of Victorian Buildings,” in ICE (1989), Conservation and Engineering Structures (London: Thomas Telford Publishing), pp. 95-108. Ma, K Y, Chan, Y K and Wong, C Y (2011), “Revitalization of Historic Buildings: Conversion of Yau Ma Tei Theatre and Red Brick Building into a  Xiqu  Activity Centre,”  Presented at 5th Cross-strait Conference on Structural and Geotechnical Engineering, Hong Kong, China, 13-15 July 2011. Wong C T, Leung M K, Liu K M and Ma, K Y (2007), “The Blake Pier Pavilion: Just a Memory?” The Structural Engineers, 85(20), pp. 38-43 (available: www.istructe.org/thestructuralengineer ; accessed: 24 November 2011). Wong, C T and Chan, P W (2007), “Refurbishment of Kom Tong Hall as Dr Sun Yat-sen Museum,” The Structural Engineers, 85(20), pp. 31-7 (available: www.istructe.org/thestructuralengineer ; accessed: 24 November 2011). Ross, P (2002), “Case Histories,” in Ross, P (2002),  Appraisal and Repair of Timber Structures (London: Thomas Telford) pp.160-210.

馬冠堯 (2011), 《香港工程考: 十一個建築工程故事 1841-1953》(香港: 三聯 書店)。

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Annex A Examples of Structural Forms of Graded Historical Buildings

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Former Montgomery Block of Victoria Barracks at Kennedy Road (now Mother’s Choice Home) •  built between 1840 and 1874 • concrete slab on filler joists supported on brick columns and walls

As-surveyed section in 2011

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Lei Yue Mun Park and Holiday Village Block 10 (former Lei Yue Mun Barracks) th th •  built in late 19  Century or early 20  Century • original block with jack arch concrete slab on filler joists supported on brick columns and walls • extension block with concrete slab on filler joists supported on brick columns and walls

As-surveyed section in 2011

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Former British Military Hospital at Bowen Road •  built between 1903 and 1906, and opened in 1907 • concrete slab on steel beams supported on brick columns and walls • structural steel roof truss covered with Chinese style double layer tiles

As-surveyed section in 2011

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Hong Kong Museum of Medical Science in Mid-Levels (former Pathological Institute) •  built in 1906 • concrete slab on brick walls with some later strengthened by steel beams • timber roof truss with timber purlins covered with Chinese style double layer tiles

As-surveyed structural layout in 2011

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Former Clubhouse of Royal Yacht Club at Oil Street, North Point •  built in 1908 • timber floors on steel beams supported on brick walls • clay roof tiles on timber roof truss

As-surveyed framing plan in 2010

Section

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Elevation facing internal courtyard (Source: AMO)

Elevation facing Electric Road (Source: AMO)

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Old Supreme Court Building •  built in 1912 • filler steel joists floor • structural steel roof trusses on north wing, south wing and west pediment with timber purlins, and structural steel roof dome over the central portion • masonry backed with brick walls and masonry columns as vertical elements • founded on timber piles

Bird’s Eye View

Front Elevation

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Side Elevation

Section

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Former Kom Tong Hall •  built in 1914 • rc slab with wire mesh as steel reinforcement on steel beams encased in concrete supported on steel columns (occasionally on brick walls)

As-surveyed typical floor framing plan in 2005

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Section

North Elevation Structural Engineering Branch, ArchSD Issue No./Revision No. : 1 / First Issue Date : August 2012

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Mongkok Police Station •  built in 1925 • timber floor on timber joists on brick walls, with the corridor slabs recast with rc • timber roof truss with steel channel purlins covered with steel profile sheet

As-surveyed section in 2012

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Former Central Fire Station • completed in 1926 and demolished in 1982 • rc slab with wire mesh reinforcement on steel beams supported by steel columns clad with brick • founded on pad footings

Typical part framing plan

Wire mesh slab details

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Section

(Source: AMO)

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Lei Yue Mun Park and Holiday Village Block 30 (former Lei Yue Mun Barracks) •  built in 1936 • rc slab on rc beams supported by brick walls and rc columns

As-surveyed section in 2011

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Queen Mary Hospital Main Block • completed in 1937 • rc slab with wire mesh reinforcement on steel beams supported by steel columns clad with brick • founded on pad footings

Photo 24. Queen Mary Hospital (built in 1937) (Source: Hospital Authority)

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