assessment and repair of fire-damaged structures: case study of tai shing street market

Structural Engineering Branch, ArchSD Page i of ii - 1 -Page 1 of 50 File code : FireAssessment

Assessment and Repair of Fire-Damaged Structures:

Case Study of Tai Shing Street Market

Edition No./Revision No. : 1/-

MKW/CTW/MKL/YFC/CYK/TWC/MFY

First Edition: February 2015

Information Paper



STRUCTURAL ENGINEERING BRANCH

ARCHITECTURAL SERVICES DEPARTMENT

February 2015

Structural Engineering Branch, ArchSD Page ii of ii - 2 -Page 2 of 50 File code : FireAssessment






Contents

1. Introduction .................................................................................................................. 1

2. The Site ........................................................................................................................ 2

3. Initial Site Visit and Preliminary Inspections .............................................................. 5

4. Preliminary Assessment ............................................................................................... 6

5. Detailed Assessment .................................................................................................. 11

6. Assessment of Residual Strength ............................................................................... 21

7. Structural Appraisal ................................................................................................... 24

8. Repair Proposals ........................................................................................................ 25

9. Concluding Remark ................................................................................................... 27

References .......................................................................................................................... 27

Appendix A Architectural Layout of Kai Tak Garden Phase I

Appendix B Structural Framing Plans of Kai Tak Garden Phase I

Appendix C Drawings for Repair Works

Copyright and Disclaimer of Liability

This Paper 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 any information storage and retrieval system, without the written permission from

the Architectural Services Department. Moreover, this Paper is intended for the

internal use of the staff in the 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 Paper at the time of publication, no guarantee is given

nor responsibility taken by the 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. The 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 Paper does not itself

confer immunity from legal obligations.

Structural Engineering Branch, ArchSD Page 1 of 48 - 1 -Page 1 of 50 File code : FireAssessment






1. Introduction

1.1 On 20 April 2013, a Level 3 fire broke out at Tai Shing Street Market in Wong Tai Sin. The fire (Figure 1) lasted for seven hours before the blaze was put out.

News reporting the incident are available at the following URLs:

Hong Kong Boardband: https://www.youtube.com/watch?v=QkvD32BQX0U

(accessed: 4 October 2013)

Now TV: http://news.now.com/home/local/player?newsId=65700 (accessed: 4

October 2013)

As a result, the market had to be closed down temporarily, with more than 400

stalls being affected. The fire had caused substantial fire damage to the

structural elements to the market, including extensive concrete spalling of the rc

slab, and cracks on the beams, columns and walls, though the fire did not cause

severe damage to the external elevation (Figure 2).

(Source: 22 April 2013, Ta Kung Pao) (Source: 21 April 2013, The Sun)

Figure 1 Fire at Tai Shing Street Market

Figure 2 Damage to external faade after the fire







1.2 SEB had promulgated SEBGL-OTH7 Guidelines on Structural Fire Engineering

Part II: Design of Structural Elements and Assessment of Fire-Damaged

Structures (SEBGL-OTH7) (available: http://asdiis/sebiis/2k/resource_centre/), in which the procedures in Figure 3 for

assessment and repair of fire-damaged structure are recommended. As SEB has

been responsible in the assessment of the fire on the structural integrity of the

building, and was also responsible for devising the repair methods to restore the

building to a sound condition. This Information Paper illustrates the details of

the assessment and proposals for repair based on the procedures in Figure 3.

Initial site visit Verify if structure is safe to enter Take action to secure public safety

Preliminary inspections Identify the scale of damage and

the follow-up areas including the

need of closure of potential

dangerous areas

Note area with maximum temperature

Detailed evaluation Computational modelling of fire

scenario using CFD method, e.g.

modelling using CFAST

Non-destructive tests Destructive tests

Structural appraisal

Repairs Identify extent of repair Prepare details and specifications

of repair

Figure 3 Procedures of assessment of fire damaged structure

(Source: modified from Gosain and Choudhuri 2008)

2. The Site

2.1 Tai Shing Street Market, completed in 2001, is located within the compound of

Kai Tak Garden in Wong Tai Sin at the junction of Tai Shing Street and Choi

Hung Road (Figure 4). Kai Tak Garden (Figure 5) consists of two phases with

a total of five nos. of 26-36-storey residential blocks sitting on a common

podium (which serves as a garden for the residents of Kai Tak Garden (Figure

6)). The market is of two storeys situated underneath the common podium of

Kai Tak Garden Phase I with a single storey basement serving both the market

and Kai Tak Garden. The structural design of Kai Tak Garden Phase I was

prepared by Wong & Ouyang (HK) Ltd in 1995-96, and the developer was

Hong Kong Housing Society. Under the Government Lease, the market is







owned by the Hong Kong SAR Government, and the podium is owned by the

Incorporated Owners of Kai Tak Garden. Food and Environmental Hygiene

Department (FEHD) is responsible for the daily management of the market, and ArchSD is responsible for the maintenance of the market. Figure 7 gives a

schematic section across the compound showing the relationship of the market

and the residential blocks.

Figure 4 Location plan of Tai Shing Street Market

(Source: www.centamap.com)

Figure 5 View of Kai Tak Garden from the junction of

Choi Hung Road and Tai Shing Street

(Source: www.Goolge.com.hk)







Figure 6 Podium Garden of Kai Tak Garden

Figure 7 Section across compound of Kai Tak Garden

2.2 The as-built architectural layout (at Appendix A) and structural framing plans

(at Appendix B) of the market have been retrieved, and the whole compound is

an rc construction with lateral stability provided by core walls. The market

itself is an rc framed structure with typical rectangular grid. Slabs are of

150mm thick spanning 3.417m on secondary beams of 750mm(D)500mm(B)

spanning 12.9m maximum. Primary beams are of 800mm(D)700mm(B) with

a maximum span of 10.25m. A FRR of 2 hours has been allowed in the original

design. The foundation of the whole compound is founded on driven steel H-

piles.







3. Initial Site Visit and Preliminary Inspections

3.1 The fire occurred in the mid-night of 20.4.2013 on the dry goods area on 1/F of

the market, and was only put off in the afternoon of 21.4.2013. After the fire,

SSE/APB immediately visited the site to make a preliminary assessment of the

structural integrity of the building. As the fire occurred on 1/F, extensive

damage was caused to the underside of the podium (i.e. 2/F slabs and beams).

SSE/APB, after consulting the then CSE/1, advised PSM and the management

office to cordon off part of the podium in order to restrict the imposed load onto

the podium, and props were then installed on 1/F as temporary support to 2/F

slabs before restoration. Of course, the market was temporally closed.

3.2 The investigation team headed by the then CSE/1 arrived at the post-fire scene

in the afternoon of 22.4.2103. During an initial inspection (Figure 8), the

debris had not yet been removed and this provided very useful information on

the spread and severity of the fire. Spalling, the flaking of the concrete, the

formation of major cracks and the distortion of the construction were identified

so as to assess the structural integrity. As the concrete surfaces of the structure

were blackened and visibility in the absence of artificial lighting was poor, it

was difficult to ascertain the extent of damage. However, the investigation team

was still able to examining the most conspicuously damaged elements and

identifying the extent of damaged elements in order to give an indication of the

likely scale of the damage and the areas to be under detailed investigation.

Figure 8 Conditions of building after the fire

3.3 In the initial inspection, SSE/APB also got the contact of fire fighting officers,

and this later served as a valuable and reliable source of information on the

history of the fire, e.g. where and when the fire started, the spread route of the

fire, whether flashover occurred, the length of time taken to fight the fire, the

operation of any automatic fire detection, and the degree of effort required to

fight the fire. Hence, in assessing fire damage, the contact point of the

responsible fire fighting officers should be obtained. Management office of Kai

Tak Garden was also contacted, and their witnesses gave information, such as

the severity of the fire, the damage to the podium, the length of time between

the fire being noted and the arrival of the fire brigade, etc.







4. Preliminary Assessment

4.1 An initial assessment of the gas temperature at the time of the fire was required

to determine:

(a) whether structural damage had been resulted; and

(b) whether detailed structural investigation was required.

4.2 Conditions of fittings after fire

Table 1(a) and Table 1(b) list the effect of elevated temperature on and the

ignition temperature of common construction materials. A quick guide was

therefore referenced to the position, the condition, the melting and the charring

of materials (including non-structural materials) (Figure 9). It was noted that

the iron fresh water pipes, steel drain pipes and aluminium air ducts were

unaffected by the fire, and it might be deduced that the maximum temperature at

such locations during the fire was less than 500oC.

Figure 9 Condition of fresh water pipes, drain pipes and air ducts after fire

4.3 Debris and Combusted Materials after Fire

To study the fire severity and scenario of the fire incident, observation on

remaining debris and combusted materials within the affected area is crucial.

Hence, it is important to carry out an initial inspection as soon as the fire

damaged area can be safely entered before the removal of debris. Those

remaining combustible materials (Figure 10) are also a fuel to combustion

process so that the observation can give a general idea how much fire load was

given in this fire incident. In this fire, it was noted that except for those severe

damaged areas with longest duration exposed to the fire, only minimal damage

was observed in most areas. For example, BS trunking was not distorted and

melted, and in some areas, even polystyrene fittings remained intact. Maximum

attainable gas temperature can be simulated by computer program with

estimated the fire load and further assist PSE to study the effect of the fire to

existing structures in detailed assessment.







Figure 10 Debris and combusted materials after the fire

Table 1(a) Effect of elevated temperatures on

common construction materials

Approximate

temperature

(oC)

Substance Examples Condition

100

150

Paint Deteriorates Destroyed

120

120-140

150-180

Polystyrene Thin-wall food

containers, foam, light

shades, handles, curtain

hooks, radio casings

Collapse

Softens

Melts and flows

120

120-140

Polyethylene Bags, films, bottles,

buckets, pipes

Shrivels

Softens and melts

130-200

250

Polymethyl

methacrylate

Handles, covers,

skylights, glazing

Softens

Bubbles

100

150

200

400-500

PVC Cables, pipes, ducts,

linings, profiles, handles,

knobs, house ware, toys,

bottles

Degrades

Fumes

Browns

Charring

200-300

240

Cellulose

wood

Wood, paper, cotton Darkens

Ignites

250

300-350

350-400

Solder lead Plumber joints,

plumbing, sanitary

installations, toys

Melts

Melts, sharp edges rounded

Drop formation

400

420

Zinc Sanitary installations,

gutters, downpipes

Drop formations

Melt

400

600

650

Aluminium

and alloys

Fixtures, casings,

brackets, small

mechanical parts

Softens

Melts

Drop formation

500-600

800

Glass Glazing, bottles Softens, sharp edges rounded

Flowing easily,

Viscous

900

950

Silver Jewellery, spoons,

cutlery

Melts

Drop formation

900-1000

950-1050

Brass Locks, taps, door

handles, clasps

Melts

Drop formation

900

900-1000

Bronze Windows, fittings,

doorbells, ornamentation

Edges rounded

Drop formation

1000-1100 Copper Wiring, cables,

ornaments

Melts

1100-1200

1150-1250

Cast iron Radiators, pipes Melts

Drop formation

(Source: IStructE 2000 and Concrete Society 2008)







Table 1(b) Ignition temperatures of common construction materials

Material Ignition

temperature (oC)

1 Auto-ignition

temperature (oC)

2

Wood 280-310 525

Wool 240 -

Paper 230 230

Cotton fabrics 230-270 255

Polymethylacrytate (Perspex) 280-300 400-600

Rigid polyurethane foam 310 410

Polyethylene 310 415

Polystyrene 340 350

Polyester (glass-fibre filled) 350-400 480

PVC 390 455

Polyamide 420 425-450

Phenolic resins (glass-fibre filled) 520-540 570-580 Notes: 1 The temperature to which material has to be heated for sustained combustion to be initiated

from a pilot source. 2 The temperature at which the heat evolved by a material decomposing under the influence of

heat is sufficient to bring about combustion without application of an external source of

ignition.

(Source: IStructE 2000)

4.4 Concrete spalling and cracks

4.4.1 Severe concrete spalling was found on some of the slabs, and minor concrete

spalling was also noted on walls and columns (Figure 11). SEBGL-OTH7

summarises detailed information on the causes of concrete spalling during a fire.

There are three common types of spalling, namely: explosive spalling,

aggregate spalling, and corner spalling (Concrete Society 2008)). Explosive

spalling occurs early in the fire (typically within the first 30 minutes) and

proceeds with a series of disruptions, each locally removing layers of shallow

depth. Aggregate spalling also occurring in the early stage, involves the

expansion and decomposition of the aggregate at the concrete surface causing

small pieces of the aggregate flying off the surface. Such type of spalling will

only result in superficial damage. Corner spalling occurs in the later stage of

the fire, and is due to tensile cracks developing at planes of weakness. However,

this type of spalling occurs in the later stage, when the concrete is already

significantly weakened, and will not usually affect structural performance.







Figure 11 Concrete spalling during fire

4.4.2 One major effect of spalling is that it may significantly reduce or even eliminate

the layer of concrete cover on the reinforcement bars, thereby exposing the

reinforcement to high temperatures, leading to a reduction of strength of the

steel and hence a deterioration of the mechanical properties of the structure as a

whole. In the present case, a few areas of the exposed spalled surfaces were

smoke blackened, indicating on such areas, the reinforcement might have

subjected to direct fire exposure. Detailed investigation of these areas was

therefore warranted. However, in the majority of the spalled areas, the exposed

surfaces were not blackened (Figure 12), suggesting that spalling might have

occurred due to quenching effect by the cold water from firemens hoses. In addition, moisture content measurement on slab soffit by moisture meter was

carried out and showed that the readings taken are in normal range (Figure 13),

This further eliminates the possibility of the spalling resulted by excessive

moisture content over the concrete surface so that the quenching effect is likely

to be a cause of the extensive spalling.

Figure 12 Exposed concrete surfaces







Figure 13 Moisture Content Measurement

4.4.3 Besides spalling, surface cracks (Figure 14) appeared on most of the beams

adjacent to the spalled slabs. It is fortunately found that the cracks were only of

a few mm depth, and showed the patterns of the shear stirrups of the beams,

suggesting that they might have been resulted from the thermal expansion of the

stirrups.

Figure 14 Cracks on beams after fire

4.4.4 Based on the observations during the initial site visit, the duration of the fire,

and the extent of damage, especially the extensive concrete spalling on slabs,

the degree of damage at some areas was severe and major structural repair

would be required. It was therefore decided that detailed structural assessment

of the structure was required.







5. Detailed Assessment

5.1 A detailed assessment programme was then devised to study the effect of fire to

the structural integrity of the market and to devise the repair proposals.

Moreover, as Tai Shing Market situated underneath Kai Tak Garden, which is

controlled by Buildings Department, the assessment will have to be submitted to

Buildings Department. The main steps of the assessment programme are listed

as follows:

1. Measurement of the extent of damage

2. Assessment of maximum temperature during the fire

3. Computer modelling of the fire and its effect on the structure

4. Preparation of the assessment report and repair proposals

It was expected that the assessment would take about two months to be

completed after the clearance of the debris from the site. SSE/APB therefore

advised the PSM, which coordinated with FEHD to inform the stall lessees of

the progress of the assessment and repair. Dr Y L WONG of The Hong Kong

Polytechnic University was also engaged to assess the maximum temperature

during the fire, and to prepare the submission to Buildings Department.

5.2 Measurement of extent of damage

FEHD took about one month to clear the site from the debris and to install

necessary temporary props and access platforms to 2/F slabs (Figure 15). SEB

staff then measured the extent of damage, and this information was put onto a

drawing (Figure 16). In order to measure the depth of spalling, automatic self-

leveling rotary laser was employed to determine a reference of horizontal level

for measuring the depth of spalling. The laser beam is projected to a measuring

ruler vertically placed at soffit of slab to be measured. Measurement can be

taken from the laser line to the soffit of slab.

Detailed survey on cracks, especially defect location with water seepage was

extensively carried out and recorded on a drawing (Figure 16) for subsequent

repair by grout injection. All defects including cracks and spalling were

recorded on the drawing as a basis for deciding the most appropriate repair

strategy.

Figure 15 Access platforms for measurement







Figure 16 Measured extent of damage

5.3 Assessment of maximum temperature during fire

5.3.1 Colour of concrete at fire

Concrete is made from aggregate, and its colour changes when subjected to heat.

The change of colour is due to the presence of ferrous components in the

cement paste, coarse and fine aggregate. At above 300oC, a red discolouration is

important as it coincides approximately with the onset of significant strength

loss. However, this change of colour is most pronounced for siliceous

aggregates but not so for granitic aggregates, since the red colour change is a

function of the ferrous content which varies with different types of aggregates.

This modification in colour is permanent: it is therefore possible, on the basis of

the colour of the concrete, to make an approximate assessment of the maximum

attainable temperature and temperature profile reached during the fire. Figure

17 shows the colours of the concrete at different heating temperatures, and

Table 2 provides an overview of the colours of concrete at different temperature

ranges.

Figure 17 Colours of concrete at different heating temperatures

(Source: Hager 2013)

Table 2 Summary of colours of concrete in different temperature ranges

Heating

Temperature Colour Description

300 to 600C pink or red

600 to 900C whitish gray

over 900C buff

(Source: International Federation for Structural Concrete 2008, Felicetti 2004)







This means that it is possible to assess maximum attainable temperature of

concrete at the fire by observing the colours of the concrete. Figure 18 shows

that the colour changes gradually from heating face to inner of the concrete. In

practice, any concrete that turns pink is suspicious. A temperature of 300C

corresponds, more or less, to concrete that has lost a permanent part of its

resistance (Concrete Society 2008). A greywhite colour indicates concrete that is fragile and porous. Furthermore, a permanent distortion of the construction

indicates an overheating of the reinforcement. However, colour changes are

most pronounced for siliceous aggregates and less so for granitic aggregates,

which are predominant in Hong Kong. Also, due consideration should

always given to the possibility that the pink/red colour may be a natural feature

of the aggregate rather than heat-induced (Concrete Society 2008).

Figure 18 Change in colour of concrete heated from the left face

(Source: Short et al 2001)

5.3.2 Petrographic examination

Originally, it was intended to carry out petrographic examination of the concrete

thin sections cut from the core in order to determine the maximum temperature

attained and deduce the depth to which the concrete has been damaged.

However, there is a lack of experts in petrographic examination in Hong Kong,

and Public Works Central Laboratory can only provide interpretation on

petrographic images related to the alkali-aggregate reaction and alkali-silica

reaction. Moreover, it should also be noted that colour changes are most

pronounced for siliceous aggregates and less so for granitic aggregates which

are commonly found in Hong Kong, and as such, such option was not available.

5.3.3 Colour image analysis

5.3.3.1 In order to study the maximum temperature during the fire, Dr Y L WONG of

The Hong Kong Polytechnic University was engaged to develop a baseline

colour chart from a set of control samples obtained from in-situ concretes in

non-fire damaged areas in different elevated temperatures. This set of control

samples was similar to the in-situ damaged concrete in respect of mixing

proportion, concrete grade, age and effects from external environment. A pair







of concrete slices was heated in different elevated temperatures, e.g. 200C,

300C, 450C, 600C and 800C.

Figure 19 Colours of sliced concrete cores taken

5.3.3.2 A chart (Figure 19) showing colours of the concrete samples in different

temperatures together with colours of the concrete sample at ambient

temperature was established as reference to determine the depth of damage of

the in-situ concrete in fire. In order to minimise the subjective approach of

using visual observation, an objective approach is to use colour description

systems using RGB and HSI colour spaces was tried (Figure 20). RGB colour

space is a system most commonly used in most devices displaying images.

Every colour can be represented by three elements in terms of amounts of Red

(R), Green (G) and Blue (B). It is now also possible to convert the temperature

distribution in a concrete element by using colour image analysis in HSI

colour space. The colour image analysis aims at determining the temperature

of concrete by the change in hue (H) (), saturation (S) () and intensity (I) () when concrete is heated. In order to convert the RGB colour space into HSI colour space, the values of H,

S and I can be calculated mathematically as follows:

}B)B)(G(RB)(R

B)](RG)[(R0.5{cosH

I

B}G,min{R,-1S

B)G(R3

1I

2

1







(i) RGB colour space (Source: Blue Lobster Art and Design)

(ii) HSI colour space

(Source: Black Ice Software)

Figure 20 Colour description systems

5.3.3.3 Lin et al (2004) further carried out colour image analysis on a number of

mortar specimens by using an ordinary digital camera and his own developed

image colour intensity analyser, and obtained the variation of H, S and I of

three primary colours R, G and B (Figure 21) at different elevated

temperatures. They observed that the numerical values of H decrease as

temperature increases, but the variation is not significant. Unlike the results of

Short et al (2001), they observed that S shows a marked increase with

increasingly temperature. I shows little changes in the range 0200C, decreases with increasing temperatures in the range 200800C, and increases with temperatures in the range 8001000C. The variation of these three properties with temperature therefore serves as a useful way to deduce the

temperature gradient across concrete cross section.

Figure 21 Variation of H, S and I with temperature

(Source: Lin et al 2004)

5.3.4 ImageJ, a free Java-based image processing program (available:

rsbweb.nih.gov/ij) developed at the National Institutes of Health in the US,

can be used to carry out the colour image analysis. This program is capable to

analyse 3D live-cell imaging and radiological image by user-written plugins

originally in medical and health care industry. Since the user-written plugins

allow adding special features in this Java-based program, the program has then

been widely applied in other industries to analyse images. By using ImageJ, a

particular location, layer or element of concrete samples can be selected to

analyse the colour properties in respect of R, G, B, H, S and I (Figure 22).







Figure 22 Colour image analysis using ImageJ

5.3.5 Figure 23(a) plots the variation of R, G, B, H, S and I of the colours of the

concrete sliced samples in different temperatures in the present case.

Unfortunately, the correlation between the colours and temperature cannot be

observed from the colour image analysis results of the samples from this fire.

Trials were carried out to polish the surface of the sliced samples by Public

Works Central Laboratories to see whether better correlation can be observed,







and Figure 23(b) shows the colours of the polished surfaces. Though the

correlation of the colours and temperature could be improved, it was noted that

the crack densities increase with high temperatures. The relationship of crack

density and temperature may therefore worth further investigation.

Figure 23(a) Results of colour image analysis for fire-damaged concrete

sliced samples at Tai Shing Street Market

Figure 23(b) Colours and cracks in polished sliced concrete samples with

temperature at Tai Shing Street Market







5.4 Fire Modelling

5.4.1 With the colour image analysis, the maximum temperature at the most severe

fire-damaged areas was estimated. Site visits and measurements also gave

information on the history of and the spread rout of the fire, and the extent of

damage. However, the extent of damage by this fire was quite large, and it was

impractical to determine the maximum temperature of every structural member.

To aid the damage appraisal and the development of a cost-effective repair

schedule, a fire model using CFD method was therefore used to estimate the fire

intensity (gas temperature) and the resultant approximate isothermal surfaces.

Consultation and discussion with Fire Services Department confirmed the

ignition point of and spread route of the fire. Photos taken during the initial

inspection formed vital part in the modelling, as these photos gave rough idea of

the fire load on the spread rout of the fire. The observations during the initial

inspection were very useful in validating the fire model, as the results should

tally with the observations in terms of the spread and the maximum gas

temperature.

5.4.2 A zone model (Figure 24) using CFAST was built, and each compartment was

divided using a system of differential equations that express the conservation of

mass and energy, assuming valid the ideal gas law and defining the density and

the internal energy. Figure 25 shows the results of the fire modelling, which

tallies with the spread route as per the information from Fire Services

Department. Moreover, the maximum fire temperatures predicted by the model

also tally with the damage to the market.







Figure 24 Zone model for Tai Shing Street Market







Figure 25 Computer simulation of the fire







6. Assessment of Residual Strength

6.1 SEBGL-OTH7 provides detailed information on the residual strength of the

structural materials after the fire. Figure 26 shows the residual strength of

Grade 20 and Grade 30 unstressed concrete upon cooling with the

corresponding changes of its colour. Usually, the residual strength for concrete

exposed to temperatures above 300C (Concrete Society 2008). Figure 27

shows the residual strength of steel reinforcement. The original yield stress of

hot rolled steel bars is almost completely recovered on cooling from

temperatures of 500C to 600C, and on cooling from 800C it is only reduced

by 5%. That means that it may be assumed that there is no loss in residual

strength for hot-rolled steel reinforcement for a temperature up to 600oC

(Concrete Society 2008).

Figure 26(a) Residual strength of

concrete

(Source: Concrete Society 1978)

Figure 26(b) Recommended residual

strength of concrete after a fire


Figure 27(a) Residual strength of steel

reinforcement and prestressing wires

(Source: IStructE 2000)

Figure 27(b) Recommended residual

strength of hot-rolled steel

reinforcement after a fire








6.1.2 Besides correlating the strength of concrete and steel reinforcement using the

colour image analysis and the computer modelling, tests were carried out to

determine residual strength of concrete and steel reinforcement by respectively

compressive tests on concrete cores from the fire-damaged zone and tensile tests

on steel reinforcement. However, it should be noted that strength tests on cores

suffer a major limitation that they average the strength of concrete throughout

the core, which may contain both damaged and undamaged concrete. Table 3

summarizes the results of these tests.

6.2 Moreover, Schmidt hammer tests on the concrete surface had been carried out.

Though the tests could not provide accurate measurements of the concrete

residual strength, they provided a first, quick monitoring of the severity of the

effect of fire on a concrete structure, and allowed engineers to recognise the

most impaired parts of a member. Furthermore, in the case of concrete

members with thermal gradients, Felicetti (2005) found that the hammer tests at

the heated surface can indicate the average strength of the concrete located at

about 15 to 25 mm depth.







Table 3 Summary of compressive tests on concrete cores

and tensile tests of steel reinforcement

1. Residual compressive strength of cores from rc slabs

Sample No. Mean Diameter (mm) Estimated In-Situ Cube Strength (MPa)

2S2 53.4 40

2S22 53.4 41.5

2S6 53.9 45

2S10 53.4 36.5

2S11 54 44.5

2S12 53.9 41.5

2S28 53.8 29

2. Residual compressive strength of cores from rc beams


2B11 79.9 24.5

2B12 76.4 22.5

2B15 76.7 32

2B25 76.1 33

2B26 76.4 28.5

2B39 76.5 45.5

3. Residual compressive strength of cores from rc walls


W1 76.4 41

W2 76.4 50.5

W3 76.3 31.5

W4 76.3 37.5

4. Residual tensile strength of rebars from rc slabs Sample No. Yield Strength (MPa) Tensile Strength (MPa) Elongation %

2S20 530 662 23

2S14 464 621 31

2S37A 540 672 20

2S37B 454 583 25

2S9 360 539 33

2S13 440 576 32

2S12 530 637 25

2S35 487 645 29

2S25 510 664 28

A01 419 575 28

A02 472 656 28







7. Structural Appraisal

7.1 Table 3 shows that the average strengths of the cores of rc slabs and beams are

39.7MPa and 31.0MPa respectively, which are greater than the original concrete

strength of 30MPa. The residual concrete strength of fire damaged structures

demonstrates that effect of the fire is minimal to the structural adequacy of

existing structures. The minimum cube strength of 24.5MPa from one

individual sample was then adopted to check the structural adequacy of the

existing concrete slabs and beams within the fire damaged area.

7.2 For corewall and columns within the fire damaged area, an average strength of

40MPa was obtained from samples from the rc core walls, which is slightly

lower than the original design strength of 45MPa. Since concrete core samples

were retrieved from the outer layer of core wall (less than 100mm from

concrete face) on 1/F and the fire effect is usually limited to the surface

zone, the result is expected. Hammer rebound test on all existing structural

elements including rc core wall and columns were also conducted. The results

of all these hammer rebound tests show that the correlated concrete strength is

over 50MPa. Thus, it was concluded that residual concrete strength of lower

than 45MPa was only localised at the surface zone.

7.3 For the selected reinforcement bars, it was found that the average yield strength

is about 473MPa, which is higher than the original strength of 460MPa.

Average measured elongation at the tensile strength of the selected samples of

about 27% shows that the reinforcement after the fire performs more ductile,

compared with 12% specified in BS 4449.

7.4 Assessment of Structural Adequacy

With the establishment of the temperature profile and distribution, and the

strength of the concrete, steel reinforcement and structural steel, calculation was

carried out to assess structural capacity and the need for repairs. Usually,

member design (unless the stability of the structure is in doubt) is adequate.

Calculation using the residual strength was carried out to assess the structural

integrity of the market after the fire. It was concluded that with assumed

concrete strength of 24.5MPa from original strength of 30MPa, the rc beams

and slabs are adequate to support original design imposed load of 10kPa on 2/F,

and that with the assumed concrete strength of 40MPa, the rc corewalls and

columns are capable to sustain original design loads from superstructure. Thus,

no extensive repair work was required for rc columns, walls, and beams. Only

removal of loosen concrete and surface preparation on existing concrete were

required in the repair strategy.







8. Repair Proposals

8.1 Given the fact that there were locations with severe damage to the slabs, the

most cost effective solution at these locations should be partial demolition

followed by recast. However, this would seriously affect the continuous

operation of the market on 1/F and G/F, and the podium above. Repair was

therefore adopted. The following information was required:

the extent of breaking out of fire damaged concrete and removal of fire damaged steel reinforcement;

requirements for preparation of concrete surfaces that are to receive repair concrete, including special requirements to prevent feathered

edges;

details of new steel reinforcement including lap length and splicing with original bars, mechanical anchorage, cover etc;

any fabric reinforcement or wire mesh that may be required to hold the repair concrete in place in the temporary condition, including means of

supporting the fabric/wire mesh and the required concrete cover; and

the thickness and the properties of the repair materials.

8.2 Based on the extent of damage, the following three methods were use to repair

the damaged areas:

(a) Areas with damage limited to the concrete surface zone: the damaged concrete removed followed by patch repair by using repair mortar;

(b) Areas with cracks, where the concrete had been heated up to 500oC: removal of the damaged surface to a depth of about 15-20mm followed by

spraying (Figure 28);

(c) Areas with extensive cracks or concrete spalling, where the concrete might have been heated up to 700

oC: all damaged and/or loose concrete

removed followed by spraying with local thickening (Figure 29).

Additional steel reinforcement had been provided to the thickened slabs so

as to increase its structural capacity.

To provide the required key of the repair material to the existing concrete, all

concrete surfaces after removal of damaged concrete should be roughened.

Appendix C shows the details of the above repair proposals. Figure 30 shows

the repaired soffits with BS installed.







Figure 28 Spraying of concrete

Figure 28 Completed repaired areas before BS installation

Figure 30 Completed repaired soffits of 2/F slabs







9. Concluding Remark

Reinforced concrete structures have a very good fire resistance. Fire-damaged

concrete members can therefore be repaired by inexpensive repair methods.

This paper has demonstrated the procedures to assess the damage and the

residual load carrying capacity by combining site inspections, investigations,

testing combined with computer simulation and design calculation for a fire

damaged structure at Tai Shing Street Market.

References

Felicetti, R (2005), TR 1/05: New NDT Techniques for the Assessment of Fire

Damaged RC Structures (Milano: Politecnico di Milano).

Concrete Society (1978), TR 15: Assessment of fire-damaged concrete

structures and repair by gunite (Camberley: Concrete Society).

Concrete Society (2008), TR 68: Assessment, design and repair of fire-damaged

concrete structure (Camberley: Concrete Society).

Felicetti, R (2004), Digital-camera colorimetry for the assessment of fire- damaged concrete, Proceedings of the Workshop: Fire Design of Concrete Structures, Milan, 2-3 December 2004, pp. 21120.

Hager, I (2013), Colour Change in Heated Concrete, Fire Technology, 49, pp. 1-14.

IStructE (2000), Appraisal of existing structures (London: IStructE, 3rd

ed.).

Gosain, N K, Drexler, R E and Choudhuri, D (2008), Evaluation and repair of fire-damaged buildings, Structure Magazine, September, pp. 18-22.

Lin, D F, Wang, H Y and Luo, H L (2004), Assessment of fire-damaged mortar using digital image process, Journal of Materials in Civil Engineering, 16(4), pp. 383-6.

Short, N R, Purkiss, J A and Guise, S E (2001), Assessment of fire damaged concrete using colour image analysis, Construction and Building Materials, 15(1), pp. 9-15.







Appendix A

Architectural Layout of Kai Tak Garden Phase I







Basement Plan







G/F Plan







1/F Plan







2/F (Podium) Plan







3/F (Transfer Floor) Plan







Typical Floor Plan







Appendix B

Structural Framing Plans of Kai Tak Garden Phase I







Basement Framing Plan







G/F Framing Plan







1/F Framing Plan







2/F (Podium) Framing Plan







3/F (Transfer Floor) Framing Plan







Appendix C

Drawings for Repair Works

assessment and repair of fire-damaged structures: case study of tai shing street market

Documents

information paper assessment

fireassessment assessment

preliminary assessment

detailed assessment

information storage

damaged structures

structural appraisal

repair proposals