Assessment and Repair of Fire-Damaged Structures: Case Study of Tai Shing Street Market

Information Paper

Assessment and Repair of Fire-Damaged Structures: Structures:

Contents

1.

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

2.

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

3.

Initial Site Sit e 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

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 elevati on (Figure 2).

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

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 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 6 Podium Garden of Kai Tak Garden

3.

Initial Site Visit and Preliminary Preliminary Inspections

3.1

The fire occurred in the mid-night of 20.4.2013 20.4.2013 on the dry goods area on 1/F of the market, and was only put off off in the afternoon of 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 CSE/1 arrived at the post-fire scene in the afternoon of of 22.4.2103. 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.

4.

Preliminary Assessment

4.1

An initial assessment of the the gas 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 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 t han 500oC.

Figure 10 Debris and combusted materials after the fire Table 1(a) Effect of elevated temperatures on common construction materials Approximate temperature o ( C) 100 150 120 120-140 150-180 120 120-140

Substance

Examples

Paint Polystyrene

Polyethylene

Condition Deteriorates Destroyed

Thin-wall food containers, foam, light shades, handles, curtain hooks, radio casings Bags, films, bottles, buckets, pipes

Collapse Softens Melts and flows Shrivels Softens and melts

Table 1(b) Ignition temperatures temperatures of common common construction materials materials Ignition Auto-ignition Material o 1 o 2 temperature ( C) temperature ( C) 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: Source: IStructE 2000)

Figure 11 11 Concrete spalling during fire 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 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 firemen’s 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),

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.

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 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. repair. Dr Y L WONG of The Hong Kong 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

Figure 16 Measured extent of damage

5.3

Assessment of maximum temperature during fire

5.3.1 Colour of concrete 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

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 300°C corresponds, more or less, to concrete that has lost a permanent part of its resistance (Concrete Society 2008). A grey ‐white 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).

of concrete slices was heated in different elevated temperatures, e.g. 200°C, 300°C, 450°C, 600°C and 800°C.

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 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.

(i) RGB colour space (ii) HSI colour space (Source: Source: Blue Lobster Art and Design) (Source: 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 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 range 0 – 200°C, 200°C,

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 densities increase with high temperatures. The relationship of of crack density and temperature may therefore worth further investi gation.

Figure 23(a) Results of colour image analysis analysi s for fire-damaged concrete sliced samples at Tai Shing Street Market 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 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 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 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 model

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 changes of its colour. Usually, the residual strength for concrete exposed to temperatures above 300°C (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 500°C to 600°C, and on cooling from 800°C 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).

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 Sample No.

Mean Diameter (mm)

Estimated In-Situ Cube Strength (MPa)

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

7.

Structural 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 t he fire damaged area.

7.2

For corewall and columns within the fire damaged damaged area, an average average strength of 40MPa was obtained from samples from the rc core walls, which is slightly lower than the original design strength 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

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. 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 on the extent of damage, the the following three methods were use to repair the damaged areas: (a)

Areas with damage limited to the concrete surface zone: the damaged

Figure 28 Spraying of concrete

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: 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: structure (Camberley: Concrete Society). Felicetti, R (2004), (2004), “Digital-camera colorimetry for the assessment of firedamaged concrete”, concrete”, Proceedings of the Workshop: Fire Design of Concrete Structures, Milan, 2-3 December 2004, 2004 , pp. 211 20.

Appendix A Architectural 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

Structural Engineering Branch, ArchSD Assessment and Repair of Fire-Damaged Structures: Case Study of Tai Shing Street Market

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File code : FireAssessment MKW/CTW/MKL/YFC/CYK/TWC/MFY First Edition: February 2015


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Assessment and Repair of Fire-Damaged Structures: Case Study of Tai Shing Street Market

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