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Information Paper
Assessment and Repair of Fire-Damaged Structures:
Case Study of Tai Shing Street Market
STRUCTURAL ENGINEERING BRANCH
ARCHITECTURAL SERVICES DEPARTMENT
February 2015
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of 50 File code : FireAssessment
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Case Study of Tai Shing Street Market
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First Edition: February 2015
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.
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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
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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
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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)
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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.
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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.
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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.
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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)
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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.
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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
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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.
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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
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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)
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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
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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
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(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).
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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,
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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
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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.
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Figure 24 Zone model for Tai Shing Street Market
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Figure 25 Computer simulation of the fire
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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
(Source: Concrete Society 2008)
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
(Source: Concrete Society 2008)
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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.
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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
3. Residual compressive strength of cores from rc walls
Sample No. Mean Diameter (mm) Estimated In-Situ Cube Strength
(MPa)
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
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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.
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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.
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Figure 28 Spraying of concrete
Figure 28 Completed repaired areas before BS installation
Figure 30 Completed repaired soffits of 2/F slabs
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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.
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Appendix A
Architectural Layout of Kai Tak Garden Phase I
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Basement Plan
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G/F Plan
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1/F Plan
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2/F (Podium) Plan
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3/F (Transfer Floor) Plan
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Typical Floor Plan
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Appendix B
Structural Framing Plans of Kai Tak Garden Phase I
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Basement Framing Plan
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G/F Framing Plan
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1/F Framing Plan
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2/F (Podium) Framing Plan
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3/F (Transfer Floor) Framing Plan
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Appendix C
Drawings for Repair Works
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