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FR of tall UK residential buildings
Danny Hopkin & Eoin O’Loughlin
OverviewIntroduction– Emerging high-rise trends– Prescriptive guidance – stretched?– The need for a more considered approach
Expression of design goals– Review of contemporary approaches– Identifying limitations
An alternative approachSummary
introductionfire resistance & tall residential buildings
Something in common?
All considered unusual (un-common);SFE deployed;Predominantly office occupancy;PFP optimised;Presumably more cost effective than…
Emerging trends
263 towers (>20 storeys) proposed in LondonMostly apartment buildingsMany will be designed prescriptively
“intended to provide guidance for the more common building situations…”
Prescriptive FR – a health warning
“need to take into account the particular circumstances of the individual building…”
The need to do betterWhy do we accept tall offices as being ‘unusual’ but design most tall residential buildings ‘prescriptively’?Is prescription good enough to deliver “stability for a reasonable period” for most of these 236 towers?What are the alternatives?How do we define what we’re trying to achieve?
defining the design goalstructural design for fire safety
What are we trying to achieve?B3 - “Stability for a reasonable period”Defining ‘reasonable’ in terms of a design confidence level (or reliability)Expression of resilience in terms of % of fires resistedA consistent level of risk….
‘Scale’
Frequency Consequence
‘Risk’
Reliability – Kirby, et. al
Risk = frequency x probability of failure x consequence
𝑅𝑖𝑠𝑘=(1− 𝑟𝑒𝑙𝑖𝑎𝑏𝑖𝑙𝑖𝑡𝑦100 )h2
Defining the goal
𝑅𝑒𝑙𝑖𝑎𝑏𝑖𝑙𝑖𝑡𝑦=100(1− 64.8h2 )
Inherent crudeness:– Area proportional to height
• Floor to ceiling?• Foot-print?
– People affected proportional to height• Occupancy? Evacuation mode? Location?
– Why adopt 80% at 18m???
Kirby, et. al – relevance to resi?Height (m) Reliability
(office) %Reliability (resi) %
0-5 20 46.4
5-11 46.4 80
11-18 80 92.8
18-30 92.8 98.2
30-60 98.2 99.6
60+ 99.6
Prob of fire number of dwellings;Conseq of failure depends on the number of occupants and people in the vicinity;Is height the sole metric by which reliability should be measured?
an alternative way of thinking?tall residential buildings
Resi - reliability revisitedWhat impacts the likelihood of resi fires?– Number of dwellings (duplexes
vs. single),– Size of dwellings,– Demographic?!
What impacts consequence in the event of failure?– The number of people in the
building at the time,– The extent of collateral.
Height (m) Reliability (office) %
Reliability (resi) %
0-5 20 46.4
5-11 46.4 80
11-18 80 92.8
18-30 92.8 98.2
30-60 98.2 99.6
60+ 99.6
Necessary simplifications (1)
𝑹𝒊𝒔𝒌=𝑵×(𝟏−𝒓𝒆𝒍𝒊𝒂𝒃𝒊𝒍𝒊𝒕𝒚 )×𝒄𝒐𝒏𝒔𝒆𝒒𝒖𝒆𝒏𝒄𝒆
Necessary simplifications (2)
𝑹𝒊𝒔𝒌=𝑵×(𝟏−𝒓𝒆𝒍𝒊𝒂𝒃𝒊𝒍𝒊𝒕𝒚 )×(𝒅𝒊𝒓𝒆𝒄𝒕+𝒊𝒏𝒅𝒊𝒓𝒆𝒄𝒕)
𝑹𝒊𝒔𝒌=𝑵×(𝟏−𝒓𝒆𝒍𝒊𝒂𝒃𝒊𝒍𝒊𝒕𝒚 )×(𝑵𝒙 𝟏+
𝑯𝒙𝟐 )
Selecting a baselineCorrelation without context is meaningless,Defining a statistically common building:– It should have flat typologies consistent with
current and historical demand,– It should have flat proportions consistent with
historical trends, planning constraints, etc.,– It should deliver an acceptable level of safety:
• A simple building, representative of most,• Designed wholly in accordance with prescriptive
recommendations
Baseline – a statistically common building
2000/01
2001/02
2002/03
2003/04
2004/05
2005/06
2006/07
2007/08
2008/09
2009/10
2010/11
2011/12
2012/13
2013/14
2014/15
0
5
10
15
20
25
30
35
401 bedroom
2 bedrooms
3 bedrooms
Year
Perc
enta
ge o
f all
dwel
lings
com
plet
ed (%
)
28-2936-37
41-4244-45
47-4850-51
53-5456-57
59-6063-64
66-6775-76
86-8790-91
100-1010%
2%
4%
6%
8%
10%
12%
14%
16%
Gross Internal Area (m²)
Perc
enta
ge o
f sur
veye
d ty
polo
gy
Flat typology trends – source - DCLG
2 bedroom flat sizes – source - CABE
Single stair buildings – a starting point
Prescriptive guidance practically limits the number of apartments per level,The baseline should have statistically relevant apartment types and sizes delivered within the confines of limits imposed by ADB,Planning constraints – minimum floor to ceiling height 2.5m
6.3-
6.4
6.4-
6.5
6.5-
6.6
6.6-
6.7
6.7-
6.8
6.8-
6.9
6.9-7
7-7.1
7.1-
7.2
7.2-
7.3
7.3-
7.4
7.4-
7.5
7.5-
7.6
7.6-
7.7
7.7-
7.8
7.8-
7.9
7.9-8
8-8.1
8.1-
8.2
8.2-
8.3
8.3-
8.4
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
12.00%
14.00%
16.00%
Number of flats
Perc
enta
ge o
f ins
tanc
es
Baseline + the risk correlationBaseline:– 7 flats per level,– 2.5m floor to ceiling– 3.0m floor to floor
Effective occupant storeys:– N Flats / base (7) flats per
level
Effective height storeys:– Building height / base floor
to floor height (3.0m)
¿
Anchoring & societal riskCorrelation needs to be ‘tethered’ to an ‘accepted’ minimum level of performance…What range of fire severities can be expected in our statistically common apartment types?
FR45FR65
FR85FR105
FR125FR145
FR165FR185
FR205FR225
FR2450%
20%
40%
60%
80%
100%
120%
Equivalent duration of ISO834 exposure (min)
Cum
mul
ative
freq
uenc
y
FR45FR55
FR65FR75
FR85FR95
FR105FR115
FR125FR135
FR145FR155
FR165FR175
FR185FR195
FR205FR215
FR225FR235
FR245FR255
0
500
1000
1500
2000
2500
Equivalent duration of ISO834 exposure (min)
Freq
uenc
y
a la Kirby, et al / BS 9999
Fire severities
FR45
FR55
FR65
FR75
FR85
FR95
FR105
FR115
FR125
FR135
FR145
FR155
FR165
FR175
FR185
FR195
FR205
FR215
FR225
FR235
FR245
FR255
0%
20%
40%
60%
80%
100%
120%
Equivalent duration of ISO834 exposure (min)
Cum
mul
ative
freq
uenc
y
Fire resistance period (min)
Percentage of fires
60 min ≈ 18%90 min ≈ 67.5%120 min ≈ 86%
Calibration – a 90 minute buildingTallest 90min building achievable = 30m11 storeys each with 7 flats77 flats90 min = 67.5% of fires
Benchmarking
For an 18m high ‘statistically common’ apartment building:– 7 storeys– 7 flats per level– 49 flats in total– Reliability of 17.5%– FR60 (consistent with
prescriptive approach)
Percentage of fires
Fire resistance period (min)
≈ 18% 60 min
≈ 67.5% 90 min
≈ 86% 120 min
Application - tower examples
Case Total
number of flats
(N)
Qualifying storeys (-) Building height (m) RFRS (%) FR (min)
A 287 41 120 97.7 164B 205 41 120 96.3 154C 369 41 120 98.5 173D 217 31 120 96.5 156
FR45FR65
FR85FR105
FR125FR145
FR165FR185
FR205FR225
FR2450%
20%
40%
60%
80%
100%
120%
Equivalent duration of ISO834 exposure (min)
Cum
mul
ative
freq
uenc
y
4 very different buildings (of the same height) delivering consistent levels of risk
concluding remarkssummary &
SummaryTall resi buildings are no more common than tall offices – SFE will be increasingly called upon to support designs;Defining the design goal is key, current approaches are too crude;We’ve developed a concept means of expressing the design goal for tall resi buildings;We welcome your feedback/thoughts:– Dealing with multi-stair buildings?– Mixed use?– Quantifying sprinkler contribution?, etc.– Weighting of direct vs. indirect consequence?
