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?

Thanks

Danny Hopkin– 07894483449– Danny.Hopkin@trentonfire.co.uk