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EN 1992-1-2: Structural fire design

EC2 Workshop Eurocodes Moscow 2010

J.C. Walraven

Vermelding onderdeel organisatie

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Information on structural fire design - Eurocode 1: EN 1991-1-2, : Loads on structures, Part 1-2: General loads – Loads due to fire

- Eurocode 2: EN 1992-1-1: : Design and calculation of concrete structures: General rules and rules for buildings

- Eurocode 2: EN 1992-1-2:

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Control of structures subject to fire Eurocode distinghuises analysis of elements, partial

systems andstructures as a whole

Analysis of structural member

Analysis of structural system

For the design of standard fire requirements in general the analysis of structural members is sufficient.

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Alternative design procedures EC2

Prescriptive Rules

Member analysis Analysis of part of structure

Analysis of entire structure

Calculation of mechanical

actions at boundaries

Tabulated data

Simple calculation

models

Advanced calculation

models

Simple calculation

models

Advanced calculation

models

Advanced calculation

models

Calculation of mechanical actions at boundaries

Selection of mechanical

models

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Alternative design procedures Performance based design;

physically based thermal actions

Selection of simple or more advanced fire development models

Member analysis

Analysis of part of the structure

Analysis of entire structure

Calculation of mechanical

actions at boundaries

Calculation of mechanical

actions at boundaries

Selection of mechanical

models

Simple calculation

models

Advanced calculation

models

Advanced calculation

models

Advanced calculation

models

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Summary table for alternative methods

Tabulated data

Simplified calculation methods

Advanced calcu- lation methods

Member analysisMember considered to be isolated. Indirect fire actions are not considered, except those resulting from thermal gradients

Yes- Data given for standard fire only, 5.1(1).- In principle data could be developed for other curves

Yes-Standard fire and parametric fire, 4.2.1(1)

- Temperature profiles given for standard fire only, 4.2.2(1)- material models apply only to heating rates similar to standard fire 4.2.4.1(2)

Yes4.3.1(1)Only principles are given

Analysis of parts of the structureAnalysis of the entire structure. Indirect fire actions within the subassembly are considered, but no time dependant interaction with other parts of the structure

No Yes-Standard fire and parametric fire 4.2.1(1)-temperature profiles given for standard fire only 4.2.2(1)- material models only for heating rates similar to standard fire 4.2.4.1(2)

Yes4.3.1(1)POnly the principles are given

Global structural analysisAnalysis of entire structure. Indirect fire actions considered throughout structure

No No Yes4.3.1(1)POnly principles are given

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Alternative design procedures

Prescriptive rules (traditional):

Rules for minimum cross sections, minimum cover, reinforcement geometry, mostly based on ISO 834 curve

Performance based design (modern/future))

Bearing capacity should be maintained during fire (Criterion R) In case of subdivision of building in compartments: separating elements (including joints) should keep their separating function during the fire, so: - no loss of integrity due to cracks, wholes which would allow transmission of gas or flames (Criterion E) - no loss of isolating function which would lead to rise of temperature at opposite side resulting in fire (Criterion I). Mostly assumed to be satisfied if max. T < 180 K.

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Modeling the fire load

For general (standard cases) ISO 834 curve is appropriate

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More advanced heating curves (for performance based design)

Parameters• Burning capacity of materials

in room• Opening-factor• Wall-, floor and ceiling

properties• Risk factors (presence of

sprinklers or alarm system)• Ventilating conditions

Parametric temperature – time curves

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Load on the structure during fire

Accidental loading situation applies:

for buildings: 0,5 x,1 0,9 and 0,3 x,2 0,8

Accidental action Ad due to imposed deformations as a result of thermal actions in statically indeteminate structures

1

,211,1 i

kiikxdj

kkj QQAPG

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Special case of imposed deformations

1

,211,1 i

kiikxdj

kkj QQAPG

Accidental load due to restrained temperature deformations

Dotted lines: shifted moment lines due to temperature restraint!

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Basis for control of fire resistancce

Rd bearing resistance (no fire)Ed design load (no fire)

Rfi,d(t) bearing resistance (fire)Ed = fiRd governing load for fire

situationtfi,req required fire resistance in minutes (criterion R)

Rfi,d(t) can be calculated on the basis of material laws which reflect material deterioration under increasing temperature

)()( ,, tRtE fidfid

for tfi,req

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Control with tables - Based on ISO temperature – time curve- Provides design solutions for standard fire exposure up to 4

hours- Valid for normal weight concrete with siliceous aggregate- For calcareous or lightweight aggregates the minimum

dimension may be reduced by 10%- No further checks required for shear, torsion or anchorage- No further checks required for spalling up to an axis distance

of 70mm- For HSC (> C50/60) the minimum cross section dimension

should be increased- Axis distance a according to figure (nominal

values- The tables have been derived for a critical rein-

forcing steel temperature of 5000C and a loadingdegree of fi = Ed,fi/Ed

a AxisDistance

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a AxisDistance

Example of a table: Minimum thickness and axis distance for flat slabs

Control with tables

Standard fire resistance

Minimum slab thick-ness (mm)

Minimum axis dis-tance (mm)

R 30 150 10

R 60 180 15

R 90 200 25

R 120 200 35

R 180 200 45

R 240 200 50

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Control with tables

Combination with diagrams

In combination with the tables diagramscan be used, which give the reduction ofthe steel strength as a function of the increasing temperature. These relationscan be used to convert the results of the tables to degrees of loading differentfrom 0,7 and critical temperaturesother than 5000 C

0,8

1

0

1

2

3

0,6

0,2

0,4

1000200 800400 12000 600

ks(cr), kp(cr)

cr [°C]

Prestressing strands and wires

Reinforcing steel

Prestressing bars

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Control with tables

provs

reqs

s

yk

d

fidfis S

ACf

E

E

,

,0

,,

)20(

The tables have been derived for a critical steel temperature cr = 5000, a loading degree fi = 0,7, and s =

1,15.The corresponding steel stress is

for Ed,fi/Ed = 0,7; s = 1,15 and As,req/As,prov

= 1 a stress s=300 MPa is found, so s/fyk=0,6. This is confirmed by the

diagram

0,8

1

0

1

2

3

0,6

0,2

0,4

1000200 800400 12000 600

ks(cr), kp(cr)

cr [°C]

yks f/

Temp.

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Control with tables

Procedure for combined use of tables and diagrams

Example: Qk=Gk and fi = 0,7. With G=1,35

and Q=1,5 it is found that fi = Efi,d/Ed = (1/1,35 +

0,7/1,5)/(1+1)=0,6If As,req/Asprov = 0,9, then the stress in the

rein-forcing steel under fire conditions is:s = fi{fyk(200)/s}(As,req/As,req) =0,6(500/1,15)0,9 = 235 MPa. So

=235/500 = 0,47. From the diagram (andcorresponding mathematical relations) it

is readthat the critical temperature is cr = 556

MPa.So the axis distance (Tabulated value)

can bereduced by:

a = 0,1(500-556) = - 5,6 mm

0,8

1

0

1

2

3

0,6

0,2

0,4

1000200 800400 12000 600

ks(cr), kp(cr)

cr [°C]

yks f/

Temp.

yks f/

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Control with tables

NRd = Asfyd + Acfcd

= Asfyk+Acccfck/c

Rd

Edfi

Rd

fiEd

N

N

N

Nn

,

Example of table: dimensions for columns

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Design with tables

Standard fire resistance

Slab thickness(mm)

One direction

2 directionsly/lx 1,5

2 directions1,5 < ly/lx 2

R30 60 a = 10 mm a= 10 a= 10

R60 80 a = 20mm a= 10 a= 15

R90 100 a= 30 mm a= 15 a= 20

R120 120 a= 40 mm a= 20 a= 25

R180 150 a= 55 mm a= 30 a= 40

R240 175 a= 65 mm a= 40 a= 50

Minimum slab thickness and axis distance a for slabs spanning in one and 2 directions

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bfi

b

dfi

50500 °C

d

C

T

dfid

bfi

b

500 °C

500°C isotherm method (Annex B1)(Anderberg)

• Determine the position of the 500C isotherm for the specific fire exposure.

• Determine the values of dfi and bfi.

• Determine the temperature and reduced strength of reinforcing bars in the tension and compression zones.

As

As'

z' dfi

bfi

z

fcd,fi(20)

xbfifcd,fi(20)

As1fsd,fi(m)

z'

Fs = As2fsd,fi(m)

Fs = As'fscd,fi(m)

Mu2Mu1

xx

+

• Use conventional calculation method to determine ultimate load capacity.

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M1 aZ2

w2

M2

w1

aZ1

aZ1

Zone method (Annex B2)(Hertz)

• Section is divided into zones. The mean temperature and mean compressive strength, fcd( ) of each zone is determined

• The fire situation is represented by a reduced cross section ignoring a damaged zone of thickness az.

• The value of az is determined by assessing the mean properties of the concrete at point M

• The example shows the combination of two sets of calculations. One for the flange and one for the web.

kc( M)

kc()

kc( )

kc( 2)

kc( 3)

w w

kc( 3)

M

• The point M is an arbitrary point selected on the centre-line of the section

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Mfi = f(1/r) (N = NEd,fi)

M2,fi

M0Rd,fi

M

1/r

MRd,fi

M2,fi = NEd,fi (1/r) l0 /c2

• Divide cross-section into zones with mean temperature of 20C, 100C, 200C, 300C ... up to 1100C.

Buckling effects on columns (Annex B3)(Isquierdo)

Ac,i,j

yi,j

xi,j sup

i,j

s1

s2

s3

inf

• Determine temperature of each reinforcing bar.

• Determine ultimate moment capacity, MRd,fi for NEd,fi and nominal second order moment, M2,fi for corresponding curvature.

• Integrate to determine moment-curvature diagram for each zone and reinforcing bar for NEd,fi

• Determine ultimate first order moment capacity, M0Rd,fi and compare with M0Ed,fi

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Fire in praxis

Fire in Faculty of Architecture, Delft

University of Technology

13 May 2008, 10h30

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TU Delft, Faculty of Architecture, May 13th 2008, 13h00

Fire in praxis

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TU Delft, Faculty of Architecture, May 13th

2008 16h00

Fire in praxis

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TU Delft, Faculty of Architecture, May 13th

2008 17h00

Fire in praxis

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TU Delft, Faculty of Architecture, May 13th

2008 19h00

Fire in praxis