Fire Resistance of Metal Framed Historical Structures

Chrysanthos Maraveas

Supervisors: Professor Yong Wang, Thomas Swailes

Steel in Fire Forum meeting Institution of Structural Engineers, London

21 April 2015

Research Background

• Strict Building Codes -> High fire resistance requirements -> applied in refurbishment of historical structures

• Many Cast Iron Structures still in use in UK, Central/West Europe and USA

• Metal structures vulnerable to fire attack

• No systematic research on the subject exists

Typical 19th century metal structures

Cast iron columns

Beams: Cast iron Wrought iron Mild steel

Masonry arches: Brick + Mortar

“Early Concrete” (great variety of materials): Aggregate: coke, clinker (incl. coal or coke), breeze, cinders, broken brick, burnt clay, limestone or rubbish. Cement: lime, Roman or Portland cement.

Cast iron Brittle material – asymmetric σ-ε diagram

Graphite flakes

Flaws open in tension Thermal conductivity:

Different depending of flakes direction

Structure of work

• Elevated Temperature Material Properties

– Material properties from literature

– Sensitivity analysis

– Experimental investigation

• Behaviour of Structural Elements in Fire

– Moment capacity of cast iron beams exposed to fire, material safety factor estimation

– Fire resistance of cast iron columns

Literature Review

• Collates thermal and mechanical properties of metals & thermal properties of the ‘‘insulation’’ from literature. Data compared with Eurocode’s expressions for contemporary materials to assess the applicability of the Eurocode expressions for the 19th century fireproof flooring systems.

Thermal properties of cast iron

Mechanical properties an thermal expansion of cast iron

Thermal properties of masonry units

Sensitivity analysis

• Effect of material properties variations

• Variations placed with upper and lower bounds

• Thermal properties->Thermal analysis -> temperature variations->structural response

• Mechanical properties->Structural analysis -> structural performance/resistance

• Estimate the governing material properties

Thermal properties of metals

Mechanical properties of metals

Analysis of arch jack system

Effect of thermal properties of insulation

Effect of thermal properties of cast iron

Analysis number Thermal

Conductivity Specific Heat

1 Upper bound EC3-1-2

2 EC3-1-2 EC3-1-2

3 Lower bound EC3-1-2

4 EC3-1-2 Upper bound

5 EC3-1-2 Lower bound

Structural response Effect of thermal properties and thermal

expansion

Structural response Effect of mechanical properties

Conclusions-1

• The thermal properties of steel per EN1993-1-2 can be used for all metals

• Sensitive to mechanical properties of metals (especially of strength) and thermal expansion

• Thermal properties of insulation can be taken as concrete per EN1992-1-2 (lower bound thermal conductivity)

Experimental investigation of elevated temperatures mechanical

properties of cast iron

For temperatures >400oC moderately brittle fracture

Stress-strain diagram in tension

Proposed model for tension

Stress-strain in compression

Proposed model in compression

Coefficient of Thermal

Expansion

Conclusions -2

• A stress strain temperature relationship for cast iron has been proposed

• Large uncertainties remain – material safety factor should be estimated

• The thermal expansion relationship of steel per EN1993-1-2 can be used for cast iron

Cast iron columns in fire

• Six fire tests performed in US has been simulated

• FEA software: Abaqus, FE brick DC3D8

• Thermal properties and thermal expansion according EN1993-1-2 for steel

• The proposed σ-ε-Τ model used

• Applied fire curve according the fire test report

Thermal response

Structural response

Structural response

Utilization and critical temperature (γfi=3.0 for cast iron column)

Design buckling resistance

Conclusions -3

• The fire resistance of cast iron columns is higher than steel columns

• The cast iron columns have similar behaviour with steel columns and EC3-1-2 procedures can be used (where fy is the 0.2% proof stress of cast iron)

Moment capacity of cast iron beams exposed to fire

• For cross section analysis, a fiber model developed and verified against FE models

• 4 cross sections studied:

Fiber analysis results

Unprotected sections

Simplified method

Temperature profile can be estimated with use of equations for slim floors (Zacharia and Franssen (2012))

εc and εt from empirical equations

Conclusions -4

• The arch jacked cast iron beams are R60+ in terms of moment capacity

• The EN1993-1-2 can be used for unprotected cast iron beams

• A simplified method which is giving quite accurate results have been proposed

Material safety factors estimation

• For arch jacked cast iron beams (non linear temperature profile)

• A reliability method used for target reliability index 3.8 • 8 random parameters -> statistical/analysis-distribution

-> monte carlo simulation -> randomized σ-ε-Τ relationship

• Calculation of Moment capacity distribution for various cross sections with use of the fiber analysis model

• Material safety factor for fire design of arched jack cast iron beams estimated

• For failure probability 10-1, 10-2, 10-3, 10-4 the corresponding safety factors are 1.5, 2.5, 4.5 and 5.5

Detailed information:

•C. Maraveas, Y.C. Wang, T. Swailes, G. Sotiriadis, An experimental investigation of mechanical properties of structural cast iron at elevated

temperatures and after cooling down, Fire Safety Journal, Vol. 71, 2015, pp 340-352.

• C. Maraveas, Y.C. Wang, and T. Swailes, Fire resistance of 19th century fireproof flooring systems: a sensitivity analysis, Construction and

Building Materials, Vol. 55, pp 69-81, 2014.

• C. Maraveas, Y.C. Wang, T. Swailes, Thermal and mechanical properties of 19th century fireproof flooring systems at elevated

temperatures, Construction and Building Materials, 48: 248-264, 2013.

• C. Maraveas, Y.C. Wang, T. Swailes, Moment capacity of cast iron beams in jack arched construction exposed to fire, Constructional Steel

Research.

•C. Maraveas, Y.C. Wang, T. Swailes , Elevated temperature behaviour and fire resistance of cast iron columns (in preparation)

•C. Maraveas, Y.C. Wang, T. Swailes, Probabilistic determination of material safety factor for cast iron beams in jack arched construction

exposed to fire, CONFAB 2015-The First International Conference on Structural Safety under Fire & Blast, Glasgow, UK.

•C. Maraveas, T. Swailes, Y.C. Wang, Modeling of insulation in 19th century metal framed structures, 2nd International Conference on

Protection of Historical Constructions, Antalya, Turkey, pp 257-261, 2014.

•C. Maraveas, T. Swailes, Y.C. Wang, Sensitivity of fire resistance of 19th century fireproof flooring systems to thermal and mechanical

properties of masonry, 2nd International Conference on Protection of Historical Constructions, Antalya, Turkey, pp 475-481, 2014.

Thank you!