Design of steel structures under the aspect of fire protection measures TU BRAUNSCHWEIG iBMB Dr.-Ing. E. Richter Institute for Building Materials, Concrete Structures and Fire Protection Technical University of Braunschweig

iBMB Contents l Introduction l Fire exposure u Standard fire, natural fire l Properties of steel in fire l Protective materials l Steel temperature u Protected and unprotected steelwork l Simple calculation model u Critical temperature, Eurocode 3 Part 1-2 l Conclusions

iBMB Introduction l General objectives of fire design u Load-carrying capacity: members in a structural assembly should resist the applied loads in a fire u Insulation: limitation of temperature of 140 K (average) or 180 K (peak) on the unexposed side of a wall or floor u Integrity limitation of breaks or cracks to avoid passage of smoke or flame to the unexposed side of a wall or floor

iBMB Standard temperature/time curve time [min] temperature [°C]

iBMB Natural fire - Design curves l Fully developed compartment fire Fire load density Opening factor = 0.12 m 1/2

iBMB Steel temperature development Unprotected steelwork  a,t = · h net,d ·  t A m /V c a ·  a A m /Vsection factor [1/m] A m exposed surface area per unit length [m 2 ] Vvolume per unit length [m 3 ] c a specific heat of steel [J/kgK]  a density of steel [kg/m 3 ] h net,d design value of the heat flux per unit area [W/m 2 ]  ttime intervall [  5s ] · ·

iBMB Section factor A m /V Open section exposed to fire on all sides A m perimeter V cross-section area = Tube exposed to fire on all sides Open section exposed to fire on three sides A m /V for unprotected steel members A m 1 V t = A m surface exposed to fire V cross-section area = Flat bar exposed to fire on all sides A m 2·(b + t) V b·t =

iBMB Steel temperature time [min] temperature [°C] ISO 834 A m /V  small A m /V  large

iBMB Unprotected steel column l Damage after fire exposure Buckling length: l fi  0.5 LBuckling length l fi  0.7 L

iBMB Structural steel l Thermal elongation Example (beam): l = 5 m,  a = 600 °C:  l  1.4*10 -5 *( )*500 = 4.1 cm

iBMB Unprotected steel construction l External steel frame

iBMB Fire protective materials l Traditional materials (  heavy) u concrete (normal, lightweight) u brickwork l Modern materials (  light) u sprays: Perlite-cement, Vermiculite, glass- or mineral fibre-cement sprays u fire boards: fibro-silicate, gypsum, vermiculite u mineral fibre or other mat materials u intumescent coatings

iBMB Steel temperature development Protected steelwork  a,t = ·  t - (e  /10 -1) ·  g,t with  = d p · A p /V p · A p /V (  g,t -  a,t ) d p · c a ·  a ( 1+  /3) A p /Vsection factor for steel members with fire protection material [1/m] A p area of fire protection material per unit length [m 2 ] Vvolume of per unit length [m 3 ] c a specific heat of steel, from [J/kgK] c p specific heat of the protection material [J/kgK] d p thickness of the fire protection material [m]  t time interval [  30s ]  a,t steel temperature at time t  g,t ambient gas temperature at time t  g,t increase of the ambient gas temperature during  t p thermal conductivity of the fire protection material [W/mK]  a density of steel [kg/m 3 ]  p density of the fire protection material [kg/m 3 ] c p ·  p c a ·  a

iBMB Section factor A p /V A p /V for protected steel members Contour encase- ment of uniform thickness Hollow en- casement of uniform thickness Contour encase- ment of uniform thickness, exposed to fire on three sides steel perimeter steel cross-section area 2·(b+h) steel cross-section area steel perimeter - b steel cross-section area ApVApV = ApVApV = ApVApV =

iBMB Steel temperature time [min] temperature [°C] ISO 834

iBMB Structural steel l Stress-strain relationship

iBMB Critical steel temperature crit  a = f (utilisation factor  0 )  a,cr = ln[1/(  0 3,833 ) - 1] critical temperature [°C] utilisation factor  0

iBMB Steel temperature time [min] temperature [°C]  a,cr ISO 834

iBMB Protected steel construction l Box protection with fire boards

iBMB Protected steel beam l Intumescent coating Before fire exposure After fire exposure (35 min ISO-curve)

iBMB Intumescent coating l Fire resistance thickness fire resistance A/V = 200 A/V = 291 variation A/V d = thickness of intumescent coating

iBMB Protected steel construction l Composite steel and concrete structure

iBMB Composite cross-sections l Columns l Beams

iBMB Protected steelwork l Water cooled structure

iBMB Protected steelwork l Main columns with water tank