Keywords: fire, steel, resistance, buckling, numerical simulations, tests.

Traditionally, the fire resistance of load-bearing members has been assessed by means of standard fire tests. These do not describe the behaviour of load-bearing members in building structures subjected to real fires particularly well. There are differences arising from the applied loads, restraint conditions and fire exposure characteristics [1,2,3]. Analytical methods are nowadays more widely accepted for determining the behaviour of structures in a fire. The high cost of carrying out real fire tests on full-scale structures and the advances made in computational methods justify that preference [6,7,8,11].

The recent approval of the parts of the Eurocodes relative to structural fire design, give the designer the possibility to use not only experimental way to determine the fire resistance of elements but also simplified and advanced calculation methods. Whilst the experimental tests and the advanced calculation methods allow the analysis of single elements and parts of the structure, the simplified calculation methods could be used only for single elements [4].

The advanced calculation methods are normally applied by computer programmes. These methods are more realistic, because the parameters involved in structural behaviour in case of fire can be better modelled. Parameters such as the combination of axial force and bending moments, variable restraint conditions and second order effects due to thermal elongation can be considered. Methods of this kind can simulate parts of, or whole building structures [11].

In this paper a computer programme used to simulate the behaviour of steel columns at high temperatures when inserted in a building frame is presented. This programme named ZWAN, calculates the restraining forces that arise when the column forming part of a structure with certain stiffness is heated. The evolution of the restraining forces results from the permanent compatibility between the axial displacements of the top of the heated column and the displacements of a spring (surrounding building structure). In each step an internal equilibrium between the axial forces acting on the column and on the spring is realised. The basis of calculation used by ZWAN programme is described in this paper [5].

The ZWAN programme was then used to simulate the results of a large series of fire resistance tests on small steel elements with restrained thermal elongation carried out at Instituto Superior Técnico in Lisbon. Some of the results of these tests and their comparison with the ones obtained with ZWAN programme are presented [9,10].

1

Faris A., O'Connor D., "Structural performance of rotationally restrained steel columns in fire", Fire Safety Journal, vol. 36, England, pp. 679-691, 2001. doi:10.1016/S0379-7112(01)00017-0

2

Faris A., Shepherd A., Randall M., Simms I.W., O'Connor D.J., Burgess I., "The Effect of Axial Restraint on the Fire Resistance of Steel Columns", Journal of Constructional Steel Research, vol. 46, No. 1-3, pp 305-306, England, 1998. doi:10.1016/S0143-974X(98)80036-9

3

Franssen J.M., "Failure Temperature of a System Comprising a Restrained Column Submitted to Fire", Fire Safety Journal, vol. 34, England, pp. 191-207, 2000. doi:10.1016/S0379-7112(99)00047-8

4

Neves I.C., Valente J.C., Rodrigues J.P.C., "Thermal Restraint and Fire Resistance of Columns", Fire Safety Journal, vol. 37, England, pp. 753-771, 2002. doi:10.1016/S0379-7112(02)00029-2

5

Neves I.C., "The Critical Temperature of Steel Columns with Restrained Thermal Elongation", Fire Safety Journal, vol. 24, England, pp. 211-227, 1995. doi:10.1016/0379-7112(95)00026-P

6

Olawale A.O., Plank R.J., "The Collapse Analysis of Steel Columns in Fire Using a Finite Strip Method", International Journal for Numerical Methods in Engineering, vol. 26, England, pp. 2755-2764, 1998. doi:10.1002/nme.1620261212

7

Poh K.W., Bennetts I.D., "Analysis of Structural Members Under Elevated Temperature Conditions", Journal of Structural Engineering, American Society of Civil Engineers (ASCE), No. 4, vol. 121, USA, pp. 664-675, April, 1995. doi:10.1061/(ASCE)0733-9445(1995)121:4(664)

8

Poh K.W., Bennetts I.D., "Behaviour of Steel Columns at Elevated Temperatures", Journal of Structural Engineering, American Society of Civil Engineers (ASCE), No. 4, vol. 121, USA, pp. 676-684, April, 1995. doi:10.1061/(ASCE)0733-9445(1995)121:4(676)

9

Rodrigues J.P.C., Neves I.C., Valente J.C., "Experimental Research on the Critical Temperature of Compressed Steel Elements With Restrained Thermal Elongation", Fire Safety Journal, vol. 35, England, pp. 77-98, 2000. doi:10.1016/S0379-7112(00)00018-7

10

Rodrigues J.P.C., "Fire Resistance of Steel Columns with Restrained Thermal Elongation", PhD Thesis, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal, 2001.

11

Valente J.C., Neves I.C., "Fire Resistance of Steel Columns with Elastically Restrained Axial Elongation and Bending", Journal of Constructional Steel Research, vol. 52, England, pp. 319-331, 1999. doi:10.1016/S0143-974X(99)00033-4

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