Fire resistance of solid steel columns

Fire Safety Journal, 4 ( 1 9 8 1 / 8 2 ) 237 - 241

237

Fire Resistance of Solid Steel Columns W. K L I N G S C H

Institute for Materials, Concrete Constructions and Fire Safety, Technical University, Beethovenstr. 52, D-3300 Braunschweig (F.R. G.) *

SUMMARY

THERMAL ANALYSIS

The resistance o f massive steel columns is significantly better than for ordinary steel columns because o f their much smaller aspect ratio. Under full service load they can attain a fire resistance o f greater than 30 minutes for an ISO DIS 834 fire w i t h o u t any fire protection, and more than 90 minutes with a thin intumescent coating. Results o f numerical thermal, and load bearing analysis, material investigations, and fire tests are summarized for practical application in fire engineering.

Steel is a material with very good temperature conductivity. This is the reason for simplicity in c o m m o n fire engineering design methods such as the relations between failure times tu and profile aspect ratios U/A. However, the validity of this interaction involves an assumption of a uniform temperature distribution over the whole cross section o f the profile. This condition applies with sufficient accuracy to all I-profiles, but for extremely heavy steel profiles the condition cannot be achieved without restriction because of their large mass. An experimental and numerical investigation has been undertaken to study the thermal behaviour of solid steel sections. Figure 1 shows the measured temperature increase in the core area of a column of 180 mm square solid section during a fire test according to ISO DIS 834, and the results o f a numerical thermal analysis associated with the test. Experimental investigations of the time dependent temperature distribution in solid steel sections pose certain problems associated with the introduction of thermocouples within the cross section; these can

INTRODUCTION

Solid steel columns show a high load bearing capacity for a small cross section. This leads to a slender and space saving column construction. For example, a typical storeycolumn of 3.70 m in length with a square cross section of 180 mm edge length can be used for up to a 3000 kN service load with normal steel (St 37, ~y = 240 N/mm2). A number of buildings have been erected in the last few years using solid steel columns. For fire safety they can have surface insulation, or a low load level when uninsulated. Both procedures reduce the economic advantage compared with possibilities under normal temperature conditions. A research program at the Technical University, Braunschweig, was started in 1979 to derive a theory for the realistic fire engineering design of solid steel columns [ 1 ]. This research on the fire resistance of solid steel columns is a development of the basic research in this field formerly undertaken in Canada [2]. * H e a d : Prof. Dr.-Ing. Dr.-Ing. E. h. Karl K o r d i n a . 0379-7112/82/0000-0000/$02.75

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238 easily cause incorrect measurements or, indeed, totally fail. F o r two dimensional numerical analysis o f cross section heating a Finite Element program has been used with respect to the t e m p e r a t u r e d e p e n d e n c e o f thermal material parameters {Fig. 2). The t e m p e r a t u r e difference over the cross section b o t h measured and c o m p u t e d was less than 50 K for this profile. In Fig. 3 calculated t e m p e r a t u r e distributions for different types o f solid steel profiles are plotted against a scale o f normalized geometry. The Figure shows a rapid increase, with increasing bulk, in the t e m p e r a t u r e difference be t w een the fire exposed surface and the core area. This result calls into question the unrestricted

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application of simple and c o m m o n design m et hods for the fire resistance of steel structures.

FIRE RESPONSE

To obtain information on the load bearing behaviour of solid steel columns under fire conditions, b o t h experimental and numerical investigations were undertaken. The objective in this research was to obtain data on temperature distribution across the section and to define the engineering applications of solid steel columns. Figure 4 shows the d e f o r m a t i o n behaviour during an ISO DIS 834 fire test of t w o solid steel columns of 180 mm square cross section and 3.70 m col um n length. The test load was calculated as t he m a x i m u m design load according t o German Standard DIN 18 800, Part 2, St 37 steel quality (~,, = 240 N/mm2), acting with a constant load eccentricity of 5 mm at each hinged end. The significant difference in fire response between these two samples is a result o f their different mechanical t r e a t m e n t during manufacture, i.e..~ forged or hot rolled. One of the remarkable results was the failure time of the hot rolled column at a t e m p e r a t u r e for which no ther-

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Fig. 4. D e f o r m a t i o n c h a r a c t e r i s t i c s o f solid steel c o l u m n s (St 37 q u a l i t y ) in fire t e s t ( 1 8 0 x 180 mm 2 profile, 3600 mm column length, both ends hinged, 2700 kN test load, 5 mm eccentricity).

239

mally induced strength reduction had occurred. Material investigations and numerical analysis of cross sections and complete columns provided an explanation for the different behaviour o f these columns under fire conditions. It may be summarized as the influence of geometrical and physical imperfections and their interactions [3]. Hot rolled columns show higher pre
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Figure 6 summarizes the results of a number of fire tests on massive steel columns under full service load compared with theoret. ical values based on aspect-ratio theory. For very heavy sections, i.e., small U/A-values, some loss of fire resistance seems to be indicated. This may be caused b y increased temperature gradients and corresponding overproportional thermal strain gradients. As a result, a compression stress increase on the outer part of the cross section will be caused according to the theory of plane remaining cross-sections. Because of this, and additional high temperatures in this area, the yield point of the steel will be reached at lower temperaI - Prolils

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240

tures. Under equilibrium conditions this results in increased curvature and earlier column failure (Figs. 3, 5, 6). Thus a less bulky section, though taken into account by design load calculation, shows a significant negative influence on fire response (Fig. 6). CONCLUSIONS

Solid steel columns combine an extremely high load bearing capacity with small cross sections, and have been used successfully in the construction o f both industrial and administrative buildings in different countries. To meet fire resistance requirements these columns are either provided with external fire-insulation or are considerably overdesigned to raise their critical temperature. New research results suggest that unprotected solid steel columns of up to one hour's fire resistance, with no or only little overdesign may be used. Load eccentricities when taken into account at the design stage have no influence on failure time. Also there is no dif-

ference in fire response between circular and square cross sections of the same aspect ratio; for rectangular cross sections, the aspect ratio should be calculated by 4/a-minimum. For fire engineering design, graphs such as Fig. 6, or similar graphs based on a correct U / A - t u relationship should be used. Some commercial plots in general circulation are substantially on the unsafe side. Computer programs for a realistic and complete calculation o f the load bearing capacity of solid steel columns in a fire situation are available and should be used for cross sections larger than 240 mm square and slenderness ratios greater than l / a = 20. A tabulated calculation guide with respect to these parameter dependencies is in preparation. A notable improvement in fire resistance may be obtained with very light insulation. Using an intumescent coating classified for 30 min only, a solid column of 180 mm square cross section reached a failure time of 109 min compared with 37 min for an unprotected column (Fig. 7). This effect, resulting from light protection will be useful

Fig. 7. Solid steel c o l u m n o f t e s t no. 5 w i t h i n t u m e s c e n t c o a t i n g b e f o r e t e s t (left), a n d (right) at 100 m i n o f ISO-fire t e s t .

241

when designing solid steel columns of up to one hour's fire resistance when using more slender cross sections. The foregoing results may, however, only be used when designing forged sections. At present this is mandatory. For hot rolled columns of limited mass, progress can only be expected after the inclusion of an additional annealing treatment, developed as an industrial process in combination with rolling.

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ACKNOWLEDGEMENTS

This research has been sponsored by "Studiengesellschaft fiir Anwendungstechnik von Eisen und Stahl e.V.", Diisseldorf, with financial support from the European Coal and Steel Community (ECSC), Brussels (Projekt P 35, ECCS-7210/SA/1/108).

NOMENCLATURE

T Tc Tcrit

To MII

No U/A

Temperature (°C) Temperature at column's core (°C) Critical or failure temperature (°C) Normal temperature (+20 °C) Bending m o m e n t of second order theory (including influence of lateral deflection v) (kN m) Normal force (kN) Aspect ratio (U: periphery, A: crosssection area) (m -1) Cross section dimension (mm)

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Thermal diffusivity of structural steel (a = ;~/pc~) (mmZ/min) Specific heat capacity (kJ/kg K) Column length (mm) Time (min) Failure time (min) Vertical elongation (mm) Horizontal deflection (mm) Coordinates Yield strength (N/mm 2) Strength, activatable by load (o) (N/mm 2 ) Thermal conductivity (W/m K) Density (kg/m 3) Acting stress (N/mm 2)

REFERENCES

1 W. Klingsch, Fire resistance of composite columns and solid steel columns, Research Reports (in German, intermediate reports not published, final report for publication in preparation), Technical University, Braunschweig, F.R.G. 2 W. W. Stanzak and T. T. Lie, Fire resistance of unprotected steel columns, ASCE J. Constr. Div., 99 ST5 (1973) Proc. paper 9719. 3 W. Klingsch, Fire resistance of massive steel columns, Proc. Int. Seminar on Fire Resistance o f Steel and Composite Columns, Technical University, Delft, Netherlands, Nov. 1980, (report in German). 4 K. Roik and P. Schaumann, Tragverhalten yon Vollprofilstiitzen -- Fliessgrenzenverteilung an Vollprofilquerschnitten, unpublished research report, Ruhr-Universit~it Bochum, F.R.G., 1980.