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Canadian Consulting Engineer, May/June, pp. 39-40, 1990-05
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Concrete filling: fire protection for steel columns
Lie, T. T.; Chabot, M.
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https://nrc-publications.canada.ca/eng/view/object/?id=dc86763e-ce09-4698-9bce-1f8f37cb4212 https://publications-cnrc.canada.ca/fra/voir/objet/?id=dc86763e-ce09-4698-9bce-1f8f37cb4212Concrete Filling: Fire Protection for Steel Columns
by T.T. Lie and M. Chabot
T.T. Lie is a principal research officer in the National Fire Laboratory of the Institute for Research in Construction.
M. Chabot, at the time of writing, was a steel industry fellow at IRC, sponsored by the Canadian Steel Construction Council.
Originally published in "Canadian Consulting Engineer" May/June 1990, p. 39-40
Hollow structural-steel sections are the most efficient of all structural sections in resisting compression loads. By filling these sections with concrete, the load-carrying capacity of such columns can be increased substantially. Another benefit of doing this is high fire resistance without additional fire protection for the steel surface. Eliminating surface protection increases the usable space in a building.
For a number of years, the Institute for Research in Construction (IRC) has been engaged in research to predict the fire resistance of concrete-filled hollow steel columns. This research, which is supported by the Canadian Steel Construction Council, consists of theoretical and experimental studies.
Several factors influence the fire resistance of the columns, the most important of which are: the load intensity, cross-sectional area and length of the column, the type of concrete used as filling, and the presence of steel reinforcement in the concrete. Hollow steel columns filled with plain concrete, steel-fibre-reinforced concrete and bar-reinforced concrete were studied. In addition, IRC has been developing calculation methods that can be used to predict the fire resistance of such columns, thus avoiding the high cost of testing.
Figure 1. Expansion of concrete-filled steel column during fire exposure
Experiments and Predictions
Over the years, many fire tests have been conducted by exposing the columns to heat in a furnace, specially built for the testing of loaded columns under fire conditions. During exposure to fire in this furnace, the steel of a concrete-filled steel column initially carries most of the load. It quickly expands, and then gradually yields because of decreasing strength until, usually after 20 to 30 minutes, the concrete takes over and carries a progressively increasing portion of the load. Figure 1 shows a typical example of this phenomenon. It is this contribution of the concrete that provides fire resistance to the column.
The strength of the concrete also decreases with time and ultimately the column can no longer support the load and collapses. The length of time the concrete is capable of supporting the load can be substantial. It can provide the column with a fire resistance of several hours, depending on such factors as the load on the column and the size of the cross-section of the column.
Effect of Concrete Filling
Filling hollow steel sections with concrete is a cost-effective way to obtain columns with a high load-carrying capacity and a high fire resistance. Typical fire resistance values of hollow steel columns (cross-section including void about 50 000 mm2) with and without concrete filling are shown in Table 1. It can be seen that unprotected steel without concrete filling has a fire resistance of about 15 minutes, which is low in comparison with those of the columns with concrete filling. For the concrete-filled columns, fire resistance values are given for plain concrete filling, for steel-fibre-reinforced concrete filling and for bar-reinforced concrete filling. In no case is external fire protection applied to the steel.
The most economical arrangement, from the point of view of fire resistance, would be filling the column with plain concrete without any steel reinforcement. The increase in cost is
approximately the price of the concrete itself. However, because of strength-loss in the steel casing at elevated temperatures, without reinforcement and containment of the concrete core, early cracking and failure may occur. Therefore, the fire resistance of these columns is typically limited to between 1 and 2 hours.
A method to prevent early cracking and thus increase the fire resistance is to add a small amount of steel fibre to the concrete (about 2% by weight). The additional cost of this over the concrete is approximately the cost of the steel fibre. For these same columns, fire resistances of 2 to 3 hours have been obtained under substantially increased load.
The fire resistance and load-bearing capacity of columns at elevated temperatures can further be increased by reinforcing the concrete with steel bars. For these columns, fire resistances of over 3 hours have been obtained in full-scale tests. There is, however, the additional cost of installing bar reinforcement in the columns.
Table 1. Typical fire resistance of
hollow-steel columns (50 000 mm2) with and without concrete filling
Filling Fire Resistance (hours)
None 0.2-0.3
Plain concrete 1.0-2.0 Steel-fibre reinforced
concrete 2.0-3.0
concrete
Calculation Methods
The main objectives of the experimental studies are to generate fire resistance data for
immediate use by industry, and to provide information for the development of general methods of calculating fire resistance of concrete-filled steel columns for a wide range of applications. Determining fire resistance by testing full-size columns is costly and time-consuming.
Calculating fire resistance, on one hand, can be done at a fraction of the cost and time involved in testing.
To develop methods to predict fire resistance, the conditions during exposure to fire, e.g., fire severity and load, are simulated by mathematical models. These models are then programmed for computer-processing and the computed results are verified by comparing them with results from the full-scale tests.
The experimental studies for prediction of the fire resistance of concrete-filled steel columns and the development of associated mathematical models are at an advanced stage. For many cases, fire resistance can now be predicted using computer models developed at IRC. It is anticipated that within the next two years development of the calculation methods needed to cover most practical cases will be completed. In the interim, these models are being used, on a contract basis, to assist designers and regulators to calculate the fire resistance of these efficient structural columns.
