Unprotected hollow sections can attain up to 2 hours fire resistance when filled with concrete. When the combined section is exposed to fire, heat flows through the steel into the concrete core which, being a poor conductor, heats up slowly.
As the steel temperature rises its yield strength steadily decreases and the load is progressively transferred to the concrete. The steel then acts as a restraint to restrict spalling of the concrete. BS5950 part 8 contains a calculation method for checking the axial and moment capacities of square and rectangular columns in fire. However, the standard does not include any guidance on the fire resistant design of unprotected concrete filled circular hollow sections.
Three types of filling are possible, plain, fibre reinforced or bar reinforced concrete. Plain and/or fibre reinforced concrete performs well under compression loading but performs less well when a column is subject to moments. As a result, BS5950 part 8 section 184.108.40.206 requires that two relationships which limit the moments about the major and minor axes must both be met when using plain or fibre reinforced concrete. Compliance with both will ensure that the column remains in overall compression under the combined fire limit state axial load and moments. When moments above these limits are present, the capacity of the concrete filled column can be further enhanced by the addition of bar reinforcement. The calculation method for checking the axial and moment capacities is given in BS5950 Part 8 section 220.127.116.11.
As an alternative, a concrete filled hollow section column can be designed to its full composite capacity and then be protected by a board, spray or intumescent coating system. In this case it is still possible to exploit the improved thermal properties of the filled column to reduce the level of external protection used. For board and passive spray systems, this is determined by calculating the passive protection requirement based on the empty hollow section and then reducing the thickness by a modification factor using a tabulated method given in BS5950 part 8 section 4.6.3. Similar reductions are also possible with an intumescent coating. However, each individual product must be assessed separately to ascertain these allowable reductions. At present, only one product has been fully evaluated. Further information is available in the Corus Tubes publication Intumescent Coatings and SHS concrete filled columns.
Most of the above can also be found in greater detail, together with information on the advantages, limitations and methodologies of achieving fire resistance using concrete filled tubes in the Design Manual for Concrete Filled Columns, Part 2, Fire Resistant Design.
Design guidance for square and circular columns to Eurocode 4 Part 1-2, prepared in accordance with the base document, but covering only mainly axial loads, is available in a CIDECT Design Guide No. 4.
Moreover, freely available fire design software to Eurocode 4 has now been developed which also includes moments. A copy will be placed on the new Corus Construction Centre CD. Further information can be obtained from Corus Tubes, Corby, Northants (Telephone 0500 123133).