COLUMNS 9. 1 TYPES, LOADS, CLASSIFICATION AND DESIGN CONSIDERATIONS 9.1.1 Types and Loads Columns are structural members in buildings carrying roof and floor loads to the foundations. A column stack in a multi-storey building is shown in Fig. 9.1(a). Columns primarily carry axial loads, but most columns are subjected to moment as well as axial load. Referring to the part floor plan in the figure, the internal column A is designed for predominantly axial load while edge columns B and corner column C are designed for axial load and appreciable moment. Design of axially loaded columns is treated first. Then methods are given for design of sections subjected to axial load and moment. Most columns are termed short columns and fail when the material reaches its ultimate capacity under the applied loads and moments. Slender columns buckle and the additional moments caused by deflection must be taken into account in design. The column section is generally square or rectangular, but circular and polygonal columns are used in special cases. When the section carries mainly axial load it is symmetrically reinforced with four, six, eight or more bars held in a cage by links. It is not practical to cast vertically columns smaller than 200 mm square. Typical column reinforcement is shown in Fig. 9.1(b). General requirements for design of columns are treated in section 5.8 of Eurocode 2. The provisions apply to columns where the greater cross sectional dimension does not exceed four times the smaller dimension. The minimum size of a column must meet the fire resistance requirements given in Eurocode 2: Design of Concrete Structures-Part 1-2: General Rules-Structural Fire Design. For example, from Table 2.10, Chapter 2, for a fire resistance period of 90 minutes, a fully exposed braced column must have a minimum dimension of 350 mm with the distance from the surface to centre of the steel of at least 53 mm. 9.1.2 Braced and Unbraced Columns Lateral stability in braced reinforced concrete structures is provided by shear walls, lift shafts and stairwells. Fig. 9.2a shows a frame structure which is designed such that all horizontal load is resisted by a stiff lift shaft so that the column ends deflect

346 Reinforced concrete design to EC 2

very little. In a braced column the axial load and the bending moments at the ends of a column arise from the vertical loads acting on the beams. The horizontal loads do not affect the forces or deformation of the column. The columns do not contribute to the overall horizontal stability of the structure.