Steel Buildings in Europe

Part 2: Concept Design 2 - 49 8 PRELIMINARY DESIGN OF PORTAL FRAMES 8.1 Introduction The following methods of determining the size of columns and rafters of single-span portal frames may be used at the preliminary design stage. Further detailed calculations will be required at the final design stage. It should be noted that the method does not take account of:  Requirements for overall stability  Deflections at the Serviceability Limit State. 8.2 Estimation of member sizes The guidance for portal frames is valid in the span range between 15 to 40 m. and is presented in Table 8.1. The assumptions made in creating this table are as follows:  The roof pitch is 6  .  The steel grade is S235. If design is controlled by serviceability conditions, the use of smaller sections in higher grades may not be an advantage. When deflections are not a concern, for example when the structure is completely clad in metal cladding, the use of higher grades may be appropriate.  The rafter load is the total factored permanent actions (including self weight) and factored variable actions and is in the range of 8 to 16 kN/m.  Frames are spaced at 5 to 7,5 m.  The haunch length is 10% of the span of the frame.  A column is treated as restrained when torsional restraints can be provided along its length (these columns are therefore lighter than the equivalent unrestrained columns).  A column should be considered as unrestrained when it is not possible to restrain the inside flange. The member sizes given by the tables are suitable for rapid preliminary design. However, where strict deflection limits are specified, it may be necessary to increase the member sizes. In all cases, a full design must be undertaken and members verified in accordance with EN 1993-1-1.