The Structural Engineer > Archive > Volume 74 (1996) > Issues > Issue 1 > Load-Strength Charts for Pitched Roof, Haunched Steel Portal Frames with Partial Base Restraint
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Load-Strength Charts for Pitched Roof, Haunched Steel Portal Frames with Partial Base Restraint

Load - strength charts have proved useful for preliminary sizing of members in pitched roof portal frames. They also provide an independent means of checking simple plastic collapse strength derived from computer analysis. Unlike previous charts, those presented here allow for the effect of partial column base restraint and enable the minimum length of eaves haunch to be determined directly. The new charts are applicable to a wide range of frame geometry and rafter/column strength ratios. Nominal base restraint is often used in the UK to prevent inward collapse of frames and consequent damage to external walls, in the event of a fire. More substantial restraint can also be used to improve sway stability and eaves deflection. The basis of the charts is defined and may be incorporated into a more direct computer-aided design scheme if desired, but the real intention of the charts is to provide a computer-independent means of determining member sizes. Three examples of their use are given. The first sets out with preconceived rafter/column strength ratio, assuming pinned bases. The second imposes a restricted set of preferred sections, also using pinned bases, and seeks to improve on the previous total weight of steel. In the final example, use of base restraint equivalent to 20% of the column plastic moment leads to a further 7.5% saving in total weight of steel. J.O. Surtees and S.H. Yeap

Author(s): Surtees, J O;Yeap, S H

Keywords: loads;strength;diagrams;portal frames;steel;restrained;roofs;checking;applications;worked examples;design