Is it worth using the EQUIVALENT FRAME method in modern design of structures?

07 January 2017

It is a subjective comment on benefits and problems caused by the equivalent frame method in modern design of structures. I would like to put an emphasis on educational merits of this method, particularly with reference to education of young designers. This post also suggests how to shorten laborious calculations.

What point is there in using the equivalent frame method nowadays?

This question comes to mind of many engineers of civil structures, who know the equivalent frame method and are aware of its laborious use in practice. Considering the modern possibilities and common use of the finite element method (FEM), the concept of using the equivalent frame method as the fundamental and the only calculation method used by engineers seems rather irrational. Moreover, the frame equivalent method has significant limitations - it can be only used in case of a regular arrangement of columns. Also, the impact of openings is excluded from the calculations. It is interesting that the above analysis, regardless of its significant limitations, is specified in the current standard on design of concrete structures EC-2 [1], Annex I, point I.1.2.

The equivalent frame method can be definitely regarded as an alternative tool for verifying calculations made for flat-slab structure using the FEM. Teaching aspect is an additional and important reason for learning that calculation method. It is particularly important at the educational stage of a design engineer-to-be. Analysing and understanding the effect of rigidity of crucial nodes in the structural system on distribution of internal forces, particularly in points where columns pass through flat floor slabs is appaently instructive for a future design engineer.

How to minimize laborious of calculations?

Nowadays, modelling of the equivalent frame seems quite odd. Moreover, it would not be acceptable to calculate the bending moments at midspan and support for, separated in mind, frame on the basis of the so-called distributors and transmitters using e.g. the Cross method. The detailed algorithm for such calculations was presented by Prof. Włodzimierz Starosolski in his multi-volume handbook [2]. Naturally, some calculations can be made using the modern applications for structural analysis.