The '80s have seen an increased interest of the international community of structural concrete in the analysis and design of structures for actions which induce extreme non proportional and often reversed or even cyclic response. Such an action, be it quasi-statie, such as extreme waves, or dynamic, such as earthquakes, explosions or impact, is accidental, causing strongly inelastic response. This interest in the design of structural concrete for extreme alternate actions has produced further an interest in the inelastic behaviour of members, subassemblages of members and entire structures, under strongly non-proportional or even cyclic load histories, and in the modelling of this behaviour for the purposes of the analysis of the response.

C.E.B. responded very early to this interest by creating at the time Genera! Task Group 10: "Response of Reinforced Concrete Critica! Regions under Large Amplitude Cyclic Actions". This Task Group produced in 1983 Bulletin d'lnformation No. 161, with the title above, which presented the State-of-the-Art of that time in the experimental behaviour and analytical modelling for cyclic loading, at the material and member level.

Because of the very fast evolution of knowledge and the intensification of interest in th s area, the conclusion of this activity was followed by the establishment in 1985 of Genera! Task Group 22: "Behaviour and Analysis of Reinforced Concrete Structures under Alternate Actions lnducing lnelastic Response". The new Task Group, under the chairmanship of Prof. P.E. Pinto, focused on producing a State-of-the-Art document of fundamental nature, covering the behaviour and constitutive modelling of concrete under multiaxial generalised stress histories, of reinforcing steel and bond under uniaxial cyclic loading, and of crack interfaces under generalised multiaxial loads, as well as the assemblage of the constitutive models of the individual materials and of their interaction into Finite Elements. This document was presented in 1991 to the 28th Plenary Session in Vienna, as Bulletin d'lnformation No. 2 10: "Behaviour and Analysis of Reinforced Concrete Structures under Alternate Actions lnducing lnelastic Response. Volume 1: Genera! Models". Follow-up documents were envisaged, the first dealir1g with frame structures.

The publication of Bulletin d'lnformation No. 210 and early work on the first follow-up document, coincided with the restructuring of C.E.B .. So Task Group 111/6 was established, under Commission 111: "Design", to continue and complete the task of Volume 2: Frame Structures. The new group comprised a nucleus from the membership of its predecessor as well as several new members.

As it refers to a specific class of concrete structures, the scope of the new Task Group is less general and more applied than that of the previous. lt still focuses though on extreme cyclic or in general reversed loading and the associated inelastic response, of which the thoroughly studied and well known proportional loading and response is only a special and rather uncommon in practice case. For frames, which are usually found in building structures, alternating actions are typically lateral and dynamic, as, e.g. seismic or wind actions.

The present document builds up from the experimental behaviour and the analytica!/ numerical modelling of individual members, to that of frames. Individual frame members are considered for the type of cyclic loading to which they are subjected in laterally loaded frames: Slender, flexure-dominated beams in uniaxial bending without axial load; slender columns in uniaxial or biaxial bending with constant or varying axial force; mono-dimensional elements of low shear ratio, such as deep beams and short columns, the behaviour of which is dominated by shear, in uniaxial or biaxial flexure and shear with or without axial force; and last but not least, beam-column joints in cyclic shear. The global inelastic behaviour and the associated modelling of bare reinforced concrete frames is also considered, focusing on aspects of the behaviour and the modelling which are not a direct and obvious outcome of those of the members. Finally, the peculiar features of the behaviour of masonry-infilled frames subjected to cyclic lateral actions, and the special techniques which have been proposed to-date to model this behaviour are also reviewed.

Although far from exhaustive, the coverage of the various subjects in this document is rather broad. In this respect the present Bulletin may be considered as a reference document, both for specialists and for newcomers in the field. Readers should be cautioned, however, especially if they belong to this latter category, that because of the very rapid on-going developments in research in all the subjects covered herein, this document runs the risk of becoming soon obsolete. Counterbalancing this handicap, the Bulletin is not limited to a critica! and unifying review of the State-of-the-Art, but includes also several original contributions and extensions of the available knowledge.

During the last year of its work the Task Group established and maintained liaison with ACI-ASCE Committee 442 "Response of Concrete Buildings to Lateral Farces", and in particular with the key people of its ad hoc Subcommittee on "Inelastic Response of Concrete Structures". This latter subcommittee has completed a nine-year effort of developing a State-ofthe- Art report on "Earthquake-Resistant Concrete Structures Inelastic Response and Design", published in 1991 as ACI Special Publication SP-127. Although the scope of this latter document was different than that of the present Bulletin, focusing more on seismic design, there is a partial overlapping of the subjects of these two documents. With this in mind, the Task Group made a concerted effort to go beyond the previous ACI contribution and to prepare a document of more fundamental and analytica! character. In doing so the Task Group believes that it is making a significant contribution to the international literature on this subject.

Last but not least, liaison and cooperation of the Task Group with Task Groups 11/1, "Constitutive Modelling of Material Behaviour" and llI/l, "Seismic Design of Reinforced Concrete Structures", was achieved through multiple double memberships.