Keywords: FRP thin-walled members, pultrusion, lipped channels, generalised beam theory (GBT), buckling, local plate mode, flexural-distortional mode, distortional mode.

Although composite materials have been extensively employed in the aeronautical industry, their application to civil engineering structures only became significant in recent years. Presently, there exists a growing demand for composite structural members, namely members manufactured by pultrusion from a polymer plastic matrix reinforced with unidirectional fibers. The mechanical behaviour of such members is characterised by linear elastic stress-strain relations, absence of ductility and a distinct kind of orthotropy, often termed "special orthotropy". These mechanical properties clearly indicate that the ultimate strength of FRP pultruded members may be strongly affected by instability phenomena. Therefore, in order to assess the structural efficiency of such members, namely columns, beams and beam- columns, it is essential to investigate their local and global buckling behaviour.

The "Generalised Beam Theory" (GBT) was shown to be a rather powerful analytical tool to study the buckling behaviour of thin-walled (cold-formed) steel structural members. Recently, the authors extended GBT in order to enable its application to the stability analysis of members with special orthotropy, therefore obtaining the generalised GBT fundamental equation:

The main objective of the paper is to investigate, by means of GBT, the local and global buckling behaviour of FRP thin-walled lipped channel members (Figure 29.1(a)) displaying the special orthotropy just mentioned. Depending on the applied stress distribution, the members are designated as columns (), beams () or beam- columns () (Figure 29.1(b)).

First, the GBT technique is briefly reviewed and applied to study the buckling behaviour of a C-section column (channel with outward lips) in detail. Then, the technique is employed to assess the influence of the applied stress distribution on the buckling mode nature and corresponding bifurcation stress value. In particular, anti-symmetric distortional and mixed flexural-distortional (FDM - see Figure 29.1(b)) buckling modes are identified and shown to be relevant for the characterisation of the buckling behaviour of FRP structural members.

Then, the effect of fiber material properties is investigated. Four types of fibers are dealt with (e-glass, kevlar, graphite and boron) and it is shown that (i) boron fibers provide the highest local and global buckling resistance and (ii) e-glass and kevlar pultruded members display a similar buckling behaviour. Moreover, this study revealed that the local bifurcation stress values are fairly proportional to the fiber volume fraction, provided that only realistic values are taken into account.

Finally, the paper addresses how the cross-section shape and dimensions affect the member local buckling behaviour. The relevance of the cross-section shape is assessed by comparing the behaviours of identical hat and C-section columns and beams and it is concluded that their local plate and distortional buckling behaviours can be markedly different. The combined influence of the flange and lip widths on the buckling mode nature and corresponding bifurcation stress value of hat and C- section members is also investigated.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £122 +P&P)