J M Davies

Design of thin-walled purlins for distortional buckling

Abstract Roof purlins and sheeting rails are generally loaded through the cladding members that they support and this provides both rotational and translational restraint. This restraint reduces the tendency to lateral torsional buckling and thus increases the importance of distortional buckling in the design procedure. This paper shows that the fundamental behaviour of restrained purlins under both downward and uplift load can be best understood with the aid of ‘Generalized Beam Theory’ (GBT). GBT also provides a yardstick by which approximate design methods can be assessed. The existing approximate methods are evaluated and an improvement is proposed. The proposed design approach is then validated by comparison with test results.

An experimental and numerical study of the behaviour of glass fibre reinforced plastics (GRP) short columns at elevated temperatures

Abstract This paper presents the results of experimental and numerical studies to determine the compression strength of short glass reinforced plastic (GRP) C-shaped channels at different elevated temperatures. Several possible compressive testing methods were explored to obtain the compression strength of the material. Eventually a pair of grooved steel end plates was used to restrain the test specimen under compression. Using the measured longitudinal compressive strengths of the material from this study and the measured longitudinal Young modulus from a previous study by the same research group, a numerical modelling study utilising the commercial finite element package ABAQUS has been performed to simulate the compressive behaviour of the test columns at elevated temperatures. Estimation of other material properties has been made to enable the numerical simulations. Comparison between the numerical and test results confirms that it is suitable to use ABAQUS and the adopted material properties to model GRP columns at elevated temperatures.

An analytical modelling technique for predicting the stiffness of 3-D orthotropic laminated fabric composites

Abstract A new analytical modelling approach for the prediction of the stiffness of 3-D orthotropic laminated composites is given. The composite, which consists of stacked orthotropic layers which are in turn composed of a number of parallel unidirectional stripes, is assumed to be homogeneous and orthotropic macroscopically. The technique introduced is to discretise the representative unit cell of the composite into slices (layers) and then stripes (elements). The stiffness of each slice can then be obtained under the condition of isostrain or isostress. The final stiffness of the composite is formulated analytically by combining these slices. The model eliminates the inconsistency between macro- and micro-level strains and gives more realistic distributions of strain for the representative unit cell. The results demonstrate that the present model, which is both simple and computationally efficient, can give a very accurate prediction compared with data from experiments and some existing models.

Mechanical analysis of a cracked beam reinforced with an external FRP plate

A theoretical method to predict the loading capacity of a cracked FRP reinforced concrete flexural beam is developed. The beam subjected to three-point bending is externally reinforced with unidirectional FRP plate near the bottom surface of the tensile zone. No slip between the FRP plate and plain concrete is assumed. A fictitious crack approach which has been used previously in conjunction with finite element method in the fracture analysis of concrete is adopted here to estimate the equivalent bridge effect of the fracture process zone of concrete. The predicted results of loading capacities are then shown graphically.

Design procedures for profiled metal sheeting and decking

When designing profiled metal sheeting and decking at the ultimate limit state, the conditions at the internal supports, where the bending moment interacts with the support reaction force, are critical. Design code provisions for estimating the moment of resistance over an internal support are known to be inadequate and the problem is aggravated if account is taken of the redistribution of bending moment accompanied by post-yield buckling. This paper shows that the performance at the internal support can be predicted by finite element analysis and that continuing the analysis into the drooping post-yield range allows a relatively simple pseudo-plastic design method to be developed.

Shear strength of empty and infilled cassettes

Abstract This paper considers the shear buckling of light gauge steel cassette sections both with and without an infilling of relatively rigid thermal insulation. In cassette construction, in-plane shear stresses usually arise as a consequence of stressed skin (diaphragm) action and, in this context, it is local buckling of the wide flange that usually governs the design. Although there are some rudimentary equations for the shear strength of plain cassettes in Part 1.3 of Eurocode 3 (EC3) (Stahlbau 7 (1987) 197), this is a subject that is not fully understood and which raises some fundamental questions such as: What are the boundary conditions for plate buckling—is it sufficient to consider individual plate elements or is it necessary to consider the whole section? Is it sufficient to neglect the post-buckling strength or should this be incorporated in the design equations, as it is for plate elements in compression? How can the favourable interaction with a relatively rigid, thermally-insulating infill be incorporated in the design equations? This paper addresses these fundamental questions and illustrates the principles involved in designing for local buckling in shear.

Thermal and structural performance of GRP panels and short columns under fire conditions

This paper presents the results of an experimental study of the behaviour of different types of GRP structures under fire conditions. The experiments were conducted in two stages. The first part is concerned with the thermal performance of GRP sandwich panels and stringer systems when exposed to fire on one side. The objective of this study is to provide experimental information of temperature developments in GRP structures in fire. The products examined in this investigation include: glass reinforced phenolic laminate sheets, lightweight phenolic foams, sandwich panels made from GRP sheets and phenolic foams, sandwich panels made from plasterboards and phenolic foams, stringer systems made from GRP sheets and pultruded GRP channels insulated with mineral wool and stringer systems made from plaster boards and pultruded GRP channels with mineral wool. For comparison, fire tests were also carried out on conventional steel stud systems consisting of plasterboards and cold-formed-thin-walled steel channel sections insulated with mineral wool. In all these tests, the core materials were 100 mm thick. Plasterboards were 12.5 mm thick and the GRP sheets were 3 mm thick. Fire exposure was according to the standard BS: 476 Part 20 – Cellulosic fire temperature–time relationship. The second part gives results of compressive tests on pultruded short GRP channel sections at elevated temperatures to evaluate their residual strength in fire. The objectives of these tests are to assess the feasibility of using GRP based products as wall construction in low storey buildings such as accommodation units and also to provide experimental data for the calibration of numerical models of heat transfer and structural analysis. Results of these tests indicate that the low-density phenolic foam used in this study was involved in burning very early in the fire test and would not be suitable for fire resistant construction. Initially the fire exposed thin GRP sheet could not provide as much insulation as the plasterboard, however, the GRP sheet prevented direct heating of the core material and was able to reduce the core material temperatures considerably. At 30 min of the standard fire exposure, temperatures in large parts of the GRP stringer systems were less than 100 C. For compressive tests, pultruded channel sections 100 x 30 x 4 mm by 400 mm long were heated in an electric kiln furnace to different levels of temperature and then loaded to failure in compression in a specially constructed loading facility. The test temperatures were 20, 60, 90, 120, 150, 200 and 250°C. The results of these tests indicate that GRP materials lose their strength and stiffness at relatively low temperatures, however, due to their low thermal conductivity, temperatures in large parts of the pultruded GRP channel sections were very low (less than 100°C) enabling them to carry some loads (up to 70%) under fire conditions.

RESEARCH STUDIES OF THE FIRE PERFORMANCE OF COMPOSITE MATERIALS AT THE UNIVERSITY OF MANCHESTER

Owing to their lightweight and good corrosion resistance, Glass fibre Reinforced Plastic (GRP) composites are increasingly being used as an alternative offshore construction material to steel and concrete. However, their applications in the building industry are limited due to their perceived weakness under fire conditions and a lack of design information. The fire performance of GRP composites is being studied in a number of places world-wide and this paper reviews some of the research studies carried out by the Structures and Fire Research Group at the University of Manchester, UK. This research has so far been only concerned with the experimental and numerical study of the thermal properties of a variety of composite materials and the thermal performance of different offshore composite systems including panels and pipes. This paper describes the completed experimental programme and their results and discusses the numerical heat transfer modelling technique and the selection of appropriate thermal properties of GRP composites for numerical modelling.