CRITICAL LOCAL STRESS OF POLTRUDED RHS SECTIONS
- Introduction
- Background
The increasing popularity of pultruded FRP sections is attributed to two factors: corrosion resistance and high strength to weight ratio. These two factors have made pultruded FRP sections perfect for use in civil infrastructure for design of long-life structures such as bridges. The main drawback of this method of construction is that design guidance has not been fully developed as in traditional construction materials such as concrete and steel. Furthermore, the high strength of pultruded FRP sections limits them to only small section thicknesses, which makes them prone to instabilities caused by local buckling. The anisotropic material response of pultruded FRP sections further compromises their structural response, resulting in increased local buckling. This increase arises from the fact that the post-buckling of slender pultruded sections is directly influenced by three factors: the Poisson’s ratios, Young’s modulus in the horizontal and longitudinal directions, and the in-plane shear modulus.
Providing reasonable estimates of the buckling load of pultruded sections requires the use of advanced design techniques that are based on cross-section slenderness. One example of this technique is the Direct Strength Method. There needs to be a definition of the slenderness of the section considered, depending on the action applied, such as major axis bending, minor axis bending, and compression. This definition assists in determining the elastic critical local buckling stress. Various studies conducted on steel cross-sections have given rise to the development of explicit empirical equations that relate to the effect of the elemental interactions between the flange and the web on the critical bulking stress. The aim of this project, therefore, is to extend these studies onto pultruded RHS sections.
1.2 Aims and Objectives
The main aim of this project is to develop explicit empirical equations that can be used to determine the elastic critical local buckling stress of RHS sections. To achieve the above aim, several objectives need to be completed. First, a literature review has to be carried out to understand better the behavior of pultruded sections failure by local buckling. Then, a Finite Strip model using CUFSM has to be created to help determine the elastic critical buckling stress of channel sections subjected to compression and major axis bending. The next objective is to conduct parametric studies to investigate the effect of parameters such as section aspect ratio and material properties on the elastic critical local buckling stress. Finally, empirical equations for the buckling coefficient of pultruded channel sections that are guided by geometric and material parameters have to be developed.
- Outline of Dissertation
Chapter 1: INTRODUCTION
- Background
- Aim and Objectives
- Dissertation Outline
Chapter 2: CRITICAL LOCAL STRESS OF POLTRUDED RHS SECTIONS
2.1 Local Buckling of Plates and Governing Equations
2.2 Local Buckling of Cross-Sections
2.3 Pultrusion and Material Orthotropy
2.4 Past Experimental Studies on Pultruded Sections Failing By Local Buckling
2.5 Database of Available Test Results
Chapter 3: METHODOLOGY
3.1 Finite strip analysis and CUFSM
3.2 Development of the model
3.3 Model validation
3.4 Parametric studies.
3.4.1 Geometry
3.4.2 Material Properties
Chapter 4: RESULTS AND DISCUSSION
4.1 Sections in compression
4.2 Sections in major axis bending
4.3 Recommendations
Chapter 5: CONCLUSIONS
5.1 Conclusions
5: CONCLUSIONS
5.1 Conclusions
The need for new structural solutions to increase construction speed and more durability gave rise to the use of GFRP pultruded materials. These materials are advantageous because of several factors: easy installation, low maintenance, higher durability, low self-weight, fabrication versatility, electronic transparency, and high strength-to-weight ratio. The main disadvantage of these materials is the lack of design codes for easy access and application. The designs of GFRP pultruded profiles are governed by two deformation constraints: low Young’s modulus and low slenderness. Studies conducted show that GFRP pultruded beams exhibit elastic behavior up to a point, after which they fail every test, illustrating post-buckling and first-order buckling behaviors.
Previous experimental data used to determine the behavior of cross-sections of pultruded elements show that there are two slenderness ratios. The high ratio emphasizes the global buckling load, while the low ratio suggests the local buckling and critical loads are close. Researchers have concluded that in pultruded sections, compressive strength and local flanges buckling are the failure modes.
The FSM method is used to determine the instabilities of thin-walled members through longitudinal stress. This method focuses on the distortional, flexural-torsional, and local I-beams buckling modes about their major axis. The main disadvantage of FSM implementations is their single applicability to simply-supported boundary conditions. However, it is advantageous over FEM in that it is more computationally coefficient.
On sections in compression, there is a good agreement achieved between the method proposed by Cardoso and the finite strip method, where the coefficient of variation is 10%. Bending in sections in the major axis show that there is good agreement between the modified critical buckling coefficient and the CUFSM critical local buckling coefficient.
A good agreement between Cardoso equations on the estimation of the critical local buckling in axial and CUFSM results suggest that Cardoso’s equation is more applicable in estimating local buckling. In the estimation of the local buckling stress for orthotropic material bending moment major axis of RHS sections, the equation
is the most suitable.