Structural Performance of YSt-310 Cold-formed Steel Tubular Columns

INTRODUCTION

BACKGROUND

Based on the production route chosen at the time of the production process, steel tubular sections can be mainly classified into two groups: hot and cold rolled tubular sections. With the advancement of cold forming techniques, thicker steel sections up to 25 mm are now being produced, see Guo et al.

POPULARITY OF TUBULAR STRUCTURES IN INDIA

During cold forming process, stress-strain behavior of the stomach (parent) material (identified with clearly defined and generally distinct yield point, followed by plateau region) was observed to change to a rounded stress-strain material behavior with a certain amount of strain hardening (see in Ringle, 1969; Afshan et al., 2013). However, most of the contemporary international design codes do not incorporate the strength improvement due to cold forming and mainly rely on the elastic-perfectly plastic stress-strain material model, which is usually applicable for hot-rolled steel sections.

PERFORATED TUBULAR MEMBERS

Furthermore, the structural performance of cold-formed YSt-310 steel tube sections has not been reported so far, and there are no test data to evaluate the applicability of international design standards currently available in the literature. It may be noted that, although Tata Structura YSt-355, with nominal yield and tensile strength of 355 and 490 MPa respectively, was recently introduced by Tata Structura Pipe Division (2019); however, it has not yet been available in the domestic market.

RESEARCH MOTIVATION

Furthermore, it has been noted that extensive research has been conducted on steel plates since the late 1950s (e.g. it is apparent that most published experimental and numerical studies have focused mainly on plates and open cold-formed steel profiles, and there is an apparent lack of a systematic investigation of perforated rectangular tubular (closed) steel sections subjected to axial compression.

OBJECTIVES

OUTLINE OF THESIS

In Chapter 4, the structural performance of YSt-310 cold-formed steel tube stub columns has been investigated through experimental and numerical programs. First, previous research works on the mechanical properties of YSt-310 cold-formed steel tube sections are briefly reviewed.

LITERATURE REVIEW

INTRODUCTION

Second, a summary of previous research works on the cross-sectional load-bearing study of YSt-310 cold-formed steel tubular columns was presented, including test and numerical programs, codified design expressions, and design suggestions recommended in the literature. Based on a literature review, research gaps in the analysis and design of cold-formed steel tubular sections are then identified.

MATERIAL PROPERTIES

Material properties at ambient temperature
Material properties at elevated temperature
Material properties after post-fire

This section presents a brief review of previous studies on the post-fire properties of cold-formed steel. Kesawan and Mahendran (2018) conducted a study on the mechanical properties of G350 and G450 grade cold-formed steel after fire.

STUB COLUMNS

Unperforated stub columns

Design rules
Design Methods

PERFORATED STUB COLUMNS

Design rules

Based on the test results, a linear decrease in the ultimate buckling load was reported as the perforation diameter increased. Shanmugam and Dhanalakshmi (2001) conducted a numerical study on the ultimate column capacities of perforated cold-formed steel channel stub columns.

SUMMARY

Therefore, in this research project, an attempt has been made to establish the exact material characteristics as well as column performance database for square and rectangular hollow sections of YSt-310 cold-formed steel. The test results are then used to evaluate the applicability of design guidelines that have been developed based on hot-rolled steel materials and recommend effective practical design guidelines for cold-formed steel tubular sections. In this chapter, a comprehensive experimental program to characterize the SHS and RHS material properties of Tata Structura YSt-310 cold-formed steel is studied under three different temperature conditions, namely ambient temperature, elevated temperature and post-fire conditions. .

Section 3.2 of this chapter provides a detailed description of the steel material considered for this project. In Section 3.3 of this chapter, the chemical composition, metallography, and mechanical properties of YSt-310 cold-formed tubular steel sections were investigated at room temperature using standard test procedures.

MATERIAL CHARACTERISATION

INTRODUCTION

Accurate estimation of basic mechanical properties is the main and fundamental step in the analysis and establishment of design equations for structural members. A detailed description of the test material, test procedure, equipment used, test results and discussion are reported in this chapter.

DETAILS OF MATERIAL SUPPLY

AMBIENT TEMPERATURE MATERIAL PROPERTIES

Chemical composition
Microstructure
Mechanical properties

Flat coupon tests
Corner coupon tests
Weld coupon tests

Test results and discussion

Flat, curved and weld coupon samples were extracted from each of the flat, corner and weld sections of each steel section in the direction parallel to the direction of rolling process (see Figure 3.1). The dimensions of flat coupon specimens reported by Huang and Young (2014c) were adopted in the present study provided, see Figure 3.5 (a). The thickness and width of the flat coupon samples were measured to determine the actual cross-sectional area using a digital Vernier caliper before the tensile test.

Corner coupon samples were taken from the corner section (see Figure 3.1) with a nominal size of 6 mm wide and 25 mm gauge length. Tensile tests on weld coupons were carried out with the intention of estimating the strength of the weld.

ELEVATED TEMPERATURES MECHANICAL PROPERTIES

General
Test specimens
Test apparatus
Test procedure
Test results and discussion

Stress-strain curves
Elastic modulus
Yield strength
Stresses at 0.5% and 1.5%, and 2.0% strains
Ultimate strength and strain
Fracture strain

Design reduction factors
Reliability analysis
Comparison of design reduction factors

Elastic modulus
Yield strength
Stress at 0.5% and 1.5% strain
Stress at 2.0% strain
Ultimate strength
Ultimate and fracture strains

The reduction factors proposed by Balarupan (2015) (for temperatures up to 700 °C) and Imran et al. 2018) are in good agreement with the current test results. Figure 3.27 shows that the current test results agree well with the yield strength reduction factors previously reported by Outinen et al. The yield strength reduction factor proposed by Balarupan (2015) and Imran et al. 2018) predicts well with the test results.

The reduction factor for the 0.5% yield stress from the present test results are compared with those reported by Outinen et al. The design predictions proposed by Balarupan (2015) are in good agreement with the actual test results.

POST-FIRE MECHANICAL PROPERTIES

General
Test specimens
Heat treatment
Tensile coupon test
Microhardness test

General
Hardness test

Experimental outcome

Tensile coupon test

Comparison with previous investigations

Reduction factors

Prediction formulae

Present test results
Combined test results

Reliability analysis

In the current study, a Vickers microhardness test was conducted to measure the post-fire hardness value, Hv*, of all coupons that. Therefore, the 0.2% test stress was considered as the yield point for all coupon samples tested in this study. The figure shows that a similar trend to that of the yield and final strength reduction factors was observed.

A similar trend in the tensile strength reduction factor is observed as that for the yield strength reduction factor, but the reduction in tensile strength (~ 66% at ~ 800 °C) is found to be lower than that of the yield strength (~ 50% at ~800 °C) for all cold-formed steels considered. A separate set of design prediction formulas has been developed, taking into account all previously studied steels and current test results, to predict the post-fire reduction factors for the modulus of elasticity, yield strength, tensile strength and percent elongation at break.

SUMMARY

Ambient temperature material test
Elevated temperature material test
Post-fire material test

STUB COLUMNS

INTRODUCTION

A column is an important structural element which primarily carries compressive loads along the main axis of the element. Generally, the structural performance of columns under various loading conditions is estimated experimentally or numerically (using finite element tools) and design equations are developed for efficient use of the material and safety. In this chapter, the structural capacity of YSt-310 cold-formed steel tube stub columns under concentric loading is explored through experimental and numerical studies.

Full range of stress-strain curves and key material parameters (such as Elastic modulus, 0.2% proof stress, ultimate stress and corresponding strain, etc.) at ambient temperature evaluated in Chapter 3 of this thesis were used to develop finite element (FE) models. The applicability of current up-to-date international design standards such as EC AISI S following Effective Width Methods (EWM); and design rules such as Direct Strength Method (DSM) presented in AISI S and Continuous Strength.

EXPERIMENTAL INVESTIGATION

Preparation of stub columns
Local geometric imperfection
Stub column test

A representative measurement for each cross-section was performed on one of the stub column specimens. The imperfection amplitude was measured along the center line of all four planes of the section, as shown by the red line in Figure 4.8 (b). A typical measured two-dimensional (2D) longitudinal section profile of the four planes of SHS butted column specimen is shown in Figure 4.9.

Concentrically loaded stub column tests were performed on both the SHS and RHS. The results of the column stub tests are shown in Table 4.1, in the form of ultimate loads (PTest) and the associated displacements (δu).

FINITE ELEMENT MODELLING

General
Finite element type
Flat and corner material properties
Weld material properties
Local geometric imperfection and residual stress
Loading and boundary conditions
Validation of FE procedure
Parametric study

In this study, the distribution of face and corner material properties around the cross section has been considered based on the microhardness test reported in Section 4.2.2. However, it is seen from Table 4.2 that there is a slight improvement in the predicted values when the properties of the weld material are taken into account in the FE models. For the parametric study performed in the following part of this thesis, weld material properties have been ignored.

The material properties included in the FE analysis for parametric investigation are the measured plane and corner material properties of SC1 sections, as the validation of the FE modeling for this section is accurately captured in the test result. The length of the stub column L in the finite element models is set equal to four times the minimum cross-sectional size.

CURRENT DESIGN CODE AND RULES

General

European code
Continuous strength method
Direct strength method
Modified DSM

The thickness, t of the diameter considered in the present study varies from 2.0 mm to 50.0 mm, to cover a wide range of cross-sectional slenderness. The cross-sectional elastic buckling stress, cr,cs was estimated from Abaqus (2010) based on centerline geometry, allowing element interaction proposed by Seif and Schafer (2010). Originally developed for the design of non-slender stainless steel clad sections, the applicability of CSM has been extended to design for slender clad cross section by researchers such as Ahmed et al.

Two upper bounds are placed on the deformation capacity in equation 4.10 to limit the extent to which deformation of the non-thin cross section can occur. The column capacity according to CSM is estimated based on the gross cross-sectional area of Ag.

RELIABILITY ANALYSIS

The proposed model was found to give good prediction for SHS and RHS made of different stainless steel materials – Austenitic, Ferritic, Duplex and lean Duplex stainless steel. Weighted average ultimate and yield stress material properties from tensile coupon tests were used for generating the design curves since the ultimate and yield stresses for test and FE results are different for each section. The mean  Pm and coefficient of variation  Vp of the ratio of test and FE results to the design predictions were estimated and are shown in Tables 4.4–4.9.

For direct comparison and consistency purposes, a load combination of 1.2DL + 1.6LL and constant resistance factors (ϕ) of 0.85 were adopted to evaluate the reliability of all design equations. Similar load combinations and resistance factors have been adopted by previous researchers such as Zhu and Young (2012); Huang (2013) etc.

COMPARISION OF TEST AND FE STUB COLUMN STRENGTHS WITH

General
Comparison with Eurocode
Comparison with CSM
Comparison with DSM
Comparison with modified DSM
Proposed modified DSM

In Figure 4.19, the ultimate load normalized by the compressive load, Agfy, is plotted against the slenderness of the cross-section. In this section, the applicability of CSM (see Zhao et al., 2017) for the design of cold-formed YSt-310 structural steel columns for both non-slender and slender sections was evaluated. They observed a slightly conservative prediction of about ~ –16% for the non-slender section, although a fairly accurate prediction is seen for the slender section.

From Figure 4.23, it can be observed that ~50% of the data points are below the curve predicted by DSM for thin cross section fll 0.776, leading to non-conservative predictions. The resistance of the test and the FE column normalized by the pumpkin load are plotted against the cross-sectional slenderness fl in Figure 4.24.

SUMMARY

PERFORATED STUB COLUMNS

INTRODUCTION

EXPERIMENTAL INVESTIGATION

General
Test material
Stub column preparation
Local Geometric imperfection
Strain measurement
Stub column tests

RESULTS AND DISCUSSION

Local geometric imperfection

Unperforated plate elements
Perforated plate elements

Compression test results
Effect of d/w on the capacity of stub columns
Strain distribution at perforations location

FINITE ELEMENT ANALYSIS

Finite element modelling
Validation of FE perforated model
Parametric study
Effect of perforation size ratio on the ultimate capacity of stub column using

DESIGN METHODS FOR PERFORATED STEEL COLUMNS

Design equations by Shanmugam et al. (1999)
Design equations by Dhanalakshmi and Shanmugam (2001)
Design equations by Shanmugam and Dhanalakshmi (2001)
Design equations in AISI Standard

Determination of global buckling
Determination of local buckling

Design equation proposed by Miller and Peköz (1994)

Calculation of effective design width based on European,
Calculation of effective design width based on British

RELIABILITY ANALYSIS

DESIGN OF PERFORATED STEEL STUB COLUMNS

Assessment of current design methods for perforated steel members against

For perforation size ratio: 0.1 ≤ d/w ≤ 0.9

Assessment of current design methods for perforated steel members against

Proposed design equation

CONCLUSIONS

RESEARCH SUMMARY

Material characterisation

Ambient temperature tests
Elevated temperature material tests
Post-fire material tests

Unperforated stub column tests
Perforated stub column test

SUGESSIONS FOR FUTURE WORK

Extension to present research work
Other thoughts