Chapter 1 INTRODUCTION - The Sydney eScholarship Repository: Home

457 Pages·2000·4.16 MB·English

by  Tim Wilkinson

Chapter 1
INTRODUCTION
1.1 BACKGROUND
There has been increased use of hollow structural sections (HSS) in recent years. In particular,
rectangular hollow sections (RHS), square hollow sections (SHS), and circular hollow sections
(CHS) are used for their aesthetic appeal. Modern architecture in Australia is dominated by
tubular structures (eg Stadium Australia, Sydney Football Stadium, and Darling Harbour - all in
Sydney, Australia). However, the market for steel extends well beyond such famous examples.
While the cost of cold-formed hollow sections has been decreasing with advances in
manufacturing technology, the cost per tonne is still greater than that of hot-rolled open steel
products (eg I-beams). Hot-rolled open sections are more commonly used in industrial structures,
such as warehouses, garages and farm sheds.
Despite costing more per tonne than hot-rolled sections, hollow sections have properties (some
non-structural) which make them suitable for use in portal frame structures:
&
High torsional rigidity reduces the need for lateral bracing
&
Less surface area per length of member reduces painting and fire resistance costs
&
There is less possibility of unhealthy deposits accumulating within the structure in
agricultural applications (eg feathers in a chicken coop, or dust in a grain facility).
Plastic design is a method of design that can result in more economical structures compared to
structures designed by traditional elastic methods. Plastic design can provide the most benefit to
structures that resist load primarily by bending, and where deflections due to dead load are not
a significant design consideration. Portal frame structures (warehouses, garages and farm sheds)
can therefore benefit from plastic design.
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Plastic design methods were verified experimentally by tests on hot-rolled I-sections only
(Chapter 2), and consequently some steel design standards do not permit plastic design in cold-
formed hollow sections. In particular, cold-formed hollow sections have material properties
which are considerably different from hot-rolled sections, and fail to satisfy the material ductility
requirements for plastic design in some steel design standards.
There has been a significant research program at The University of Sydney since the 1980s into
the behaviour of cold-formed RHS (Section 2.8). The aim of the research has been to develop
appropriate design rules for cold-formed SHS and RHS for use in the limit states steel design
specification in Australia, AS 4100 (Standards Australia 1990a). Much of AS 4100 is now
applicable to cold-formed RHS, and one of the few remaining omissions is the use of cold-formed
RHS in plastic design. The main aim of this thesis is to investigate the suitability of cold-formed
RHS for use in plastic design.
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1.2 AIM OF THE THESIS
The primary aim of the thesis is to investigate the suitability of cold-formed rectangular
hollow sections for use in plastic design.
The main aim can be subdivided into further sub-sections:
1. To examine experimentally the element slenderness limits of cold-formed RHS for use in
plastic design, particularly the web slenderness limit, and any interaction between the web
and the flange, by means of simple beam tests.
2. To assess experimentally the suitability for plastic hinge formation of various connection
types for the beam-rafter knee joint in cold-formed RHS portal frames, and suggest
alternative connection designs.
3. To study experimentally the behaviour of cold-formed RHS portal frames under simulated
gravity and combined simulated gravity and wind loading conditions, specifically the
ability of the frames to form a plastic collapse mechanism and the rotation capacity
required to achieve the plastic collapse mechanism.
4. To simulate the behaviour of the RHS beam tests described in (1) via finite element
analysis, and extend the finite element analysis to consider the effects of imperfections,
varying material properties, and specimen dimensions not considered in the experimental
program.
5. To simulate the behaviour of the portal frame tests described in (3) via various types of
structural analysis
6. To review existing design recommendations and standards and compare the relevant
clauses with results obtained from (1) - (5) above and suggest improvements.
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1.3 OUTLINE OF THE THESIS
1.3.1 GENERAL
The main body of the thesis contains the introduction, a review of previous research, the
experimental and numerical (finite element) investigations, recommendations, and conclusions.
There are several appendices which include more comprehensive results.
1.3.2 EXPERIMENTAL INVESTIGATIONS
This majority of the work described in this thesis is experimental, and three series of tests were
carried out.
Chapter 3 describes a series of bending tests on a variety of cold-formed RHS. Forty four RHS
beams were tested under four point loading to examine their ability to form plastic hinges. Two
steel grades were used, Grade C350 (350 MPa nominal yield), and Grade C450 (450 MPa
nominal yield). The aim of the experiments was to establish a relationship between the rotation
capacity, and web and flange slenderness of the section. A simplified representation of the test
apparatus is shown in Figure 1.1
Loading ram
Spreader beam
RHS
Figure 1.1: Test Setup for Bending Tests
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Chapter 4 details a set of experiments on knee joints constructed from cold-formed RHS, using
the testing rig shown in Figure 1.2. A knee joint is the column to rafter connection in a portal
frame. Different types of joints were used to examine the ability of the joints to deform
plastically, primarily under either opening or closing bending moment, and a small amount of axial
force. Connections constructed from Grade C350 and Grade C450 steel were considered.
Loading ram
RHS specimen
Fixed DARTEC base
Figure 1.2: Test Setup for Connection Tests
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Tests of three portal frames are described in Chapter 5. The portal frames were constructed from
cold-formed RHS and were large scale, being seven metres wide, and four metres high at the
apex, as shown in Figure 1.3. Three frames were tested under downwards vertical and a
transverse load. Frame 1 was constructed from Grade C350 steel, while Frames 2 and 3 were
constructed from Grade C450 RHS. The experimental behaviour is compared to several different
forms of structural analysis.
MTS actuator
Gravity load simulator Horizontal load cradle
Figure 1.3: Test Setup for Portal Frame Tests
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1.3.3 NUMERICAL (FINITE ELEMENT) ANALYSIS
Chapter 6 describes finite element analyses simulating the RHS beam tests in Chapter 3. The
finite element analyses were performed using the commercial program, ABAQUS. The stages
in the development of the model are described. The main aim of the finite element analysis was
to establish significant trends in the behaviour of cold-formed RHS beams, which could be used
to help extrapolate and interpolate the experimental results. Part of a typical finite element mesh
is shown in Figure 1.4.
Loading plate
Weld
RHS
Figure 1.4: Part of a Typical Finite Element Model
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Chapter 2
INTRODUCTION TO PLASTIC DESIGN, LITERATURE
REVIEW, AND CURRENT DESIGN STANDARDS
2.0 CHAPTER SYNOPSIS
The following chapter provides an introduction to plastic design of steel frames, and examines the
relevant previous research in plastic design, including aspects such as local buckling, slenderness
limits, and behaviour of knee joints in portal frames.
In particular, this chapter investigates the development of the element slenderness (b/t) limits in
various steel design specifications. There are inconsistencies in the development of the Class 1
slenderness limit for webs in the AISC LRFD Specification.
The limits for webs in bending have been based on investigations of I-sections, but are applied to
both I-sections and RHS. While many researchers have indicated that there is interaction between
the flange and web of I-sections, which affects local buckling and rotation capacity, all current
steel design codes prescribe flange and web slenderness limits which are independent of each
other.
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