Development of inelastic design procedures for stainless steel indeterminate structures

Lead Research Organisation: University of Birmingham
Department Name: Civil Engineering

Abstract

The excellent atmospheric corrosion resistance and favourable mechanical properties of stainless steel make it well suited for a range of structural applications, particularly in aggressive environments or where durability and low maintenance costs are crucial design criteria. The main disadvantage hindering the more widespread usage of stainless steel in construction is its high material cost and price volatility. However, life-cycle costing and sustainability considerations make stainless steels more attractive when cost is considered over the full life of the project, due to the high potential to recycle or reuse the material at the end of life of the project.

The design of stainless steel structures is covered by a number of international design codes, which have either recently been introduced or or were recently amended in light of recent experimental tests, thus indicating the worldwide interest stainless steel has received in recent years. Despite the absence of a well-defined yield stress, all current design standards for stainless steel adopt an equivalent yield stress and assume bilinear (elastic, perfectly-plastic) behaviour for stainless steel as for carbon steel in an attempt to maintain consistency with traditional carbon steel design guidance.Given the high material cost of stainless steel, improving the efficiency of existing design guidance is warranted. Improvements can be made either by calibrating the existing design procedures, some of which are based on engineering judgment and limited test data, against additional experimental results, or by devising more accurate design approaches in line with actual material response. In any case more efficient yet safe design rules are desirable.

The majority of published research articles on stainless steel structures focus on the response of individual members. Due to insufficient relevant experimental data, no rules are given for plastic global analysis of indeterminate stainless steel structures in any current structural design code, even though the ductility of stainless steel is superior to that of ordinary structural steel. The controversy of not allowing plastic design for an indeterminate structure made of a ductile material is obvious in Eurocode 3:Part 1.4 where it is explicitly stated that "No rules are given for plastic global analysis" even though a slenderness limit for stocky elements is specified in the same code. Moreover, because of the lack of relevant experimental data for stainless steel frames, no specific design provisions to account for second order effects in stainless steel frames are specified in any stainless steel design code. Deficiencies in current design guidance puts stainless steel at a disadvantage compared to other materials thereby hindering its use in applications where it might be the preferred solution, had the design standards not imposed strict restrictions to its design due to a gap in current knowledge.

The proposed project aims at investigating the structural response of stainless steel indeterminate structures and developing appropriate design rules, by means of experimental studies on two-span continuous beams, as well as portal frames and advanced numerical analyses. Experimental results, suitable for the validation of numerical models, will be generated and will allow a rigorous study of the ultimate response of indeterminate stainless steel structures. The accuracy of current design procedures (i.e. not allowing for plastic design or partial moment redistribution in indeterminate structures) will be assessed and the possibility to apply plastic design to stainless steel structures will be explored. It is envisaged that the proposed project will lead to design rules suitable for incorporation in EN 1993: Part 1.4.

Planned Impact

This research proposal aims at addressing shortcomings in current stainless steel structural design codes thereby enabling designers to make informed decisions when selecting a structural material for a given project and promoting the use of stainless steel in construction with profound economic and environmental benefits. It is envisaged that the outcomes of the project will enable design code amendments thus rendering Eurocodes the most technically advanced design standards and consolidating the internationally leading position of UK research. The project is expected to generate significant short- and long-term benefits for a number of beneficiaries. These include:

1. The international academic community, which will benefit from both the experimental and numerical results. The results of the project will be reported in detail and allow other aspects of underpin future studies into the robustness of stainless steel structures and structures employing material with a highly nonlinear response.
2. The UK construction industry, which is both a major contributor to UK GDP and also a major contributor to CO2 emissions. As a result of the project, awareness of the benefits of stainless steel in construction will be raised and safe and economic design procedures will be generated. Increased use of stainless steel as a result of more advanced design guidance will be promoted thereby helping the construction industry to reduce its carbon footprint.
3. The PDRA and his/her future employers will significantly benefit from the experience that the PDRA will gain working on the project. The mentor allocated to the PDRA and the whole research environment in the University of Birmingham will nurture a talented researcher enabling him/her to take senior academic or industrial posts in his/her future career.
4. Current and prospective civil engineering students will greatly benefit from the project. Current MEng and MSc students will have the opportunity to engage in research via suitable research projects within the framework of the project. The findings of the research will inform the teaching of relevant structural engineering modules thereby delivering a long-lasting benefit for prospective students and contributing to their training.
5. School children will benefit from the dissemination of project results in university open days and during school visits. The project will serve as a showcase for structural engineering research and practice and can potentially help to raise awareness on the importance of civil/structural engineering and render it an attractive choice for the children's future studies.
6. Finally, the wider society will benefit significantly from the improvement of current design procedures and the promotion of a sustainable material in the construction industry, which will lead to reduced CO2 emissions.

Publications

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Gkantou M (2019) Plastic design of stainless steel continuous beams in Journal of Constructional Steel Research

 
Description A series of experimental and numerical studies on stainless steel beams were executed as part of this project. The obtained experimental and numerical results on the stainless steel continuous beams along with all available test data collated from the literature were used to assess the accuracy of the design procedure specified in current design codes (EN 1993-1-4) and to explore the possibility of plastic design for stainless steel indeterminate structures, i.e. allowing for the real material response instead of the assumed elastic-perfectly plastic one that is assumed customarily in design provisions for convenience
1) current design guidance is overly conservative , in some cases the structures can resist more than 2 times the load specified by the design code.
2) The level of conservatism depends on cross-sectional slenderness, i.e. structures with thicker corss-sections are more likely to be under-predicted by design standards
3) The level of conservatism depends on support conditions and load application, i.e. the current design procedure is not only conservative but also inconsistent as it allows for different safety margins for different structural arrangements
4) Accounting for both the strain-hardening (material response) and moment redistribution (system response) is of paramount importance for the design of stainless steel continuous beams. Both are currently not considered in design standards for stainless steel structures, the latter is allowed by design standards for ordinary structural steel only.
5) The continuous strength method is therefore proposed as a rational design method for indeterminate stainless steel structures with stocky cross-sections.

A series of tests on stainless steel fixed based frames was conducted and accompanying numerical studies were executed. Key findings include:
1) the first ever tests to be conducted on stainless steel frames
2) a systematic numerical study which allows the effects of strain-hardening at cross-sectional level, moment redistribution at system level, section slenderness and member slenderness to be appraised
3) Current design guidance was proved insufficient
4) The continuous strength method in conjunction with the well-established Merchant-Rankine design approach is proposed as a means to account for both sway and strian-hardening in the design of stainless steel frames.
The overarching key finding is that both financial and environmental savings can be achieved if full utilisation of the actual material response of stainless steel is incorporated in the design procedure. This will results in less material consumption without compromising accuracy, and most importantly, a uniform safety margin for all structures instead of the currently random variations in safety margins brought about by the inability of the deign methods to properly account for the observed response.
Exploitation Route The published datasets and freely available research publications stemming from this research project can be freely downloaded and utilised by anyone, researchers can valdiated their own nuemrical models against them and conduct their own studies. Furthermore the design standards drafting bodies can be supported in the creation of next generation design standards as a sufficient base of evidence exists that justifies the explicit incorporation of both strain-hardening and moment redistribution in design provisions. Further research is underway and further funding is seeked to tackle the next obstacle int he incorporation of strain-hardening in design: the design of stainless steel connections. The applications are relevant primarily to the construction with possible secondary applications in the energy sector (offshore energy sector)
Sectors Construction,Manufacturing, including Industrial Biotechology

URL https://edata.bham.ac.uk/167/
 
Description UKIERI
Amount £35,500 (GBP)
Organisation British Council 
Sector Charity/Non Profit
Country United Kingdom
Start 04/2018 
End 03/2020
 
Title Research data supporting "Plastic design of stainless steel continuous beams" 
Description he results of an experimental study on eight simply-supported and four two-span continuous beams employing austenitic and duplex stainless steel rectangular hollow sections (RHS) are reported. In parallel with the tests, finite element models were developed. Upon validation against the experimental results, parametric studies were conducted to expand the available structural performance data over a range of cross-section slendernesses, structural systems and load configurations likely to occur in practice. The obtained experimental and numerical results along with collated test data were used to assess the accuracy of EN 1993-1-4 design provisions and to explore the possibility of plastic design for stainless steel indeterminate structures, simultaneously accounting for the effect of strain-hardening at cross-sectional level and moment redistribution exhibited by structures employing stocky cross-sections. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact Too early to tell 
URL https://edata.bham.ac.uk/167/
 
Description Collaboration between University of Birmingham and Wuhan university on stainless steel structures 
Organisation Wuhan University
Country China 
Sector Academic/University 
PI Contribution Through expertise gained from this project, my research team has collaborated with a research team from Wuhan university on the behaviour of stainless steel beams and stainless steel connections. The collaboration initiated at the Stainless steel experts seminar where findings from my funded project were presented to the technical community and industrial experts on stainless steel structures. Realising that wer are working towards similar goals, we provided our research partner with both experimental and numerical results that resulted so far into 4 top tier journal publications and a Newton International Fellowship application to further strengthen our collaboration.
Collaborator Contribution Our partners form Wuhan university have provided us with both state of the art experimental facilities where instrumentation not currently available at my own institution was used to monitor the initial imperfections and initial stresses of the studied components.
Impact Yuan ?.?., Chen X.W., Theofanous M., Wu Y.W., Cao T.Y. and Du XX. (2019). Shear behaviour and design of diagonally stiffened stainless steel plate girders. Journal of Constructional Steel Research 153, 588-602. Yuan HX, Hu S, Du XX, Yang L, Cheng XY and Theofanous M. (2019) Experimental behaviour of stainless steel bolted T-stub connections under monotonic loading. Journal of Constructional Steel Research 152, 213-224. Elflah M., Theofanous M., Dirar S. and Yuan HX. (2019). Behaviour of stainless steel beam-to-column joints - Part 1: Experimental investigation. Journal of Constructional Steel Research 152, 183-193. Elflah M., Theofanous M., Dirar S. and Yuan HX. (2019). Structural behaviour of stainless steel beam-to-tubular column joints. Engineering Structures 184, 158-1785
Start Year 2017
 
Description Stainless steel in structures FIFTH INTERNATIONAL EXPERTS SEMINAR 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact The Stainless Steel in Structures Fifth International Experts Seminar was organised by The Steel Construction Institute and held in London on 18-19 September 2017. The
purpose of the Seminar was to provide a forum where researchers could share recent research into the behaviour of structural stainless steel and discuss advances in the development of national and international structural design standards. Technical experts attended the Seminar from India, China, Brazil, Australia, New Zealand, USA and many European countries. It was clearly evident from the seminar that recent research undertaken around the world has generated a huge amount of useful data, which is enabling the development of more comprehensive and less conservative structural design standards.
Year(s) Of Engagement Activity 2017
URL http://www.steel-stainless.org/experts17