General considerations

1.1 General considerations

In recent years, there have been significant developments in steel processing. Indeed, the improvements in industrial processes by the combination of rolling practices and cooling rates allowed the achievement of High Strength Steel (HSS) with very attractive properties.

Owing to the high performance, the use of HSS has a number of benefits in terms of economic, architectural, environmental and safety aspects in which the increase of strength allows a size reduction of the structural members, enabling potential benefits in terms of environments impact through energy saving and reduction of gas emissions.

The use of HSS in structural elements has been expanded to civil engineering structures.

The HSS has been successfully implemented in the automotive and ship industry due to the limitation of professional experience, standard with adequate information about the design and to a limited number of researches performed in order to investigate the use of HSS in civil structures. However, the increase in the use of HSSs for building construction allowed the creation of research programs in order to investigate the structural behaviour of these steels.

An European project has recently been started with the aim of investigating and evaluating the seismic performance of building frames using the dual-steel concept called HSS-SERF (High Strength Steel in Seismic Resistant Building Frames). The research group is composed by steel producers, design and consulting company, research institutions and universities with large and recognized experience in the field of proposed research.

The limited number of HSS applications in civil construction is also related to some lack of information regarding the behaviour and design rules in the codes. A working group within

CEN/TC250/SC3 has recently developed a proposal for Part 1-12 of Eurocode 3 (EN1993-1-12, 2007) to cover the design of steel structures using yield strengths between 460 MPa (S460) to 700MPa (S700). A new European standard with additional rules for the delivery condition of structural steel was also established in 2004 (part 6 of EN 10025:2004).

EN10025-6 states the technical delivery conditions for flat products of HSS in the quenched and tempered condition in which they are widely used in structural applications.

Concerning to costs, it is recognized that the price of HSS is superior in comparison with the Mild Carbon Steel (MCS), especially in Europe. For instance, the S690 steel grade, which it is produced in the Netherlands, is around 70-75 % more expensive than S355 per kg (Mercon Steel).

On the other hand, the cost of material represents only 25-30% of total costs of framed steel structures. The cost of fabrication and erection complete the overall costs. Therefore, an increasing of 20% of material cost would increase in about 5% the cost of the final structure (AISC, 2010).

Nevertheless, the market demands are still quite limited, and only a few fabricators are producing HSS. The trend, however, is that this scenario will change, and the increasing of market demand will result in decreasing of prices for HSS in the near future.

In Europe the use of HSS in bridges is already considered state-of-art, but the applications in buildings are few, especially in seismic zones. Some illustrative examples of its use in building application are following:

 Mafpre Tower in Barcelona, with 42 floors and a height of 150 m, where the columns of H-shaped steel grade S460M were used, resulting in a reduction of 24% of the weight of steel, compared to the S355 solution (see Figure 1.1);

Figure 1.1 – Mafpre Tower (1992)

 Europe Tower in Madrid, in which there is an inclination of 14°, in all structural elements using steel grade S460M (see Figure 1.2);

Figure 1.2 – Europa Tower – Madrid (1996)

 Tower Pleiades Brussels with the steel columns of S460N and a saving of 20%

compared to a project using steel S355J0;

 Conference Center "Espace Leopold" in the European Parliament in Brussels with composite columns of steel grade S420M and S355 in steel beams;

 The steel grade S460 was also used in the structure of Commerzt Bank in Frankfurt;

 In the Sony Center in Berlin, the S690 steel grade was used in structures elements and connections (see Figure 1.3);

Figure 1.3 – Overview of the roof structure (Sedlacek et al., 2005)

 The Airbus Hangar in Frankfurt is an example of application of steel TM S460ML constant yield up to 120mm thick;

Japan is more advanced in the use of HSS to seismic resistant buildings, and some notable buildings can be presented, all based on the dual-steel concept, i.e., columns with HSS, and, beams together with bracing made with MCS (Dubina et al., 2006).

 In Figure 1.4, Yokohama Landmark Tower located in Yokohama (with 70 floors) is an example of the use of HSS. The SA440 steel was used in the columns;

Figure 1.4 – Yokohama Landmark Yower (1993)

 JR EAST Head Office Building and Roppongi Hills Mori Tower Building located in Tokyo. Both used the SA440 steel in structural elements;

 Triton Square, also located in Tokyo, with 39 floors, used the SA440 steel in the columns.

In China, it can mention the World Financial Center as an example of the use of high strength steel, S460M class (see Figure 1.5).

Figure 1.5 – World Financial Center (2008)

In the USA, there are two examples of buildings that are composed of HSS: Hudson Colgate Center in Jersey, and Fremont building in San Francisco, in which the columns are steel type A919 grade 65 (equivalent to S460 steel) while other elements are formed by steel A919 grade 50 (similar to S355). In both cases, a reduction in weight of steel was approximately 20% when compared to a steel composed exclusively of class A919 grade 50 solution.

Based on the issued stated before, the seismic applications potentially represent the rational field to exploit the high performance of HSS. Indeed, according to modern codes the seismic

design of steel or composite buildings are based on the concept of dissipative structures, in which specific zones of the structures should be able to develop plastic deformation, mainly on ductile member, in order to dissipate the seismic energy. On the contrary, the non-dissipative zones and members should behaviour elastically under seismic action in order to avoid the brittle collapse of the building. For this reason, these zones should be designed to resist the full plastic strength of the dissipative members. Consequently, the large overstrength demands to non-dissipative zones lead to high material consumption, and sometimes, huge size of members to fulfil this design requirement.

Recent studies (Dubina et al., 2006; Dubina, 2008; Dubina, 2010) have highlighted the advantages of dual-steel concept, especially for what concerns the control of seismic response of multi-storey buildings to achieve overall ductility mechanism. In particular, Dubina (2010) showed the potential benefits of using HSS in full strength moment-resisting steel beam-to-column connections in order to guarantee the formation of plastic hinge in the beam and preserving both the connection and the column in MRFs. Some studies have also been conducted in Japan using MCS in dissipative elements and HSS in non-dissipative elements that must remain elastic especially during strong earthquakes – dual-steel concept (Takanashi et al., 2005).

This Thesis presents the results of a parametric study analysing the seismic behaviour of three structural systems using the dual-steel concept. In detail, the Moment-resisting Frame (MRF), Concentrically Braced Frame (CBF) and Dual-Concentrically Braced Frame (D-CBF) have been investigated. Relevant parameters that can influence the seismic response have been assessed, such as: the soil condition, type of composite steel-concrete column, length span, type of HSS employed in non-dissipative structural elements and the number of storey.

The examined frames have been designed in accordance with requirement preconized by EN1998-1 (2004). This code uses the capacity design approach, in which specific zones are responsible for energy dissipation under earthquake. Moreover, due to the absence of adequate information about the seismic design of the dual-system structures, the AISC-341 (2005) code has also been used in order to design the study cases with dual-system.

According to this code, the MRF subsystem should be provided with a minimum of 25%

of the total lateral strength of the structures.

Furthermore, a Performance Based Seismic Design has been used in order to investigate the benefits of dual-steel concept. In fact, the lessons learned from recent earthquakes regarding the costs of structural repair and disruptions of the functions of the buildings have evidenced a potential improvement of the current methodologies employed in actual codes. The

earthquake engineering community was able to observe that the control of damage caused by ground motion achieved its goal of life protection. However, the economic impact of excessive structural damage that disable the functionality of the building revealed to be quite high.

In this research, therefore, the seismic performance of study cases has been analysed through static and dynamic nonlinear analyses compared with three limit states as defined in EN1998-3 (2005): Damage Limitation (DL), Severe Damage (SL) and Near Collapse (NC).

After describing the results of numerical analyses, the discussion look for insights in the following issues: (i) quantification of performance parameters for each limit states, (ii) characterization of the behaviour factors in each limit states, (iii) comparison between dual-steel and single grade dual-steel, (iv) assessment of economic efficiency.

A numerical model has been implemented in order to perform both, nonlinear static and dynamic analyses. The models were developed using the Force-based (FB) distributed inelasticity elements. These elements account for distributed inelasticity through integration of material response over the cross-section and integration of the section response along the length of the elements. The cross-section behaviour is reproduced by means of the fibre approach, assigning a uniaxial stress-strain relationship in each fibre. In order to model the study cases, the SeismoStruct (2011) software has been utilized in this Thesis. The model is validated through experimental tests results. In addition, a comparison in economic and technical terms between a solution using MCS in the non-dissipative elements and HSS in these is carried out. Moreover, some guidance to design buildings using the dual-steel concept located in seismic zones have been proposed.

1.2 Objectives

This dissertation has the purpose of investigating the seismic behaviour of buildings using the dual-steel concept in Moment-resisting Frames, Concentrically Braced Frames and Dual-Concentrically Braced Frames focusing on the following topics:

 Validation of the proposed typologies for high/moderate seismicity zones, by advanced numerical simulations and consideration of different types of accelerograms;

 Development of design criteria and performance based design methodology for dual-steel structures using HSS. Criteria for assessment of ultimate building and prediction of the collapse mechanism;

 Recommendation of relevant design parameters (behaviour factor – q, overstrength factor Ω) to be implemented in further versions of EN1998-1 (2004) in order to apply capacity design approach for dual steel framing typologies.