Seismic Design of Aluminium Structures in the Second Generation of Eurocode 8 ^†

Abstract

Eurocode 8 is currently under revision, and their amended versions will be available in the next few years. One of the major novelties of the second generation of Eurocode 8 is a specific chapter for the seismic design of aluminium buildings (i.e., Chapter 15) that is missing in the current EN1998-1:2005. Thus, the second generation of Eurocode 8 is the first European set of rules for seismic-resisting aluminium structures. These new rules are mainly based on the current Japanese recommendations, but adapted to Eurocode philosophy. The main novelties of the second generation of Eurocode 8 and the requirements for seismic-resistant aluminium structures are summarised in the present article.

1. Introduction

Aluminium structures are typically used in cases where the lightness, corrosion resistance, and functionality of structural shapes are required [1,2]. In European practice, aluminium constructions are usually for (i) long-span roof systems (in which live loads are small compared with dead loads, as in the case of 3D lattice floor, geodetic domes, long span roofing); (ii) crossing structures (e.g., foot bridges, temporary bridges, military bridges, etc.); (iii) and light-weight constructions in chemically aggressive environments (in which durability and no maintenance are required, as in the case of off-shore constructions, helidecks, portal frames for traffic signs, antennas, electrical transmission and lighting towers, etc.) [3]. Therefore, aluminium alloys are the rational solution any time durable light weight constructions are of utmost importance and, consequently, reliable and ease-of-use design methods are of great importance [3,4,5]. It is also worth noting that aluminium alloys have been largely used in seismic-resistant structures for dissipative fuses and/or hysteretic dampers [6,7,8].

The design rules of aluminium structures are commonly based on those of steel structures [8]. However, due to the mechanical properties of aluminium alloys, significant differences can be recognised in terms of what constitutes the classification of members, the stability checks, and the resistance of the connections. In fact, it is well known that the stress–strain constitutive law of aluminium alloys is well modelled by Ramberg–Osgood law, so that the main material parameters are the conventional elastic limit, namely “f0.2”, corresponding to 0.2% plastic strain and the hardening exponent “n”, which can lead to very high strain-hardening effect [9]. However, aluminium alloys are also characterised by high deformability due to an elastic modulus equal to about one-third of the steel one. Therefore, aluminium members and structures may be theoretically more sensitive to local and global instability, higher sensitivity to thermal variations than steel structures, and potential high-strength reduction effects into the heath-treated zones in welded sections [10,11,12]. In addition, the buckling behaviour of aluminium plates and the local buckling of aluminium members differ from those occurring in steel elements due to hardening effects, as well as the different role of mechanical imperfections [13,14,15,16,17]. Therefore, in steel sections classified as slender, the local instability phenomena in compressed parts is analytically taken into account by means of an effective width, while reducing the thickness of the compressed parts is the convenient approach for aluminium elements [13].

In Europe, the design rules for aluminium constructions are currently given in EN1999-1-1 (also commonly referred to as Eurocode 9) [18], which mainly covers the general cases of design against static loads, and neither guidance nor recommendations are provided for the seismic design of such structures. However, specific rules and requirements for seismic-resistant aluminium structures have recently been drafted for the next generation of EN1998-1 (also commonly referred to as Eurocode 8) [19]. In fact, the first generation of all structural Eurocodes is currently under revision and amendment. This process has been regulated according to the mandate M515 “Evolution of the Structural Eurocodes”, and coordinated by the technical committee CEN/TC250 with the aim of revising and updating the existing rules, adding regulations for new systems (e.g., the aluminium structures), and improving the clarity and the ease of use of the codes.

The CEN/TC250 is organised into sub-commissions (SC) in charge of revising each Eurocode. In addition, specific working groups (WG) are established to support the relevant SC within specific competencies, while the Project Teams (PT) are devoted to drafting the normative documents. For what concerns the set of seismic codes, the subcommittee TC250/SC8 is dealing with the amendment of Eurocode 8 [19], and SC8/PT2 is the project team that drafted the material-dependent rules of the new version of Eurocode 8. The revision and update of rules for steel, composite, and aluminium structures are also supported by the SC8/WG2, which acts in synergy with the Technical Committee “TC13—seismic design of steel structures” of the European Convention for Metallic Constructions (ECCS) [20].

In the second generation of Eurocode 8, the former EN1998-1 has been split into two sub-parts, namely prEN1998-1-1, covering the material-independent rules (e.g., general philosophy, seismic actions, methods of analyses, criteria of regularity, etc.) [21], and prEN1998-1-2, covering the material-dependent rules (e.g., detailing rules and requirements for all structural materials) [22]. In particular, specific rules for the seismic design of aluminium structures are given in Chapter 15 of the latest draft of prEN1998-1-2 [22]. This set of rules is mainly based on the Japanese recommendations for the seismic design of aluminium structures developed by the Architectural Institute of Japan [23], as well as being strongly influenced by the standard provisions adopted for steel structures.

In this paper, the organisation of the second generation of Eurocode 8 is presented, also introducing the main novelties related to the general (material-independent) rules. The specific regulations for aluminium structures are discussed in Section 3, with a particular focus on material properties, ductility classes, behaviour factors, and verifications at limit states.

It is also worth noting that, at the present time, the new Eurocode 8 has not been yet formally completed; therefore, the rules discussed in the following refer to the latest draft of Part 1-1 [21] and Part 1-2 [22] of prEN1998, and further modifications concerning marginal aspects cannot be excluded.

2. The Next Eurocode 8

The organisation of the current EN1998-1:2005 [19] has been substantially revised; general rules and seismic actions have been collected in the prEN1998-1-1 [21], and specific rules for buildings have been provided by the prEN1998-1-2 [22].

The main novelties of the prEN1998-1-1 [21] are related to (i) the behaviour factors, (ii) seismic action classes, and (iii) ductility classes. In fact, the behaviour factor q has been defined as follows:

$$q=q_S·q_R·q_D$$

(1)

where q_S accounts for the design overstrength, q_R considers the redundancy of the system, and q_D depends on the ductility of the structure. The default upper limit values for regular structures q_R and q_D are provided in prEN1998-1-2 [22] as a function of the material, the structural type, and the ductility class, while q_S is assumed to be equal to 1.5 for all cases.

The seismic action in current EN1998-1 [19] depends on the reference peak ground acceleration on bedrock agR, while prEN 1998-1-1:2022 [21] introduces the “seismic action classes” that depend on the seismic action index Sδ, which is intended to be the pseudo-acceleration at the plateau of the elastic spectrum, which depends, in turn, on the consequence class (CC), the short-period amplification factor, the soil topography, and the maximum response spectral acceleration Sα,475 (5% damping, site category A, for the return period Tref; CC2).

Table 1. Seismic action classes according to prEN 1998-1-1:2022.

Seismic Action Class	Reference Spectral Accel. Sα,475	Seismic Action Index Sδ
High	Sα,475 ≥ 5.0 m/s2	Sδ ≥ 6.50 m/s2
Moderate	2.5 m/s2 ≤ Sα,475< 5.0 m/s2	3.25 m/s2 ≤ Sδ < 6.50 m/s2
Low	1.0 m/s2 ≤ Sα,475 < 2.5 m/s2	1.30 m/s2 ≤ Sδ < 3.25 m/s2
Very low	Sα,475 < 1.0 m/s2	Sδ < 1.30 m/s2

summarises the four seismic action classes defined in prEN 1998-1-1:2022 [21] and the ranges of the corresponding seismic action index Sδ.

Concerning the ductility class (DC), the new Eurocode 8 keeps the distinction of three DCs as in current EN1998-1:2005, but their meaning and field of use have been substantially revised. In fact, the current EN1998-1:2005 considers the use of ductility class low (DCL) solely in very low seismic class (see Table 1 to recognise the maximum allowed spectral acceleration), and design criteria and the limits of application of medium (DCM) and high (DCH) ductility class are the same. On the contrary, the prEN1998-1-1 [21] specifies different performance objectives, design criteria, and limits of application (expressed in terms of seismic action index limit Sδ, lim, DCi) per ductility class, whose definition has been also changed as follows: (i) DC1 (low ductility), (ii) DC2 (low-moderate/medium ductility), and (iii) DC3 (high ductility). In DC1, the system is characterised by pseudo-elastic behaviour; the overstrength capacity is considered, while the deformation capacity and energy dissipation capacity are disregarded. Therefore, the behaviour factor is assumed to be equal to 1.5 for all materials and structural systems. Concerning the design method, solely the general rules contained in prEN 1998-1-1 [21] and in chapters 4 and 5 of prEN 1998-1-2 [22] should be applied. Therefore, no capacity design criterion is considered, and structural members and connections are verified according to the relevant Eurocode for the static design of the material used for the structure (e.g., EN1999-1-1 for the case of aluminium structures). In DC2, the local overstrength capacity, the local deformation capacity, and the local energy dissipation capacity are considered. Simplified capacity design rules are provided to control the local ductility demand because the design procedure is not aimed at enforcing a global yielding mechanism. Therefore, in DC2 the recommended behaviour factors do not exceed 3.5 due to the limited ductility of the structural systems.

In DC3, the structure is designed to guarantee the activation of a global plastic mechanism. Therefore, the local overstrength and deformation capacities are considered, and stringent global and local capacity design rules are recommended. Consistently, larger values of the upper bound behaviour factor are allowed but not greater than 6.5 in the most favourable cases (e.g., multistorey steel moment frames). No seismicity limit for the use of DC3 is provided.

3. Seismic Design of Aluminium Structures According to prEN 1998-1-2

3.1. Generality

Chapter 15 of prEN1998-1-2: 2022 [22] is specifically devoted to the seismic design of aluminium buildings, thus covering specific rules for materials, connections, and capacity design as specified hereinafter.

3.2. Material Properties

Section 15.3 of prEN 1998-1-2 specifies the material properties to be used for seismic applications, as well as recommending the alloys and the tempers to be used for the primary elements of the seismic resisting systems. The selection of the proper materials depends on their mechanical properties, whose characterisation for seismic application is a crucial issue because the alloys should guarantee adequate low cycle fatigue, cyclic hardening, dissipated energy, and plastic fracture. However, limited studies have been performed on this topic. Hopperstad et al. [24] performed uniaxial cyclic tests on extruded bars made of 6060-T4 and T5 alloys and they provided an improvement of the cyclic plasticity model to consider the Bauschinger effect. Srivatsan [25] performed low-cycle fatigue tests of the 7055 aluminium alloy that showed evidence of cyclic softening to failure. Borrego et al. [26] carried out low-cycle fatigue tests on 6082-T6 and 6060-T6 aluminium alloys. These experiments showed that the 6060-T6 alloy exhibits nearly ideal Masing behaviour, while the 6082-T6 one presents significant deviations from the Masing model. Yahya et al. [27] performed constant amplitude cyclic tests of AA6063 aluminium alloy at room temperature that showed a decrease in fatigue life when strain amplitude increased. Dusicka and Tinker [28] investigated the low cyclic response of the 6061-T6511 at 2% constant strain amplitude, which was characterised by a satisfactory cyclic softening behaviour. More recently, Pisapia et al. [29] performed a comprehensive experimental campaign on 6060, 6082, and 7075 alloys to evaluate the random material variability and cyclic hardening factors to be used for the capacity design.

In light of this concise overview of the state of the art, it is rather clear that further research would be necessary to characterise the seismic behaviour of the structural alloys and temper. However, the Japanese recommendation for the seismic design of aluminium structures [23] provides a list of seismically qualified alloys and tempers that was experimentally validated at a national level. Therefore, the pr EN1998-1-2 [22] recommends material conforming to the AIJ list for the dissipative elements of ductile aluminium structures (i.e., in DC2).

Table 2. Aluminium alloys for dissipative parts of DC2 seismic resisting frames.

Structural Element	Product Form	Alloy	Temper	Thickness
Sheet, strip, and plate	_	5052	H12|H22/H32	≤40
		5049	O/H111	≤100
		5083	O/H111	≤80
		5383	O/H111	≤120
			H116/H321	≤80
		5454	O/H111	≤80
		5754	O/H111	≤100
		6061	T4/T451	≤12.5
		6082	T4/T451	≤12.5
Extruded profiles, extruded tube, extruded rod/bar, and drawn tube	ET, EP, ER/B	5083	O/H111 F/H112	≤200
	ET, EP, ER/B	5454	O/H111 F/H112	≤25
	ET, EP, ER/B	5754	O/H111 F/H112	≤25
	DT	6060	T6	≤20
	EP, ET, ER/B		T64	≤15
	EP, ET, ER/B	6061	T4	≤25
	DT		T4	≤20
	EP, ET, ER/B	6082	T4	≤25
Legend: EP—Extruded profiles ER/B—Extruded rod and bar		ET—Extruded tube DT—Drawn tube

summarises the permitted alloys for dissipative parts of DC2 systems. On the contrary, the alloys to be used for DC1 structures and DC2 non-dissipative elements may conform to EN1999-1-1 [18]. It is also worth noting that alloys different from those specified in Table 2 may be used, provided that the ratio fu/f0 is not smaller than 1.10 and the elongation at failure is not smaller than 10%, where fu is the ultimate tensile strength and f0 is the conventional elastic strength.

3.3. Structural Types, Ductility Classes, and Behaviour Factors

The prEN1998-1-2 [22] provides rules for the structural systems shown in Figure 1, namely (a) moment-resisting frames, (b) concentric bracings, (c) dual frames (in which at least 25% of the lateral force resistance is provided by moment-resisting frames that are combined with frames with concentric bracings), and (d) inverted pendulum. Eccentrically braced frames are not permitted as aluminium seismic-resistant systems in accordance with Japanese recommendations due to the limited evidence on the effectiveness of the stiffening details of aluminium links that may not guarantee adequate ductility [23].

These structural systems can be designed in either DC1 or DC2, while DC3 is not allowed. However, differently from all other structural materials, the DC2 can be adopted without any limitation to the seismic action index Sδ (see Figure 2).

The upper bound values of the behaviour factor q (as well as its relevant contributions qR and qD) for DC2 aluminium buildings are reported in

Table 3. Default values of behaviour factors for aluminium buildings.

Structural Type	DC2
	qD	qR	q
Moment-resisting frames (MRFs)
Single-storey MRFs	1.5	1.1	2.5
Multi-storey MRFs	1.5	1.3	3.0
Frames with concentric bracings	1.5	1.0	2.3
Diagonal bracings
V-bracings
X-bracings on either single- or two-storey
Dual frames (MRFs with concentric bracing)	1.7	1.2	3.0
Inverted pendulum	1.3	1.0	2.0

.

3.4. Verification to Limit States

In addition to the Japanese background [7], the seismic design rules for aluminium buildings developed within Chapter 15 are mainly based on the homologous requirements provided for the steel systems; the latter have, in turn, been updated and amended in many aspects, in light of recent research findings and critical issues widely encountered by the scientific community [30,31,32,33,34].

3.4.1. Deformation-Related Requirements

The control of lateral displacements is mandated at the significant damage limit state to mitigate damage. Therefore, the interstorey drift d_r,SD at the SD limit state should be limited as follows:

−

d_r,SD ≤ 0.020 h_s for moment frames;

−

d_r,SD ≤ 0.015 h_s for frames with concentric bracings, for dual frames, for inverted pendulum structures and for all other structural types.

where h_s is the interstorey height.

In prEN1998-1-1 [21], the sensitivity to the second order (P-Δ) effects should be verified by controlling the stability coefficient that is evaluated considering the secant stiffness at the maximum capacity of the idealised pushover response of the building [20,30,31]. Therefore, the stability coefficient is evaluated as follows:

$$\theta =\frac{P_{\mathrm{t}\mathrm{o}\mathrm{t}}{d}_{\mathrm{r},\mathrm{S}\mathrm{D}}}{{q_R{q}_{S}V}_{\mathrm{t}\mathrm{o}\mathrm{t}}h}$$

(2)

where P_tot is the total gravity load at and above the storey, due to the masses considered in the seismic analysis of the structure; d_r,SD is the design interstorey drift at the SD limit state; V_tot is the total storey shear in the seismic design situation; h is the interstorey height; q_S is equal to 1.5, and q_R is reported in Table 3.

3.4.2. Design Rules for Dissipative (DC2) Aluminium Structures

The design rules for dissipative aluminium systems have been borrowed from those of steel structures and updated on the basis of the Japanese recommendations [23].

Regarding the requirements to enhance the local ductility, cross-sectional class 1 in accordance with prEN1999-1-1 is recommended for the members which dissipate energy in compression or bending. For tension members or parts of members in tension, the ductility requirement of prEN1999-1-1 should be satisfied. The unique source of plasticity should be provided by the members. Therefore, non-dissipative connections should be designed to meet the following condition:

$${{R_{\mathrm{d}} \ge \omega }_{\mathrm{r}\mathrm{m}}{ \omega }_{\mathrm{s}\mathrm{h}} R}_{\mathrm{f}\mathrm{o}}$$

(3)

where

• R_d is the resistance of the connection in accordance with prEN 1999-1-1;
• R_fo is the nominal plastic resistance of the connected dissipative element;
• ω_rm is the material overstrength factor in the dissipative zones, which is assumed equal to 1.45 unless the National Annex gives a different value for use in a country;
• ω_sh is the hardening factor in the dissipative zones, taken as 1.3 or the value calculated in accordance with EN1999-1-1, whichever is greater for elements in plastic bending. For elements in plastic tension it is equal to 1.5 or the ratio $\frac{f_{\mathrm{u}}}{f_{\mathrm{o}}}$, whichever is the greater.

In addition, in the case of bolted joints, category C in shear and category E in tension in accordance with EN1999-1-1 should be designed for the connections of dissipative members.

For what concerns the global ductility, capacity design rules should be applied in order to prioritise the yielding in the dissipative elements. Therefore, the design effects (i.e., N_Ed, M_Ed, V_Ed) of non-dissipative members in DC2 should be calculated as follows:

$$\begin{gathered} N_{Ed}=N_{Ed,G} “+” {\Omega ·N}_{Ed,E } \\ {M}_{Ed}=M_{Ed,G} “+” {\Omega ·M}_{Ed,E} \\ {V}_{Ed}=V_{Ed,G} “+” {\Omega ·V}_{Ed,E} \end{gathered}$$

(4)

where

• N_Ed,G, M_Ed,G, and V_Ed,G are the axial force, the bending moment, and shear force in the non-dissipative member due to the non-seismic actions included in the combination of actions for the seismic design situation, respectively;
• N_Ed,E, M_Ed,E, and V_Ed,E are the axial force, the bending moment, and shear force in the non-dissipative member to the design seismic action;
• Ω is the action magnification factor given according to

  Table 4. Values of seismic action magnification factor Ω in DC2.

  STRUCTURAL TYPE	Ω	Members to Which (Equation (4)) Apply
  Moment-resisting frames (MRFs)
  Single-storey MRFs	1.8	columns
  Multi-storey MRFs	2.0
  Frames with concentric bracings	1.5
  Diagonal bracings		beams and columns
  V-bracings
  X-bracings on either single or two-storey
  Dual frames (MRFs with concentric bracing)	2.0	beams and columns of the concentric bracing; columns of the MRF
  Inverted pendulum	1.5	columns

  .

4. Conclusive Remarks

The overview of the rules and requirements for the seismic design of aluminium buildings in the latest draft of the second generation of Eurocode 8 is summarised in this paper. These rules are reported in Chapter 15 of prEN1998 (2022) and represent a major novelty of the new Eurocode 8, also being the first set of rules in Europe for the seismic design of aluminium structures.

The code-drafting process has been carried out under the mandate M515 “Evolution of the Structural Eurocodes” and coordinated by the technical committee CEN/TC250 with the aim of revising and updating the existing rules, adding regulations for new systems (e.g., the aluminium structures), and improving the clarity and the ease of use of the codes. The subcommittee TC250/SC8 deals with the amendment of Eurocode 8 [19], and SC8/PT2 is the project team that drafted the material-dependent rules of the new version of Eurocode 8. The revision and update of rules for aluminium structures has also been supported by the SC8/WG2, which acts in synergy with the Technical Committee “TC13—seismic design of steel structures” of the European Convention for Metallic Constructions (ECCS) [20].

The novel rules for seismic design for aluminium structures provide recommendations for the selection of alloys for dissipative and non-dissipative elements, the ductility classes, the behaviour factors, the verifications at limit states, and the hierarchy of resistances. These rules and requirements do not cover any structural types, and there are wide margins for improvements and investigations. Therefore, further experimental and numerical studies on the local and global seismic response are deemed necessary to assess the effectiveness of the new codes, especially because some rules have been directly adapted from those of steel structures.

At the present time, the new Eurocode 8 has not yet been formally completed, even though it is in an advanced stage. Therefore, further modifications concerning marginal aspects of the rules discussed in this paper cannot be excluded.