SEISMIC DESIGN OF A SIX-STOREY CLT BUILDING IN FLORENCE, ITALY Davide Vassallo1, Maurizio Follesa2, Massimo Fragiacomo3 ABSTRACT: This paper illustrates the design and construction of a six-storey residential CLT building in Florence, Italy. All the different stages of the project are described, with a detailed description of the structural, fire and seismic design but considering also production, transportation and construction issues. The seismic design was carried out following the capacity design rules given for CLT buildings in a recent proposal of revision of Chapter 8 of Eurocode 8. A detailed discussion is also presented about the solutions used for the uplift restraint connections, especially concerning the concrete side design strength.

KEYWORDS: Multi-storey timber buildings, CLT, Linear analyses, Eurocode 8. 1 INTRODUCTION 123

Timber systems are becoming very popular in the construction of medium to high rise buildings in different seismic areas of Italy. The capacity of storing carbon dioxide by displacing it from the atmosphere; the reduced energy consumption during production, transportation and erection compared to other construction materials; the good thermal properties of wood which, together with the use of insulation materials, lead to excellent rates in the energy performance of the overall building; and the speed of erection due to a completely dry construction process are some of the reasons for this increased use of timber, particularly for school and residential buildings. Furthermore, the excellent seismic performance is another significant advantage of timber construction, as was demonstrated by recent research results based on extensive numerical simulations and full-scale tests on multi-storey buildings [1], [2]. These reasons led to the construction of a large number of multi-storey timber buildings in Italy in the last ten years, most of them made with the cross-laminated (CLT) system. The highest CLT buildings are four 9-storey residential buildings in Milan, a low seismicity area (Figure 1).

1 Davide Vassallo, dedaLEGNO, Firenze, Italy, vassallo@dedalegno.com 2 Maurizio Follesa, dedaLEGNO, Firenze, Italy, follesa@dedalegno.com 3 Massimo Fragiacomo, Department of Civil, Construction-Architectural and Environmental Engineering, University of L’Aquila, Italy, massimo.fragiacomo@univaq.it

Figure 1: Four 9-storey CLT residential buildings built in Milano, 2012. Courtesy of Rossiprodi Associates, Florence, Italy.

The project of the six-storey residential CLT building presented in this paper is part of a larger project for the construction of two residential buildings: a four-storey (3 storeys of CLT on top of a reinforced concrete podium) designed for 6 apartments and a six-storey entirely CLT building designed for 39 apartments for social housing, managed by the public housing authority of Florence, Italy. In this paper, the main features of the project are presented with a special focus on the structural and seismic design. 2 BUILDING FEATURES

The six-storey building is composed of a reinforced concrete underground level for parking and six CLT storeys, made with Spruce and Pine CLT panels manufactured in Austria. All the structures are made with CLT panels including stairs and lift cores with

some glulam and steel beam at each level in order to provide multiple supports to the floor panels when walls could not be used. The building is composed of two portions separated by a 26 cm seismic gap, one with a regular rectangular shape and a second one with a trapezoidal shape. The total length of the building is approximately 61.5 m, the width is approximately 15.6 m and the total height is around 20 m. The floor plan is approximately 865 m2 and the total floor area is 5190 m2. Figure 2 shows the plan view of the building structure for the 1st, 2nd to 5th and 6th storey.

Figure 2: Plan view of the 1st, 2nd to 5th, and 6th storey of the building.

As it can be observed from Figure 3 on one of the two longitudinal sides of the building the longitudinal external walls from the 2nd to the 6th floor bear on the transversal side walls of the 1st storey, while on the other longitudinal external side the 2nd to 5th storey overhang with respect to the 1st storey of approximately 1.8m. On the 6th storey there is a setback of the same length so that the external longitudinal 6th storey walls are in correspondence to the external longitudinal 1st storey walls.

Figure 3: 3D view of the building structure from two opposite sides.

The timber structure is completed for the external walls on the inner side with 12.5 mm type F gypsum plasterboard, a 50 mm cavity for installations filled with low density glass fibre and on the external side with 140 mm of 115 kg/m3 rock-wool insulation and two types of external lining, plaster and ventilated wall with tiled lining. Floors are made with 160 and 180 mm thick CLT panels, 100 mm of lightweight concrete, double acoustic layer, 50 mm of lightweight concrete and tiled flooring, while the horizontal roof is made with 120 mm CLT panels, vapour barrier, 200 mm of 100 kg/m3 rock-wool insulation, waterproofing sheet, ventilated cavity and PVC waterproofing coating. The details of walls and floors construction are illustrated in Figure 4.

Figure 4: Details of walls and floors construction.

Regarding the energetic performance, the building is classified as a NZEB (Nearly Zero Energy Building) Building with an energy consumption of 20 kWh/m2 per year. A centralized system has been adopted both for winter heating and summer air-conditioning with an air-to-water heat pump. Hot water production is made by means of a boiler and a heat pump integrated with a solar thermal system with natural circulation. The production of electricity is obtained through an extended photovoltaic field placed on the roof. A controlled mechanical ventilation with heat recovery is placed in each apartment. 3 STRUCTURAL DESIGN

The building has been designed according to the Italian Building Code [3]. Specific rules for seismic design of CLT buildings and capacity based design can be found neither in the current version of Chapter 8 of Eurocode 8 [5] nor in the Italian Building Code. Therefore, the capacity design rules and seismic detailing provisions for CLT buildings included in a proposal of revision of the Chapter 8 for the seismic design of timber buildings of Eurocode 8 recently presented [4] were followed. 3.1 STATIC DESIGN

The first step was the static design of the CLT structural elements for the imposed and gravity loads assessed based on the details of wall, floor and roof panels displayed in Figure 4. Floors generally span along the short direction of the building. To minimize their thickness, approximately 15 m long single panels have been used on multiple supports, represented by the longitudinal walls and by steel beams where necessary (Figure 5).

Figure 5: Static design of a CLT floor as a single piece of approximately 15 m of length spanned over five supports.

The 1.8 m overhang in the transversal direction is supported by the longitudinal walls hatched in Figure 6, which bear the loads transferred by the external beams supporting the floor panels.

Figure 6: Transversal walls supporting the 1.8 overhang of the building from the 2nd to the 5th storey, highlighted with a hatched pattern.

Again in order to minimize the thicknesses of the structural CLT panels and reduce cost and seismic weight, different wall thicknesses have been used at the same level for each storey, depending on the load on the single walls.

Element Description Th. [mm]

P01 External plastered CLT wall12.5 50.0 var 140.0

var

Element Description Th. [mm]

A Type F gypsum plasterboard 12.5

B

C

Glass wool insulation 35 kg/m³ 45

D

CLT panel variable

E

Rock wool insulation 115 kg/m³ 140

Plaster 7

BA

CDE

var

120.0

200.0

var

ABCDE

I

A PVC waterproof coating 1.5

B OSB/3 panel 15

D

ventilated cavity variable

E Rockwool 100 kg/m³ insulation 200

F Vapour barrier -

S12 Roof

C

C Waterproof transpiring sheet -

FGH

G CLT roof panel 120

H Glasswool insulation 27

I Type A gypsum plasterboard 12.5

27.0

369.5

27.0

160.0

100.0

50.0

ABCD

FGH

A Tiled flooring 10

B Light concrete 1400 kg/m³ 50

C Double acoustic layer 5+5

D Light concrete 400 kg/m³ 100

E Separating sheet -

F CLT floor 160

G Galsswool insulation 27

E

H Type A gyspum plasterboard 12.5

S04 Inter-storey floor

Element Description Th. [mm]

P02 External ventilated CLT wall12.5 50.0 var 140.0

var

A 12.5

B

C

D

var

E

140

F

Wind and UV barrier -

BA

CDEFG

G

Ventilated cavity 30

Tiling 10

45

Element Description Th. [mm]

Type F gypsum plasterboard

Glass wool insulation 35 kg/m³

CLT panel

Rock wool insulation 115 kg/m³

External beam supporting floor panels

1.8m overhang over the 1st floor

The wall thicknesses of the 5-layer CLT panels are 180-160-140-120-100-100 mm going from the 1st to the 6th storey respectively. The lift shaft is made of 5-layer 160 mm thick CLT panels for the whole height of the building. Floors are made with 5-layer 180, 160 and 140 mm CLT panels, while roof panels are made with 5-layer 120 mm CLT panels. Special care was taken to ensure no failure will occur on the floor panels in the perpendicular to the grain direction. To this aim, the Eurocode 5 [6] formulas and the method proposed by A. Thiel, 2013 [7] were used. 3.2 SEISMIC DESIGN

3.2.1 GRAVITY LOADS AND SEISMIC WEIGHTS

Gravity loads for the seismic combination were estimated based on the structural and non-structural elements considering the specifications given in Figure 4. The permanent loads G of external and internal walls are 1.39 kPa and 1.07 kPa, respectively, for all the six storeys. The permanent loads G of the floor and roof diaphragm are 3.16 kPa for the inter-storey floors and 1.31 kPa, respectively. The imposed loads Q for the floors are 2.00 kPa for residential use and 4.00 kPa for the balconies and stairs while for the roof diaphragm no imposed load is considered for the seismic combination. Based on these gravity loads, Table 1 lists the total seismic weight of each floor of the building. The total seismic weight of the building is W = 12040 kN. Table 1 Total seismic weight for each level of the building.

Story Seismic weight, G + 0.3Q (kN)

1 2203 2 2230 3 2230 4 2230 5 2205 6 942

Sum 12040 3.2.2 DESIGN SPECTRUM The design response spectrum for the city of Florence, displayed in Figure 7, for a 10% probability of exceedance in 50 years was calculated based on the Italian National Building Code [3] with the following parameters: • nominal life of the structure equal to 50 years; • design ground acceleration ag = 0.130 g; • soil factor S = 1.5 for ground type C; • amplification factor F0 = 2.40; • lower limit of the period of constant spectral

acceleration branch TB = 0.156 s; • upper limit of the period of constant spectral

acceleration branch TC = 0.469 s; • seismic force modification factor q = 2;

• reduction coefficient for the seismic modification factor taking into account the non-regularity in elevation KR=0.8.

Figure 7: Design response spectrum adopted for the ULS and DLS seismic design.

3.2.3 NUMERICAL MODEL OF THE SIX STOREY BUILDING

The three-dimensional numerical model of the six-story building was implemented in the widespread software package for structural analysis SAP2000 [8]. A pre- and post-processing software specifically developed by Tecnisoft [9] was used for the input and output phases with the procedure already detailed in [10]. As the building was divided into two different portions by the 26 cm seismic separation, two different models have been made. Due to length limitation, reference will be made only to the first, rectangular shaped, portion of the building. Figure 8 displays the numerical model of the building extracted from the pre-processing software.

Figure 8: Undeformed shape of the numerical model.

The numerical model of the CLT building for linear analysis uses shell elements for the wall panels, truss elements for the panel-to-panel connections, and beam elements for the lintels connecting the walls above openings. The model described below is based on some simplified assumptions: • floor diaphragms are assumed to be in-plane rigid

while their out-of-plane stiffness is not considered; • the connection between perpendicular walls is

assumed as rigid; • the connection between floors and supporting walls

is assumed as rigid; and • the hold-down connectors are not explicitly

modelled.

Shell elements with membrane and bending stiffness are used for the CLT wall panels with a typical mesh of 0.5x0.5 meters. These elements are defined with the same length, height and thickness as the corresponding CLT wall panels. Orthotropic material properties are defined based on the orthotropic properties of the wood boards of the CLT walls, the number of layers and their thickness. The equivalent modulus of elasticity along the two main directions is calculated as suggested by Blass and Fellmoser [11] using the transformed section method. The shear modulus in the plane of the wall is assumed the same as the shear modulus of the boards or can be reduced by 10% if no edge bonding exists between the boards of each layer [12]. A pair of horizontal cross truss elements is used to connect each wall to the foundation as well as to the wall of the upper floor. The cross-sectional area and material properties of the trusses are computed based on the horizontal stiffness of the connections used to transfer the shear force from the wall to the floor diaphragm underneath. Generally, these connections consist of angle brackets or steel plates for the base connection and angle brackets for the inter-storey connection. The horizontal stiffness of the angle bracket connections was computed according to the specifications given by the producer and based on experimental results [13]. Connections between walls and upper diaphragms are considered as rigid, thus are not modelled both because they are usually designed for the overstrength of the dissipative connectors in order to satisfy capacity design requirements and also because floor diaphragms are not explicitly modelled but just schematized using a kinematic constraint of rigid floor. Vertical truss elements are used to simulate the flexibility of the floor diaphragms perpendicular to the grain, as the walls bear on the floor panels. Thus, the modulus of elasticity perpendicular to grain is selected for the isotropic material properties of these vertical truss elements. Since the shell elements are meshed with a grid of 0.5 meters length, the cross sectional area of the vertical trusses is equal to 0.5 meters times the thickness of the wall above. At the foundation level, the modulus of elasticity of concrete is used for the isotropic material properties. Forces in the vertical truss elements of a wall are then utilized to calculate the tensile forces for the design of the hold-downs, typically installed at each end of the wall to resist uplift forces due to overturning moments from horizontal seismic loads. It should be noted that although hold-down anchors play a major role in the actual lateral stiffness and strength of the wall, they are not explicitly simulated in the linear numerical model mainly due to their marked nonlinearity. The hold-downs exhibit significantly different stiffness in compression (where there is contact between wall and floor panels) and in tension (where only the hold-downs resist). Figure 9 illustrates a typical schematization of a couple of CLT wall panels and the connections at the base of the building as well as the connections with the upper floor walls. With this schematization, the in-plane shear forces transmitted from the walls to the walls underneath can be directly obtained from the axial forces of the

horizontal truss elements, while the uplifting vertical forces are obtained from the tensile forces in the vertical truss elements. A rigid diaphragm constraint is used to constrain all nodes at the same level in Figure 9. It should be noticed that a separate constraint is used to constraint all the nodes at the bottom of the wall and all the nodes at the top of the wall underneath.

Figure 9: Wall schematization used in the linear model (after [10]).

A first preliminary design was made with the equivalent static procedure defining the horizontal stiffness of each wall proportional to its length. Although the user can define different stiffness per unit length for each wall, the same value is used for all wall elements at this preliminary stage. The actual value of the distributed stiffness is not important since only the lateral force method of analysis is conducted for the preliminary design and the fundamental vibration period is calculated from the equation provided by the Italian Building Code [3] and Eurocode 8 [5] for structures with shear walls. The estimated vibration period of the structure in this case is:

�� � 0.05 ∙ 19.6�� � 0.46� (1)

Based on the results of the preliminary analysis and design, the numerical model was built incorporating the actual stiffness contribution of the angle brackets in the material properties of the horizontal springs of each wall assembly. This model was then used to perform a modal response spectrum analysis of the structure. Table 2 lists the fundamental periods and the associated mass participation factors for each mode shape of the structure. Figure 10 displays the first three mode shapes. The first mode shape has a period of 0.71 sec that is slightly higher with respect to the 0.46 sec computed with the code equation This mode shape is related to translation along the long direction of the building. Although this may not seem logical, it is justified by the number of windows and door openings along this direction, leading to a lower lateral stiffness of the building compared to the stiffness along the short direction. The second mode shape, with a period of 0.48 sec, is related to translation along the short direction of the building.

Table 2 Modal analysis results

Mode

Period

(sec)

Mass participation

factor for translation along the

long direction (%)

Mass participation

factor for translation along the

short direction (%)

Mass participation

factor for rotation along

the vertical direction (%)

1 0.71 72.32 0.02 4.28

2 0.48 0.19 76.92 1.37

3 0.43 4.83 1.54 74.39

4 0.26 14.35 0 0.4

5 0.18 0 12.01 1.05

6 0.17 0.21 1.14 9.51

Sum 91.9 91.6 91.0

Figure 10: First three mode shapes of the structure.

3.2.4 ANALYSIS RESULTS The modal response spectrum analysis was conducted for 16 load combinations accounting for bidirectional and torsional effects. The bidirectional effects were considered by applying 100% of the design spectrum in one direction and 30% of it in the perpendicular direction while accidental eccentricity was introduced by translating the floor masses by 5% of the building length in each direction.

The required number of shear (angle brackets) and uplift (hold-downs) connections was calculated, based on the results of the modal response spectrum analysis, using the capacity design rules given in [4] for CLT structures in Medium Ductility Class. Based on these criteria, some structural elements are regarded as non-dissipative and are designed with sufficient overstrength. These elements are: (i) all CLT wall and floor panels, (ii) the connections between adjacent floor panels, (iii) the connections between floors and the walls underneath, and (iv) the connections between perpendicular walls. The connections devoted to the dissipative behaviour are (i) the shear connections between walls and the floor underneath and between walls and foundation (usually steel brackets or screwed connections) and (ii) the anchoring connections against uplift placed at wall ends and at wall openings (usually hold-down anchors). In order to achieve the desired dissipative behaviour the following equation should be applied: ����

∙ F��,� � F��,� (2)

where FRd,d is the design strength of the ductile component, FRd,b is the design strength of the non-ductile (brittle) component, γRd is the overstrength factor which, according to [4] should be taken equal to 1.3 for CLT structures, and βsd is the reduction factor for strength degradation due to cyclic loading which should be taken equal to 0.8 according to [4]. The same capacity design requirements also apply at the connection level where a ductile failure mode characterized by yielding of fasteners (nails or screws) in steel-to-timber or timber-to-timber connections should be achieved and the brittle failure mechanisms such as tensile and pull-through failure of anchor bolts and screws, and steel plate tensile and shear failure in the weakest section of hold-down and angle brackets connections should be avoided.

Figure 11: Wall ID’s for the first storey.

Table 3 show the results of the linear analysis model in terms of shear acting on each wall, maximum uplift forces at each end of the wall and maximum axial load for seismic and static load combinations. For the sake of brevity the results are referred only to the first storey walls, whose ID’s are displayed in Figure 11. As it can be observed a maximum shear force of approximately 53 kN/m and a maximum uplift force of almost 500 kN were found. For the second trapezoidal portion of the building, values of 630 kN for the maximum uplift forces were found.

Table 3 Linear analysis results

Wall Name Length

[m]

Seism. Shear

[kN/m]

Seism. Uplift 1 [kN]

Seism. Uplift 2

[kN]

Seism. Axial

[kN/m]

Stat. Axial

[kN/m]

PTX-1-1 1.43 33.4 49.2 200.9 249.1 274.2

PTX-1-2 0.60 17.1 33.9 33.8 46.3 130.0

PTX-1-3 1.43 31.7 136.0 71.2 254.5 413.5

PTX-1-4 1.49 33.4 47.6 178.0 297.7 410.3

PTX-1-5 0.60 17.1 33.9 33.8 46.3 130.0

PTX-1-6 3.21 40.1 206.5 200.3 261.5 375.0

PTX-1-7 0.60 17.1 33.9 33.8 46.3 130.0

PTX-1-8 1.51 33.6 185.3 48.0 304.0 409.4

PTX-1-9 1.49 32.5 79.4 143.5 266.5 410.4

PTX-1-10 0.60 17.0 33.7 33.6 46.3 130.3

PTX-1-11 1.46 33.9 212.1 43.1 317.9 325.3

PTX-2-1 0.61 27.1 19.4 0.0 221.7 350.1

PTX-2-2 0.72 27.0 0.0 124.2 546.9 471.6

PTX-2-3 1.39 32.7 103.0 40.7 239.1 313.4

PTX-2-4 1.55 44.3 37.0 147.4 133.9 146.9

PTX-2-5 1.55 42.7 136.6 46.1 117.4 145.6

PTX-2-6 1.47 35.0 37.0 159.6 262.9 319.0

PTX-2-7 0.70 28.5 93.7 59.0 609.6 521.7

PTX-2-8 0.70 29.6 0.0 10.3 202.5 374.0

PTX-2-9 0.69 29.3 9.2 0.0 202.5 374.3

PTX-2-10 0.70 28.5 59.1 93.5 609.5 521.2

PTX-2-11 1.49 34.9 159.0 39.3 263.5 318.2

PTX-2-12 1.55 43.3 45.3 141.4 118.8 146.0

PTX-2-13 1.55 43.8 143.9 39.2 135.1 146.1

PTX-2-14 1.43 34.0 46.5 139.6 242.5 320.5

PTX-2-15 0.70 28.0 90.1 55.2 586.0 506.5

PTX-2-16 0.70 28.9 11.1 36.1 197.9 314.4

PTX-3-1 1.27 37.1 81.0 83.9 41.5 72.7

PTX-3-2 1.09 34.2 75.1 67.8 48.9 74.0

PTX-3-3 4.70 52.7 135.8 135.5 11.7 26.1

PTX-3-4 1.19 36.2 74.5 80.4 45.4 70.5

PTX-3-5 1.31 38.1 85.8 75.9 47.2 75.4

PTX-3-6 1.29 37.7 77.0 85.0 47.2 76.1

PTX-3-7 1.21 36.6 81.4 75.4 45.1 69.9

PTX-3-8 4.70 52.7 135.8 135.6 11.8 26.1

PTX-3-9 1.24 37.1 77.3 82.7 44.1 68.9

PTX-3-10 1.21 36.2 81.4 76.5 43.9 74.3

PTY-1-1 15.02 28.1 291.3 439.8 121.3 143.1

PTY-2-1 5.96 34.7 204.4 0.0 138.5 223.9

PTY-2-2 6.09 41.7 0.0 0.0 225.2 273.9

PTY-3-1 2.19 18.3 137.2 138.7 48.4 126.6

PTY-4-1 2.19 17.8 132.6 134.2 48.4 126.6

PTY-5-1 5.96 32.6 138.4 0.0 119.9 218.0

Wall Name Length

[m]

Seism. Shear

[kN/m]

Seism. Uplift 1 [kN]

Seism. Uplift 2

[kN]

Seism. Axial

[kN/m]

Stat. Axial

[kN/m]

PTY-5-2 6.09 34.5 0.0 0.0 192.1 278.2

PTY-6-1 12.95 37.3 189.7 126.7 127.7 253.5

PTY-7-1 5.96 36.7 169.0 0.0 121.8 218.1

PTY-7-2 6.09 43.6 0.0 1.2 204.9 277.5

PTY-8-1 2.19 20.6 158.6 160.7 48.4 126.6

PTY-9-1 2.19 21.7 169.0 171.2 48.4 126.6

PTY-10-1 5.96 42.0 275.0 0.0 137.6 218.0

PTY-10-2 6.09 50.2 0.0 15.5 229.0 275.1

PTY-11-1 13.05 36.4 320.3 478.2 127.7 150.0

3.2.5 DESIGN STRENGTH OF THE

ANCHORING CONNECTION TO THE CONCRETE BASEMENT

The design of the connections to the concrete basement was carried out according to the analysis results reported in §3.2.4. The first problem was to found a solution in order to resist the maximum uplift forces of 500~600 kN, also taking into account the capacity design rules at connection level given in §3.2.4. Following the specifications given by the main producers of steel connection for timber structures, the maximum design uplift forces which could be resisted by “standard” hold-down anchors are around 122 kN and 56 kN for non-cracked and cracked concrete applications respectively, calculated with a φ27 threaded steel bar inserted at a depth of 300 mm into the concrete basement. If an overstrength factor of 1.3 is considered in order to satisfy the capacity design rules given in §3.2.4, the resistances are even lower. The design strength of the anchorage to the concrete substructure, usually made through chemical bonding anchors, is calculated according to the ETAG 001 guideline [14]. According to this document, the characteristic strength is given by the lower value among all the following failure modes: (i) steel failure, (ii) combined pull-out and concrete cone failure, (iii) concrete cone failure and (iv) splitting failure. Usually modes (ii) and (iii) produce the lowest values, and even when increasing the steel bar diameter and the depth inside the concrete, the improvements in terms of design strength are minimal. This is the case especially in the case of cracked concrete, which is very common when the timber building is erected over one or two concrete underground levels, as it is the case of this building. Even increasing the number of hold-down anchors does not bring any significant advantage, since the governing failure mode is in most of the cases the concrete cone failure. In order to double up the resistance of the hold-down connection and avoid at the same time this type of failure, the two hold-downs should be placed at very large distances (more than 500 mm), thus losing their uplift restraint capacity. To overcome the aforementioned problems, a special type of hold-down anchor made with thick steel plates (5 to 8 mm) was specifically designed for this project. This

hold-down was placed on one side or both sides and connected to the CLT panel with annular ringed nails. The connection to the concrete was achieved with a purposely designed concrete base beam of the same width of the CLT panel. In order to avoid the concrete cone failure or any type of failure mode in the concrete beam, anchoring reinforcing bars were placed in the concrete beam and, in case of higher tension loads, the anchorage was provided by steel beams inserted in the concrete base beam. Figure 12 displays the solution adopted for the uplift restrain connection with the concrete basement. Similar solutions were adopted for the shear restrain connection, made with steel plates placed on one or both sides of the concrete base beam.

Figure 12: Purposely designed hold-down and shear steel plate connection at the ground floor and view of the hold-down connection at the building site.

The same type of solution was adopted for the uplift restraint connections at the upper floors (2nd and 3rd) where in some cases high uplift forces (even if lower

than at the 1st floor) were found from the linear dynamic analysis, i.e. tie-down anchors made with thick steel plates anchored to the CLT panels with annular ringed nails. This solution was adopted for both the external and the internal wall uplift connection. 3.3 FIRE DESIGN

The requirements concerning the fire design for residential buildings are ruled in Italy by a Ministerial Decree dated 1987 [15], containing the fire safety design criteria, regardless the type of structural material used in the construction. According to this Standard, requirements are given for building having a “fire height” (i.e. the maximum height measured from the lower level of the opening of the highest livable floor with respect to the lowest external level) of more than 12 m. According to the value of the fire height, buildings are classified into 5 different classes (A, B, C, D and E), and requirements are given concerning the maximum fire compartment area, the minimum value of the fire resistance of the structural components, the number of stairs and lift shafts, and the reaction to fire of the internal finishing materials of common areas. Even if the fire height of the building (17 m) was above the minimum height of 12 m, the maximum fire compartment area and the reference area for each stair case were below the minimum required, therefore no specific requirements concerning the fire resistance of the structure were given. Nevertheless, all the walls are protected with a 12.5 mm type F gypsum plasterboard (on both sides for internal walls) on cavities filled with mineral wool which, summed with the own fire resistance of the CLT panels, gives a fire resistance rating ranging from 30 to 45 minutes for all the walls. Moreover, the gypsum plasterboard finishing is classified as type A2-s1-D0 concerning the reaction to fire according to EN 13501-1 [16]. 4 CONSTRUCTION OF THE BUILDING

The construction of the six storey building started on December 14th, 2015 and ended on April 29th 2016, with a total of 71 effective working days. Considering the location of the building site, in a central area of Florence, near the river Arno, the reduction in construction time was crucial in order to keep the disruption of the daily city traffic to a minimum.

Figure 13: View of the building site with Google Earth showing with the 4-storey building almost completed and the six-storey building construction not started yet.

Building Site

Arno river

S. Maria del Fiore Cathedral

Regarding this latter aspect and considering the limited space within the building site for the material storage, the delivery of the structural elements to the building site was accurately organized so that the CLT panels were directly put in place from the truck, avoiding the storage on the building site. Nevertheless it was chosen to integrate the holes for all the installations and the metal connectors directly in the production drawings in order to reduce the construction time but also to control the structural implications and interferences. For the entire construction a total of 1700 m3 of CLT panels (corresponding, more or less, to 2500 Spruce and Pine trees), 40 m3 of glulam beams, 9250 kg of steel beams and 300.000 nails were employed. Considering both the capacity of timber of storing carbon dioxide and its reduced embodied energy, about 3400 tons of CO2 were saved with the CLT building construction. Figure 14 shows the construction stages of the rectangular portion of the 6-storey building, while Figure 15 displays the completed rectangular portion of the 6-storey building and an internal view of the structure during construction.

Figure 14: Construction stages of the six storey building.

Figure 15: 6-storey CLT building completed (photo of the rectangular portion taken from the seismic separation) and internal view of the structure.

5 CONCLUSIONS

The project of a 6-storey CLT residential building in Florence, Italy has been presented in this paper. The main features of the project have been highlighted and a detailed description of the structural and seismic design has been made. Due to the lack of specifications for seismic design in the current edition of Eurocode 8, the capacity design rules given in a recent proposal of revision of Chapter 8 of Eurocode 8 have been adopted,

confirming the feasibility of the proposed requirements. The issue of the low strength values of the currently adopted chemical bonding anchorage used to anchor the hold-down to the concrete basement is discussed, and possible solution to overcome this problem presented. Finally some information on the erection procedures is also given. ACKNOWLEDGEMENTS The contribution of Dr. Ioannis Chistovasilis, Structural Engineer and Technical Director at Aether Engineering, Florence, to this paper and of the builders Campigli Legnami and Imola Legno is gratefully acknowledged. REFERENCES [1] Ceccotti, A., Follesa, M. Seismic Behaviour of

Multi-Storey X-Lam Buildings. Proceedings of 426 COST E29 International Workshop on Earthquake Engineering on Timber Structures. pages 81-95, Coimbra, Portugal, 2006.

[2] Tomasi R., Piazza M. Investigation of seismic performance of multi-storey timber buildings within the framework of the SERIES Project. In International Conference on Structure and Architecture [ICSA2013]. Guimaraes, Portugal, 2013.

[3] Ministry of Infrastructures. Decree 14/01/08 – Technical Regulations for Construction. Rome, 2008 (in Italian).

[4] Follesa M., Fragiacomo M., Vassallo D., Piazza M., Tomasi R., Casagrande D., Rossi S. A proposal for a new Background Document of Chapter 8 of Eurocode 8. Proceedings of the International Network on Timber Engineering Research meeting INTER 2015. Ŝibenik, Croatia. paper 48-102-1 - ISSN: 2199-9740, 2015.

[5] Eurocode 8. Design of structures for earthquake resistance, Part 1: General rules, seismic actions and rules for buildings. EN 1998-1. CEN, Brussels, Belgium, 2004.

[6] Eurocode 5: Design of timber structures – Part 1-1: General rules and rules for buildings. EN 1995-1-1. CEN, Brussels, Belgium, 2009.

[7] Thiel A. ULS and SLS design of CLT and its implementation in CLT designer. Focus Solid timber Solutions – European Conference on Cross Laminated Timber (CLT). Graz 2013.

[8] Computers & Structures Inc. SAP2000—Integrated finite element analysis and design of structures. Ver. 14. Computers & Structures Inc.: Berkeley, CA., 2000.

[9] Tecnisoft s.a.s., “ModeSt – Version 8.5”, Prato, Italy, 2014.

[10] Follesa M., Christovasilis I., Vassallo D., Fragiacomo M., Ceccotti A. Seismic design of multi-storey CLT buildings according to Eurocode 8. In: Ingegneria Sismica, Special Issue on Timber Structures. n. 04/2013, pp. 27-53., 2013.

[11] Blass H.J., Fellmoser P.. Design of solid wood panels with cross layers. 8th World Conference on

Timber Engineering WCTE, Lahti, Finland, p. 543-8, 2014.

[12] Fragiacomo M., Dujic B., Sustersic M.. Elastic and ductile design of multi-storey crosslam massive wooden buildings under seismic actions. Engineering Structures, Special Issue on Timber Structures, 33(11):3043-3053, doi:10.1016/j.engstruct.2011.05.20, 2011.

[13] Rotho Blaas S.r.l. Metal Plates and Connections for Timber (in Italian). Cortaccia, BZ, Italy, 2015.

[14] European Organisation for Technical Approvals. ETAG 001 - Guideline for European Technical Approval of metal anchors for use in concrete. Edition 1997.

[15] Ministry Decree 16/05/1987 n.246 – “Fire safety regulations for residential multi-storey buildings” (in Italian).

[16] EN 13501-1. Fire Classification of Construction Products and Building elements – Classification using test data from reaction to fire tests +A1 : 2009