FOUR RESIDENTIAL TOWERS AS CLT TIMBER CONSTRUCTION IN THE CITY OF MILAN

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1 WCTE 2016 World Conference on Timber Engineering August 22-25, 2016 Vienna, Austria FOUR RESIDENTIAL TOWERS AS CLT TIMBER CONSTRUCTION IN THE CITY OF MILAN Andrea Bernasconi1 ABSTRACT: The timber construction of the residential building Via Cenni in Milan (erection ) is of course an ambitious project. But from the point of view of the design of the timber structure with CLT this project is "just" the application of the actual state of the art. On the other hand it is not very correct to describe this project as "timber housing"; it is more correct to describe this kind of object as "CLT for engineered large building structure". Large structures need the discussion and the solution of several not usual questions. The connections on the concrete foundation, the robustness of the structure, the earthquake design are examples for this. This remarks are just a contribution to the discussion from the really experience in Milan. KEYWORDS: multi-storeys, CLT, timber structures for buildings, CLT-Structures, building in the city 1 INTRODUCTION 123 The project of the residential area "via Cenni" in Milan consists on 4 9-storeys tower-buildings, and 4 longer 2storey buildings. The lower constructions are the connections buildings between the towers. Figure 1: Residential quarter in Milan with 3 CLT-towers The entire project includes 123 residential units and a lot of others spaces for urban services, for administration and concierge, for social activities, for garden and public area. The residential units are part of a new concept of social housing. The project is realized by Polaris Investment Italia SGR Spa, in the name and on behalf of the ethical real estate fund "Abitare Sociale". 1 Andrea Bernasconi, High School for Engineering and Figure 2: One of the four CLT-Towers in Milan The buildings of this new residential quarter near to the San Siro Stadium in Milan were in building during 18 months beginning on January This project is the result of a design competition carried out form the promoter in 2009 with the goal of promoting ongoing experimentation in innovative approaches to social housing management. The winner of the competition and the author of the project is Architect Fabrizio Rossi Prodi in Florence, Italy.

2 This project was created and designed from the beginning as a timber construction and as a timber structure with CLT. The raisons for this choice were given by the interest for innovative solutions and by the advantages of the CLT construction, so as described from the project team in the competition project. These are especially the high safety of the structure and of the construction, the high level of the comfort of the residences and the high flexibility by the organization of the spaces. important to give attention to the earthquake safety of the building. The structural project had to obtain a formal approval from the highest national building authority. This was required in Italy before January 2012 for all timber constructions with more than 4 levels. The author of this paper had the lead of the project-team, which obtained in June 2011 the approval from a special commission of experts. Figure 3: The dimension of the 4 towers The possibility to realize the construction during a short time and to reduce the risks of a longer time on the building site, or the possibility to realize with timber a construction with a high level of insulation, the reduction of the energy costs for heating in winter and for cooling in the summer, are other basically reasons for the design with CLT. 2 THE STRUCTURAL CONCEPT The main structure of the tower is a spatial, three dimensional structure, composed on CLT decks and walls. All the CLT panels are continuously connected together, in order to assure a regular, continuous flow of the forces and in order to obtain a correct stiffness of the CLT panel connections. Other structural elements - as bending beams or columns - are very rarely used and have just a local structural effect in case of openings in walls or decks or in case of locally, small irregularity of the construction. The main timber structure consists "just" on vertical and horizontal CLT panels, and of course on the structural connections between the CLT panels. The main buildings of the project are the 4 towers, with 9 storeys and ground dimensions 13.6 m x 19.1 m. The load bearing structure consists on CLT-panels; the structure can be considered as an important engineered CLT-structure and had to be designed, calculated and verified in strongly accordance to the rules and knowledge of the modern timber engineering and construction. Italy is a known earthquake area. The city of Milan and generally the northern part of Italy are not subject to strong earthquakes, but earthquake is a very sensible topic by the authorities as well by the population. So it is 3 DISCUSSION TOPICS AND EXPERIECNE FROM THIS OBJECT 3.1 CONCEPTUAL DESIGN The conceptual design and generally the design of a building of big dimension have to respect the expectation of the owner in form of professional makers of the buildings. Internal check procedures of each step of the design have to be achieved and passed successful. Solutions for each question have to be delivered and it is not possible to delay some answer or solution to the building site. In this case the structural design of the structure was object of an investigation commission with specialists on earthquake and on structures for higher residential building (in concrete). It was necessary to explain, how timber construction can be designed in order to fulfil the requested safety - and especially how this big structure was analysed. In order to simplify the work and to have a really chance to obtain the approval, it was decided to have a simple concept of the structure and to have clear and easy solution for the principal structural components. In the following chapters some of the main topic of the design concept are shortly explained. 3.2 INTERFACE TIMBER-FOUNDATON The interface of the timber structure to the foundation is the connection with the highest forces, but it is a very delicate interface of the construction too. It must be assured that this zone are free from water during the building site - and during the lifetime of the building -, that the concept are conform with concrete and with timber construction rules, that a sufficient stiffness and resistance are given and that the precision of the execution of the concrete part are acceptable for the timer part (tolerances and adjustment on building). The foundation of the tower is given by the one storey underground building. This part of the construction contains the car parking places, a lot of technical equipment (heating, water circulation engines, the place for a further cooling and/or air circulation equipment, and other more). A solid concrete deck on the level 0 (at the natural level of the terrain) is the ground basis for the timber structure. The concrete building under this level have to assure the directly continuation of the forces flow. Interruption of the continuity of the vertical load

3 bearing walls was principally not accepted. Exceptionally cases of an interruption of the concrete wall under a CLT-timber load-bearing wall were accepted by short spans, or by short opening in the concrete walls, with very strong conditions for the stiffness of the concrete structural elements. In this cases a maximal deflection ratio of 1/1000 of the effective opening were imposed and respected by the design of the concrete. Figure 4: Principle of the interface CLT-concrete The interface between the timber construction and the concrete foundation has to fulfil more, different requirements. It is of course to assure that the flow of the forces is not interrupted and that the connections are not the cause for forces concentrations in the CLT or in the concrete. But this interface is the food point of the timber construction and it defines the precision of the erection of entire timber contraction. Due to the higher tolerances of the concrete, it is to assure that a calibration and a correction of the exactly position of the timber elements can be reached. In other words it is necessary to develop a construction system, which permits to adjust the exactly position of the CLT elements before the CLT-montage. In order to assure the correct position of the first CLTwall, a completely connection and erection concept was applied. In this case of the Via Cenni in Milan, the strong version of this connection was applied, in order to assure a continuously connection between CLT-Wall and concrete foundation. In other words, a more or less full connection with the CLT and the concrete was created. In addiction, an erection procedure in order to assure an adjusted position - in order of +/- 2 mm in the vertical and in the horizontal direction - was applied. As first step a wall of 50 cm height was erected corresponding to the position of the CLT-wall-elements. The concrete wall have the same thickness as the CLT, in order to assure the continuity of the construction on the both sides of the structural wall. The anchorage bars in the concrete are part of the concrete steel reinforcement and they should be positioned at the same time as the reinforcement bars of the concrete deck. Alternatively the design of the anchorage bars has to be very detailed, in order to assure the full anchorage of the steel bars in the small concrete wall. Using of a template (as a steel plate, or a timber panel, or an other material) assure the exactly position of the anchorage bars in the horizontal direction. After the hardening of the concrete, the template can be removed and the steel plates of the connection can be positioned and adjusted in the vertical direction. Screwing on both sides of the steel plate makes this adjustment. A special mortar allows to fulfil the open space and to finish the concrete erection. The CLT-Wall can be now positioned on the steel plates. The CLT-Wall are pre-worked in order to introduce the vertical steel plate into the CLT. The compression forces are transmitted by contact compression. So it is important to assure the full contact between steel and CLT by the vertical layers of the CLT-Wall. The notches in the CLT for the final part of the concrete anchorage bars have to be placed according to the horizontal layers of the CLT-wall panels. Otherwise the compression contact area between timber and steel will not be sufficient and a higher thickness of the walls on the ground level will be necessary. Figure 5: The foot of the CLT-structure The relatively higher level of the deck between ground leel and level 1, and the requirement to use a small number of CLT-Elements, imposed to use at the ground level CLT-wall-elements with standard height and to lift the lower border of the CLT using a small CLT-wallelement. The requirement to lift the lower level of the timber form the ground floor should be always applied, in order to avoid the contact of the CLT-foot-border with the water on the ground of the building site. So it is recommended to lift the food of the CLT lower wall at least 10 to 15 cm from the concrete level. Otherwise the CLT-Elements will remain during the building time, and during a lot of days and weeks, under water. Figure 6: CLT-wall and connection on the concrete

4 After the positioning of the CLT-wall, the last step of the ground connection is given by the dowel connection between CLT and the central, vertical steel plate. Using of pre-drilling dowels is probably the easiest solution. The experience in Milan shows that by an higher number of bolts it can be interesting to use a semi-automatic drilling engine to drill several times timber and steel at the same time. Further the dowel can be introduces manually into the holes. Figure 8: Connection line and punctual connection On a similar way the stiffness in the connection can be considered. Mechanical connections are never very stiff and the stiffness of the connections has an important effect on the internal load and forces distribution. And the stiffness of the connection is an essential component of the continuity of the structure. Even if a very small resistance is required for a connection, the not sufficient stiffness of the connection will produce a to high displacement between the connected CLT-panel. Figure 7: Connection on the CLT-wall with dowel 3.3 CONNECTION OF CLT-ELEMENTS The connections have to transfer high forces, to assure the regularity of the structure and of the flow of the forces; and they have to be easy to realise. Continuously connections along the borders of the CLT-panels are needed and the modelling of the structure requires the consideration of the stiffness of the connections, because the deformation in the connections is relevant for modelling and design. Principally two different connection types [1] were used for the structure, in order to assure a sufficient resistance and a sufficient stiffness on the connection lines. It have to be noticed, that the 3-D structure is given by the compositions - as the continuity of the resistance and of the stiffness - of a lot of CLT-panels. In order to assure a continuity of the CLT-resistance through the connections, it is necessary to assure a mechanical performance of the connections similar, or in the best cases equal, to the mechanical performance of the connected CLT-panels. Otherwise the connections represent a lot of weak points of the structure; and weakpoints have to be considered principally as noncontinuity or as non-regularity of the structure. A simple check of the efficiency of the connection is possible by the comparison of the mechanical performance of the connection with the mechanical performance of the connected CLT-panel. Punctual connection have a very lower resistance than the connected CLT-panel, even if strong punctual connector are used. In addition it have to be considered, that in case of punctual connections the stress in the timber can be local very higher. On this basis it can be recommended, that by exigent CLT-structures a minimal stiffness of all the connections can be assured. A possibility to reach this is given by using of line-connections and by avoiding punctual connections. The comparison of the connection stiffness of punctual and of line connections shows clearly that the stiffness of punctual connection is always very, very small. 3.4 EARTHQUAKE DESIGN AND MODELLING The application of the usual design procedure for timber buildings (with small number of storeys) have to be checked and discussed for the application to this kind of multi-storeys structures. The question about the effect of the ductility - in case of earthquake - of a part of the connections on the distributions of the forces and on the different other structural elements is still open. In case of application of usual ductility rules for timber construction, it is probably necessary to discuss it. The ductility of timber structures is given by the steel connections, and especially by the dowel by shear forces. Usually a value of q = 2.0 to q = 2.5 is given for CLT structures, if the conditions for ductility of the connections are given. But the real impact or effect of the ductility of one or more connections on building construction with higher dimensions is probably not very known. It seems very important for an accurate earthquake design to define the effect of the ductility of the connections on the other structural elements. The ductility of the connections needs a local high deformation within the connection or along a connection line. The compatibility of this deformation with the elastic and brittle deformation of the CLT-elements is an open question. 3D CLT structures can be very adequate in case of earthquake, especially if the basically rules for

5 earthquake engineering are respected by the geometry of the construction. By this project each tower is a singular building and the conditions of regularity of the structure in the horizontal and in the vertical direction are strictly respected. More contributions to the regularity of the construction are part of the design concept of this structure. At the first, the vertical load bearing walls are not interrupted along the entire height of the structure; opening in the walls reduce the structural dimension of the wall, and the reduced dimension continue as reduction for all the upper levels. The dimension of the structural components were defined respecting the criteria of regularity: all the walls of each level have the same thickness; and the thickness of the walls is reduced, from 200 mm at the ground level, down to 120 on the top level (level 9). The walls are interrupted by the decks: the transfer of the compression forces through the decks is very similar on the entire level. Within each level, the connections for shear and traction forces are very similar. On this way the regularity of the stiffness of the connections is a contribution to the global regularity of the structure and of the forces flow. The earthquake risk in the region around Milan is not very high; other Italy's regions show higher earthquake risk. But it should be regarded that earthquake is a very delicate subject by the authorities as well as by the people. This is the first very large timber project for residential building in Italy. Therefore it was decided to avoid the consideration of the inelastic behaviour of the timber structure: the earthquake analysis was carried out as elastic analysis. In case of inelastic modelling and in case of the consideration of the energy dissipation in the timber structure, by this kind of big structures, it would be probably necessary to describe the deformations and the energy dissipation of the different connections. It would be necessary to describe the distribution of the forces on the different connections and CLT-elements depending on the inelastic behaviour. Furthermore it should be remembered, that the most connections of the tower are made with axially loaded, self-tapping screws and the assumption of a brittle failure of this kind of connection is at this time not recommended. The CLT-structure of this building was designed on earthquake without consideration of the ductility effect of the structure, or simply the elastically behaviour was considered. The numerical calculation was carried out with a value of q = 1.0, which represents a smaller value than the "legal correct value" for structure without ductility of q = 1.5. The reduction up to 1.0 is of course a supplementary way to a careful approach to a new field of application of CLT-structures in big dimensions. The approval by the national authority seems to confirm this approach. On the same way, a lot of connections on this building are dowel-connections, and by all these connection the design rules for high ductility were applied and respected. It results a CLT-structure calculated and verified on earthquake within the elastic range, but with an effective ductility potential in case of earthquake or in case of exceptional earthquake outside form the design rules. Another results of this earthquake design concept is the fact, that no damage have to be expected in case of design earthquake. This is a point of high interest for the owner and/or for the investor, because in case of ductility design not just some damages on the structure, but a lot of damages on the non structural components of the construction have to be expected an taken into the account too. Away from the structural considerations, the owners of big construction are very sensible to the requirement on maintenance activities and to all the possible restriction of the use of the building. In this case the design concept offers a higher safety (elastic behaviour in case of design earthquake, and safety reserve in case of exceptionally events), the assurance of no damage in case of design earthquake and so the warranty of the immediate and costless serviceability of the building after a design earthquake. The earthquake design concept in this case was given by the structural concept. But in the future it may be of interest, to take into the account of the economical design the risk of earthquake damage and the following maintenance and costs. And may be, the structural design on ductility with q = 2.0 or q = 2,5 and the elastically design with q = 1.5 cam be considered under other point of view. 3.5 DURABILITY AND MAINTENANCE The durability and the need of maintenance interventions is one of the interesting questions for the owners of the building and for the investor. The impact of the facades on the durability of big timber construction is a question to be investigated and to be answered, in order to give basic information for the planers. During the entire procedure of the design and of the erection time of the building, the question of the durability of the timber structures and construction was object of questions, remarks, and doubts by the owners and by the investors. It should be considered, that experiences with this kind of timber constructions are not existent at this time. And on the same way references for effective costs for the usual maintenance of this kind of timber constructions are at this time not existent. By the building in Milan it was always possible to give - and to repeat - the answers to the questions, in order to give the satisfaction and to explain that the maintenance of a building with a timber structure is principally a thing of the maintenance of the façades and of the nonstructural components. And they are not made on timber. 3.6 REQUIREMENT ON ROBUSTNESS Robustness is a design requirement. But the conditions to be fulfilled in order to demonstrate the robustness of the entire structure are not defined. By this project the requirement of the local collapse of one CLT-wall without successive collapse of other parts of the structure was defined, proposed to the authority and finally successful applied. For this a small number of connections was reinforced or modified during the final

6 design. But an exchange of the experiences and the applications ways of the robustness is needed for future buildings. Robustness is a design requirement in order to assure, that in case of an exceptionally event, the consequences for the construction are not disproportionate high. Robustness is one of the basic requirements for big structures, especially for residential buildings. By timber housing it is not usual to give high importance to the robustness because the small size of the construction. By very large building the importance of the robustness is higher and a detailed consideration is needed. In the Italian codes this requirement is explicitly described and demands that the design of the structure are able to avoid important damages due to unforeseen situations. In other words it is required to prevent progressive collapse in case of exceptional events. For this construction it was defined that it must be possible to lose a wall between two decks and between two other perpendicular walls, without progressive collapse of the structure. The principle of the design of the robustness of this construction is very simple. It have to be assumed, that a wall - as a vertical element of the load bearing structure - is lost and can't contribute to the load bearing. In this case it have to be assured, that a progressive collapse of the construction can be avoided. In other word, the damage has to remain limited and localised near to the lost wall. The loss of a wall element has two consequences. At the first the continuity of the descent of the forces is interrupted; and as a second effect, the support for the upper deck doesn t more exist, so the deck will fail. To avoid these two consequences, it have to be assured that the deck becomes an alternative support and that de descent of the forces can be assured by alternatives flows of forces. The principle to obtain an alternative support for the deck can be reached by a suspension of the deck on the upper wall, or on the upper part of the structure. In case of a continuously wall this is very easy, because the normal connections used by this building have enough capacity to assure the suspension of the deck. The alternative flow of the forces can be assured by using the upper wall to redistribute the forces on the other structural components. It has to be remembered, that in the case of an exceptional event, according to the reduced partially safety coefficient, just a reduced residence of the structural elements has to be verified. By full wall elements - without interruption and without opening - this is not a problem. Figure 10: Wall with opening as lead bearing element In case of opening in the walls, or in case of interrupted walls, the fail of a wall element have to be careful analysed. By openings, the load capacity of the wall in case of interruption of the continuity in the vertical direction can be very low. In this case it is very important, that the wall can be assumed as one structural element between two other, perpendicular wall elements. Therefore it is very important, that interruptions into the walls are avoided and the walls are made of one CLT panel. So it is important for the robustness of the structure, that CLT-panels are not composed of several small parts, but are - if possible - so long as the distance between the main vertical components of the structure. 3.7 ERECTION AND BUDILING SITE The connections of the structural elements have to be able to absorb the tolerances of the erection of the construction, but they have to respect the maximal tolerances for the connections - for example for dowels with predrilling. It can be recommended, to reduce the pre-working with high precision at the absolutely minimum. On the best way, pre-working of the CLT elements for connections should not affect the precision of the erection. Adjusted predrilling or predrilling for adjusted dowels should be avoided. In case of dowels, self-drilling dowels are a good way to avoid problems with tolerances. Or, as described bellows, adjusted holes can be worked during the erection and after the positioning of the structural components. Screws connections don't require any preworking of the timber and are very interesting in order to adjust tolerances during the building. The discussion about the erection of timber building with big dimensions has to consider the weather protection during the building site. In the case of the towers in the Via Cenni in Milan a concept for the timber protection was prepared. At the first, the covering of the entire building site during the erection was excluded, because to expensive. Figure 9: Alternative load transfer in case of loss of one wall

7 Figure 12: Fire protection on the wall surface Figure 11: Raw timber construction Therefore plastic films assure the protection of the top of the erected walls. On the same way, the exposed timber surfaces were covered by a plastic film at the week-ends and in case of exposition longer than 24 hours. All openings in the walls were closed during the building time by plastic films. The outside part of the raw timber construction was partially protected by the scaffolding and partially by plastic films. The decision to change the concept of the fire safety was imposed by the owner, in order to avoid the risk of a damage of the fire protection. It has to be remarked; that by so a big building is simply not possible to avoid that rainwater comes to the timber elements. The described protection elements are absolutely necessary, in order to reduce the quantity of water on the timber to an acceptable level. But it must be accepted, that water comes to the timber. And so it is very important to assure the ventilation and the quickly drying of the timber. 3.8 FIRE SAFTY By this building the fire safety requirement imposed for the timber structure overall the requirement of REI60. So all the structural element has to fulfil this requirement. The design concept was very easy: all the CLT wall have to be protected by a double gypsum fire panelling on both sides. By the decks the panelling was applied only on the lower side, because on the upper side the floor construction assure a sufficient fire protection. All the walls have on both sides an interspace with 40 mm thickness as installation place. In the original project, this interspace was between the fire protection panelling and the CLT; so the interspace for the technical installations was a part of the fire safety. This is the usual and normally applied solution in timber building. By the building in the Via Cenni in Milan it was decided to apply the fire protection panelling - 2 times gypsum fire protection panelling - directly in contact with the CLT-walls. The interspace for the technical installations was created outside of the fire protection, by a third gypsum panel. On this way the fire protection is positioned at the right place and assure a directly protection of the timber. From the point of view of the owner of the building, the installation interspace is now outside form the fire protection of the structure and the risk of a damage of the fire protection is near to zero. Figure 13: Effective, formal and possible fire protection Finally the effective and formal fire protection of this building is given by the double fire protection gypsum panelling. In addiction to this, the third gypsum panel is of course a partially contribution to a higher fire resistance. And last but not least is should be remarked, that the CLT elements can contribute to the fire safety. So the fire safety of the timber decks of this building can reach 60 minutes, and the fire resistance of a part of the walls can reach at least 30 minutes. The design of the fire safety of this building was not affected from special conditions for timber constructions, but it had "just" to fulfil the "normal" fire safety requirements for 9-storeys buildings. 3.9 COSTS It is not always simple to compare the costs of a building. So it seems not correct to give detailed information about the effective costs of the building. But it is important to remark, that this project was realised by a big building investor, based on founding resources. It was from the beginning of the project of the timber construction a strictly condition given: the project have to be realised as a timber construction, but only if the timber construction is possible without majored costs.

8 Therefore it can be assumed, that the costs for this building with the timber CLT-structure are not higher as a similar construction with other materials. 4 CONCLUSION The concept, design and erection of timber residential building with this dimensions is not usual. A lot of topics are very different from the normal situation on timber building. The structural concept and design are an important part of the challenge, because the normal procedure to modelling - for example without the modelling of stiffness of the connections - will give wrong result. But some other questions have to be checked carefully. The described topics are just an overview of the open questions by this kind of projects. But they are probably the demonstration that the realisation of timber building with 9 or 10 levels are of course possible and can be regarded as the state of the art. This project shows and confirms that it is well possible to realize this kind of building with timber and especially with CLT - as multi storeys building and in a country with earthquake risk - under the following conditions: - the vertical and horizontal regularity of the construction have to be strictly respected - the continuity of the force flow have to be assured; for this continuously connections on the edge of the CLT panels are required - connections have to be designed with regarding the stiffness conditions, and not just the resistance - the robustness of the construction has to be considered as an important design principle. 5 REFERENCES [1] Bernasconi A.: Concept and design oft he CLTstructure oft he four residential towers with 9 storeys in the city of Mialn, Proceeding of the WCTE 2016 The building of the 4 towers in the Via Cenni in Milan is an example for the effective performances of this kind of building. Figure 14: View of the new quarter of the Via Cenni