Overcladding of Existing Large Precast Concrete Panel Buildings Using Steel

Transcription

1 Bauhaus Summer School in Forecast Engineering: Global Climate change and the challenge for built environment August 2014, Weimar, Germany Overcladding of Existing Large Precast Concrete Panel Buildings Using Steel FLORICEL, Andra Department of Steel Structures and Structural Mechanics, Politehnica University Timisoara, Romania UNGUREANU, Viorel Department of Steel Structures and Structural Mechanics, Politehnica University Timisoara, Romania Abstract A large part of the Romanian urban population, as that of Eastern Europe, lives in collective residential buildings made out of large prefabricated concrete panels, mostly built between 1960 and 1989, most of them with major structural and energetical deficiencies. However, the refurbishment of these buildings had to comply with these neccesities, without compromising the structural integrity and stability of the building. The solution of overcladding these buildings has become not only an inevitable approach, but rather a mandatory process in achieving the enlargement of the living space, as well as the refurbishment of the existing building stock in terms of energy consumption and environmental aspects, without bringing much extra weight to the structure. Present paper presents and compares three types of overcladding structural solutions based on intensive use of steel elements: a) hot rolled steel; b) rectangular hollow sections; c) cold-formed steel profiles. The solutions are analysed and compared from structural point of view, but also from environmental impact perspective, in order to determine the optimum solution for these large precast concrete panel buildings. Introduction Most of the large prefabricated panel buildings from Romania were built in the period , according to data provided by the same census. In the period prefabricated buildings were built which represent more than 81% of the total existing buildings of this type. In order to achieve large cost advantages in the construction of these apartment blocks, the national design institutes delivered a series of standardized projects applicable over the national territory. The most popular IPCT projects were the low rise 5 storey, as follows: (a) P+4: 770; 744; 994; ; 1340; 1586; 2926; 1399; (b) P+8: 772. An insight analysis for the city of Timisoara shows that the housing made of precast concrete panels were executed in three major stages using different standardised project depending on the applicable state decrees during time, which were in accordance with the evolution of the design and urban systematization stages (Botici et al, 2012): (1) Period the most used standard project was T744R-IPCT ; (2) Period the most used standard project was 770-IPCT ; (3) Period the most used standard project was IPCT. Existing solutions for building overcladding The refurbishment of this type of structures by adding a usable living space level is a usual method of enlarging the existing space. Although in the last decade a lot of standards and guidelines concerning

2 FLORICEL, Andra-Daniela, UNGUREANU, Daniel-Viorel / FE the structural, technological and energetic efficiency for these interventions have been elaborated, some aspects regarding urban planning and exterior aesthetics of new-added storeys have been ignored when applied in reality. Therefore, in present context, there is a large variety of geometries, volumes and aesthetics for the overcladdings even in the boundaries of the same neighborhood, degrading the overall visual image (see in Figure 1). Also, the main errors pointed out in the execution of these overcladdings can be divided in two categories: the first regards the bearings and anchorage of the overcladding system to the buildings structure of resistance while the second is related to the actual structural errors. Figure 1. Interventions by overcladding in Timisoara Steel solutions for overcladding The proposed solutions for overcladdding were considered for the block typology E744.R (built in the period ), a representative typology for these large prefabricated blocks, with a great appliance not only in Timisoara, but on the entire country. The structural model was analysed in the finite element program Etabs 9.7, by considering diaphragm element type, of 11 cm thickness for external and respectively 14 cm for the internal ones. In a first step the existing structure was checked from resistance and stability perspectives, considering the current loading and checking norms. In a second step the building was considered together with the extra-storey and again checked for both existing and new elements. As conclusion it could be stated that the overcladding system proved to be light enough as to not overload the existing structure. This was mainly due to the lightness of the material chosen for its construction by application of steel solutions. It is also to be noted that the building had been previously unloaded after the process of removing the hydro insulation and thermal insulation layers of the terrace, facilitating the process of choosing the steel structure solution as the optimum one for this kind of blocks. The overcladding system was analysed by using the same program. The following geometry was considered: 3 m height, 9.6 m span and 7 longitudinal frames of varying lengths between m. In order to have a homogenous aspect, the spans and interior compartments of the building were preserved (see Figure 2). The concrete structure and overcladding were designed in accordance with the following standards: (1) EN "Design code. Basis of structural design"; (2) EN "Design code. Evaluation of load actions on structures"; (3) EN "Design of concrete structures"; (4) EN "Design of steel structures"; (5) EN "Design of structures for earthquake resistance". For the seismic analysis of the overcladding system, the design gravitational acceleration specific to the region of Timisoara was chosen, a g =0.20g and a behaviour factor q = 1, specific to non-dissipative structures. For the chosen case study (block type E 744.R), three types of solutions of overcladding based on intensive use of steel were chosen: (a) Overcladding solution based on the use of hot-rolled profiles

3 FLORICEL, Andra-Daniela, UNGUREANU, Daniel-Viorel / FE (IPE beams and HEB columns); (b) Overcladding solution based on the use of rectangular hollow sections; (c) Overcladding solution based on the use of cold-formed sections. Figure 2 also shows the total weights of the steel elements and connecting devices (steel structure). Case: Case 1: Hot-rolled profiles Case 2: Rectangular hollow sections Case 3: Cold-formed sections Total weight of structure (kg) Figure 2. Block E744.R + overcladding (3D view) and total weight for the 3 cases of overcladding solutions Regarding the base connection, a pinned solution was chosen for all of the 3 cases. (see Figure 3a and 3b). The wall covering is placed on the attic and supported by a cold-formed C profile pier framework (see Figure 3). (a) (b) Figure 3. Column-base connection details case 1 and case 3 respectively The structure for the external walls is realised by cold-formed C profile, while for the roof Z purlins were considered. The overcladding system is enclosed by using OSB panels for both walls and roof. Moreover, the choice of stratifications for both the roof and wall macro-components was based on the need of ensuring a proper thermal and hydro insulation, such that an optimum interior thermal comfort would be assured, condense being avoided. The roof and exterior wall macro-components are detailed in Table 1. Table 1. Material quantities macro-component (per square meter) Thickness Weight/area Total weight Element type No. [mm] [kg/mp] (kg) 1 Roof Interior rendering + Plaster board , Vapour barrier Mineral wool Aluminium foil ,096 5 Oriented strand board (OSB) Anti-condense foil , Timber frame Steel tiled sheet ,352 9 Cold formed steel elements - 5 1, Interior rendering + Plaster board ,868 11,622 No. Wall Element Type Thickness Weight/area Total weight

4 FLORICEL, Andra-Daniela, UNGUREANU, Daniel-Viorel / FE [mm] [kg/mp] (kg) 1 Interior rendering +Plaster Board ,265 2 Vapour Barrier Oriented strand board (OSB) int Mineral Wool Oriented strand board (OSB) ext , Thermal insulation (polystyrene expanded) 2 Polyester wire lattice ,311 (glass fibre)+baumit rendering 8 Cold rolled steel profiles ,373 7,072 Numerical analysis of overcladding connections In order to prove the efficiency of the base connections chosen for the study, a numerical simulation was realized in the finite element program Abaqus The two solutions modelled were the ones illustrated in Fig. 3 above : Semi-rigid connection based on hot rolled profiles; Semi-rigid based on cold-formed sections. In both simulation scenarios, the column-floor connection assembley was modelled in the program Abaqus 6.11 (see Figure 4). The material used in the connection was presumed to be elastic perfect plastic, so that it would have a plastic deformation, and a maximum displacement of 100 mm was imposed on the model, trying to obtain the maximum capable stress of the connection, and a forcedisplacement curve. The calculus was non-linear and a dynamic analysis was used. The efforts resulted were normal and tangential. a) b) Figure 4. Column-base assembley

5 FLORICEL, Andra-Daniela, UNGUREANU, Daniel-Viorel / FE The first numerical model is a pinned connection with two base plates, connected with bolts M16 and M20. The first plate is welded on the contour to the base plate connection. The bolts that make the connection along the web of the profile are sink bolts. The entire connection is linked to the existing building through the base plate of 200 mm length and a thickness of 15 mm and through angle profiles. The column is a hot-rolled HEB180 profile. The second numerical model is also a pinned connection but with only one base plate. The column is a cold-formed section of two back to back C profiles of 300 mm height and a 2 mm thickness. Two rows of M16 bolts were used along the web. The geometrical characteristics of the base plate and the angle profiles are the same. A rectangular hollow section of 6 mm thickness and 150 mm height was placed between the two sections of the column for stability purposes, and was welded to the base plate on its contour. The following partial results were obtained : - The welded plate of the hot-rolled profile presents o slight plastic deformation (see Figure 5); - The maximum effort for the hot-rolled connection was reported in the sunk bolts on the direction of loading (see Figure 5); - The maximum effort for the cold-formed connection was reported in the upper flange of the section on the direction of loading (see Figure 5); - Both connections were modelled as semi-rigid but present an articulated behaviour ; - The performance of both case scenarios was obtained under lateral loading ; - After numerical analysis the load transfer mechanism was identified ; - The weak components were singled out, which are presumed to yield first ; - Plastic efforts and deformations were obtained (see Figure 5) ; a) b) Figure 5. Stresses in the connections The scope of the numerical program is a better understanding of the behaviour of the two types on node connections and to find the optimum solution for a base connection for this kind of overcladding systems that will not bring additional efforts to the existing concrete panel building. This numerical study is actually a process of finding the optimum solution and it will be followed by a process of parametric study, and last but not least a laboratory study of these kind of connections.

6 FLORICEL, Andra-Daniela, UNGUREANU, Daniel-Viorel / FE Environmental impact analysis Sustainability of the built environment The concept of sustainability refers to the evolution of all branches of society without irreversibly affecting the environment, ideally resulting in equilibrium between the social, economic and environmental issues. According to The Chartered Institute of Buildings in Great Britain, around 45% of global energy is used for maintaining the buildings and only 5% of global energy is used during the construction stage of the structures. The current legislation regarding the sustainability in buildings includes the following European standards, which present the general methods of performing a life cycle assessment (LCA): EN 15643: Sustainability of construction works Assessment of buildings; EN 15804: Sustainability of construction works - Environmental product declarations; EN 15978: Sustainability of construction works Assessment of environmental performance of buildings. Figure 6 presents as comparison the environmental impacts of the three structural systems considered with the impact of the envelope. The results show by far a higher environmental impact resulting from the latest than that of any of the three steel solutions, and the most affected impact category is the one of resources. Figure 6. Comparison steel structures vs. cladding considering major impact categories Figure 7. Environmental impact of the three steel structures and the secondary steel structure skeleton In order to depict the impact due to the steel elements from the envelope, Figure 7 presents the same results of the impact analysis for the three steel structures but compared with the impact caused by the steel elements from the covering (light gauge C section piers and Z purlins). The results show the fact that the secondary steel structure skeleton brings only a relatively small contribution to the global impact of the entire overcladding. Taking into account that the material used was steel for all four compared items, the importance of quantity in this analysis was highlighted once again. In consequence, referring to the envelope macro-component, it results that the contribution of the nonsteel elements of the envelope (roof + ext. walls) can be pointed out as having the greatest environmental impact. This resolution is especially interesting as it proves that the choice of materials for the roof, walls, thermal insulation, etc. could be more important than the choice of steel structure from an environmental impact point of view. Into an inner analysis of the envelope system, Figure 8 offers a comparison between the roof and the exterior walls macro-components. The highest environmental impact is given by the materials from the roof sub-assembly, almost double compared to the materials from the wall sub-assembly. The inert materials contained in higher amounts in the

7 FLORICEL, Andra-Daniela, UNGUREANU, Daniel-Viorel / FE roof (mineral wool, polystyrene, plasterboard, etc.) bring the highest environmental impact, especially due to their low recycling capacity. Figure 8. Roof vs. wall cladding environmental impact comparison considering the main impact categories Conclusions The study presented in this paper shows the possibilities of overcladding the existing large precast concrete panel buildings by using steel-intensive solutions. The structural and environmental impact analyses show the following: (1) the structural systems are robust and safe even considering the new generation of codes and loading conditions; (2) steel-intensive solutions are ideal systems for overcladding the existing large precast concrete panel buildings due to their lightness, reversibility and clean sites. Also, they can adapt to existing structural systems and several structural typologies can be thought; (3) the environmental impact analysis have shown the fact that the recyclability of the steel elements lowers the overall impact of the overcladding systems. In this manner, the impact is due mainly to thermal and hidro-insulation materials, which for now are with low recyclability and or reusability. Acknowledgement This work was partially supported by the strategic grant POSDRU/159/1.5/S/ (2014) of the Ministry of National Education, Romania, co-financed by the European Social Fund Investing in People, within the Sectoral Operational Programme Human Resources Development References Botici A.A., Ungureanu V., Ciutina A., Botici A. and Dubina D. (2012). Sustainable retrofitting solutions for precast concrete residential buildings. Proceedings of the Third International Symposium on Life-cycle Civil Engineering: Life-Cycle and Sustainability of Civil Infrastructures - IALCCE 2012, Viena, Austria, , CRC Press, Taylor & Francis Group, CD ( ). EN (2012), Sustainability of construction works. Assessment of buildings. Framework for the assessment of social performance, European Committee for Standardization CEN. EN (2012), Sustainability of construction works. Environmental product declarations. Core rules for the product category of construction products, European Committee for Standardization CEN. EN (2011), Sustainability of construction works. Assessment of environmental performance of buildings, European Committee for Standardization CEN. NP Normativ pentru proiectarea mansardelor la clădiri de locuit (in Romanian) / NP , Romanian standard for design of overcladding residential buildings. Project Type 744; ; Institute for Standardized Building Design IPCT Bucharest. Simapro tutorial manual; Simapro Database and Methods Manual The Chartered Institute of Building- CIOB, Sustainability and Construction,