Structural Design of a Habitation Tower in Sines

Transcription

1 Structural Design of a Habitation Tower in Sines Projeto de Estruturas duma Torre Habitacional em Sines Nuno Miguel Queiroz Florindo Extended Abstract October 2013

2 Structural Design of a Habitation Tower in Sines Lack of freedom in terms of optimal positioning of vertical elements to ensure torsional rigidity. Nuno Miguel Queiroz Florindo IST, Technical University of Lisbon, Portugal Key words: Eurocodes; Pre-design; Prestressed slabs; Seismic analysis; Slab-column connections; Torsionally flexible. Figure 1 Cantilever slab area (green shaded) on typical floor plan 1. Introduction The purpose of this dissertation is a practical application, of part of the knowledge achieved during the IST MSc in Civil Engineering, through the structural design of a residential building. The design process started with the conception of the structural solution, based on the architectural design. The next step was the definition of cross section dimensions, for all the structural elements, taking into account the actions and the materials properties as defined in the European regulation (Eurocodes). Lastly, through the utilization of the threedimensional finite elements program SAP2000, it was possible to quantify the effects of the loads on every concrete element, allowing the design of the steel reinforcements and prestress solutions needed to satisfy Limit States (Ultimate Limit States and Serviceability Limit States) and safety conditions. 2. Structural Conception The architectural project showed from the beginning its strong influence on the structural conception, mainly dictated by: Cantilever slab areas (Fig.1) with important span (over 3,50m) in 1st to 13th floors. Front panels (Fig.2) representing a load on the slabs edges; Architectural constraints relatively to the adoption of beams; Figure 2 Architectural project s South elevation (typical view of front panels) Once that all the conditions above were understood, they had to be conciliated with a viable reinforced concrete structural solution: Flat slabs (Fig.3) for all the floors (panels thickness 0,22-0,29m), with drops on the columns, in parking floors and ground floor (drop total thickness 0,37-0,42m), and prestressed 1st to terrace floors. Continuous (foundation-terrace) columns, in the central part of the building, with three cross-section variations, at levels coinciding with floors 0, 6 and 12. Continuous (foundation-terrace) walls without cross-section variations and constant 0,40m thickness. Mat foundation, due to the nature of soil (allowable bearing capacity of 1

3 400kPa) and the fairly heavy load induced on it by the 13-storey building. Figure 3 Flat slabs without and with drops 3. Design Criteria and Actions All the design process, as already mentioned, was developed following guidelines from European and Portuguese regulation: NP EN 1990:2009 [1] Eurocode0 Basis of structural design; NP EN :2009 [2] Eurocode1 Actions on structures, part 1-1 (general actions); NP EN :2010 [3] Eurocode2 Design of concrete structures, part 1-1 (general rules and rules for buildings); NP EN :2010 [4] Eurocode7 Geotechnical design, part 1-1 (general rules); NP EN :2010 [5] Eurocode8 Design of structures for earthquake resistance, part 1-1 (general rules, seismic actions and rules for buildings); DL nº 349-C/83 [6] REBAP Regulamento de estruturas de betão armado e pré-esforçado ; DL nº 235/83 [7] RSA Regulamento de segurança e ações para estruturas de edifícios e pontes. Safety verifications, of Ultimate Limit States (ULS) and Serviceability Limit State (SLS), were performed comparing effects on the structure Ed, of relevant combinations of actions, with resistance Rd and admissible values of serviceability parameters Cd. The general structure of the combinations of actions is presented in (1) and (2): E d = E{ γ G,i G k,i + γ Q,j Ψ j Q k,j }; (1) E d = E{ G k,i + A Ed + Ψ 2,j Q k,j }; (2) where: γ G,i - Partial factor for permanent action i; G k,i - Characteristic value of the permanent action i; γ Q,j - Partial factor for variable action j; Ψ j - Factor for appropriate value of the variable action j; Q k,j - Characteristic value of the variable action j; A Ed Design value of seismic action; Ψ 2,j Q k,j Quasi-permanent value of the variable action j. The structural materials choice (Table 1) was mainly influenced by the strong seismic action (according to [5]) on vertical elements. The lightweight concrete, of front panels and rooftop, satisfied the limitation of slabs vertical deflection, while reduced torsional seismic forces. Concrete C35/45 Lightweight Concrete Reinforcement bars Steel Prestress strands LC35/45 A500NR Y1860S7 Table 1 Resistance classes of structural materials according to European regulation 4. Structural Pre-Design The goal of pre-design process was the choice of the location, size and arrangement of different structural elements, respecting the architectural project. The definition of cross-sections dimensions intended to obtain a reasonable estimation of structure final geometry. A good pre-design was critical in the pursuit of ductile sections, for vertical elements, and controlled deflections of prestressed slabs. Slabs were pre-designed based on appropriate span/thickness ratios for every case [8]. Once obtained all necessary thicknesses (Table 2), it was possible to calculate exact permanent loads on the slabs. The last pre-design step consisted of estimation, of bending moments design values, through the equivalent frame analysis [3]; for every slab was calculated the necessary rebar percentage, based on bending moments of respective beam model (Fig.4), that allowed to confirm the practicability of chosen slab thickness. 2

4 Slab thickness/drops total thickness Floor (m) -2 and -1 0,22/0,37 0 (Ext.) 0,27/0,42 0 (Int.) 0,25/0,40 1 to Terrace 0,27 However, due to torsional flexibility of the building, the condition, previously cited, regarding seismic walls was clearly insufficient, leading to the necessity of a complex iterative design process based on the three-dimensional finite elements model. Table 2 Slabs thicknesses coming from preliminary design Figure 4 Equivalent frame (red and blue shaded) and corresponding beam model for typical residential floor The basis of vertical elements (Fig.5) pre-design was the quest for sufficient ductility in response to seismic action. Therefore it was necessary to classify vertical elements according to their participation in the building resistance to earthquakes. Then, for every column and wall (based on their influence areas and neglecting self weight), was calculated the design value of the applied axial compression NSd that conduced to cross section area Ac through: A c N Sd 0,85 f cd For vertical elements neglected in seismic design; A c N Sd 0,70 f cd For columns poorly resistant to seismic action; A c N Sd 0,65 f cd For primary seismic columns [5]; A c N Sd For primary seismic walls 0,40 f cd [5]. Figure 5 Identification of vertical elements (red) on underground floors simplified architectural plan The vertical elements pre-design ended with the earth retaining walls, surrounding underground floors. A thickness of 0,30m was chosen, based on recommendations [9] for such structural elements. Further preliminary verifications were made, regarding shear and bending moment resistance, based on a beam model (Fig.6), loaded with the relevant combination of actions including soil pressure. Figure 6 Beam model representing earth retaining wall Once known the design value of the axial force NSd at the base of all vertical elements (including their self weight), it was possible to calculate (3) the minimum area Amin, of standalone foundations, able to ensure the ability of soil to support the transmitted loads: 3

5 where: A min = N Sd σ adm ; (3) σ adm - Allowable bearing capacity of soil (400kPa). From previous calculations (3), resulted that standalone foundations should cover 45% of total area of building implantation. Facing that high percentage, it was considered more convenient (according to [10]) to design a mat foundation. A simple pre-design rule [10] provided the slab thickness: where: h = 10L + 30; (4) 5. Structural Modelling Modelling of pre-designed structural elements and acting loads was performed through the three-dimensional finite elements program SAP2000. The structural analysis, especially of dynamic behaviour of the building, provided accurate results in relatively short time, allowing the necessary adjustments, of cross-sections geometry and prestressing solutions, to the Limit States requirements fulfilment. The first step of modelling process was the definition of properties relative to all the materials employed in structural elements. Then the reference grid was created, based on structure alignments (Fig.8). h - Mat slab thickness (cm); L Largest span between vertical elements founded through the mat (m). The result of (4) was a thickness of 1m, which was applied to the central area loaded by all the floors (Fig.7). The remaining foundation was considered 0,50m tick due to the relatively moderate loads received from the columns P1 and P2. Figure 8 Reference grid Vertical elements and beams were simulated as 3D bar finite elements with two nodes (Fig.9), whereas slabs and walls modelling adopted finite shell elements with 3 and 4 nodes (Fig.10-11). Figure 7 Different thicknesses areas of foundation slab Parking access ramps (0,22m thickness) and stairs slabs (0,20m thickness) were pre-designed just to quantify the loads transmitted by them to the main structural elements. Once again simplified beam models were used. Figure 9 Finite bar element 4

6 Figure 13 Solid view of connection bar elements Figure 10 Mesh discretization of foundation slab All the properly modelled structure was linked to two different types of foundation slab model, according to design needs. The first model results, with foundation displacements restraints for all the vertical elements (Fig.14), were used in the design of all the structure, except foundation slab and soil retaining walls. Figure 11 View of west retaining wall mesh Lifts and stairs cores demanded particular attention in order to be correctly simulated. The main concern was the interaction between the elementary walls of each resistant set. The solution was to create horizontal connection elements, between the walls bar elements (Fig.12), with (Wall Tickness Storey Height ) cross- section (Fig.13). Figure 12 NC2 core walls bar elements (grey) and corresponding horizontal connection elements (red) Figure 14 Simply supported (green) vertical elements, with monolithic connection to the foundation slab The second configuration, of the foundation level, presented continuous elastic support (Fig.15) bellow the mat slab (area springs function in SAP2000 ); so, the soil-structure interaction was duly considered in the design of foundation slab and soil retaining walls. The stiffness of area springs (k=5000 kn/m), representing modulus of subgrade reaction ks, was obtained according to Bowles [11] recommended methodology (based on theory of elasticity). 5

7 Figure 18 Portion of front panels (shaded) acting on 6th floor slab Figure 15 Vertical elements founded on elastically supported (light blue) mat The modelling of actions and combinations obeyed the criteria defined in the already cited European norms. All the permanent and variable loads had to be properly simulated except the self weight of concrete elements (slabs, columns and walls), which was automatically calculated by the computer program. Stairs were simply represented by their actions on walls and slabs (Fig.16-17), as well as front panels (Fig.18-19). Figure 19 Linear loads on 6th floor slab edge, equivalent to front panels action Roof slab support panels needed a different type of modelling (Fig.20) due to the necessity of load transmission to lower floors. Figure 20 Portion of front panels represented by shell elements (roof slab support) Figure 16 Modelling of stairs action on core walls Prestressing action on slabs was converted in an, easily applicable, equivalent load system calculated, for each tendon set, based on cable profile (Fig.21). Figure 21 Typical cable profile and equivalent load system (beyond axial force P) Figure 17 Modelling of emergency stairs action on floor slab Then, total actions ( M1, M3, F1, F2, F3 and P), representing a cable set, were uniformly distributed in its influence width (Fig.23). 6

8 P u P máx P e (kn/cable) (kn/cable) (kn/cable) Table 3 Prestressing forces Therefore the employed design value of effective prestressing force Pe was 600kN/cable. Figure 22 Typical plan view of a cable set The aim of cable profiles conception was to maximize prestressing efficiency while respecting VSL design recommendations. So, plan and elevation layouts simplicity was combined with maximization of eccentricities emax,x (Fig.24) for cables disposed along the X direction (due to the location of major deflection problems). Figure 23 Modelling of vertical loads ( F1, F2, F3) equivalent to Fig.22 cable set Once modelled all the loads, corresponding to the building mass, it was possible to define seismic action. The application of EC8 [5] response spectrum, to the computer analysis of structural dynamic behaviour, gave the sought results. 6. Prestressing The necessity of prestressed slabs for 1st to terrace floors was clear since the pre-designing phase; however the definition of the final solution, with allowable deflections, led to an extensive iterative process. All the prestressing components technical features were obtained from the manufacturer VSL International Ltd. [12]. A bonded posttensioning system was chosen, with steel flat ducts, each one containing four strands (Table 3). Losses in prestress (immediate and time dependent) were simply estimated [8], assuming two consecutive 15% decrements applied to initial load Pmáx (corresponding to 80% of breaking load Pu [3]). Figure 24 Interaction between cable ducts and ordinary reinforcement bars in maximum eccentricity zones Slab thickness (cm) e max,x (cm) e max,y (cm) Table 4 Maximum cable eccentricities Afterwards, equivalent loads (Fig.21) were defined for all the cable groups (Fig.22) that formed the final prestressing solution. As already referred this was an evolutionary process (Fig.25-27), in search of slabs deflection control, which started with few cable sets, just for column bands, and ended with two slabs thicknesses along with twelve different tendon configurations. With the purpose of verifying deflection serviceability limit state, calculation of allowable values was performed (Table 5). It was necessary to convert long-term deflection (limited by regulation [3] [6]) in an equivalent instantaneous value, of such parameter, comparable with elastic floor deformation, coming from finite elements analysis; so the 7

9 effects, of shrinkage, creep and cracking, were considered able to triplicate (prestressed floor system) or quintuplicate (conventionally reinforced floor system) instantaneous deflection [13] (Table 5). Allowable long-term deflection (m) Parking floors and garden floor 0 to terrace floors 0 to terrace prestressed floors Long-term effects multiplier Allowable elastic deflection (m) L/250 5 L/1250 Figure 27 7th floor deflection problem solved by final prestressing solution 0, ,0030 0, ,0050 The prestressing design was completed when allowable elastic deflection was not exceeded in any point of the model. An increase, of slabs thickness, to 0,29m was also needed for 13th and terrace floors. Table 5 Allowable deflections (due to quasi permanent load combination) admitted in the serviceability limit state verification Next step consisted of identifying critical zones where conventional floor system failed to guarantee deflection control (Fig.25). 7. Seismic Analysis The seismic action was quantified according to Eurocode8 [5] recommendations; it was accordingly necessary to identify and quantify all the parameters, needed to a proper earthquake representation. Figure 25 7th floor deflection problem without prestressing Figure 28 Configuration of elastic response spectrum [5] Figure 26 7th floor deflection with prestressed X direction column bands (intermediate solution) In the regulation context [5] the earthquake motion at a given point of the surface is represented, for a reference return period TNCR 8

10 (475years), by the elastic response spectrum Se(T), where T is the vibration period of a linear single degree of freedom system. Therefore that kind of spectrum was the first to be defined (Table 6), for both types of seismic action (Type I or II representing far or near-field ground motion). Type I spectrum Type II spectrum Seismic zone a gr (m/s 2 ) 1,50 1,70 γ I 1,00 a g (m/s 2 ) 1,50 1,70 Ground type S max 1,60 1,60 S 1,50 1,46 T B (s) 0,10 0,10 T C (s) 0,60 0,25 T D (s) 2,00 2,00 Table 6 Parameters values, applied to elastic response spectra definition The capacity of structural system to resist seismic action in the non-linear range, dissipating energy through ductile behaviour of its elements, was taken into account by performing an elastic analysis based on a reduced elastic response spectrum (Sd(T) design spectrum) [5]; such decrease was accomplished by introducing the behaviour factor q, which is an approximation of the ratio of the seismic forces that the structure would experience if its response was completely elastic with 5% viscous damping, to the seismic forces that may be used in the design. Consideration of non-linear resistance recruitment, also compelled to take into account cracking effects on primary elements, through the reduction of flexural and shear stiffness properties to one-half [5] of the uncracked elements stiffness. C Mode T (s) f(hz) Translation along X dir. Translation along Y dir. Rotation % % % % % % 1 2,979 0,336 3,54 3,54 0,07 0,07 23,92 23,92 2 2,011 0,497 4,45 7,99 35,26 35,33 0,53 24,45 3 1,992 0,502 34,93 42,92 4,58 39,91 2,14 26,58 4 1,015 0,985 0,73 43,64 0,03 39,94 3,73 30,31 5 0,638 1,566 6,47 50,11 0,10 40,04 0,50 30,81 6 0,570 1,754 0,37 50,48 0,05 40,09 1,03 31,85 7 0,510 1,963 0,24 50,72 10,60 50,68 0,01 31,85 8 0,386 2,591 0,03 50,74 0,00 50,69 0,66 32,52 9 0,345 2,898 2,10 52,84 0,05 50,74 0,08 32, ,288 3,474 0,03 52,88 0,01 50,75 0,32 32, ,242 4,134 0,66 53,53 0,14 50,88 0,00 32, ,238 4,197 0,07 53,60 2,39 53,27 0,00 32,92 Table 7 Periods, frequencies, and mass participation factores of all modes of vibration contributing significantly [5] to the global structural response In the identification of relevant modes of vibration (EC (3) [5]), it was important to consider as total mass of the structure just the one above ground floor level; so, 50,72% of the mass, including basement floors, corresponded to 95% of the mass effectively involved in oscillatory motion. To respect the condition, above described, seismic analysis could have been based on the response of nine modes of vibration; however it was considered appropriate to include twelve modes (12th mode presented a 4,8% mass participation factor for translation along Y direction). Prior to behaviour factor calculation, a spatial modal analysis was performed in order to obtain intrinsic dynamic properties of the structure (Table 7) and preliminary understanding of its seismic response (Fig.29-31). Figure 29 1st mode of vibration plan view 9

11 k w - Factor reflecting the prevailing failure mode in structural systems with walls (EC (11)-(12) [5]). q 0 k w q 2,0 1 2,0 Table 8 Calculation of the behaviour factor Once determined all the required parameters (Tables 6 and 8), design spectra were defined (Fig.32). Figure 30 2nd mode of vibration plan view Figure 32 Design spectra Figure 31 3rd mode of vibration plan view In order to quantify the behavior factor q, structural regularity was tested, firstly in plan (EC [5]), due to raise greater concerns (Fig.29), then in elevation (EC [5]). The aim of structural classification, regarding to regularity, is to taking into account geometrical and stiffness distribution characteristics that may aggravate the demand on seismic members, reducing their ductility. The building, as already suspected, revealed its irregularity in plan, due to lack of minimum torsional rigidity (EC (6) [5]), which also led to a torsionally flexible structural system classification. According to these results, the basic behaviour factor q0 was obtained (EC8 Table 5.1 [5]) for the DCM class building (Medium Ductility Class). Lastly the value of ULS design behaviour factor q was calculated (5): where: q = q 0 k w ; (5) The obtained results (Fig.32 and Table 7) allowed to conclude that, for relevant modes of vibration, the type I seismic action was superior, and thus should be the one adopted in the design process. The complete definition of design seismic action, also required accidental torsional effects consideration (due uncertainties in the location of masses and in the spatial variation of the seismic motion EC [5]). Therefore, at each floor level, a torsional moment was applied (6), representing such effects: where: M ai = e ai F i ; (6) M ai Torsional moment applied to floor i; e ai - Accidental eccentricity of storey mass i from its nominal location, applied in the same direction at all floors (EC (1) [5]); F i - Total horizontal force, acting on floor i, due to design seismic action (from design response spectrum modal analysis). X direction eccentricity, allied with Y direction horizontal forces, presented the higher values of Ma, which were duly combined with spectral effects (Fig.32). 10

12 Second order effects proved to be inferior to 10% of horizontal seismic action (according to EC (2) [5]), and therefore could be neglected. After the, so far described, quantification of ULS design seismic action, damage limitation requirement was studied, checking (EC (1a) [5]): where: d r ν 0,005h; (7) d r - Design interstorey drift, evaluated as the difference of the average lateral displacements d s (EC [5])) at the top and bottom of the storey; ν - Reduction factor, which takes into account the lower return period (95 years) of the seismic action associated with the damage limitation requirement (ν=0,4 according to EC8 National Annex table NA.III [5]); h- Storey height. Even considering the attachment of nonstructural elements of brittle materials to the structure, damage limitation did not represent a problem. Figure 33 Core configurations P5, NC1-3 and NC2-3 walls clearly failed in such condition (8) and, due to architectural constraints to cross-section dimensions enlargement, had to be classified as secondary walls. Once individuated primary walls, according to EC8 [5] procedure, design envelopes (Fig.34), of shear and bending moments, were defined for such elements (Fig.35), aiming to consider the effects of uncertainties regarding the moment distribution, along the height, and increase in shear forces, due to base yielding. 8. Ultimate Limit States Design All the design assumptions adopted in this paper were based on material properties, and sections/elements resistance characteristics, recommended by the Eurocodes 2 [3] and 8 [5]. Throughout this process, final validation of predesign choices was obtained, with the definition of reinforcement bars configurations, ensuring sufficient resistance and suitable ductility of designed members. Needed verifications were performed relatively to bending (with and without axial force), shear and punching Ultimate Limit States. The design began with the analysis of walls (Fig.5 and Fig.33), and more specifically with their classification (8) into primary and secondary members (EC (2) [5]): where: ν d 0,40; (8) ν = N d - Normalised axial load of a (b h) bhf cd section of concrete, with design compressive strength f cd, submitted to maximum compression N d (for design seismic action). Figure 34 Seismic design envelopes (EC [5]) 11

13 prevent premature crush and detachment of compressed concrete in boundary zones. Figure 36 Confined boundary element in critical region of NC2-1 wall By the combination of the different results, obtained for all primary walls, the first two floors were treated as critical region of the core, in order to maximize constructive simplicity. Outside critical region, and in secondary walls design (based on moment and shear diagrams from analysis), EC2 9.6 [3] detailing rules were applied. Figure 35 Diagrams from analysis and design envelops of NC2-1 wall Necessary longitudinal (manual calculation through proper concrete design interaction diagrams [10]) and shear reinforcement bars were designed (EC2 6.1 and [3]) based on calculated envelopes. It is important to stress that in shear reinforcement calculation, of vertical primary members potential regions for plastic hinge formation (EC (1) [5]), a value of 38 was assumed (30 was generally used out of critical regions), for the angle θ (between the concrete compression strut and the element axis perpendicular to the shear force), in order to prevent fragile sliding brittle failures [14]. Beyond resistance verification, local ductility was achieved (EC [5]) through the application of confining reinforcement detailing requirements (Fig.36); the purpose of such providences was to improve walls resistance to cyclic earthquake action, by Regarding the columns design, as well as for walls, the first step was the verification of suitability to primary elements limitations, in the axial load domain (EC (3) [5]): ν d 0,65. (9) Such condition was verified for all the columns; however P6 elements (Fig.5) showed to be impossible to reinforce, under design seismic action, and were therefore classified as secondary members. To represent early loss of resistance (EC (1) [5]), under earthquake action, secondary members (columns and walls) were modelled with parameters, of flexural and shear resistances, one hundred times inferior to those of uncracked elements. Once properly modelled all the vertical elements, SAP2000 capabilities (concrete design for biaxial bending with axial force situation) were used to obtain longitudinal reinforcements of columns and secondary walls. Columns shear failure 12

14 prevention, according to capacity design rule (EC (1) and [5]), led to design values of shear forces; from the combination of shear and confining demands, transverse reinforcement was designed within (EC [5]) and out (EC2 NA.9.5.3(3) [3]) of critical regions. Secondary columns were designed according to EC2 9.6 [3] recommendations and shear values from analysis, such as primary ones in the zone below ground floor. Slabs design began with bending reinforcement calculation using, once again, the potential of SAP2000 (design considering bending and prestress axial force); based on nodal reinforcement areas, provided by the computer program, average values were calculated for every band (Fig.37-38) with predictably homogeneous detailing [8]. based on most unfavorable M d,x - Md,y - Vd combination for each slab-column connection (Fig.39). Shear reinforcement was designed where needed (EC [3]). Figure 39 Actions involved in punching resistance assessment In order to provide further security guarantees, for the performance of slab-column connections, preventing situations of progressive collapse of the structure [3], postpunching reinforcement (Fig.40) was conceived (Fig41). Figure 37 Typical floor design bands along X direction Figure 40 Model of slab-column connection during punching failure [15] Figure 38 Typical floor design bands along X direction The reinforcement areas, thus obtained, were compared with empirical values, associated with the good behaviour of slabs [8], and limit values (EC2 9.3 [3]), showing to be perfectly feasible. During the detailing phase, cracking control had to be considered (EC [3]), leading to 16mm maximum bar size and 200mm maximum bar spacing. The punching resistance was then verified (EC2 6.4 [3]) Figure 41 Anchorage lenghts of post-punching rebars [16] Foundation slab was also designed according to the algorithm described above; the main 13

15 difference was in detailing phase when rebar configuration, needed for the more stressed bands (for each direction and slab thickness), was extended to all the slab, obviating to soil related heterogeneities and uncertainties. Additional shear resistance assessment was performed along thickness transitions. Soil retaining walls were treated as one way slabs, assuming cylindrical bending, whose design values of bending moment and shear, for each relevant level, coincided with the highest ones (along the wall length) coming from finite elements model. It is important to note that, desirably, shear resistance did not require the adoption of specific reinforcement, confirming the validity of pre-deign. Horizontal reinforcement was designed according to EC [3] recommendations. 9. Conclusions The realization of this work proved to be a constant challenge of intermediate design targets fulfilment, overcoming the involved difficulties. However the inspiration came from the thought engineers would not exist without problems to solve. So, it was possible to provide a valid solution to the problem, mainly consisting of architectural constraints to a simple seismic design. Firstly is important to stress that, in a real case, the original architectural configuration, which was not modified in the present work, would probably be subject of debate with the architect in order to allow the design of a less expensive and more efficient (greater torsional stiffness) structural solution. The flat slab system, then, would be avoided due to the lack of energy dissipation capacity (slab-column joints) [5]. The special emphasis, given to pre-design phase, was subsequently crucial, leading to final structural definition without major changes in cross-sections. The only exception is represented by the core walls whose design process consisted of an iterative process based on the finite elements model. Some issues arose regarding consultation and interpretation of existing regulations, particularly EC8 [5]. In addition to the complexity and ambiguity in the definition of some parameters, it is noteworthy the absence of specific guidelines for the seismic analysis of buildings with flat slabs systems. Finally, it is worthy to evidence the importance of software in structural analysis, especially in the seismic effects characterization field, which was personally witnessed during this project. 10. References [1] NP EN 1990:2009. Eurocode0 - Basis of structural design, [2] NP EN :2009. Eurocode1 - Actions on structures, part 1-1 (general actions), [3] NP EN :2010. Eurocode2 - Design of concrete structures, part 1-1 (general rules and rules for buildings), [4] NP EN :2010. Eurocode7 - Projeto geotécnico, parte 1 (regras gerais), [5] NP EN :2010. Eurocode8 - Design of structures for earthquake resistance, part 1-1 (general rules, seismic actions and rules for buildings), [6] DL nº 349-C/83. REBAP - Regulamento de estruturas de betão armado e préesforçado, [7] DL nº 235/83. RSA - Regulamento de segurança e ações para estruturas de edifícios e pontes, [8] C. Marchão e J. Appleton, Betão armado e pré-esforçado II (Folhas de apoio ás aulas), IST, 2011/2012. [9] P. Lança, Pré-dimensionamento de elementos estruturais em betão armado, ESTIG, [10] P. Jiménez Montoya, Á. García Meseguer e F. Morán Cabré, Hormigón Armado, Gustavo Gili, SA, [11] J. E. Bowles, Foundation Analysis and Design, McGraw-Hill,

16 [12] [Online]. [Accessed on ]. [13] C. Marchão e J. Appleton, Betão armado e pré-esforçado I (Folhas de apoio ás aulas), IST, 2011/2012. [14] M. Lopes (coord.), Sismos e edifícios, ORION, [15] ACI R89 - Recommendations for Design of Slab-Column Connections in Monolithic Reinforced Concrete Structures, [16] A. Ramos e V. Lúcio, Estruturas de betão armado II - Lajes fungiformes, UNL,