Industrial type precast buildings Effects of roof diaphragm and frame continuity

Transcription

1 Industrial type precast buildings Effects of roof diaphragm and frame continuity Pedro Carvalho Costa Instituto Superior Técnico, Lisboa, Portugal October 2015 Abstract In this paper, it is studied the theme of precast reinforced concrete buildings of the industrial type, focusing on its peculiarities and differences compared to the traditional building techniques, in particular, to structural design and detailing of the connections between structural elements Regarding the building s strength and structural behavior upon occurrence of seismic events, it is studied the importance of achieving a global behavior of the structure through a stiffening of the roof and solidarization of all vertical structural elements. It is proven that a more independent action between columns it is unfavorable in terms of global response. This second type of behavior features a large number of such solutions in Portugal, It s studied the influences of these two types of design in the design for rupture and evaluation of their performance in service, with special interest on improving the structure s robustness, redundancy and structural reliability, but also in the control of differential displacements between columns, which have an impact on a deformability reduction. This reduction allows greater freedom in the choice of materials of non-structural elements of facade and roof. The stiffening of the roof plane is achieved by a truss system designed at that level, and its efficiency is discussed based on a case study. It s also studied the influence of creating continuity in the beam-column connections and its positive consequences in the structural behavior. The structural connections between elements that are required to achieve the different idealized structural behaviors are also designed. Key-words Structures, Precasting, Seismic Behavior, Structural Connections 1. Introduction The term "precast element" refers to any structural concrete element not produced in their final destination, it may be made in the construction site and subsequently put in place, or typically in technical facilities built for the purpose, which are usually protected adequately from adverse weather conditions. [1] Prefabrication as a construction technique has numerous advantages among which are emphasize the reduction of the construction time and the fewer need for workers at the construction site, which lead to a lower overall cost of the work. These advantages are directly dependent on the simplicity and ease with which the various precast elements are connected and put into place. Industrial type buildings are the one of the largest markets for precast structures in Portugal. Although prefabrication already has several decades of use in the country, some carelessness in design has caused a delay and a weak growing of the activity that was not initially expected In particular, the implementation of structural connections between elements has been affected by both a great resistance to change by the precast companies and a continuous pursuit for a constructive method as simples as possible, which have contributed for less credibility and expansion of this construction technique. This weakness, combined with constructions with less degrees of freedom where rarely there are structural connections transferring bending moments, causes that less robust and redundant buildings are designed, something not very advisable, especially in areas with seismic activity. 2. Background The connections between prefabricated elements are what distinguishes a precast structure from a cast in-situ structure and they are also a key part of the behavior of this type of buildings. The resistance of a structure will be fully affected by the resistance of its connections, one that since there is no point having a very robust structure if the transmission of stresses between the various structural elements is not possible or close to what the designer intended.

2 2.1 Transmission of stresses Compressive forces Any precast element must be supported at one or more sites, so it can transfer its own weight and the loads it is subjected to the foundations of the structure to which it belongs. The main aspect of the transmission of such efforts is the area that transmits from one piece to the adjacent ones. This transmission may be by direct contact between the concrete elements (Figure 1), by means of mortar or concrete joint, or even through rigid or flexible support devices. [2] Typically in industrial buildings there aren t heavy loads on the roofs, so that the transmission compressive forces by direct contact is widely common. Figure 1 Slab supported by direct contact [3] Figure 2 Role of ties against an explosion event [4] Tensile forces The concrete, as its known, has itself very little to resisting tensile stresses, typically its small contribution to this resistance is neglected. As such, in precast structures, connections should be designed assuming that the sections are cracked. The strength of a connection to tensile forces is not only determined by its rebar, but also by how it is conceived and by the resistance provided by anchors, if any. This type of connection is common in precast buildings when they have locking systems or when you want to provide more robustness and structural integrity in a framework, through the so-called "ties"(figure 2) that usually only are activated due to accidental actions, creating alternative load paths. [4] In connections transferring bending moments, the transfer of tensile stresses is also made with special careful for the anchorage length Shear Forces The shear force can be transmitted in a connection through many ways. The best known are the friction between faces of elements or between "indented" surfaces, the dowel action or through steel devices, such as plates and bolts. Figure 3 Dowel behavior [3] The dowel action (Figure 3) it s not mentioned in EC2 and can only be considered as reinforcement if it is only resisting to one kind of stress. [5] There are several connection failure modes when it is based on this resisting method. The simplest of them all is the shear rupture of the rebar.

3 The resistant force for this case can be obtained with the expression of von Mises for the yielding tension Equation 17 [3]: F vr = 1 3 f ya s (17) However, the Model Code 90 [1] suggests a slightly different expression, which takes into account whether the rebar is simply attached or with some type of anchoring device, and this expression is Equation 18. F vr = α d f yd A s (18) In which the α d may be 0.6 in the so-called normal cases or 0.75 if the bolt has a more complex anchorage. The general expression which provides the connection s strength in the general case, taking into account a mechanism with two plastic hinges (Figure 4) and a possible eccentricity is Equation 19. Figure 4 2 plastic hinges mechanism [3] This mechanism is formed when the rebar is sufficiently strong in relation to the surrounding concrete and there is a significant settlement of the dowel bar in the surrounding concrete that is crushed under high compressive stresses. By this formula, we arrive at the conclusion that an eccentricity with the same order of magnitude of the rebar, causes a reduction of about 40-60% of the connection resistance, thus discouraging designs containing higher eccentricities. On the other hand, the manual for seismic design of frames with industrial elements in concrete from CERIB [6], suggests a different expression for the dowel rebar design (Equation 20), it considers only the rupture of the steel by stress, being however, less conservative than the one suggested for the double plastic hinge mechanism. (3G + N) A < f yk (20) In this expression, G stands for the shear affecting the dowel and N stands for the normal stress that should be considered as positive in case it s a tensile force. Usually, this last one only appears when there is torsion in the beam. 2.2 Bending Moment The connections transferring bending moments are typically designed to stabilize or to increase the robustness of structures may also provide greater structural integrity and redundancy to improve resistance to progressive collapse. Another advantage may be a better and more equilibrated distribution of stresses between columns and beams, which may lead to the design of structurally slender elements. Making a connection between pre-fabricated elements, in a monolithic solution can also solve problems of lack of knowledge and connections design guides for seismic actions, since it is a way for the connections to behave as if the structure was casted in-situ. However, the fact that a connection transfers bending moment does not indicate by itself that it becomes more ductile [2]

4 2.3 Seismic Design Norms In terms of specific regulations for this type of structures there is still much to develop, only existing a few articles, in particular Article 10 EC2 [7], and Article 5.11 of EC8 [8] About specific criteria for the seismic design of connections, there is also a lot to evolve and there aren t many documents concerning this subject. Another subject is the choice of the behavior factor for this kind of structures. The EC8 has two main categories which may adjust to this case, such categories are as an inverted pendulum system or as a frame system. The problem is that for industrial type buildings, that typically have only one floor, the difference of applying choosing one category or the other is quite big, from 1,5 to 3,3. In this sense, the Italian Norm [9] is wider in the categorization of the types of precast structures, also assigning different maximum values for q 0 to additional categories considered (Table 1). Table 1 - Values of q0for precast structures (Adapted from [9]) Structural Type q 0 CD B CD A Strutures with wall panels 3,0 4,0 α u /α 1 Cell structures 2,0 3,0 Structures with isostatic collumns 2,5 3,5 In particular, the structural type of structures with isostatic columns seems to adjust to the characteristics of industrial type buildings and with a value of 2,5 stays in the middle of the values suggested by the EC8 In the CERIB manual [6] there is also a classification for this type of buildings (Table 2). Table 2 - Values of q0for precast structures (Adapted from [6]) Structure Concrete Columns with medium ductility 1 floor, rigid roof 3 1 floor, flexible roof 2 1 floor +total mezzanine 3 1 floor +partial mezzanine 2,4 This classification focuses on the rigidity of the roof and in the existence or not of an intermediate floor that could span over the entire area of the building or just partially. 3 Implementation 3.1 Introduction For the case study, it is analyzed a precast building of the industrial type, constructed in Portimão, south Portugal, by the company Concremat. This first analysed structure, from now on, named as Base Structure (BS) will be studied in that seismic zone and designed according to EC8 whenever possible. There will also be studied two other variants of these structure. The fist one is called Variant 1 (V1), where through a truss system in the roof we will mobilize a diaphragm effect on that level. The second one, called Variant 2 (V2) is basically the same structural model as the one before, but with a slight change in the connections of the frame system, that now will also resist to bending moments. The purpose of these study is to study the structural behavior of the three structures and to obtain a small economic appreciation of the different structural designs. Furthermore, the conceptual design of the newly introduced connections will be analysed. 3.2 Base Structure (BS) The Base Structure is composed by various frames with spans that can go up to 20 meters (Figure 5). There are 6 different types of columns, defined like so, do to the similarities of the funtions of each one. The connections between the beams and the columns are assured by a dowel, placed on a small cantilever in the top of each column, as shown in Figure 6.

5 Percentage of mass mobilized In Figure 6, we can see the I-shaped beams represented in green. These are the ones who assure the biggest transmission of vertical loads from the roof to the columns. This, as for the other beams, will not be specifically analyzed in this study. Figure 5 Finite element model of BS Figure 6 Beam-column connection [10] In terms of finite element modelling, the software used was SAP2000, and the basic aspects to remark are that all elements were created as frames and that the connections where conceived as pinned, so that they don t transmit any bending moments. For the foundations, the original project uses precast shoes, founded directly into the ground, so it is assumed that the columns are fully restrained at the base. For the seismic design, the method used was the one suggested by EC8, through a response spectrum analysis. The only parameter that has a value unclear is the behavior coefficient, chosen after the modal analysis of the structure. As one can see by looking at Figure 7, the mass mobilization only reaches values superior than 86% at around the 22 nd mode of vibration, which is due to the lack of a group response by all the columns. 100,00% 80,00% 60,00% 40,00% 20,00% Uy Ux 0,00% Modes of vibration Figure 7 Accumulation of mobilized mass per mode of vibration By looking at Figures 8 and 9 we can observe the two types of vibration modes that characterize this structure. Figure 8 shows the 1st mode of vibration, where only the central frame in the X direction is vibrating, not mobilizing any movement from the other frames. On Figure 9 we can observe the 9 th mode, characterized by different behavior, where groups of columns, only linked on one direction on their top, move independently. Figure 8-1 st Mode (f=1,3227 Hz, T=0,756 sec) Figure 9 9 th Mode (f=1,5463 Hz, T=0,6467 sec)

6 The existence of these two different types of vibration modes tells us that the structure does not work as a whole in responding to lateral forces, such as seismic events. To define a coefficient behavior for this structure, applying EC8 and considering it as an inverted pendulum, we would get a value of 1,5, which seems to be very penalizing for the building. Assuming this value is being too conservative, once it is not taking into account the group behavior of the frames, but considering that all the columns work independently. Taking into account the Italian Code, this structure can be classified as a structure with isostatic columns, its maximum value should be 2,5. On the other hand, the CERIB seismic manual suggests that for a flexible roof structure with only one floor, the coefficient adopted should be 2,0. Considering all the sources, the coefficient chosen for the design was 2,0. This way, the frame effect is taken into account, and it is not as penalizing for the design as the value suggested by EC8, coming close to the other values suggested by the Italian Code and the CERIB manual. At this point, the design of the columns is already possible through a response spectrum analysis. In terms of longitudinal rebar, the values obtained for the various typologies of columns where the following: Table 3 Longitudinal rebar and area percentage Column Longitudinal Rebar ρ = A s,adopt [%] Adopted A pilar P1 6Φ32+2Φ20 2,181 P2 4Φ32+8Φ25 1,984 P3 4Φ25+6Φ20 2,138 P4 4Φ32+6Φ20 2,041 P5 6Φ25+4Φ20 1,681 P6 4Φ25+4Φ20 2,013 For the transversal rebar, the most conditioning aspect was the minimum rebar amount defined by EC2, which makes us believe that it won t change in the other 2 structures analyzed. The only connections analyzed in this structure are the ones between columns and beams, assured by the dowel effect. Taking into account the different methods above referred, the following values of rebar where obtained for the dowels, considering only the most affected columns in each category. Column Table 4 Dowel reinforcement for beam-column connections FIB Von Mises (Equação 17) [cm 2 ] FIB 2 plastic hinges mechanism (Equação 19) [cm 2 ] CERIB - (Equação 20) [cm 2 ] Rebar Adopted P1 3,02 5,53 4,55 2 Φ 20 P2 3,66 6,70 5,52 2 Φ 25 P3 2,05 3,75 3,09 1 Φ 25 P4 3,00 5,50 4,53 2 Φ 20 P5 2,27 4,16 3,43 1 Φ 25 P6 2,46 4,50 3,70 1 Φ 25 We can see, by looking at Table 4 that the mechanism calculation method is more conservative than the others.

7 3.3 Alternative solutions Considering the changes mentioned above for the two variant structures, the finite element model for both of them can be seen in the Figure 10. The blue beams represent the new structural elements introduced into the roof in order to assure a diaphragm effect. According to Appleton [11], by connecting elements with different functions in a structure, the global response will be more homogeneous, like the resulting stresses and displacements. That was the objective of these conceptual changes in the Base Structure. These newly introduced elements where chosen to be made in concrete. There were two reasons for this choice. First, the whole structure is made of concrete casted in a factory, so it would be cheaper and easy to produce it alongside with the other structural elements. Second, using a concrete element and not a steel one, decreases the sensibility to buckling. The results of the modal analysis for V1 where the following: Table 5 - Accumulated modal participation (V1) And for V2: Mode Period [sec] Frequency [Hz] Accumulated Modal Participation Ux Uy 1 0,696 1,437 0,001 0, ,692 1,445 0,869 0, ,644 1,553 0,873 0, ,589 1,698 0,873 0,865 Mode Figure 10 Finite element model for the variant solutions Period [sec] Table 6 - Accumulated modal participation (V2) Frequency [Hz] Accumulated Modal Participation 1 0,518 1,932 0,731 0, ,437 2,288 0,876 0, ,416 2,404 0,884 0, ,377 2,652 0,886 0,856 Ux Uy In both cases, after 4 modes, the majority of the mass has already been mobilized, which means the main objective of the different conceptual design was accomplished and the structures have a more uniform behavior. Recurring to the same method of seismic design as before, the only parameter where there are changes relatively to the Base Structure is the behavior coefficient. In these cases, the structure responds to lateral actions as a whole, having a more redundant and reliable behavior. This way, the absorption of energy provoked by the seismic event is made in a more equilibrated way for all the columns. In these conditions, we can chose to adopt a value of 3,3 as a behavior coefficient, following the EC8 classification of a frame structure.

8 According to the same code, a more redundant structure should have a higher behavior coefficient. Although, recent studies in the European project Safecast [12] show that the behavior of a cast-in-situ frame is very close to the behavior of a pinned precast frame. Based on this and in the fact that this coefficient is not defined in a very precise way, the choice is to use 3,3 for both variants. It is now possible to compare the difference between the various structures analyzed in terms of seismic exposal, by looking at Figure 11. It is clear that there is a great reduction of the intensity of the seismic ground acceleration. This will induce less stresses into the vertical structural elements, and it will allow a more economic detailing for both variants. In fact, in Table 7 we can actually see that the percentages of rebar decreased almost generally in the two structures. Column Figure 11 Design response spectrum for the different structures Longitudinal Rebar Adopted Table 7 - Longitudinal rebar and area percentage Variant 1 Variant 2 ρ = A ρ s,adopt Longitudinal Rebar [%] A pilar Adopted = A s,adopt A pilar [%] P1 6Φ25+2Φ16 1,339 6Φ25+2Φ12 1,268 P2 4Φ25+4Φ20+4Φ16 1,118 6Φ25+2Φ20 0,993 P3 4Φ25+4Φ12 1,342 4Φ20+2Φ20+1Φ12 1,565 P4 6Φ25+2Φ16 1,339 6Φ25+2Φ12 1,268 P5 6Φ25+2Φ16 1,339 4Φ25+2Φ16+2Φ12 1,036 P6 4Φ25+2Φ16+2Φ12 1,619 4Φ25+4Φ12 0,966 Another important thing to remark is that on both alternative solutions, the displacements decreased, especially in the second one. A key aspect to those results was the bigger stiffness of both structures, which also reduces the differential displacements between consecutive columns. As for the roof trusses, considering the element with the highest stresses, the results where the following: Table 8 Key values for truss design N sd [kn] M sd [knm] V sd [kn] Astot Asw 298,18 72,04 20,40 4Φ16+1Φ12 Φ8//0,25 It is important to say that the rebar adopted for the trusses is about the double of the code minimum rebar required for such an element. Also important is to refer that as the bending moments in the columns reduced, also the shear decreased, so as the needs for rebar ate the dowel connections on Variant 1. On Variant 2, as the connections are almost monolithic, the transmission of shear is made like in a cast-in-situ frame.

9 3.4 Economic Analysis Taking into account the concrete used on the columns and in the truss system and considering the detailings above exposed as continuous without curtailments of the rebar in all the elements, it was possible to estimate a cost of this parts of the structure. For concrete it was assumed a price of 80 /m 3 as for the steel 0,80 /kg. Table 9 Approximate costs of all structural solutions Quantity Steel [kg] Concrete [m3] Cost [ ] BS 11495,7 61, ,50 V1 8090,0 81, ,35 V2 7531,3 81, ,39 As it is shown in Table 9, the prices of the variant structures are lower. Moreover, this decrease can even be bigger in a complete project, once lower bending moments in the columns will lead to smaller and less reinforced foundations. 3.5 Connections Detailing In this chapter, both the conceptual design of the truss connections to the roof and the design of almost monolithic joints will be analyzed Truss connection For the first ones there are two choices, the first (Figure 12) is to use rebar with bolted ends on the trusses and bolt them to the beams in the roof, the nearest possible to the column. Doing the connection directly on the columns would be more difficult due to the number of elements converging into the same space. The second one is to use a gousset plate to make the connection, with similar principles (Figure 13). Figure 12 First option for truss connection Figure 13 Second option for truss connection By construction simplicity, apparent higher robustness, economic factors and less sensibility to buckling issues, the suggested solution is the first one Monolithic joints connections According to Câmara et al [13], a structure s behavior is not jeopardized for having continuity joints between precast elements and cast-in-situ concrete, not even in zones characterized by high stresses. To make stiffer connections between beams and columns, creating continuity, some aspects have to be taken into account. First, for these solutions to be competitive in relation to the original ones, the scaffolding used in the final casting procedures should be the less possible and only lateral, in order to have a simple and fast casting process. Another important issue is the conceptual design of the connections so that they have positive and negative moments continuity, which is not easy to achieve when facing diverse types of beams in the same connection. A joint design concerning this last aspect is prepared for stronger seismic events, giving the joint the ability to resist to positive bending moments in the beam s extremities, which is better to withstand such actions. Considering the simplest case, where two beams meet in the same alignment, there are two possible solutions.

10 In Figure 14, it is possible to see the first option for this type of connection. In this case, only the upper continuity is assured. The column supports both beams and then continuity rebar is placed through holes in each beam s ending area. This way, there is a bonding length of at least 50cm between the longitudinal rebar of the column and the continuity reinforcement. Figure 14 First option, concerning only upper continuity In Figure 15 we can see a more complex design, featuring positive bending moment resistance in the beam s endings. For this to be possible, some small concrete cantilevers have to be added to the column, in order to support the beams during the coupling processes. This connection is very similar to the previous, but has rebar in the lower edge of the beam that is bent and tied right before the casting. Figure 15 Second option, concerning upper and lower continuity Despite being more efficient, the type of connection is much more complex than the previous one. For the other connections in the case study, the solutions are similar, having some differences when there are more beams in the joints and especially when there are beams with different sections converging in the same column. However, generally the principles are always the same.

11 4 Conclusions The use of precast solutions has multiple solutions from the constructive and structural points of view that make them competitive in relation to the more traditional methods. The quality control and the speed are the major benefits of this technique. On the other hand, there are still a lot of developments to do, especially in terms of connection design. It s in this specific matter that despite having already some documentations focusing this aspects, there is still a lack of orientation and a certain resistance to change by some precast companies. These factors have jeopardized the credibility of these solutions in Portugal and have not allowed a greater growth of this activity. In this study it was verified that there are advantages in having a more homogeneous structural behavior through solidarization of all vertical elements in the resistance to horizontal actions. It was analyzed a well designed structure by Concremat, which as its usual, had the smallest number of structural elements possible and pinned connections. Further on it was introduced a trussed system in the roof that allowed us to achieve better results in terms of column differential displacements and lower levels of reinforcement. The adoption of continuity joints in beam-column connections also proved to be better in these same aspects Structurally speaking, both variants are more redundant and robust, proving to be more efficient, especially in dissipating seismic energy and allowing more stress redistributions. The reduction of differential displacements between columns achieved in both studied variants grants higher integrity for the roofing system and allows a greater freedom of choice for the non-structural material to place in both façades and roof. However, despite being a structural efficient solution, continuity joints should be adopted in a project right from the start, so that there is not a great variety of dimensions and shapes, which create a great number of different joints. Only this way we can take advantage of one of the main advantages of prefabrication, speed of construction. References [1] CEB-FIP Model Code 1990, Comite Euro-International du Betón, Lausanne, Switzerland, 1993 [2] Silva, António., Ligações entre elementos pré-fabricados de betão,master s Thesis, IST, 1998 [3] FIB, fib bulletin 43: Structural connections for precast concrete buildings, February 2008 [4] FIB, fib bulletin 43: Structural Integrity of Precast Concrete Structures under Accidental Actions May 2011 [5] Câmara, José., Construção em Betão Pré-Fabricado Um desafio para o futuro, Presentation at Ordem dos Engenheiros, February 2010 [6] Centre d Études et de Recherches de l Industrie du Béton (CERIB), Guide de vérification et de dimensionnement des ossatures en éléments industrialisés en béton pour leur résistance au séisme, January 2009 [7] CEN, Comité Européen de Normalisation, Eurocode 2: Design of concrete structures Part 1-1: General rules and rules for buildings, EN , April [8] CEN, Comité Européen de Normalisation, Eurocode 8: Design of seismic resistant structures Part 1: General rules, seismic actions and rules for buildings, EN , March [9] - Norme Tecniche per le Costruzioni D.M., Itália, January 2008 [10] Proença, J., Apontamentos sobre estruturas reticuladas pré-fabricadas de betão armado, March 2012, Lisbon [11] Appleton, João., Solidarização de estruturas prefabricadas de grande vão, LNEC, 1981 [12] Toniolo, Giandomenico, Past and ongoing research, SafeCast project presentation [13] Câmara, José., Cavaco, Eduardo., Pacheco, Ilton., Comportamento de juntas com continuidade em betão préfabricado, IST, 2008