COMPARISON BETWEEN NUMERICAL AND EXPERIMENTAL CYCLIC RESPONSE OF ALTERNATIVE COLUMN TO FOUNDATION CONNECTIONS OF REINFORCED CONCRETEC PRECAST STRUCTURES Ettore Fagà, Dr, EUCENTRE, Pavia, Italy Lorenzo Bianco, Dr, Peikko Group, Milano, Italy Davide Bolognini, Dr, EUCENTRE, Pavia, Italy Roberto Nascimbene, PhD, EUCENTRE, Pavia, Italy ABSTRACT The cyclic response of a column to foundation connection system based on the mechanical connection between steel shoes embedded into the column base and protruding steel bolts anchored into the foundation, is examined in this paper. This connection is to be considered as an alternative solution compared to the traditional reinforced concrete precast pocket foundations. The hysteretic behaviour and the global collapse mechanism of the connections, to be compared to an equivalent monolithic solution, are studied through three full scale specimens subjected to a constant axial load and to a quasi-static cyclic horizontal top displacement history at increasing drift levels. Contemporaneously, two and three-dimensional finite element models, characterized by non-linear properties of materials and geometry, are developed and calibrated in order to analyze and capture the local experimental response of the connections and to compare it to the behaviour of a cast-in place equivalent connection. The aim of this research is to develop a global numerical model able to carefully predict the response of similar connection typologies to cyclic loading Keywords: Precast connection, Column-foundation connection, Quasi static cyclic test.
1 INTRODUCTION The column-foundation connection system described in this paper has been developed to be considered as an effective alternative of the use of traditional precast pocket foundations. This system is based on the mechanical connection between steel shoes (Fig. 1(c)) embedded into the column base and protruding anchor bolts anchored into the foundation (Fig. 1- ). The use of nuts and washers attached to the anchor bolts permits to control the vertical position, the height level of the column and the fixity of the connection. An additional injection of cement mortar into the void below the column is required to complete the construction of the system. (c) Fig. 1 Columns shoes and anchoring bolts in column connections; details for additional steel reinforcement in square columns (welded bars and lap splice along the height); (c) a single type of shoe. The stress transfer from the anchor bolts steel shoes system (Fig. 1(c)) is based on a couple of longitudinal bars welded at the top of each shoe and on additional overlapped bars, which represent the steel reinforcement of the column (Fig. 1). The seismic performances of the system have been evaluated by means of a numerical and experimental research subdivided into the following main arguments: - reference structure for the design of the specimens; - main characteristics of the specimens; - arrangement of the experimental tests; - results and critical comments. The main objectives are: evaluation of the real response of the welded connection between steel shoes and reinforcing bars; definition of the real global collapse mechanism, the displacement ductility and the dissipation capacity resources; (c) the capacity of the base section of the column to resist shear actions and the initial stiffness of the column compared to the stiffness of other precast structural typologies. 1
2 REFERENCE STRUCTURE AND CHARACTERISTICS OF THE SPECIMENS A reference RC precast three-storey rectangular building was considered in order to design the specimens. In particular, the considered case-study is characterized by monolithic connections and span lengths equal to 14 m and 8 m, respectively along the two principal directions. The total height is 12 m, subdivided into three interstorey heights equal to 4 m. (c) (d) Fig. 2 Geometrical dimensions and details of the steel reinforcement used in the connection; -(c) base and mid-height (d) column details of the specimens. The assumptions of high ductility class, peak ground acceleration value of 0.25g and soil composed of medium dense sands were considered during the development of the design phase. Square columns with cross sections of 40 x 40 cm were considered; it has been hypothesized that the connection between the columns and the foundation are made through the classical steel shoes (called in the paper n1) together with standard anchor bolts. The additional steel longitudinal and transversal reinforcement along the height of the column has 2
3 been designed to resist the distribution of bending moment and shear action, respectively, without a strong interruption of longitudinal bars. The amount of reinforcement is depicted in the Fig 2. Four ø 24mm steel pins with length of 150 mm has been utilized to avoid that the anchor bolts could be subjected to shear action during the experimental tests (Fig. 3). The steel pins embedded into the column of the specimens were used in order to resist the shear action and permit to the anchor bolts to be subjected only to axial loads. The adoption of this steel pins was essentially due to particular requirements of the test set-up. Fig. 3 Column to foundation details: 4 ø 24mm steel pins added to resist shear action. The foundation has been over-designed in order to prevent the arising of a crack pattern during the experimental test and to be a fixed support for the column, since the elements to be tested are the column and the connection. EXPERIMENTAL TEST SETUP The experimental loading history consists of a series of horizontal displacement cycles at the top of the column with increasing target drift as reported in Table1. Table. 1 Drift and displacement levels at the top of the column. Drift level Displacement in [mm.] ± 0.40 ± 8.60 ± 0.80 ± 17.20 ± 1.20 ± 25.80 ± 2.40 ± 51.60 ± 3.60 ± 77.40 ± 4.80 ± 103.20 3
4 The vertical distance between the axis of the actuator and the base of the column is 2150 mm; it is expected that the shear component of the displacement profile of the specimen will be negligible, due to the value of the ratio between the height and the depth of the cross section of the column (value of 5.37). A constant vertical load will be imposed at the top of the column during the tests: the planned values are 200 kn, 400 kn and 600 kn, respectively and equal to 5%, 10% and 15% of the non-dimensional axial force. (c) (d) Fig. 4 Test set-up; specimen at a drift of 4.8 % (displacement 103.20 mm); (c)-(d) instrumentation of the connection perpendicular and parallel to the load direction. The test set-up and instrumentation are shown in Fig. 4 -(c) and (d); furthermore the connection corresponding to a drift level of 4.8 % (Table 1) is represented in Fig. 4. The instrumentation depicted in Fig. 4(c)-(d) has been arranged in order to measure relative displacements, deformations and curvatures of the cross section at different height levels. 4
5 (c) Fig. 5 Base shear and horizontal top displacement of the specimen subjected to a vertical load of 200 kn, 400 (kn) and (c) 600 kn; damage pattern at the end of the test. 5
6 EXPERIMENTAL RESULTS AND NUMERICAL COMPARISONS The results obtained from the experimental campaign on three specimens are summarized as follows: - the behaviour of the anchor bolts governs the collapse mechanisms, without any significant damage of the specimen, independently of axial load level imposed on the top of the column; - the non-linear branch of the base shear top displacement curve (Fig. 5 corresponding to the three specimens) is due to the yielding of the anchor bolts, which are the only components of the system to reach the plastic level, whilst the other components do not exceed the yielding condition and are not significantly damaged; - the hysteretic behaviour, evaluated up to 4.8 % drift level, is characterized by negligible strength degradation and by global ductility level always greater than 5; - the total damping (given by the sum of the viscous and the hysteretic part) is about 8.5% in the elastic branch (up to a drift level equal to 1.2%), then it increases and is included in the range 16% - 20% for the first cycles of the following drift levels; - the incipient yielding condition of the examined columns is attained at a drift level included in the range 1% - 1.3%; in addition, since the column does not exceed the elastic branch and it is not subjected to significant damage, a more rational design than the case of traditional RC precast structures characterized by monolithic columns and pinned beams can be obtained. During the experimental campaign, two different type of finite element models have been defined: 1. a two-dimensional numerical force-based fibre element model using Opensees 2 computer platform. This is in order to numerically simulate the cyclic response of a cast-in place equivalent connection and to do a comparison with the system in terms of total damping capacity; 2. a three dimensional model using Midas FEA (Fig. 6) general purpose code using tetrahedrical 4 node element and contact surface element in order to capture the local behaviour at the interface between mortar and concrete (246438 degrees of freedom corresponding to 459676 number of elements). The concrete has been modelled using and elastic modulus of 34500 MPa,, Poisson ratio of 0.15 and weight density of 2.5e-5 N/mm 3 ; a total strain crack approach with fixed scheme has been used together with a secant stiffness calculation during the iterative approach; the lateral crack effect uses the Vecchio and Collins 3,4 approach and the confinement effect has been taking into account using the Selby and Vecchio 5 method. The steel has been model with a classical Von Mises approach: elastic modulus E=210000 MPa. The aim of this model is to extend the result obtained for the shoes n1, using three-specimens subjected to a series of horizontal displacement cycles, to bigger steel shoes (called n2) avoiding more tests. The calibration of the numerical coefficients related to the behaviour of the concrete (linear tension softening model) and of the steel reinforcement (Giuffrè-Menegotto-Pinto model) resulted in a effective representation of the cyclic response of a cast-in place column fixed at the base and free at the top. The comparison between experimental and numerical data is depicted in Fig. 7. The main information coming from the numerical model is that such model can be considered an effective reference in terms of total damping only for great drift 6
7 (c) (d) Fig. 6 Midas FEA model: column, foundation, n1 steel shoes, mortar and steel bars. levels (e.g. drift > 1.00%). A new analysis has been developed on an equivalent cast-in place column with the already calibrated parameters of concrete and steel material. The equivalent column is characterized by the same main longitudinal reinforcement of the real column and 4 M30 S500 continuous steel bars instead of the anchoring bolts and the steel shoes. A comparison between the pushover numerical curve with the experimental response in Fig. 7. The last step, i.e. the comparison between the numerical simulation of the cyclic response of the equivalent cast-in place column and the experimental results of the system, gave the results depicted in the Fig. 7. The comparison developed for higher drift levels ( 2.4%) states that the numerical prediction of the cast-in place solution is associated to greater damping values (28% vs. 22% for the drift level 4.8%). Summarizing, the difference is lower than 30% for the first cycles, 7
8 whilst it increases for the second and the third cycles. In any case, the absolute values of the damping associated to the connection seem to be effective, since they are included into the range 18.7-22.4%. Fig. 7 Base shear top displacement curve of a cast-in place column: comparison between the experimental (blue lines) and the numerical data (red lines); a drift level of 1% corresponds to 13.5 mm; Comparison of pushover curve with the experimental hysteretic response. 8
9 CONCLUSION In this experimental research programme it has been deeply investigated: the real global collapse mechanism, the displacement ductility and dissipation capacity resources of the connection; the capacity of the base section of the column to resist shear actions and (c) the initial stiffness of the column compared to the stiffness of other structural typologies. The good seismic response of the examined specimens can also noticed through the damping displacement ductility relationship (Fig. 8), where the first experimental cycles obtained from the tests are characterized by a results very close to the typical behaviour of cast-in-place frames (red dashed line) for medium-low axial load values and close to the behaviour of RC bridges for higher axial load values. A future development of this research will be the evaluation of the behaviour factor in a three-dimensional frame building using a numerical approach starting from the experimental results outlined in this paper. Fig. 8 Damping as a function of the displacement ductility, comparison between the examined system and characteristic curves of other structural systems 1 ACKNOWLEDGEMENTS The authors wish to thank the Peikko company (www.peikko.it) for the contribution to the experimental research highlighted in this paper. REFERENCES 1. Priestley, M.J.N., Calvi, G.M., and Kowalsky, M.J. Displacement-based seismic design of structures, IUSS Press, 2007. 9
10 2. Mazzoni, S., McKenna, F., Scott M.H., and Fenves, G.L OpenSees Command Language Manual, Pacific Earthquake Engineering Research Center, University of California, Berkeley, 2006. 3. Vecchio, F. J., and Collins, M. P., Compression response of cracked reinforced concrete, J. Str. Eng., ASCE 119, 12 (1993), 3590 3610. 4. Vecchio, F. J., and Collins, M. P., The modified compression field theory for reinforced concrete elements subjected to shear, ACI Journal 83, 22 (1986), 219 231. 5. Selby, R. G., and Vecchio, F. J., Three-dimensional Constitutive Relations for Reinforced Concrete, Tech. Rep. 93-02, Univ. Toronto, dept. Civil Eng., Toronta, Canada, 1993. 10