A FIVE STORY PRECAST BUILDING WITH HYBRID CONNECTIONS

Transcription

1 13 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, Paper No. 118 A FIVE STORY PRECAST BUILDING WITH HYBRID CONNECTIONS (*) PATRICIO BONELLI 1 AUGUSTO HOLMBERG SUMMARY A demonstrative building was built at "Expo Hormigón ICH in Chile, to show innovative connection systems and the erection of such structures containing precast concrete elements. Special hybrid moment frames of discretely jointed precast concrete beams post-tensioned to concrete columns were used as earthquake resistant systems in the longitudinal direction and rocking walls with energy dissipators in the transversal direction. The design had to meet the current Chilean Code NCH33Of96. An alternative design made according to ACI T1. procedures, leading a lower resistance system, is also presented in this paper. Dimensions of members were chosen to satisfy a target drift demand. Both systems were designed to resist severe earthquakes without structural damage, being capable to sustain drifts larger than 3.5%. The building response to major earthquakes was assessed using two records of the Central Chile Earthquake on 3 March Comparison with a monolithic equivalent building was made. The dynamic inelastic analysis shows that ductility demands are similar in the three study cases, the hybrid system, the alternative design and the monolithic building. Hybrid connections open and monolithic beam column joints yield at lateral displacements about.5% of building height. Columns yield at 1%. Calculated roof lateral displacement demands do not exceed 1% of height. Columns remain elastic only in the building designed according to ACI T1. with lower lateral resistance than the other two structures. Rocking walls behave satisfactorily with very small rotations at the base. 1 Universidad Técnica Federico Santa María, Casilla 11 V, Valparaíso, Phone-fax Chile, patricio.bonelli@usm.cl Instituto del Cemento y del Hormigón de Chile, Chile, aholmberg@ich.cl INTRODUCTION A building with hybrid frames and precast walls was built in Santiago de Chile, in October, to show modern prefabrication and assembly systems. The first three levels were erected during the EXPO HORMIGON ICH, Feria Internacional de Prefabricados y Sistemas Tilt-Up. The building was designed according the Chilean Seismic Design Code NCh33Of.96 emulating a traditional building. The seismic response before yielding of a building with hybrid frames and a cast-in-place building with reinforced concrete frames are very similar, but within the non-linear range of response, the hybrid system dissipates energy mainly at joints, opening and stretching the post-tensioned joints without significant damage in the structural elements.

2 Alternatively, another design was proposed for the hybrid frame, maintaining the same dimensions of the elements to control lateral displacement demands while designing the joints to achieve two objectives: 1) the ability to resist the gravitational actions if the prestressing force is lost, and ) the capability to close the joint after the seismic activity. These objectives can be achieved with less resistance than required by the Chilean Code NCh33Of96, because, on one hand, less reinforcement results in easier concrete placement; and on the other hand, lower bending resistance affords lower resistance demands.three alternative designs, a hybrid frame using the Chilean Code NCh33Of96, a monolithic traditional frame using NCh33 and a hybrid frame using ACI T1., with the same dimensions, were evaluated with a nonlinear dynamic analysis using two records of the March 3, 1985 Earthquake occurred in Central Chile. BUILDING STRUCTURE The building has two frames in the longitudinal direction and two walls and two frames in the transversal direction. As floor systems, precast slabs were used with a seven cm thick reinforced topping to assure the horizontal rigidity of the diaphragm. Figure 1, left, shows a general view of the building during the assembly at the inauguration of the Exhibition. Figure 1, right, shows the position of special reinforcement bars and central post-tensioned tendons at one of the joints. Figure 1.- General view of the assembly of the building and details of a joint Figure shows one of the plants of the building. There is a wall on each side in the transversal direction and two interior frames that take mainly gravitational loads. Walls could be traditional reinforced concrete walls or alternatively they can be built as rocking walls, with or without special steel bars to dissipate energy and central prestressed tendons to ensure the return to the original position. Responses of a rocking wall and a traditional reinforced concrete wall are compared. Rocking walls with central PT tendons and mild steel bars at each end were used in the actual building.

3 Figure, Plant view of the building NON-LINEAR DYNAMIC ANALYSIS To evaluate seismic demands, two records were chosen from the earthquake of March 3, 1985, which occurred in the central zone of Chile. These records have characteristics that are consistent with the design spectrum of the Chilean Seismic Code NCh33Of.96 used in the design of these buildings, although these records were obtained at the coast, 1 km away from the area where the building was built, since closer records to the building of that earthquake were not obtained. For the analysis, the RUAUMOKO computer program, developed in the University of Canterbury, New Zealand, was used. Longitudinal Direction. Figure 3, left, shows the moment-rotation relationship calculated for the joint beam-column of the second and third floors, designed to meet the Chilean Code specifications. Contribution of prestressed steel and the special mild reinforcement to bending moment resistance of the joints were introduced in the analysis using two springs working in parallel at each joint Mprob Mprs Ms Mprs idealizado Ms idealizado Piso N 3 Viña del Mar Llolleo Rotación [%] Desplazamiento [cm] Figure 3. Left: prestressed steel and special mild reinforcement contribution to bending moment resistance (second and third floors). Right: Lateral deformations of the frame for the instants of maximum shear in the base of the column and for maximum lateral displacement at the roof, for Viña del Mar SW and Llolleo N1E records.

4 Figure 3, right, shows the lateral deformation of the frame for the maximum lateral displacement of the roof calculated with the records of Viña del Mar and Llolleo, approximately 18 cm and 15. cm, respectively. At this level of lateral displacements, the structure has yielded, at least in some of its critical sections. In the hybrid frames, the joints opened. In the same figure, the deformed shape was drawn for the instant of maximum shear in the columns to illustrate that the maximum shear demand does not necessarily occur simultaneously with maximum roof displacement, but it depends on the deformed shape that produces the major curvature in columns. In this case, a 8 tons shear force resistance is required at the third floor, being the prediction of an incremental analysis of only 18 tons. For the other floors, the maximum shear demands occur in different times with different deformed shapes. Rotation demands in the hybrid joints. Figure, left, shows the bending moment-rotation relationship for an interior joint beam-column of the hybrid frame calculated with the record of Viña del Mar. At the left of the figure, rotations are shown for a joint designed to satisfy the Chilean Code NCh33Of.96. demanded resistance. Rotation demands were the same for all joints of the same floor. The joints of the first floor had demands similar to those of the second floor. For the Viña del Mar record, maximum rotations occur in the first floor, with a maximum value of 1.%. The demands of rotation decrease in the upper floors. of The joints the fourth and fifth floors behaved elastically. At the right of the figure, rotations are shown for the same joint but designed just to accomplish the two abovementioned criteria. Demands of rotations are slightly greater for the less resistance joint, within satisfactory limits. Viña del Mar SW Rotación [%] -3 Rotación [%] Figure. Hybrid frame. Moment-rotation relationship for interior joints in the second floor. Left: Design according to NCh33Of.96; right: design based on displacements.

5 Curvatura [1/m] Curvatura [1/m] Figure 5. Moment-curvature relationships for critical section in a column at the base in a hybrid frame. Left: Design according to NCh33Of.96; right: design based on displacements. Figure 5, left, shows the moment-rotation diagram for the base of one of the columns. Columns remained elastic in height. At the left of figure 5, the moment-curvature relationships for a critical section in a column at the base in a hybrid frame designed according to NCh33Of.96, shows non linear behavior associated to some considerable damage, but within acceptable limits according to the current seismic design philosophy. The figure at the right shows the moment-curvature relationship for the same column for a frame designed to satisfy only displacement demands, that resulted in an almost elastic response, with minor damage. The demands of rotation at the base of the columns are very similar to the ones calculated for a similar frame, but with monolithic joints. For the record of Llolleo, the demands of rotation are less than those of Viña del Mar. The main rotations occur in the first floor with a maximum value of.83%. The rotation capacity of the joints, designed for a 3.5% rotation of the beam in relation to the column, overcomes great rotation demands of the two records, not over 1%. Figure 6 shows the results for a monolithic similar reinforced concrete frame, moment-curvature relationships. At the left part, the relationship for a beam at first level is shown. This response can be associated with some level of damage while beams of hybrid systems remain undamaged. At the right part of the same figure, the relationship for a critical section at the base of a column shows similar response to columns of hybrid systems shown at figure Curvatura [1/m] Curvatura [1/m] Figure 6. Moment-curvature relationships for critical sections for a cast- in-place reinforced concrete frame Left: a beam at first level; right: base of a column.

6 Transversal Direction Two cases of study were considered: a monolithic reinforced concrete wall with all the reinforcement anchored to the foundation and a rocking wall, both walls with the same dimensions than the rocking wall used in the building. Monolithic walls were analyzed using frame elements working in parallel with the two gravitational frames; figure 7. Figure 7. Walls and gravitatory frames in parallel. For rocking walls the model shown at figure 7 was modified adding a rotational spring in the base of the wall to include uplift effects. Figure 8 shows the considered constitutive law for the rotational spring. Figure 8, Hysteresis Cycle of the rocking wall.

7 Lateral Displacements and drifts. Lateral Displacement Envelope Drift Envelope Height [m] Rocking (Llolleo) Monolithic(Llolleo) Rocking(Viña del Mar) Monolithic(Viña del Mar) Height [m] Displacement [m ] Relative Displacement Divided by Height Figure 9, Lateral displacements and drifts. Lateral displacements are not very large. Drifts do not reach.5 in monolithic reinforced concrete walls, and are between.5 and.1 in rocking walls. Lateral displacement demand for Llolleo are slightly greater than for Viña del Mar record. Figure 9 shows lateral displacement envelopes for the two considered records for a conventional wall and for the rocking wall. Shear Envelope Bending Moment Envelope 18 Rocking(Llolleo) Monolithic(Llolleo) 16 1 Height [m] Rocking(Viña del Mar) Monolithic(Viña del Mar) Height [m] Shear [ton] Shear and Bending Moment Capacities Monolithic W. Shear and Bending Moment Capacities Rocking W Bending Moment [ton-m] Figure 1, Shear and Bending Moment Envelopes. Figure 1 shows Shear and Bending Moment Envelopes for Llolleo and Viña del Mar records for the monolithic wall and for rocking walls. Rocking walls have restricted bending moment demands since they can uplift, obviously. Resistance demands at upper floors are similar in all cases. Shear demand in walls are very low.

8 Moment curvature relationships for the Monolithic Wall Llolleo Record Viña del Mar Record Moment [ton-m] Curvature [1/m] Moment [ton-m] Curvature [1/m] Figure 11: Monolithic walls - Moment curvature relationship at the base, Left. Llolleo record, Right: Viña del Mar record. Figure 11 shows moment curvature relationships for Llolleo and Viña del Mar records for the monolithic wall. The wall yields at the base for the two considered records. The rocking wall responds raising in the base remaining supported in one of the corners. Rotation Spring (Llolleo Record) Rotation Spring (Viña del Mar Record) Rotation [rad] Rotation [rad] Time [seg] Time [seg] Figures 1. Rotation spring by two record. Figure 1 shows rotation in base springs representing rocking wall uplift. Very small raisings are obtained with these records. Before earthquakes of larger demand, the wall would turn around to its bound crushing the concrete with high stress concentrations, remaining the most part of the foundation raised with important rotations. The design rotation was.3.

9 CONCLUSIONS From of the analysis made of the building, some conclusions arise: The frame was designed for a rotation in the joint of 3.5% that occurs in a lateral displacement of 3.% of the height. The global yield occurs at a lateral displacement of the roof of.5% of the height. In case of seismic action, as Viña del Mar (SW) and Lolleo (N1E) in the earthquake of March 3, 1985, there are expectations of relative lateral displacements between the consecutive floor between a 1.% and 1.6% of the height of the correspondent floor with maximum rotations in the joints with an order of 1%, much less than the capacity of rotation of the joint (3.5%). The main demand of rotation was in the joints in the first floors, practically maintaining the elasticity in the superior floors. The beams in the hybrid frames indeed remain elastic, concentrating the damage in the mortar of the joint, while in a monolithic frame, the damage spreads through the plastic region, later requiring repairs in the elements. Expected demands are smaller than supplied capacities. Hybrid frames designed with ACI T1. but with less lateral strenght than required by the Chilean Code presents less damage than hybrid frames designed according to the NCh 33 Chilean Code. Damage is better controlled in hybrid frames than in monolithic frames. Rocking walls can be considered as a feasible solution. The building can be visited in the Parque Riesco, Avenida El Salto 5, Ciudad Empresarial, Santiago. ACKNOWLEDGEMENTS The project and the construction of the building were financed by El Instituto del Cemento y del Hormigón de Chile. The analytic studies shown in this paper were developed by the students Gianella Morelli, Claudio Freire and Carolina Vergara as part of their thesis for the Civil Engineering Degree at Universidad Técnica Federico Santa María. REFERENCES Priestley. J.N., Kowalsky, MJ., Direct Displacement-Based Seismic Design of Buildings. Bulletin of the New Zealand Society for Earthquake Engineering, Vol.33, No., Restrepo. J.I., Toranzo. L.A. 1, Displacement based design of rocking walls incorporating hysteretic energy dissipators. 7th International Seminar on Seismic Isolation, Passive Energy Dissipation and Active Control of Vibrations of Structures. Assisi, Italia. -5 Octubre 1. ACI Innovation Task Group 1. Special hybrid frames composed of discretely jointed precast and post-tensioned concrete members (ACI T1.-XX) and commentary (T1.R-XX). ACI Journal, Septiembre-Octubre 1