Behaviour of Precast connections subjected to Lateral Loading

Abstract

The concept of precast construction includes structures, where the majority of structural components are produced in plants under controlled environment and then transported to the site for assembly. The precast elements are connected either by mechanical means or by embedding reinforcements in preformed ducts which are subsequently filled by grouting. Accordingly, the precast connections are known as dry or wet connections, respectively. This leads to faster construction, reduced formwork and scaffolding, less requirement of skilled labours, massive production with reduced amount of construction waste, better quality and better surface finishing as compared to typical reinforced concrete construction. Because of such advantages, the precast concrete construction is considered as sustainable construction technology and being adopted rapidly world-wide, during recent past. In precast concrete construction, connections are the critical elements of the structure, as they plays significant role in transferring forces from one element to another element. The connections between precast elements affect the load distribution, strength, stability and constructability of the global structure. In past, it was observed that, major collapse of precast structures took place because of connection failure. Therefore, it is very important to evaluate the performance of precast connections through experimental study.

This study presents experimental studies conducted on precast portal frame and precast space frame under the effect of lateral loading. Experiments are conducted on reduced 1/3rd scale specimen of portal frame and space frame, which is extracted from assembly hall building having overall plan dimension 16 meter × 6 meter with 4 meter c/c spacing between portal frames. Precast portal frame considered for the study is having wet connection provided away from the beam column junction. Reinforcement bars projecting from the beams on both the sides are overlapped together and subsequently empty space is filled with cast-in-place micro concrete, for precast wet connections considered in portal frame. Experiments are also conducted on monolithic portal frame, to compare the performance with that of precast portal frame.

Monotonic lateral load is applied at the beam level with the help of hydraulic jack of 250 kN capacity. Response of test specimens are measured in terms of load carrying capacity, deflection profile of columns and measurement of strain at critical locations on concrete surface as well as steel reinforcement bars and failure pattern. From the results of experimental studies, it is observed that behaviour of precast portal frame is almost similar as that of monolithic portal frame. Mainly, failure is observed at beam column junctions without any significant failure within the beam and column. For precast portal frame, any failure is not observed within connection region, which indicates that, forces are effectively transferred from one element to another element through connection region.

This study also addresses experiments carried out on precast space frame subjected to dynamic loading using shock table, which is an economical alternative to servo controlled shake table. Shock table used in present study is having 3 m × 6 m steel-RCC composite floor resting on rollers moving on horizontal surface and subjected to impact through 1.5 tonne pendulum. The movement of shock table is restricted using reaction beam on other side, to generate rebound of floor, when in-plane impact is applied through pendulum. Such movement under impact, simulates effect of earthquake on model constructed on floor of shock table.

One storey and one bay reduced scale precast space frame is constructed by adopting simple mechanical bolted connections between individual precast elements such as footing & column and column & beam. Precast column elements are having rectangular corbels projecting in two orthogonal directions at top and in all the four directions at the bottom. Connection between precast footing and column is carried out using 16 mm diameter bolts, after inserting column into gap provided in the footing. For connection between precast beams and column, two 16 mm diameter bolts and 5 mm thick steel plate is used. Rigid diaphragm action and mass at slab level is achieved through concrete cubes placed in steel framing on the top of beams. Behaviour of precast space frame is also compared with that of monolithic space frame. Both the test specimens are placed side by side on the floor of shock table and are subjected to impact loads through striking of pendulum with the floor of the table. A number of shocks are applied to the test specimens through pendulum, released at different angles such as 15˚, 22.5˚, 30˚ and 45˚. Accelerations are measured at the floor level of table and roof levels for both the test specimen using accelerometers.

Results of experimental study showed inferior performance of dry connection considered for precast space frame, as compared to monolithic connection. Failure is mainly observed at the beam column junctions in form of crack formation and spalling of concrete. Higher acceleration is experienced by monolithic space frame without significant failure, as compared to precast space frame. During experiment, any significant failure is not observed at junction of footing and column. At higher acceleration, test specimens experienced uplift of footing indicating need of proper fixity at base. Based on the experimental study, it is felt that when precast connections are provided away from junction can perform better, due to continuity of reinforcement.