SEISMIC BEHAVIOUR OF PRECAST COLUMN-TO-FOUNDATION. GROUTED SLEEVE CONNECTIONS
Structural Engineering, Department of Design and Technology, University of Bergamo, Italy E-Mail: paolo. riva@unibg. it
The results of a set of experimental tests concerning the cyclic behaviour of prefabricated column-to – foundation connections is presented. The tests allow to compare the response of cast-in-place connections against pocket foundation and grouted sleeve solutions. The results demonstrate that grouted sleeves ensure a ductility similar to the one of cast in situ column-foundation connections and of pocket foundations, although a slightly smaller dissipation capacity is observed. It is found that in grouted sleeves connections the damage is localized at the column base, in the thin grout layer existing between the prefabricated column and the foundation. As a result, very little damage may be observed in the column, allowing an easier post-seismic column repair.
Warehouses and commercial malls in Italy are generally built using precast reinforced concrete elements. The typical structural layout consists of cantilever columns, connected by simply supported precast and prestressed beams, supporting prestressed concrete roof elements. The foundations are usually made of isolated precast cup-footings, in which the columns are inserted and grouted in-situ. Such a structural layout is extremely cost effective, and sensibly reduces the construction time. However, its effectiveness is seriously hampered when it is intended for construction in seismic areas, particularly if Capacity Design (CD) based codes are adopted in the design process.
According to the European Code (EC8, 2003) or to the new Italian Code (OPCM 3274, 2003), the described layout is defined as an “inverted pendulum” system, which has the characteristic of being statically determinate, with potential plastic hinge regions located only at the base of the columns.
The design of the column footing is carried out by: i) assuming that a plastic hinge might develop at the column base section for the design earthquake event; ii) preventing the formation of any inelastic mechanism in the footing. The latter goal is reached by designing the foundation to resist to the design axial force and to the maximum possible resisting bending moment of the column base section, computed by considering an appropriate overstrength factor.
Adopting a CD approach, the foundation base becomes easily very large, also for medium-sized columns. This issue might be hardly relevant for cast in-situ structures, but could seriously limit the cost effectiveness of pre-cast concrete structures such as those previously described. In fact, due to the dimensions of the foundations, the footings have to be cast-in-situ, making the use of isolated cup – foundations less attractive.
Hence, mat foundations become often more convenient than isolated footings. In this case, the column-to-foundation connection is usually done: i) by using steel base plates; ii) by adopting column pockets, grouted in-situ; iii) by means of grouted sleeves.
Both from the point of view of prefabrication and of seismic response, steel base plates appear to be the least attractive of the three connection types, as they require small tolerances for the on-site placing, and adequate ductility might not be easily ensured in the column base section.
M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 121-128.
© 2006 Springer. Printed in the Netherlands.
Pocket foundations are the most used, at least in Italy, as they ensure ease of placement, and adequate ductility of the column base section after grouting. As a matter of fact, the behaviour of the connection is very much similar to that of cast-in-situ structures. On the other hand, either a thick mat foundation or a collar are required to accommodate the column, with consequent increase of costs related to the construction of the pocket.
Grouted sleeves appear to be the cheapest, hence most convenient, type of connection. However, the seismic response of such column-to-foundation connections is not well documented. In fact, while a large amount of experimental tests have been carried out on columns subjected to cyclic loading (e. g. CEB, 1996), no experimental results concerning the cyclic response of grouted sleeve column-to – foundation connections are available in the literature, at least to the Author’s knowledge.
In order to investigate the cyclic behaviour of grouted sleeve connections and to compare the behaviour of such connections with cast-in-situ and grouted pocket column-to-foundation connections, an experimental campaign was set-up at the University of Brescia. The response of five columns (section 400x400mm, height 3200mm), subjected to a cyclic top horizontal displacement history, was investigated, considering different connection details. A constant axial force equal to 600kN was applied to all columns.