On improving the seismic performance of precast concrete frames

Abstract

In New Zealand, over the past three decades there has been a complete shift away from constructing buildings using case-in-place concrete. Nowadays, to speed up the construction process, most multi-storey buildings are constructed using various precast concrete systems. It is therefore not surprising that following the 1994 Northridge earthquake in California, there was much consternation amongst structural engineers as quite a number of the structures that collapsed in that earthquake were of the precast concrete variety. Thus research programmes commenced at both Auckland and Canterbury Universities to investigate whether such a problem would exist with the New Zealand style of precast concrete construction. At the University of Canterbury, work focused on the hollow-core system through a series of full-size super-assemblage experiments coupled with companion analysis.

Initial work focused on conducting experiments on a full-scale slice of a representative precast concrete building with precast hollow-core floors. Based on companion computer simulations, loads and displacements were applied to the physical experimental specimen that in the first instance were representative of an earthquake that is likely to be seen within the lifetime of such a structure. This was then followed by a more extreme loading, less likely to occur within the lifetime of the structure, but nevertheless possible. Normal design objectives for these two types of loading are that the structure should survive the first with some repairable damage, and not collapse leading to loss of life in the second.

This original experimental specimen failed to survive both types of applied earthquake loading criteria. Indeed, the collapses seen in the field in California were replicated in the laboratory. Companion analysis confirmed that some 20 percent of such buildings would be expected to collapse in the rare but strong earthquake event.

As failures in the existing form of precast concrete construction were confirmed both experimentally and through advanced computer simulation, it was thus considered necessary to investigate what remedial actions and design improvements were necessary for the new generation of precast concrete structures with hollow core floor systems. Based on consultation with the design and construction fraternities, several detailing improvements were proposed for experimental investigation. As a result, two further large-scale experiments were conducted.

The first of these investigated a simple (flexible) floor-to-support beam connection. The second investigated a reinforced (rigid) hollow core floor-to-support beam connection. For both experiments, a more onerous loading protocol was adopted to ensure the most adverse form of seismic loads and displacements could be resisted.

Both experimental specimens performed well under the imposed simulated seismic loading. Although damage was observed, it was of the sort one would expect for a well designed cast-in-place concrete structure. Through post-experiment analysis it was shown that all of the damage patterns were explainable and could be predicted using customary engineering theories. For both proposed construction details it was confirmed that the ultimate goal of achieving life-safety in extreme earthquake events was met, as the specimens were capable of deflecting well beyond the most adverse design limits. An example of the appearance of the last experiment, while under test, at an extreme limit is shown in the figures below.

On the strength of the research conducted, the New Zealand Concrete Design Specification (NZS3101:1995) has been officially amended (Amendments 3; 2004). Within that specification there are now improved seating and detailing requirements for precast concrete hollowcore floor systems. These details are also included in a forthcoming re-issue of this specification. The reinforcing design and construction details are of a nature that are deemed to be “acceptable solutions” because they have been specifically validated through large-scale research.