Understanding the need for retrofitting and the possible means to do so
In recent years, the construction industry has seen an increasing demand to reinstate, rejuvenate, strengthen and upgrade existing concrete structures. This may be attributed to various causes such as environment degradation, design inadequacies, poor construction practices, lack of regular maintenance, revision of codes of practice, increase in loads and seismic conditions etc. In India most of the structures are designed for gravity loading as per IS 456:2000. These structures are susceptible for damage during earthquake. During a severe earthquake, the structure is likely to undergo inelastic deformation and has to depend on ductility and energy absorption capacity to avoid collapse. Such buildings designed for gravity loading need to be strengthened to increase strength, stiffness and ductility.
Factors leading to Degradation of Structures
For convenience, the deficiencies can be broadly classified as global deficiencies and local deficiencies. Global deficiencies refer to the deficiencies which are observed in the building as a whole. Global deficiencies are the attributes that degrade the lateral load resisting mechanism of a building subjected to an earthquake. Some of the deficiencies are caused by ‘irregularities’ in the structural configuration (IS 1893: 2016). Plan irregularity can be torsional irregularity, re-entrant corners, diaphragm discontinuity, non-parallel system, out of plane offsets etc. Vertical irregularity includes stiffness irregularity, mass irregularity, geometric irregularity, strength irregularity etc.
Local deficiencies refer to individual members. Each beam and column in a frame should have not only the required strength, but also sufficient deformation capacity. A slab should be properly connected to the frames. The deficiencies can be detected by checking the capacities and studying the detailing of reinforcement in the slabs, beams, columns and joints as shown in the as-built drawings. The capacities can be compared with the force demands calculated from a building analysis. The detailing of reinforcement in the drawings can be compared with the requirements of the code IS 13920-2016.
When a building has deficiencies that have not been accounted for in the structural design, it needs retrofit. The different retrofit strategies can be grouped under global retrofit strategies or local retrofit strategies. A global retrofit strategy targets the performance of the building as a whole under lateral loads. Addition of new walls, frames or braces, reduction of any irregularity or mass of the building are grouped under global retrofit strategies. These improve the lateral strength and stiffness of the building. If a building is significantly deficient in resisting seismic forces, a global retrofit strategy should first be investigated.
Global Retrofitting Strategies
Local Retrofitting Strategies
All the retrofitting methods described above are effective, each with its own advantages and limitations. Concrete is labour intensive, steel requires high maintenance during the life of the structure, and composites have high initial cost. Global methods retrofit the entire structure at once by adding shear walls or steel braces, or by using base isolation. Shear wall are labour intensive and expensive. Steel brace can be easier to implement but present some connection problems. Base isolation is effective and works well but cannot be applied to all type of structures. The choice of the method depends on the building, on its specific requirements, as well as its condition, location, and geometry. Several methods should usually be considered and compared to find the appropriate best one. To provide greater flexibility in the retrofit scheme, hybrid methods should be used in which several methods can be combined and implemented together, combining the advantages of each.
To start with your fully-compliant concrete-to-concrete connection design, use a software like our PROFIS Rebar for quick and reliable support: you can calculate rebar anchorages and splicing with Hilti injection systems according to the most-updated state-of-the-art design codes and Hilti methods.
In case you need more support to understand which qualification is most suited to your application requirement, please leave a comment on this article or post your question in our Q&A section.
For further information, please contact a Hilti Field Engineer or write to us at: