The repair or strengthening strategy must be appropriate to the type of structure. For example, during an earthquake, very tall reinforced concrete portal frame structures are relatively flexible and work in bending, whereas low concrete wall structures work mainly in shear. Somnath Biswas, Managing Director, Freyssinet Menard India Pvt. Ltd, a leader in concrete repair, reinforcement and protection techniques, delves on seismic repair and strengthening of existing buildings.
Safeguarding users is the main objective of the various types of rehabilitation. Other objectives also come into consideration depending on the socioeconomic importance and the residual service life of the structure; these include maintaining emergency services (hospitals, barracks etc.), protecting equipment (major industrial facilities), conserving the national heritage (historic monuments), safe evacuation of the building, etc. Freyssinet helps contracting authorities translate their objectives into technical performance requirements to be met by the strengthened or repaired structure. The level of protection is the intersection between a level of seismic stress and a damage limit state of the structure.
The repair or strengthening strategy must be appropriate to the type of structure. For example, during an earthquake, very tall reinforced concrete portal frame structures are relatively flexible and work in bending, whereas low concrete wall structures work mainly in shear.
Strengthening solutions
To reduce the vulnerability of low-ductility buildings, it is often more cost-effective to make them stronger so that they can withstand greater seismic forces, rather than trying to improve their ductility. Strengthening solutions are often accompanied by an increase in the weight and stiffness of the existing structure, and therefore increase the seismic forces to which it is subjected. However, rigidity can help to protect non-structural accessories, which cannot tolerate significant deformation of the building.
Flexible structures can be strengthened by increasing their strength, particularly by means of additional cross-bracing, or by increasing their ductility; for example, by introducing plastic hinges. Increasing ductility consists of making the building more deformable before failure, without increasing the forces to which it is subjected, in such a way as to distribute the seismic action over the entire building and make better use of its resilience. When plastic hinges are introduced, they help to increase the structure’s dissipation capacity.
Whether rigid or flexible, structures can be fitted with damping devices that are able to dissipate a large part of the seismic energy transmitted to them. Harnessing forces of several tens or even hundreds of tonnes over strokes measured in centimeters can contribute to significant financial savings by reducing the work carried out on the structure.
The seismic action placing stress on a building can be reduced by dynamic isolation of the structure from its foundations, advantageously combined with a damping device. This solution amounts to placing a “filter” between the ground and the building that only lets through part of the energy resulting from the seismic action. The dynamic isolator offsets the frequency of the structure, which works in the horizontal direction as a relatively low-frequency oscillator. This arrangement is particularly effective for rigid structures.
The level of protection that can be obtained in this way is significantly higher than the level required by the seismic rules for normal-risk structures. Structures remain operational, even after violent earthquakes. There is little or no damage to non-structural elements and equipment, which can be extremely costly (in the case of hospitals, for example.
Dry process shotcreting is used to strengthen wall ties in structures, create shells or increase the strength of existing elements with or without the addition of reinforcements, by encasing columns and beams, strengthening shear walls, increasing floor thickness etc.
Reinforcing elements
Reinforcing elements are used to strengthen reinforced concrete columns by confinement, beams in bending, in-plane shear walls etc. They are also used to reinforce existing wall ties. These reinforcing elements have the advantage of not adding weight and only slightly stiffening the structure, which prevents amplification of the seismic forces experienced by the structure and overloading of the foundations in particular.
Structures damaged after an earthquake often show some degree of cracking and peeling. The concrete is restored locally by grouting the cracks and reprofiling it.
Freyssinet produces prestressed diaphragm walls by means of external prestressing tendons anchored in additional headers or capping. The tendons can be arranged horizontally, in which case they connect opposite sides of the building, or vertically in order to compress the transverse shear walls of a façade. The vertical prestressing tendons are also use to anchor the structure to its foundations when it is subject to an overturning moment. The introduction of additional forces in the structure and foundations due to prestressing may require reinforcement, and must therefore be checked.
Bracing with reinforced concrete buttresses on one or both sides of the building enables it to withstand horizontal seismic forces and ensure that loads are transferred to the foundations. Then buttress footings also serve to strengthen the structure’s foundations. The benefit of this type of reinforcement is that it only requires external work, avoiding disruption to activities inside the building. It can be advantageously combined with the installation of horizontal prestressed wall ties.
New reinforced concrete shear walls must often be built in addition to modifying the block diagram of the structure during an earthquake, in order to ensure the stability of load-bearing elements and continued load transfer. Transverse shear walls can be outside or inside the building, and can be advantageously used in conjunction with prestressed wall ties to improve performance.
Bracing in the horizontal plane using struts makes it possible to transmit the lateral actions experienced by the building to the vertical bracing elements and distribute them more evenly. The struts must withstand the horizontal forces in that plane on every storey of the building, and transfer the dynamic loads to the foundations.
Freyssinet’s solution consists of combining additional bracing with TranspecTM FVD anti-seismic devices. These viscous fluid energy-dissipation devices have very high damping capacity and are particularly efficient over very short strokes. Their damping capacity offsets the increased stiffness resulting from the addition of braces.
When a multi-storey building is equipped with additional bracing over its entire height, struts working in parallel have to resist the same force, whatever their respective movements. When the foundation system used to transmit the vertical and horizontal forces from the building to the ground is inadequate, it can be strengthened by underpinning:
- Reinforcing the footings, by linking the stay plates or piles with reinforced concrete stringers, or by brace and pre-stressed tie rod systems;
- With Freyssinet micropiles, even in very confined spaces through the use of compact drilling equipment;
- With anchor tie rods when the weight of the structure is insufficient to ensure stability.
Ductility
The ductility of the structure is increased by strengthening joints working in the elastic range but having weaknesses, and confining plastic hinge zones. In order to overcome inadequate reinforcement layout or design faults in the beam-column joints (lack of transverse confinement reinforcements), joints can be reinforced with hoop rings. The ductility of reinforced concrete columns in portal frame structures can be increased without introducing additional stiffness by means of confinement encasing carbon fibre reinforcing strips.
The horizontal flexibility of banded elastomeric bearings and their high distortion capacity under vertical loads makes it possible to isolate the structure from movements in the foundations. Due to their elasticity, these bearings recentre the structure after an earthquake. Additionally, dynamic isolators can also have a damper function to dissipate a portion of the seismic energy. In this case, they are either made from a material with a high degree of internal damping (HDRB type) or fitted with a lead insert (LRB type), which deforms under distortion. These types of bearing require no maintenance; they must be inspected after strong earthquakes.
In order to increase the energy dissipation at the connection between the structure and its foundations, dynamic isolation can also be combined with TranspecTM FVD viscous fluid dampers, making it possible to achieve internal damping coefficients in excess of 50 per cent.
Pendulum bearings allow the structure to move relative to the foundations along a spherical surface, the radius of which determines the natural frequency of the isolated structure. After an earthquake, the bearings recentre the structure. In addition, the friction between the bearing disc and the sliding surface dissipates some of the seismic energy.
In order to house the dynamic isolators under the building’s floor, if the structure cannot be raised permanently, special arrangements must be made to insert and load them. Freyssinet installs temporary structures to relieve the load from the areas to be treated without destabilising them, creates the recesses and performs cutting operations, locally reinforces structural elements, installs the seismic isolation devices and then loads them by means of precision jacking of the structure using specialist equipment.
The isolated building may sometimes require bracing in the plane of the columns, or even the installation of a sufficiently rigid, strong floor in order to evenly distribute the movements at the base.
The best strengthening and/or repair solution can only be chosen following a detailed diagnostic process, which provides information about the strength of the existing building or its residual seismic capacity.
