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Column Jacketing: Techniques and Advantages

How to reinforce concrete structures? Column jacketing is certainly the most commonly used intervention for effectiveness and cost containment

In the field of civil engineering and architecture, structural strengthening is a fundamental aspect to ensure the safety and durability of buildings. Among the various techniques used, column jacketing stands out for its effectiveness in improving the load-bearing capacity and seismic resistance of existing columns. Its implementation is subject to careful study of the actual conditions of the structure and to the calculation and verification of the new intervention.

What is column jacketing?

Column jacketing, or jacketing in English, is a structural reinforcement intervention that involves applying an additional layer of material around existing columns to increase their strength or improve their performance. This may be necessary for a variety of reasons, including increasing the load that the column must bear to adapt to new usage requirements, correcting structural defects, or enhancing the seismic resistance of the building.

The materials used for jacketing may vary depending on the specific project requirements but can include concrete, steel, fiber-reinforced polymer composites (FRP), or a combination of these materials. The main goal is to provide reinforcement that enhances the structural characteristics of the column without compromising its stability or functionality.

It is moderately invasive, relatively quick to implement, relatively inexpensive, and allows for radical improvements in terms of safety. In particular, it allows to increase:

  • load-bearing capacity, according to the so-called “confinement” phenomenon of concrete. Both the presence of the new concrete (with better characteristics than the existing one) and the steel ensure a lateral pressure that opposes the expansion that occurs in response to axial loads;
  • bending and shear resistance, selective interventions can also be chosen. If the reinforcement aims to increase only the bending strength, the longitudinal steel bars should also be anchored in the adjacent structural elements, adding also transverse connection reinforcements between the jacket itself and the existing beams. Conversely, to increase only the shear strength, the jacketing should not be anchored, leaving a joint with the slab of about 1 cm (both for concrete and for the reinforcement);
  • deformation capacity, i.e., the ability to respond to stresses through non-structural deformation (essential for absorbing energy).

Column jacketing is a traditional and definitive technique, having demonstrated its efficiency over the decades.

It finds application in a wide range of scenarios, from the restoration of historical buildings to the correction of structural defects, from responding to additional loads to mitigating seismic risks. In contexts where demolition and reconstruction are impractical, not cost-effective, or not sustainable, jacketing offers an effective solution to improve structural performance without compromising the integrity of the building.

The materials used in column jacketing depend on the specific project requirements and the conditions of the existing structure. Reinforced concrete is a common choice for its versatility and strength, while steel and fiber-reinforced composite materials offer alternative solutions in terms of lightweight and ease of installation. Application methods vary from traditional casting to prefabrication, allowing for customization based on structural and design needs.

The only real drawback of concrete jacketing is the increase in “masses,” i.e., the width of the columns themselves, which could reduce usable floor space. However, this disadvantage could be overcome by opting for jackets made with reinforced fibers, a less common intervention compared to concrete jacketing and steel brackets but equally efficient, especially for improving tensile strength without adding additional loads and masses to the existing structure.

Phases of steel jacketing implementation

The process of implementing column jacketing may vary slightly depending on the specific project requirements and materials used. The way the intervention is carried out can make a difference in its actual success.

The basic steps to be followed for column jacketing are:

  • structural evaluation: before starting the intervention, a thorough evaluation of the existing structure is necessary, including tests and investigations on materials to define the level of knowledge of the building. This will help identify the current structural conditions, any existing defects or damages, and determine the necessary reinforcement requirements;
  • jacketing design: based on the results of the structural evaluation, the type of material to be used, the thickness of the jacket, and other technical details are determined. The design should also take into account local regulations and safety standards;
  • shoring of the affected columns: a preventive measure to preserve the structure of the element during activities;
  • assessment of the conservation status of the concrete of the column to be intervened and possible demolition of deteriorated concrete portions;
  • in case of deep lesions, proceed with mortar injections;
  • straightening of longitudinal bars, in case of deformation;
  • preparation of the laying plan by cleaning the surface and humidifying it;
  • installation of additional steel bars and stirrups;
  • application of concrete: this can include casting, manual application, or the use of prefabricated forms, depending on the chosen method and the dimensions of the column;
  • removal of shoring.

It is important to remember that in existing structures, knowledge of the structure and materials used is crucial. Defining the level of knowledge and the consequent confidence factors, starting from the tests and inspections carried out, allows to identify the average strengths of the materials to be used in the verifications.

When is structural reinforcement of columns necessary?

In general, structural reinforcement of columns through jacketing is always useful, especially in cases where damage to existing structures is found, under-dimensioning of columns, use of poor quality or deteriorated materials over time. However, there are also cases that require urgent action.

Here are the different scenarios in which column jacketing is more frequently used:

  • load increase: if a building undergoes a change in its intended use or a conversion that results in an increase in the structural load on the columns, it may be necessary to reinforce them to ensure they can safely support the new loads;
  • structural deterioration: over time, columns can deteriorate due to factors such as corrosion, fire damage, chemical attacks, erosion, or seismic actions. In these cases, it is essential to intervene with reinforcement measures to restore structural integrity and ensure the safety of the building;
  • regulatory updates: building regulations and codes may be subject to changes over time, introducing new safety and seismic resistance standards. In response to such regulatory updates, it may be necessary to reinforce existing columns to ensure compliance with current standards;
  • renovation, expansion, and elevation: during renovation or expansion works of an existing building, it may be necessary to reinforce the columns to adapt them to new design requirements and ensure the stability of the entire structure during and after construction works;
  • collapse prevention: in situations where there is an imminent risk of structural collapse due to significant damage or weaknesses in the columns, structural reinforcement can be used as a preventive measure to prevent the building from collapsing and protect public safety;
  • improvement of the building’s economic value: regardless of the actual needs, proceeding with column jacketing means improving the safety guarantees of the property and consequently its economic value. An important detail if the perspective is to sell the property, perhaps after a few years.

In general, structural reinforcement of columns is necessary when there is a compromise in the load-bearing capacity or stability of the columns themselves, which could jeopardize the safety of the building or the people living in or frequenting it. Accurate assessment of structural conditions and the adoption of appropriate measures are essential to ensure the durability and safety of constructions over time.

However, there are other cases where it is possible to intervene to improve the strength of columns with less invasive techniques:

  • restoration of the external jacket: rather than applying a new jacket, it is possible to intervene in the restoration of the existing one. The purpose is to “clean” the metallic elements, reinforce them with fast-release materials, and contain any deformations;
  • application of FRP laminates: often, the laying of very thin and minimally invasive materials such as fiber-reinforced polymer composite laminates (FRP) is enough to increase the strength of a column. These interventions generate an effect similar to what would be achieved with increased reinforcement;
  • application of modified hydraulic lime: even hydraulic lime, if properly integrated with natural pozzolans, can ensure greater resistance to columns.