A building complex in central Athens, gradually built in the 1960s, which houses Greece’s Telecommunications’ Organization center.

It consists of 15 statically independent buildings, made of reinforced concrete, each featuring a basement, ground floor and 2, 5 or even 12 overlaying floors.

The buildings total a surface of 53,000 m2.

The Telecommunications’ Organization needs to change use in several building spaces and redesign the buildings’ interiors as well as their external views.

To this end, architectural upgrading, static and earthquake-resistance strengthening and new E / M installations are required.

In addition, the technology centers inside the buildings need to remain in operation during the interventions works and stay protected against vibration, dust, dirt, water, sparks, fire and any kind of detrimental factors.

The strengthening design provides for the turning of the columns – at their full height – into “strong columns”, through steel-sheet jacketing and high-strength concrete” infill”.

Most of the new composite columns that are created, have an octagonal cross-section to confine the existing and new concrete they encase.

The steel-sheet jacketing and the confinement it provides, increases the strength and plasticity-deformability of the concrete and makes it virtually “unbreakable” under seismic stresses.

Along their height, the composite columns are connected to the floor beams, which are also strengthened with steel-sheet jackets, where required.

In some cases, where there were not enough suitable positions to implement the above described technology, the columns are connected by steel sheet profiles, erected as a composite construction of steel sheet and concrete.

New composite walls are also constructed with steel-sheet “jackets” all around the existing structural elements and fully attached to them.

Each “jacket” is connected with its wall, through additional transverse steel rods on the elongated sides, in a dense grid layout, at the full height the walls, so that they act as confined, composite walls, having adequate strength under compression and shear.

At the same time, the steel “jacket” acts as longitudinal reinforcement of the composite structural elements – walls –  and the steel rods as transverse reinforcement.

The “jacket” also acts as a “remaining formwork”.

In addition, the concrete infill and the steel transverse rods protect the steel sheets of the “jackets” from buckling.

In some high technology areas of the Telecommunications’ Center where welding is not permitted, the steel connections are established by means of bolting.

Furthermore, the buildings are connected to each other – every two or three – and converted from 15 to 7 static independent buildings.

The connections take place at the joints between the buildings and on every floor level.

This reduces earthquake displacements and drifts and eliminates the risk of pounding, especially between the two 12-storey buildings.

On two buildings, where it is technically impossible to intervene at the interior, the strengthening interventions are carried out exclusively at their exterior, with shear walls and reinforced concrete frames, fully attached on the facades.

The connections to the existing building are made with chemical anchors.

The solution applied has the advantage of speed, cost, safety, flexibility of the in phases construction, as well as avoiding dust, water, vibration and nuisance in the technology spaces.