Zakynthos Piraeus Bank
If you exclude robberies, being a bank clerk is a relatively safe and stress-free occupation, well... not in some cases. When a large piece of concrete suddenly detached from the ceiling of Piraeus bank branch in Zakynthos, it landed on a desk exactly in front of the stunned cashier! So, it was time for a little trip to the Ionian sea, at the island of Zakynthos, to perform a structural integrity evaluation.
At first sight, it was a beautiful two storey building, designed and built in 1956, conforming harmonically with the local architecture.
The building’s interior also seemed in a very good condition.
Geographically the building resided upon the highest seismic acceleration zone in Greece (0.36g). Just three years before it was raised, a major catastrophic seismic event devastated the whole region. Islands Zakynthos, Ithaca, Cefalonia were left in ruins: 83% of the buildings were destroyed, 476 citizens dead and 2,412 wounded, a forced large scale migration followed.
The building, except for the future earthquake danger had accumulated fatigue from the previous vibrations in its long history. Furthermore, concrete’s original compression strength had been reduced due to the exceedance of its life span (fifty years). Last but not least, the corrosive marine environment (chlorides and carbonation) had surely taken a toll from the steel reinforcement.
Our initial thought was to design structural strengthening for the building: remove carbonated concrete layer, apply treatment to the existing reinforcement, place reinforced gunite around columns and beams, support slabs with fibre-reinforced polymers and steel plates, also design new reinforced concrete shear walls, maybe add some cathodic protection into the recipe.
The building was stripped naked to its core structural frame and material samples were sent to the laboratory. It was time for our second visit to have a better grasp of the situation. Well, things were way worse than we anticipated:
The building had very little brickwork, it was constructed as a concrete box, probably as a precaution against the earthquakes. Concrete class was C12/15, which, nowadays is forbidden for earthquake design, it is only used as blinding concrete! Moreover, it was produced with sea water (chlorides) and, of course, was carbonated. More than 60% of the building lacked proper concrete consolidation due to inadequate vibration during concreting. Seaweeds had been added to the mix for extra reinforcement, a notorious technique of its era.
The building’s steel reinforcement was in an advanced stage of corrosion, with column rebars having lost about 30% of their total cr0ss-section area. Slabs, at some parts, had lost all their reinforcement. The laboratory results determined about 25% of the beams and 17% of columns as non-repairable.
Most of the interior shear walls had been demolished for architectural reasons.
The building’s foundation was a mystery in itself. It was found to be some kind of 2m high and 300mm thick concrete walls, aligned in a mesh pattern, with the columns fixed at their cross-sections.
Obviously, the building was barely standing and given that it could not be demolished, since its external facades were of protected monument status, an elaborate solution had to be devised. We started working on some ideas about a new steel building inside the old one. We did not want to remove existing beams and columns, so a kind of square honeycomb structure had to be implemented.
Four HEB columns, placed around existing columns, supporting a horizontal frame of beams, each one pinned to the concrete slab. The steel frame would fit exactly inside the panels created by the concrete beams.
The mathematical simulation was done with SCIA engineer software. Below you can see the final model:
The existing building has been considered of negligible stiffness. The new steel frame has been loaded with most of the existing structure’s weight and all the dead and live loads.
The idea is that the purlins will be pinned to the concrete slabs and they will transfer the seismic force to the main beams, which in turn will pass it to the columns and bracing to release it to the raft slab and then the ground.
All of the existing columns and most of the perimeter walls have not been added as static loads, since there are -and will be- of adequate strength to withstand their self-weight. Nevertheless, their mass has been added so that it generates horizontal earthquake loads, received by the new steel frame.
The steel building has been designed with 1st order theory, non-sway bracing systems in both directions, in accordance to Eurocodes 1,3,8.
After calculations and proper drawings, we proceeded to the construction stage which was no picnic either. An experienced contractor and engineer Andreas Fermelis with his crew came in and assembled this perplexed design. The mechanical-electrical installation was designed by engineer Yorgos Souris. The whole construction stage was supervised by Piraeus Bank engineer Stavros Papakonstantinou.
At first, the existing slab on grade was demolished and a brand new raft slab was constructed:
The raft slab is in general 400mm thick, with areas reaching even 900mm to resist punching shear from columns and most importantly from the bracing system.
The top part of the existing foundation walls was demolished and the rest was connected with the raft slab.
Cathodic protection was applied thoroughly to the new foundation.
Holes had to be opened at the external walls in order for the new beams to be connected with the interior steel columns. The beams are pinned to the ceiling with many bolts. The total number of the whole building’s bolts has been calculated in order to transfer the seismic force, from the slabs to the beams.
The columns where assembled in three pieces, you can see the splicing formed with welded plates (300χ80mm).
The columns extending to the first floor are assembled in three pieces, the splicing is formed with bolted plates. Stiffeners have been welded to all the columns and beams to compensate for lateral-torsional and flexural buckling.
All the columns are fixed together with SHS beams which are pinned to the existing columns. In this way buckling length decreases drastically and seismic force is delivered from the old columns to the new ones. The columns are fixed to the new foundation.
The columns must be continuous along the total building’s height, therefore holes have been drilled through the first floor slab.
In the above photo you can see the bracing system at the ground floor and in the following photo it emerges at the first floor (right corner):
The above bracing system had to be devised in the first floor, so that the window would stay intact.
A difficult, but vital, installation of bracing through the staircase:
The existing balcony was reinforced with pinned steel plates to retrieve tension on the upper side:
After we were done, Piraeus Bank Architect Elia Prevedourou rushed in to make the building functional and habitable. See below the finished building:
Finally, a safe earthquake resistant, elegant building:
