MAGNETI MARELLI – SEISMIC UPGRADING OF THE OFFICE BUILDING
Office building post earthquake 2012 – initial state before intervention
Office building post earthquake 2012 – works completed
THE SEISMIC EVENT AND THE DAMAGES SUFFERED
The office and service building annexed to the industrial sector of Magneti Marelli in Crevalcore (BO) is one of the many buildings damaged by the seismic events of May 2012. Therefore, the structural safety was verified and seismic upgrading interventions were designed. The final work adjusts to 100%, through a “steel exoskeleton in hierarchy of strengths”, the significant seismic vulnerability of a building designed in the early ’70s to withstand only vertical loads.
The office building, together with the entire production area, was built between 1973 and 1974. Over the years, it has not undergone any substantial modifications or significant seismic events.
The building consists of 2 floors above ground and a basement, rectangular elongated planimetric conformation with dimensions of 56mx12.25m for the mezzanine floor with basement and 55.55mx13.75m for the 1st floor.
Following the two seismic events of May 2012, the following damages have developed:
- Structural damage at the two stairwells
- Lesions on the external infill due to compression against the pillars
- Injuries on about 70% of the masonry partitions
- Lesions on the head of a pillar on the 1st floor
- Lesions on the external panels in reinforced concrete
- Hammering phenomena on the external panels adjacent to the walkway connecting the production building.
THE LOCAL SEISMIC RESPONSE STUDY OF THE INDUSTRIAL SITE
Teleios decided to perform a local seismic response analysis specific for the site of the plant. The analysis criteria that would be used were chosen, since only by knowing the geotechnical parameters would it be possible to design the geognostic investigation campaign in a coherent and effective way.
Therefore, an extensive and specialized on-site and laboratory testing campaign was performed, which included the execution of:
- Probing to a depth of 31m
- Static penetro metric tests with piezocone CPTu
- Geophysical surveys: down-hole DH test in borehole, surface geophysical surveys type MASW, micro-tremor measurements type HVSR
- Laboratory tests: direct shear test, udometric tests, triaxial consolidated drained tests TX CD, consolidated and undrained tests TX CIU, cyclic tests in resonant column RC
Once the analyses were carried out, the corresponding accelerograms and response spectra acting at ground level were obtained from the seismic input signals. The aim of the RSL analysis in question was not only the accurate seismic characterization of the site but also, and above all, the obtaining of tools for the design of interventions that would have affected Magneti Marelli’s factory.
The seismic adaptation project: a steel exoskeleton in “hierarchy of strengths”
The structural conception
In light of the vulnerabilities highlighted in the structural safety verification, the structural conception of the intervention was based on the idea of deputing the existing structures to the only static function for which they were actually designed, i.e. the prevailing resistance to vertical loads only. Invec for actions not appropriate to the existing structure it was decided to place them side by side to an “exoskeleton” of steel, based on valved micropiles, seismic-resistant to 100% of the performance required by the Ministerial Decree 14/01/2008 as well as by the will of the client who has not limited itself to 60% required by the Legislative Decree 06/06/2012. Since a structure, in spite of itself, reacts for stiffness, we tried to further respect its own attitudes, excluding the involuntary reactive capacity towards seismic forces through the modification of the degree of constraint at the base of the pillars of the two floors in elevation carried out with the formation of Mesnager type hinges. In such a way the seismic responses of the two structural typologies constituted by the new structures and by the existing ones have been untied. The new seismic resistant systems have been dimensioned also in function of the damage limit state (SDL) assuming as limit of interstorey displacement for the existing columns that of 0,5% of the height even if it has been foreseen to substitute all the rigid panels and the brick counter walls with more flexible elements and less subject to damages. Moreover, it has been decided to demolish and replace the two damaged internal staircases in reinforced concrete with new, lighter metal staircases, hanging from the main reinforced concrete beams or from new steel beams.
The geotechnical conditions found and the mainly flexural stresses in the seismic field to be transmitted to the ground do not allow the use of surface foundations. The design choice was therefore to use plinths on piles arranged in a punctual manner at the seismic-resistant structures.
The exoskeleton has two types of seismic-resistant trusses. For each of the two types a different foundation has been provided: plinths with two piles for the “longitudinal” ones and plinths with 12 piles for the “transversal” ones.
Given the geotechnical conditions and the need to operate adjacent to existing structures, it was decided to use drilled micropiles equipped with valves in the final section, so as to increase the resistance.
Design of structures
The seismic-resistant structures consist of steel trusses, dissipative type, arranged according to the two main orthogonal directions identified by the major and minor sides of the building. Each truss is made of CHS round tubes and has been designed as a single welded assembly that arrives at the site hot-dip galvanized and painted in the workshop, ready to be housed on the ties.
The structural scheme of the trusses is such as to be able to dissipate both as a frame structure and as a concentric braced structure, thanks to the formation of the Vierendel type mechanism between columns and transoms and of the diagonal active tension type of the St. Andrew’s cross bracing.
The arrangement of the reticular baffles on the longer sides of the building coincides with the interaxis of the existing main reinforced concrete frames so that each baffle absorbs the seismic rate competent to the single reinforced concrete frame. The distribution of the baffles on the two short sides is instead designed to block the movements of the frames both at the heads and at the bases of the columns, and in an intermediate position at the main trusses.
In order to relieve the floors from parasitic plane actions and to make the distribution of seismic forcing uniform in the plane, systems of struts and small flat reticulars have been provided, immediately under the floors, connected to the main beams and columns.
All new seismic bracing systems possess the same external geometries so as to have architectural consistency. The different requirements in terms of strength, deformability and hierarchy of resistances have been solved by playing on the thickness of the tubes and the quality of the materials.
The basic connections of the reticular partitions are made by means of pins and anchor bolts embedded in the reinforced concrete plinths. The connections between new and existing structures are made with tubular profiles working as struts or tie-rods thanks to hinge-type connections and anchorages with contrasting or restoring systems with existing reinforcements. The choice of using for the basic connection the perfect pivot hinges allows on the one hand to assign to each reticular septum the specific seismic direction of work and on the other hand to optimize the joint when applying the rules of GdR thus avoiding significant differences between the stress resulting from the analysis and the calculation stress resulting precisely from the criterion of over-resistance.
The reticular septa need to be stabilized orthogonally to their plane and for this purpose additional wall bracing systems (located on the building elevations) have been provided by means of horizontal beams and S.Andrea cross bracing. The corner closure between the levels of the six-sept crossbeams around the building has only an aesthetic function.
The elliptical shapes of the unions optimize the distances from the edges provided by Eurocode 3 – UNI EN 1993 for the unions with pins providing also a certain aesthetic value.
From what has been reported, it can be seen that the aesthetic appreciation received by the work is derived predominantly from an adequate design response to its structural needs.