The Conservation Problems and the Structural Intervention for the Improvement of the Seismic Safety
The Great Hall Vault of the Trajan Markets is one of the largest and very impressive among the remaining original roman vaults. It is made by roman pozzolanic concrete with a very thick shape which allows a nearly monolithic behaviour, just reduced by the possible negative effects of many cracks. But the weaker structural elements, in case of seismic actions, are the supporting structures. These last are today not sufficient and/or not sufficiently laterally counteracted to resist to the horizontal actions associated to seismic effect on the Great Vault mass.
On site investigations have been devoted to the identification of the geometry of the main structural parts and elements as well as of the mechanical features of the constituting materials of the Great Hall Vault and of its supporting structures. These surveys have suggested to carry out some numerical analyses which have shown the weak behaviour of the supporting structures. Thus it was designed, numerically verified and finally applied an adequate retrofitting intervention, based on the use of reversible techniques.
THE STRUCTURAL BEHAVIOUR BEFORE THE RETROFITTING
The transversal behaviour and the crack patterns
The Great Hall structures, that surround and contain the Great room, only apparently form a thick body with a squared plan; on the contrary they are two bodies, separated by the Great room itself (figures 1-3). These two buildings develop their plan parallel to the Great Vault axis, in the NE-SO direction. Thus, both of them are weaker in the transversal NO-SE direction.
Among them, the northern one appears more sound as it is less high and transversally thicker. Vice versa, the Southern one is thinner and higher as it starts from the level of Via Biberatica (figure 4).
The weaker conditions of the Southern building is shown by the crack pattern also, with a clear tendency to the detachment of the Southern façade on Biberatica Street. Moreover, it is necessary to take into account that these two buildings have to support the big mass of the Great Hall vault, under static and seismic actions too. From this point of view, it is important to notice the weakening of the transversal wall, in the Southern building, caused by the doors placed near the Southern façade, at the same level of the Great Hall pavement. The seismic action of the past, are the causes of the cracks on the arches over the doors said before and of the cracks on the transversal walls, in the lower level, just under those doors and near the southern façade; cracks that show a clear weak condition under the Great Vault thrust (in NO-SE direction) with also a clear tendency to a detachment of the Southern façade on Biberatica Street (figure 5).
It must be taken into account that, before the retrofitting, the transversal seismic acceleration of the Great Vault mass is alternatively supported only by the Southern building and only by the Northern one (changing the sign of the acceleration itself); as it is easy the arise of hinges in the key and in the springing of the Great Vault (figure 5). Moreover, this behaviour may be accentuated by the different transversal stiffness of the two building, as this difference can easily cause opposition of phase in the transversal oscillations of the two buildings.
The longitudinal behaviour, parallel to the Hall axis
The seismic action longitudinal component founds a very weak structural configuration in the vault supports at the “matronei” level. All the supporting pillars and the counteracting lateral arches, have their main stiffness planes in the transversal direction while the weaker ones are in the longitudinal direction (figure 6).
It is important to notice that the present masonry structural configuration is due to the restoration works carried out in the twenties and thirties of the last century, when they were demolished all the not original roman masonries added along the centuries and especially in the XVI century.
Thus, and especially at the “matronei” level (figure 6), the structure is weaker than in the period from the XVII up to the XIX century and also weaker than the original configuration, as some roman structural elements (some secondary vaults) are disappeared, along the past centuries.
The numerical analyses
The analytical study of the vault and its surrounding structural elements was carried out by means of a numerical 3D model developed for the static and dynamic structural behaviour evaluation, using the Algor program produced by Algor Inc.
The 3D Finite Element mesh is refined in such a way to describe with an adequate accuracy all the constructive details, using 3D “brick” finite elements.
In Table 1 they are reported the material mechanical characteristic (specific weight, Young modulus and Poisson coefficient) used for the different parts of the structure.
About the seismic spectral acceleration, the present Italian Code states a ground acceleration of around a = 0,192g at the building foot, which means an amplified acceleration of around a = 0,260 g at the Great Vault level.
In figures 7 and 8 are reported the results of the seismic static equivalent analysis in the transversal direction, while in figures 9 and 10 are reported the static equivalent analysis in the longitudinal direction.
In figure 7, all along the intrados of the vault key there are tensile stresses that reach the 210 kPa and justify the deep and large cracks visible before the last restoration.
It is important also to notice in figure 8 the strong compression stresses in the foot of the short pillars supporting the Vault: the minimum principal stresses reach the 1822 kPa.
Table 1. Material mechanical characteristics
Material weight Young mod. Poisson mod.
Caementicium 15 2.000.000 0.15"
Travertino 24 20.000.000 0.10"
Cocciopesto Mortar 18 200.000 0.20"
However the worst situation arise with the seismic action in the longitudinal direction.
The static equivalent analysis reported in figure 9 shows the risk of overturning for the pillars engaged along their weaker section axis: the vertical stresses reach 1142 kPa in the compressed side; while reach 311 kPa in the side on tensile stress. The little arches that laterally counteract the vault (figure 10), are unable to resist to the longitudinal seismic action, as in this case they are bent horizontally, out of the proper arch working plane, reaching tensile stresses up to 350 kPa.
THE REINFORCEMENT INTERVENTION AND RETROFITTING
The intervention philosophy
Evaluating the opportunity to “improve” the seismic behaviour of an historical building, it is important to study its global structural behaviour, but it is also necessary to check if each structural element may compromise, with localized failures, the structure as a whole.
In the case of the Trajan Markets Great Hall, there is a clear “global” weakness in the transversal structural behaviour, due to the weaker configuration of the Southern building, in case of seismic actions in NO-SE direction; but, at the same time, there is a “localized” weakness of the pillars supporting the Great Vault in case of seismic actions in NE-SO direction.
The failure of only one of these pillars may cause the collapse of all the Great Vault.
In the case of historical buildings, the seismic behaviour improvement has to be obtained with the minimal alteration of the original structure.
Thus it is better to apply a “diffused” and “reversible” intervention instead of a more strong and concentrated one, which last is necessarily more invasive and, thus, also less reversible.
A “diffused” intervention has to be extended as more as possible to all the structure, in such a way to better connect the different structural elements, to guarantee their collaboration and, thus, to use more efficiently their original strength.
On the contrary, too localised interventions may cause the alteration of the original global behaviour, more higher stress concentrations and, thus, also possible local damages.
In the case of the Great Hall, for the transversal (NO-SE) seismic component, it was necessary a “diffused” reinforcement of the transversal shear walls, mainly in the Southern building.
At the same time, for the longitudinal component (NE-SO) of the seismic actions it was decided to not to try the reinforcement of each pillar supporting the Great Vault; on the contrary it was designed a shear braced horizontal stiffening to connect, on both the longer sides, the Great Vault mass to the Northern and to the Southern buildings.
The transversal reinforcement
The intervention is a system of horizontal ties, distributed on each transversal wall of the two buildings supporting the Great Vault, in such a way to improve their shear strength in the NO-SE direction.
More in detail, in the weaker Southern building these ties are distributed not only on each shear wall but also on each level, as shown in figure 11.
Moreover, as shown in figure 12, for each shear wall it is placed a couple of bars nearby each side of the wall itself, instead a single one, in such a way to be less invasive, avoiding to drill horizontally those walls for all their length.
To guarantee the collaboration of both the buildings in counteracting the Great Vault mass thrust, during a seismic action, they are placed horizontal connections over the two series of lateral arches among the two buildings and the Vault itself. Then they are placed also some ties, across the Vault, inside its thickness, also to counteract the effect of possible not in phase transversal oscillations of the two buildings.
Thus it is placed a system of horizontal distributed ties also in the Northern building, but only at the III and IV level, in such a way to involve its transversal shear walls all along their length.
The distribution and the number of these ties placed in the two buildings and in the Vault, allow to reduce their diameter down to 22 mm.
The longitudinal diagonal braced shear reinforcement
The intervention is a system of nearly horizontal stainless steel diagonally counterbraced shear reinforcement, placed in the free spaces among the great vault and the lateral buildings, just over the “matronei” level (figures 11 and 12).
This shear reinforcement is designed in such a way to transfer to the two lateral buildings, parallel to the Hall axis, the main part (around the 65%) of the longitudinal seismic action involving the Great Vault mass, reducing the overturning moment on the pillars supporting the Vault itself.
Four free spaces on each side are occupied by the diagonal counterbraced reinforcements and each diagonal is made up by two tie bars with 22 mm of diameter (figure 14). Thus during a longitudinal seismic action 16 diagonal braced tie bars work together at the same time.
The numerical analyses
The numerical model, which simulate the reinforcements through stiffening boundary elements along the two longer side of the Great Vault, show a clear improvement in the Vault structural behaviour.
Particularly in figure 13 is reported the stress reduction in the pillars supporting the Vault, in case of longitudinal seismic action: compared to the case without reinforcements, the static equivalent analysis shows as the vertical stresses are reduced from 1142 kPa to 810 kPa, on the compressed side, while the tensile stresses are reduced from 311 kPa to 174 kPa.
The Trajan Markets Great Hall shows a high sensibility to seismic actions.
This fact is due to the weakness of its supports: the weak structural behaviour of the Southern building, in case of transversal actions, and the weak behaviour of the pillars at the “matronei” level, in case of longitudinal actions.
While in the first case there is an indirect risk of collapse for the Vault, related to the possible partial failure of the Southern building, in the second case, with the longitudinal component of the seismic action, there is an immediate risk of collapse af the Vault as a whole, related to the easily overturning of the pillars.
The intervention designed and already applied, with its “distribution” calls the collaboration of all the supporting structures, reducing the efforts of the single structural elements.
In this way, avoiding stresses concentrations, they are not present alterations of the original structural conception.
Moreover, the reversibility of this intervention typology is a warranty for the possibility to use the future probable improvements in the retrofitting techniques.
[by Giorgio Croci and Alessandro Bozzetti]