Many Brazilian stadiums have presented structural problems in their stands, due to excessive vibrations in days of big events. These problems are most often due to infrequent structural maintenance. In addition, the stages are not designed to support the dynamic loads to which they are being submitted.

At the time they were built, the dynamic efforts induced by the public were of a much shorter duration and intensity, and not as it currently is during great musical performances, when the bleachers must endure for a long time the rhythmic impact of an audience.

Even in cases where safety related to collapse of the structure is guaranteed, it is necessary to verify that the vibrational levels are within the limits allowed by the international norms, in order to ensure comfort to the public, thus avoiding situations of panic.

The Cícero Pompeu de Toledo Stadium (Morumbi), for example, was banned by the CONTRU – Department of the Municipality of São Paulo, which verifies and approves building safety conditions – at the beginning of 1995, and underwent several renovations since that date, with the objective of solving structural problems in their bleachers.

The São Paulo Futebol Clube, owner of this stadium, made reforms that included the recovery of concrete from the stands and the hiring of renowned Brazilian consultants who proposed some alternatives that have been analyzed. In order to verify the efficiency of the reforms carried out, São Paulo Futebol Clube contracted the Italian company Ismes Spa, associated in Brazil to the Estudos Técnicos e Projetos ETEP Civil Ltda., of which we are directors, for the execution of the first dynamic test of forced vibration realized in the Brazil. The work was done in May of 1995 and its results were presented by the club to the Contru that, in June of 1996, partially liberated the stadium.

Intended to determine the dynamic properties of the structures (natural frequencies, vibration modes, damping) and to calculate the vibration levels induced by the public, this test consists of exciting the stands by means of a mechanical vibration generator called Vibrodina, which simulates the rhythmic effect of the crowd, without the presence of the same. Inside the Vibrodina they turn two discs in the opposite direction, with two eccentric masses in each disc, that produce a centrifugal force, controlled and varied gradually, in order to reproduce all the frequencies that a twist transmits to the bleachers. The test is performed in three steps, which correspond to the application of the force produced by the Vibrodina in the vertical, horizontal radial and tangential horizontal directions, through the repositioning of the equipment at the same point.

A network of 21 sensors, installed at strategic points in the bleachers, captures the response of the structure to this excitation and transmits it to a system of filters and amplifiers that will debug these signals and file them. This “answer file” is processed in ISM (Ismes Signal Analysis) software, which converts these signals into values ​​analyzed by the associated companies – Ismes and Etep Civil – also responsible for issuing the final report. In the case of the Morumbi Stadium, the test in a single module of the bleachers was sufficient to carry out the analysis of the behavior of the structure, since the stadium is composed of 72 pillars of support (“giants”) that are repeated. In this way, the results obtained in only one module could be considered representative of the others. In addition to providing all the dynamic characteristics of the structure, the test tested an alternative study that aimed to reduce the vibrations of the stands. This alternative consisted of a metal pillar located between the lower ring (lower stand) and the intermediate (higher stand). In this way, the test was performed twice: with and without the abutment. After 10 days of dynamic tests and calculations, the final results showed that:

1) the structures of the stands did not present any plastic deformations; 2) the alternative considering the metal abutment did not show significant vibration reductions; 3) with full capacity, vibrational levels exceed the limits allowed by international standards. In order for these limits to be respected, the following condition must be obeyed: total occupancy in the lower and higher numbers, corresponding to 40,000 spectators, and in the stands only a third of the capacity, corresponding to an additional 14,000 spectators.

Currently, the stadium is released to the maximum capacity of 34,000 fans, as the lower ring is under construction. Another stadium that used the same Italian technology to test its stands was Palestra Itália, from the Sociedade Esportiva Palmeiras. With the same problems of excessive vibrations presented in the previous case, the stage was released with partial stocking.

In Palmeiras it was necessary to perform the test in five distinct modules, since the structures were built at different times, besides not being repetitive, as is the case of the Morumbi stadium. The results showed that parts of the curved stretch of the stands have vibrational levels higher than those allowed by international guidelines.

This same technology used in these Brazilian stadiums was used by Ismes during the adaptation of several Italian stadiums to the World Cup of 1990, among them the San Siro of Milan, Delle Alpi of Turin and Olympic of Rome.

Currently, Ismes (Istituto Sperimentale Modelli e Strutture), considered the largest engineering technology center in the world, has been developing services and research in the fields of computer science, large structures and soil and environmental protection. With headquarters in Bergamo, Italy, since 1947, Ismes has accumulated extensive experience in maintenance, automatic monitoring and structural recovery projects for tunnels, bridges, historic monuments, stadiums, dams, earthworks and buildings. Among his main works are the automated monitoring of the Tower of Pisa, the Bruneleschi dome in Florence, the Mexico City Cathedral, the Humber Bridge in Great Britain, the Zarate-Brazo Largo cable-stayed bridge in Argentina, 300 dams and 11 dams for the Electricity Generating Authority of Thailand (Egat). At the beginning of its activities, Ismes dedicated itself exclusively to the control of the safety of Italian dams to meet its majority partners, Enel (Ente Nazionale per l’Energia Elettrica).

In Brazil, the works carried out during the great impulse of the civil construction, from the 50’s, began to show signs of deterioration due to the time of use, lack of maintenance, aggression of the environment and, like stadiums, because they were subjected to higher than those for which they were designed.

These facts, together with the concerns, demands and greater awareness of the safety of users, led Etep civil, a company of the Etep Group, which since 1966 has been working in the field of consultative engineering in Brazil, to seek the partnership of Ismes in order to bring for Brazil technologies widely used in Europe and that only recently found conditions to be applied in our country. The main objective of the association is to practice an engineering of prevention and maintenance of the existing works, providing conditions of use in solid and safe bases.

This partnership between Ismes and Etep Civil began in November 1994, with our visit to Italy to learn about the works carried out in Italian stadiums, dams and monuments. In October 1995, engineers Paolo Pezoli and Paolo Panzeri, directors of Ismes and professors of the Polytechnic Institute of Milan, spent a week in Brazil visiting the São Paulo stadiums, as well as companies and public agencies in the areas of Energy and Transportation. During the visit to Brazil, Ismes engineers participated in the “1st International Symposium on Dynamic Characterization and Safety of Structures”, organized by Etep Civil at the São Paulo Institute of Engineering, with the support of Ibracon. On that occasion, the problems and solutions adopted in the Italian stages, as well as the main experimental techniques in the area of ​​dynamics of the structures were presented. The consolidation of the partnership took place with the works carried out in the stadiums of Morumbi and Palestra Itália.

Currently, Ismes and Etep Civil are constituting a joint venture that will have European Community funding for the implementation of a Technology Center to be called Ismes do Brazil. The initial investment will be US$ 1 million and the start of operations is scheduled for the first half of 1997. This Technological Center will be equipped with special equipment, such as those used to perform the dynamic tests of forced vibration cited. In addition to these dynamic stadium trials, the Center also intends to serve other types of structures, such as large dams, bridges, viaducts and tunnels. The monitoring for prediction and risk situations, as for the slope slope is another area in which the Center intends to act. Particularly in the area of ​​dams, Ismes has developed the Midas software, which is the intelligent system of monitoring of structures security implanted in numerous dams, inside and outside Italy. This system consists of organizing, analyzing and interpreting the periodic readings, manual or automatic, performed by sensors installed in the structures and foundations of a dam and activate warning signals in case of anomalous situations.

The extensive experience gained in the almost 50 years of existence allows the Italian company to evaluate the efficiency of the dam sensor network and resize it, if necessary, by increasing or reducing the number of existing instruments. Through the Midas system, it is also possible to develop static models, which use readings performed throughout the life of the dam, and calibrated deterministic models, starting from mathematical hypotheses regarding the geometry of the dam and the characteristics of its materials. Thus, one has the parameters resulting from the theoretical models, and the other, the manual or automatic readings obtained from the installed instruments. By means of the comparison between the theoretical and observed results, possible anomalous behaviors and triggered warning signals are defined.

In addition, this system “self-diagnoses” by periodically checking the instruments, thus defining whether a discrepant reading is due to some sensor failure or is an indication of some anomalous structure behavior.

All this technology will be available in Brazil, from the beginning of the year, in the new Ismes Technology Center of Brazil, which intends to operate throughout South America.

Eng. Liana Becocci and Eng. Marco Juliani
THE CONTRACTOR – April/1997 – nº 343