Effects of the 1969’s Earthquake Structural Analysis of a RC building Ana Luísa Pinto Vieira [email protected] Instituto Superior Técnico June 2020 Abstract This paper will focus on the earthquake that occurred on February 28, 1969. Initially, all the existing information about the phenomenon was surveyed in order to ascertain its impact in the Portuguese territory. To this end, many buildings affected in the city of Lisbon and others in the south were identified to establish damage patterns according to what was observed, and the various descriptions collected. These buildings were classified according to their period of construction and building typologies, and subsequently, using the European Macroseismic Scale (EMS-98), they were associated with a vulnerability class and a degree of damage. This assignment made it possible to determine the intensities in the districts of Lisbon, Setubal and Faro, followed by their comparison with the intensities obtained at the time, and identify the typologies that present more vulnerability in the occurrence of an earthquake of similar intensity. Afterwards, based on the record obtained by the accelerograph of the Laboratório Nacional de Engenharia Civil, a structural model of four buildings of 19 floors in height was developed, where two of the buildings were already built when the earthquake struck. Subsequently, two linear dynamic analysis were performed: one through the response spectra and accelerograms (Time-History Analysis) and the other based on the response spectrum recommended in the EC8. The vulnerability of the structure subjected to the seismic combination was evaluated. Based on the safety check performed on the structure’s project, a linear static analysis was also performed with the same objective. Key-words: 1969’s earthquake, EMS-98, vulnerability, linear dynamic and static analysis, Time-History, EC8 1 Introduction earthquake, it was severe enough to cause damage, especially in the south of the country. One of the last major earthquakes of the 20th The motivation for this work is to collect as much century occurred in the early hours of February information as possible about this phenomenon 28, 1969. Although it was not a very destructive and interpret it in the light of current knowledge 1 of seismology and seismic engineering. The injuries and, regarding material damage in damage suffered in each building was analyzed buildings, there is only to signal the collapse of and classified in order to obtain the Isoseismal an uninhabited house and the opening of small Intensity Map in the districts of Lisbon, Setubal cracks in very few buildings. and Faro, based on the European Spain and Morocco were also affected. Macroseismic Scale (EMS-98 [1]). Morocco registered 8 deaths and 11 injured, In addition, a detailed study regarding a RC and Spain 5 deaths and 6 injured. building, recently built at the time of the 2.2 Affected Areas in Mainland earthquake, was executed in order to verify its Portugal structural safety. The earthquake was felt throughout the country 2 The Earthquake of February and the damages can be summarized in the 28, 1969 opening of cracks (interior and exterior), the 2.1 Historical Context partial or total collapse of protruding elements such as chimneys, plat bands or ornaments, fall The earthquake of February 28, 1969 was felt of variable portions of plaster, displacement or in several regions of the Continent, as well as in collapse of furniture and destruction of the island of Madeira, Spain and Morocco. The tableware and glass. In the rural regions, the Portuguese seismographic network consisted collapse of buildings of older construction is of only 3 stations that registered the earthquake cited. Many churches were affected, with the at 3:41 AM (local time). The epicenter was later opening of accentuated cracks in the domes determined with the coordinates 35.98ºN and facade and the collapse or ornaments and 10.81ºW and a magnitude of Mw7.8 and Ms7.9. crosses. In some, the total or partial collapse of The only existing record is the one obtained by roofs or other structural elements were the accelerograph of the Laboratório Nacional observed. Some national monuments also de Engenharia Civil, installed on the north suffered damage. anchorage column of the Tagus bridge in Castelo Branco, Viseu, Braga and Bragança Lisbon. Due to the cut in the electricity supply, are some of the regions where the vibration was there is a total record of only 27 seconds. felt with some intensity, although no record of The earthquake caused panic in the population major material damage was observed. accentuated by cuts in communications and 2.3 Isoseismal Maps proposed electricity supply. Of the 16 fatalities registered in Portugal, only 3 were a direct result of the in 1969 earthquake. About 70 and 150 were injured in Isoseismal maps were proposed by Trêpa [3], Lisbon and Algarve, respectively, mainly due to Mendes [4] and Mezcua [5] with considerable bruises caused by falls or nervous breakdowns. differences in the intensity attributed to the In Algarve, because of the heavy rains and southern regions of Portugal. Mendes [4] thunderstorms felt right after, the damage was suggests an intensity of VIII at the end of Cape accentuated. With regards to the island of St. Vicent, while Trêpa [3] proposes a wider Madeira, the earthquake recorded at 2:43 AM area with the same intensity, covering Aljezur, (local time), did not result in any fatalities or Vila do Bispo, Sagres and Bensafrim. However, 2 the maximum intensity attributed by Mezcua [5] Table 3.1 – Damage probability matrices obtained for in the entire region of Algarve is VII. vulnerability class B (based on the EMS-98 [1]) All the studies suggest an intensity of VI and VII to the center and center-south region of the country, respectively. 3 Analysis and Treatment of the Collected Information 3.2 Lisbon’s District The damage collected in each district was The consequences in this district were vast and separated by housing/private buildings, consisted, essentially, in the partial or total churches and public buildings/monuments. collapse of protruding elements and the Based on the EMS-98 [1], each building was opening of cracks, primarily in buildings of older associated with a vulnerability class and a construction. The partial or total collapse of few degree of damage. Afterwards, the Isoseismal buildings also occurred, and some had to be Intensity Map for the districts was obtained. demolished, due to their state of deterioration. 3.1 European Macroseismic Public buildings/ monuments and churches Scale (EMS-98) were also affected. It was possible to classify 906 housing/private When applying the EMS-98 [1] it is essential to buildings, 12 churches and 21 public buildings/ access the census existing at the time under monuments. study, in order to apply the quantity definitions 3.2.1 Building types of “few” (0-10%), “many” (11-50%) and “most” In 1969, the city of Lisbon was divided into 53 (51-100%) suggested, for the buildings parishes and the building types found can be affected. summarized in stone masonry, concrete and 3.1.1 Vulnerability Classes wood. Stone masonry buildings can be divided The vulnerability classes are used to discern the in Pre-Pombalino (buildings that resisted the behavior of buildings when an earthquake earthquake that occurred in 1755), Pombalino, strikes. Consequently, the EMS-98 [1] proposes Gaioleiro and Placa buildings. 6 decreasing classes of vulnerability (A to F) The wooden buildings, assigned with class A, depending on the type of structure in question. were prefabricated wooden structures with fiber 3.1.2 Damage Class cement plates as roofs or made of cardboard The damage suffered by the building is and/or the remains of other materials. assessed by assigning a damage class To the Pre-Pombalino buildings class A or B according to its degree and extension: from D1 was assigned, depending if the building was (negligible to slight damage with no structural composed of well-kept and paired stone damage and/or slight non-structural damage) to masonry walls or poor masonry walls (adobe or D5 (total or partial collapse of the structure). clay mortar), respectively. To the Pombalino 3.1.3 Damage probability matrices and Gaioleiro buildings class B was assigned. Damage probability matrices can be created, as The main difference between the two is that the the example presented in Table 3.1, for class B. first one is characterized by having a “cage” structure on the interior walls on the floors 3 above the ground floor, used to improve the structure’s capacity to resist horizontal actions. With a maximum of 4 floors, the ground floor, essentially built by arches and masonry vaults, is used for commercial areas or warehouses. Figure 3.1 - Vulnerability classes affected and respective The Placa buildings, assigned with class C, are degrees of damage in the city of Lisbon distinguished by the existence of floors in weakly RC slabs that discharge directly into the walls. The RC buildings were assigned with a vulnerability class C or D, depending if it had Figure 3.2 – Degree of damage for class B in the city of earthquake-resistant design or not. Lisbon 3.2.2 Summary of damages observed For the present work, the method applied for For housing/private buildings, a total record of attribution of intensities is the one showed in 1286 occurrences were obtained, where the Table 3.2, for the parish of Anjos. It was highest incidence related to damage to considered that the best adjustment is given by chimneys and cracking. The most severe choosing the lowest value of the interval, which, damage was seen in downtown, where there is for this parish, corresponds to an intensity of VII.