Structural Studies, Repairs and Maintenance of Heritage Architecture IX 291 Seismic behaviour of the historical structural system of the island of Lefkada, Greece E. Vintzileou1 & P. Touliatos2 1Faculty of Civil Engineering, National Technical University of Athens, Greece 2Faculty of Architectural Engineering, National Technical University of Athens, Greece Abstract In this paper, the main characteristics of the historical structural system of the island of Lefkada, Greece, are presented. The typical damage caused by a recent strong earthquake (August 2003) are briefly described, since they allow for the seismic behaviour of the system to be studied. The pathology of the structural system seems to confirm that the structural system was conceived to sustain earthquakes. Keywords: earthquake, stone masonry, timber framed masonry, damages. 1 Introduction The island of Lefkada, one of the Ionian Islands, is situated in one of the most earthquake prone regions of Greece. A local structural system was developed before the 19th century in Lefkada. The strong earthquake occurred in 1821 proved the adequacy of the system to sustain seismic actions. Thus, the British Authorities (ruling the Ionian Islands at that time) imposed rules for the construction of new houses following the main characteristics of the local structural system. The rules, further developed and completed, constituted the Code for Construction issued in 1827. That Code provided guidance on the selection of building materials, on the thickness of stone masonry in the ground floor, as well as on the maximum storey height. In addition, minimum distance is required between adjacent buildings, to allow for better protection against fire and earthquakes. A considerable number of buildings, built according to this structural system are still in use in the city of Lefkada. WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) 292 Structural Studies, Repairs and Maintenance of Heritage Architecture IX In August 14th, 2003, a strong earthquake (Mw 6,2) occurred in Lefkada. A thorough investigation of damages observed in structures built according to the historical structural system, together with a detailed survey of the typical characteristics of the system allowed for the identification of the main features of its seismic behaviour (Vintzileou et al. [1]). The results of this investigation are briefly presented and discussed in this paper. 2 Description of the structural system 2.1 Generalities The historical part of the city of Lefkada, situated by the sea, is developed both sides of a central street that separates the city into two parts of approximately equal areas. Lateral narrow streets start from the central street and are directed to the sea (N-S or S-N direction). This arrangement allows for drainage of rainwater towards the sea. In addition, the-most frequent-north, northwest winds offer favourable conditions for reduction of humidity in the timber parts of the houses. Typical buildings (one- to maximum three-storey buildings, fig. 1) consist of a stone masonry ground floor plus one or two timber framed brick masonry storeys. Intermediate floors and roof are made of timber. Openings (typically rectangular, approx. 1m wide) are symmetrically arranged along the facades of the buildings. Few decorative elements (mainly, in entrances, in the corners of the building, as well as around the openings) characterize the typical building. The roof is covered with tiles. To protect timber framed masonry from humidity, the upper storeys used to be covered along their perimeter by timber planks (fig. 2a). The high cost of replacement of this cover (decayed with time) led to their replacement by plane or corrugated metal sheets (fig. 2b). Covered walkways (fig. 3) constitute a typical morphological and functional element in a city with frequent rainfalls. Figure 1: Typical building. WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) Structural Studies, Repairs and Maintenance of Heritage Architecture IX 293 Figure 2: Protection of timber elements, (a) by timber planks, (b) by metal sheets. Figure 3: Covered walkway. Figure 4: Grid of trunks (under the foundation). 2.2 Foundation The city of Lefkada is founded on low strength alluvia. Therefore, special care was given to the foundation of buildings. Although it is not practicable to explore the footings of existing buildings, according to descriptions (based on information provided by craftsmen that worked in the construction of similar buildings), the buildings are founded on a grid of trunks (fig. 4), at a depth of 0.6 to 1.0m. The space between trunks is filled with sand, rubble stones and hydraulic mortar. WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) 294 Structural Studies, Repairs and Maintenance of Heritage Architecture IX 2.3 Ground floor Bearing walls in the ground floor (typically, perimeter walls) are max. 3.0m high. They are 0.6m to 1.0m thick and they are made of stone masonry. The external leaf is made with roughly cut stones, whereas cut stones are used in the corners of the building (fig. 2b), as well as along the perimeter of openings (fig. 2a). Rubble stones are used in the internal leaf of ground floor walls. The space between the two leaves is filled with small size stones mixed with pieces of bricks and mortar. Natural pozzolan was used in the buildings constructed up to the end of 19th century. Later, lime was used together with straw (to improve mechanical properties of mortar), whereas in poorer structures, clay mortar was used. In addition to masonry walls, a secondary (timber) bearing system is present in the ground floor: It consists of timber columns arranged close to masonry walls. The typical distance between consecutive timber columns is equal to 2.0÷3.0m. The function of this secondary system is discussed in §3. 2.4 Upper storey timber framed masonry In fig. 5, the typical arrangement of timber elements in the timber-framed walls is shown. The timber frame of the perimeter walls is connected to the ground floor masonry, through timber beams arranged along the perimeter of stone masonry walls. Metal ties (of various geometry and size, fig. 6) are used to connect the timber elements of the floor with the stone masonry and/or with the timber frame of the upper storeys. Figure 5: Typical timber-framed Figure 6: Metal ties wall. connecting the timber floor with ground floor masonry. 2.5 Floors and roof Fig. 7 shows the typical construction of intermediate floors and roof. It should be reminded here that intermediate floors do not carry only their own (dead and WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) Structural Studies, Repairs and Maintenance of Heritage Architecture IX 295 live) loads. They also transfer the loads of the upper storey(s) perimeter masonry to that of ground floor, as well as to the secondary timber bearing system of the ground floor. 2.6 Connection between timber elements As shown in fig. 8, special care was given by the constructors to the connections between timber elements in the roof, within the timber-framed masonry of the upper storeys, as well as between timber beams and columns in the ground floor. Taking into account that Lefkada has always been an island with limited resources, one may assume that the purpose of the constructors was to enhance the overall stiffness of buildings. Figure 7: Typical construction of (a) intermediate floors and (b) roofs. Figure 8: Typical connections between timber elements. WIT Transactions on The Built Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3509 (on-line) 296 Structural Studies, Repairs and Maintenance of Heritage Architecture IX 3 Seismic behaviour of the structural system On the basis of the description presented in the previous section, a working hypothesis can be adopted regarding the conceptual seismic design of the historical structural system of Lefkada. It should be mentioned that this working hypothesis was confirmed by the typical damages observed after the 2003 earthquake (see Section 4), as well as by parameter analyses carried out within a research project (Vintzileou et al. [1]), presented also in Vintzileou et al. [2]: (a) Stone masonry walls in the ground floor are thick enough (given their limited height), well interconnected in the corners of the buildings, and pierced by a limited number of openings. Thus, they are conceived to provide the stiffness that is necessary for the building to be subjected to rather small deformations during seismic events. This favourable behaviour is enhanced by the floor, consisting of closely spaced timber beams and timber roofing, carefully connected among them, as well as with stone masonry. Thus, the box action of the ground floor is ensured. (b) The upper storey(s) are made of thin (typically, 10-12cm) timber framed masonry. In this way, a-favourable for the seismic behaviour-reduction of the total weight of the building is achieved. On the other hand, the stiffness of timber framed walls is ensured thanks to the closely spaced horizontal, vertical and diagonal timber elements, as well as by their connections (fig. 8). Furthermore, the limited dimensions of brick masonry panels (fig. 5) well confined by the timber elements, ensure their satisfactory in-plane and out-of- plane behaviour. In addition, the negative effect of failure (or even, collapse) of a number of brick masonry panels is substantially reduced. Timber framed walls are connected with the ground floor masonry through the timber floor, whereas the box action of the upper floor is completed by the rather stiff roof system (fig. 7b). It should be mentioned that the diaphragm action of both floors and roof was also proven analytically in Vintzileou et al.