Porto Historical Centre Buildings

2.1 Architectural and constructive aspects

The buildings of Porto historical centre were mostly built between the XVII and XIX centuries, evolving from older constructions. They present a rectangular plant, with a narrow front, a long depth and a variable height: an average of 3 to 4 floors and a maximum of 8 floors (the top floors being usually added afterwards); the architecture of the inside is quite repetitive, usually with staircases at the centre, toped by a roof skylight, one compartment turned to the front and one to the back of the building, Figure 3.

These constructions that normally belonged to the popular and bourgeois branch of the population aren't exclusive from Porto and exist all over Portugal and in different European countries. Porto uniqueness resides in the fact that, in the old part of the city the slender houses represent almost the totality of the civil buildings.

2.2 Building structural scheme

The structural schemes of these buildings are also quite typical, consisting of four stone masonry walls

Figure 3. Main façades of two XIX century buildings at Sao Domingos and Loios squares, Porto.
Figure 4. Beams and transversal bars of a timber floor. Sao Domingos building, XIX century (1856), Porto.

defining the building contour: the two façades and the two lateral walls. The other walls are usually wooden type panels, sometimes reinforced by a wooden strut-and-tie system that improves its in-plane behaviour and the connection between walls and floors. This internal structure, together with the timber floors and the roof trusses, strongly contributes to the buildings efficient performance under vertical, but particularly under horizontal loads. As a result, it is important to ensure a good link between these elements in order to guarantee a good and stable global behaviour.

The floors that are normally supported on the lateral masonry walls consist of a set of circular section beams (diameters varying between 0,15 m and 0,40 m), spaced 0,60 m, and a secondary transversal bar system, Figure 4. Structural elements with rectangular section can also be found, usually as the result of interventions performed after the beginning of the XX century. The most common species used on the construction of floors were chestnut, oak, pine and eucalyptus. However, floors executed with other species also exist.

The timber roofs present variable configurations, depending on the size of the building. As the spans were normally lower than 6 m, the structure was very simple, with wooden trusses constituted by two rafters and a tie beam. In other situations, trusses with two

Figure 5. Timber roof structure. Antonio Carneiro building, XX century (1916), Porto.

rafters, two struts, a hanging and a tie beam were used, Figure 5. These elements have circular sections with diameters varying between 0,10 m and 0,30 m and the wood species used in its construction are the same of the floors.

2.3 Common structural damages

Concerning timber structures, this type of buildings presents well defined structural damages. In relation to the timber floors, two main damages can be identified: shear failure at beam extremities in the stone masonry walls, due to the reduction of section from biotic attacks (insects and dry rot fungus) associated to high moisture contents; bending failure at mid-span, originated by the presence of defects, drying cracks, reduction of section due to biotic attacks, excess of load from changes of use, etc. This degradation, associated to the wood fluency, is often responsible for the excessive deformation ofthe floors. Besides this deformation, the floors can exhibit excessive vibration, normally caused by a deficient transversal bar system, insufficient beam sections or excessive spacing between beams, etc.

As for the timber elements of roof structure, besides shear failure at the beam extremities, similar to what happens to the floors, it is not unusual to find biotic attacks. As a matter of fact, the roof timber elements present a great potential of biotic attacks, as they are more exposed to the atmospheric agents. Deformation due to fluency is also observed and, if not taken into account, it contributes seriously to the entrance of water in the buildings.

Although one can not generalize, some buildings have suffered structural modifications, in particular after the middle of the XX century, with implications on the structure global behaviour. In the case of the XIX century Sao Domingos building, the wood stairs of the building at the level of the first floor were removed, with the objective of extending the room space for commercial purposes. This alteration caused an increase of load on the floors, which, together with the lack of the under stairs structure, caused important deformation of the upper floors towards the buildings central area i.e., the staircase, Figure 6.

Figure 6. Deformation of timber floor. Sao Domingos building, XIX century (1856), Porto.

Figure 6. Deformation of timber floor. Sao Domingos building, XIX century (1856), Porto.

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