Seismic vulnerability assessment of the Casbah of Algiers Viviana Iris Novelli 1, Dina D’Ayala2,
ABSTRACT The Casbah of Algiers is a world heritage site composed by complex building clusters of the Ottoman period which have been damaged by several seismic events. The first and the last reported earthquakes go back to 1365 and 2003, respectively. The most catastrophic seismic event occurred in 1716. Nowadays, the Casbah of Algiers is particularly degraded not only because several seismic events have occurred in the past but also because of lack of post-earthquake repair and lack of maintenance, which have increased the buildings’ decay. In the framework of the PERPETUATE EUFP7project, funded with the aim of defining guidelines for the evaluation and the mitigation of the seismic risk to cultural heritage assets, a vulnerability assessment of the houses of the Casbah of Algiers has been carried out. A survey has been performed on site for 150 historical buildings with the aid of a quantitative data collection form developed to record the most common typologies of the Casbah. Description of structural features and identification of the parameters which affect the seismic vulnerability are used as inputs for the application of the numerical procedure FaMIVE, to identify the collapse mechanisms of the inspected buildings. The distribution of the collapse mechanisms and of the load factor multiplier is highlight show the irregularities identified in the Algerian construction affect the seismic vulnerability of the selected case study. Keywords:
1.
Casbah of Algier, Seismic Vulnerability, Historic centre, building cluster
INTRODUCTION
The world heritage site of the Casbah in Algiers has an intense urban texture which looks like most of the typical medieval Muslim Maghreb cities such as Fez in Morocco and Tunis in Tunisia. Its shape and composition is the result of dense building agglomerates which are composed of houses, mostly from the Ottoman period, that lean against each other. The streets among the agglomerates are particularly narrow and they are often turned into galleries that support enlarged rooms which extend
1
Phd Candidate, University of Bath, BA2 7AY UK,
[email protected]
2
Reader in Structural Engineering, University of Bath, BA2 7AY UK,
[email protected]
on top of them. The presence of these architectural elements, built as vaults made in stone or bricks or horizontal structures in timber, creates a discontinuity on the façades of the buildings. The clustering of these buildings derives from a lack of space; and so new structures were usually squeezed between already existing buildings. This entails that the building typologies within the clusters can differ greatly, with adjacent buildings commonly being constructed at different times, using different techniques and layout. The first and the last reported earthquakes in the Casbah of Algiers go back to 1365 and 2003, respectively, with the most catastrophic seismic event recorded in the year1716 (Foufa et al. 2010). Nowadays, the damage in the city centre is not necessarily caused by seismic events, indeed lack of post-earthquake interventions and maintenance have greatly increased the buildings’ decay. In the framework of the PERPETUATE project, aimed at developing European Guidelines for the evaluation and mitigation of seismic risk to cultural heritage assets, the Casbah of Algiers has been selected as case study for the validation of a seismic vulnerability assessment method to be used at the territorial scale. Vulnerability assessment of masonry buildings is currently performed in many ways, with complex computer analysis leading the way in accuracy of results (D'Ayala & Speranza, 2003, Lagomarsino et al. 2009, Lantada et al. 2010, Lourenco & Roque 2005). This, however, requires a great deal of time and effort to perform. In order to find a suitable seismic vulnerability assessment approach for the Casbah of Algiers a comparison of assessment methods from the last 30 years has been undertaken identifying the following requirement criteria:
collection of typology specific data by observation in situ and by systematic survey minimum use of expert judgement minimization of epistemological uncertainties of the qualitative methods (empirical approaches), and the ontological uncertainties of the quantitative methods (analytical approaches) adaptability to different forms of construction for masonry structures exhaustive library of different failure mechanisms
Nineteen methods were reviewed ranging from largely empirical methods, such as AEDeS (Baggio et al. 2009), Simplified Index Method (Lourenco & Roque 2005) OTSO (Bosiljkov et al. 2009) reliant on observational data, through to analytical methods, such as Vulnus (Bernardini el at. 1990), DIANA(Ramos et al. 2004), SPBELA (Borzi et al. 2008), TreMURI (Lagomarsino et al. 2009),Capacity spectrum-based method (Lantada et al. 2010), SeLEna (Molina et al. 2007). In directly comparing different vulnerability assessment methods, two ‘methods best met the above criteria: FaMIVE procedure (D'Ayala & Speranza, 2003, D’Ayala 2005, D’Ayala & Paganoni, 2011) and SELENA (Molina et al. 2007). Whilst methods such as DIANA and TREMURI which rely on the frame-like approach to create capacity curves, if properly applied may produce a more accurate vulnerability assessment; they often require detailed input data (measurement of elements/ laboratory testing) which cannot be available for each building at the territorial scale, as well as being time consuming. The methods put forward do not require vastly detailed input data; however, they allow for an explicit quantification of the uncertainty on the basis of the input data, which is not usually available for empirical methods. Unlike many of the other analytical methods available, FaMIVE requires a relatively small set of parameters for a large number of buildings, in order to quantify collapse load factors for in-plane and out-of-plane failures by using limit state analysis. Moreover, the FaMIVE method has been applied to several historic districts in Italy, Turkey, Nepal, showing that the procedure is an accurate tool for the vulnerability assessment at territorial scale. Further to this, the FaMIVE procedure was considered the most effective due to the fact that it accounts for a variety of both in-plane and out-of-plane failure mechanisms. However, since the FaMIVE procedure was
initially developed to identify the seismic vulnerability of buildings characterised by regularity in plan, in elevation and in opening layouts, this methodology was not readily applicable in the case study of the Casbah since the typical Algerian constructions are particularly irregular. For this reason, in order to overcome this issue, the aim of this work is to adapt the existing seismic vulnerability method FaMIVE to the case study of Algiers by providing a more refined version of this approach which is able to take into account how irregular plans and elevations of buildings affect the damage mechanisms.
2.
ON SITE SURVEY
In order to better understand the parameters which affect the seismic vulnerability, a first visit to Algiers was conducted in November 2010 to identify the most typical structural and architectural features of the buildings in the Casbah. During a second visit, after suitably modifying the input form to tailor it to the typologies characteristics identified during the first visit, the data acquisition was performed in-situ in April 2011.Depending on the quality of the direct observation, for each surveyed building, a level of reliability is assigned for the collected information according to three possible levels, Low, Medium and High. 2.1.
Geometric and structural characteristics of the buildings in the Casbah of Algiers
The Casbah of Algiers, founded by the Romans after their invasion to the North of Africa on an hill side by the sea, had seen a significant increment of constructions during Ottoman regency. After the catastrophic earthquake of the 1716, some houses were rebuilt but others were completely abandoned to their decay. The urban fabric was also substantially altered in parts during the French colonial period. The most recent seismic event, which has considerably damaged the Casbah, was on May 21st, 2003. The main streets of the Casbah of Algiers identify several irregular blocks crossed by very narrow streets,(Figure 1),which rarely create a rectilinear grid on the map. The composition of the block determines building cluster behaviour of the entire agglomerate. However, the seismic behaviour of the single block is also particularly affected by the soil landform which rapidly slopes towards the sea and amplifies the sliding potential of each house within the block. During the first visit, the most representative blocks of the traditional building fabric were identified with the blocks A, B and D, see Figure 1 b and c which enclose around 150 buildings (reference to the official map: Groupe CNERU, Plan de Recollallment, Codification: ES 1 / DEG / 01 / 09 / CA)
a)
b)
c)
Figure 1: a) Plan of the Casbah, b and c) The most representative blocks of the urban fabric of the Casbah
Three main buildings typologies can be identified within each block (Foufa et al. 2010):
“Dar shebbak” with “wast Al-dar”, patio in the centre of the house, see Figure 2 (a) “Dar shebbak” with “shebak” in which the “wast Al-Dar” becomes a “shebak”, small vertical opening covered by a grid created to ensure ventilation and illumination to the house
“Al –Alwi” without “wast Al-dar” and “shebak”, distributed in elevation around a staircase, see Figure 2 (b) (Foufa et a. 2010)
Common buildings are from one (older and poorer) to five floor-storey height (in most of the cases the last two floors are later additions). Buildings within the blocks were in some cases built all at the same time, and it is visible from the interlocking of the masonry work among common party walls, or in other cases over time, with adjacent buildings having independent party walls and different floors height, even though no space might have been left between the facades. In a small number of cases, reinforced concrete frame buildings are also present in adjacency to masonry houses.
a)
b)
c) Figure 2: Plan and section view of traditional house (a) “Dar shebbak” with “wast Al-dar”and (b) “Dar shebbak” with “shebak, c) “Al –Alwi” (Reprinted with permission from Missoun,. Copyright 2003 INAS.
a)
b) Figure 3: Sabat and“q ‘bû”
This layout has direct implications on the seismic behaviour of the single building and the cluster, and leaves some of them at severe risk of hammering, as indeed observed. The bearing system, composed by sets of orthogonal walls, is made up of brickwork, or cut stones with randomly inserted bricks, or squared stones with regular bricks courses with a thickness that varies from 30 cm for the poorest quality to 90 cm or more for defence structures. Mortar is manly in lime, but it has been also observed that mud mortar is used in some houses. The buildings in the Casbah are often characterised by an interesting architectonic element given by a structure built between the facades of opposite buildings facing on a narrow street. This element is called “sabat” see Figure 3 (a), or “q ‘bû” see Figure 3 (b), whether it provides a total or partial roofed passageway.
Figure 4: Irregular opening layout of the facades in the Casbah of Algiers
The original alignment is frequently comprised as a result of modifications brought to the structures throughout the years and this is particularly evident from the irregular opening layout, see Figure 4. Typical alterations to the original structures are the frequent addition of one or two storeys above the original roof level. The original horizontal structures are timber floors and roofs, replaced in recently refitted buildings by concrete slabs. 2.2.
Damage and collapse mechanism identification in the Casbah of Algiers
During the survey the damage and the feasible failure modes observed on site has been classified according to an original catalogue of collapse mechanisms developed for the FaMIVE approach by D’Ayala & Speranza in 2003 which has been updated in 2005and 2011. The collapsed buildings in the blocks A B and D are around 20, most of the dwellings are ruins and only some of them are partially failed. The partial or total overturning of the façade is usually caused by the following issues, see Figure 5:
poor quality of the connections between the entire walls or part of them,
poor quality of the masonry fabric and mortar,
lack of connection between horizontal structures and bearing walls
lack of maintenance of masonry
high level of water infiltration
a)
b)
c)
d)
Figure 5: Out of plane mechanisms due to a) bad connection between walls, b) poor quality of masonry bricks, c) lack of connection between horizontal structures and baring walls, d) lack of maintenance
Differently from what usually considered in unreinforced masonry walls with regular opening, the occurrence of a recurring pattern of diagonal X shaped cracks in the spandrels or in the piers is not the most common case. This is due to the irregular distribution and size of opening which leads to uneven distribution of stiffness and shear capacity among the piers so that some might be more vulnerable than others see Figure 6. Furthermore the piers might be failing in a combination of bending and shear, rather than just shear (Casapulla & D’Ayala 2006). For these reasons a different approach to the calculation of the in plane mechanisms is proposed, by identifying the weakest load path in the façade
leading to failure, rather than simply using lateral capacity of the piers and assuming a rigid behaviour of the spandrels (or vice versa).
Figure 6: In plane mechanism
3.
VULNERABILITY ASSESSMENT METHOD AND COLLAPSE MECHANISM IDENTIFICATION
The programme FaMIVE, using the concepts of limit state analysis for non-conforming materials, correlates collapse mechanisms to specific constructional features of the external bearing walls forming a masonry building. The analysis is static equivalent and quantifies the load factor multiplier (as a percentage of gravity acceleration, g) associated with each mechanism so as to determine a lower bound of the level of shaking which will trigger the onset of a specific failure mechanism. On this basis, it is possible to produce a prediction of most probable damage modes and levels of vulnerability for individual or groups of buildings, in relation to expected levels of shaking at a site. It is also possible to analyse the reduction in vulnerability obtainable by introducing selected types of strengthening (D’Ayala 2005). As far as the current in-plane failure approach in FaMIVE, Casapulla & D’Ayala, (2006) have demonstrated that a macroelement approach can be used to define the in plane load factor multiplier and collapse mechanism, in the hypothesis that the wall is constructed of dry block masonry with a frictional behaviour. Indeed, it can be assumed that as a result of seismic loading a diagonal crack splits the wall into two macroelements: the bottom-left element that does not participate in the failure and the upper-right hand element that fails in sliding or overturning or a combination of the two (D’Ayala & Speranza, 2003). Two different equilibrium conditions at failure develop depending on the angle of the crack, αc. These are when αp≥αc and when αb≥αc ≥αp, where αb is the shape factor of the masonry unit and αp is the shape factor of the cracked portion of wall (pier or spandrel), as defined by Eqs. (1) and (2), where s is the overlap of the masonry units, h is the height of one unit, while lp and hp are the total length and height of the wall’s portion, respectively. (1)
(2)
Figure 7: Block dimensions and variable angle of crack in a pier (Casapulla & D’Ayala 2006)
Since in FaMIVE the implementation of the calculation for the in plane mechanisms has been defined by using the approach of Casapulla & D'Ayala, (2006) developed for masonry wall with regular layout of openings, in this section it will be illustrated how this model has been modified in order to be applied to the irregular opening facades of the Casbah of Algiers. The geometric quantities used in the virtual work equations for the computation of the load factor multiplier for the in plane mechanism are shown in Figure 8: lsi and lpi are the spandrel and pier width respectively; hfi and hsi are
the interstorey and spandrel height respectively, while hpi is the height of the pier over which a crack will develop; Hfi is the total height of the wall involved in the activated collapse mechanism and Hpi is the distance between the centre of gravity of the pier and the point where the cylindrical hinge forms, which coincides with the crack initiation point; qfi (or qr)is the ith floor (or roof) loading resultant, taken from the horizontal structures acting upon the portion of wall involved in the mechanism.
Figure 8: Geometric variables, crack initiation points and forces involved in a façade with irregular opening layout where αb≥αc ≥αf
The frictional behaviour of the masonry along the crack is given by frictional restraint from the pier, and frictional restraint from the spandrels. The frictional forces along the crack in the pier, Fj can be calculated by using Eqs. 3for αf ≥αc and Eqs. 4 for αb≥αc ≥αf; where γis the self-weight of the masonry units and b is the thickness of the facade. (
[
[
(
)
)
(
]
)]
(3)
(4)
The angle of the crack in the spandrel is taken as αc≤αs, where αsis the shape ratio of the spandrel. The interstorey heights, hfi, are taken as an average for the whole building, except the first floor, for which different height can be accounted for. The spandrel heights are calculated using the building and opening geometry. [
(
)
]
(5)
In the virtual work equations tanβ is used to quantify the angle of the straight line between the crack initiation and the point the crack meets the far edge of the pier accounting for the overlap s of the first masonry unit. Crack angles greater than αb are unlikely to form under the assumed conditions (Casapulla & D'Ayala, 2006). The virtual work equations for rotation about the point of crack initiation can then be written as Eqs. (6). Figure 8shows the locations of the forces contributing in a facade with an irregular opening layout when αb≥αc≥αp; n is an integer depending on the number of stories involved in the collapse, relating the relative lever arm of the overturning moment due to horizontal floor loads. Fsm is the restraining force due to friction in the spandrels. Equation 6 can be solved iteratively or with an optimization approach to define the crack layout leading to the smallest collapse load multiplier.
∑
4.
∑
∑
(
)
∑
(
)
∑
(
(6)
)
SEISMIC VULNERABILITY ASSESSMENT: APPLICATION TO THE CASBAH OF ALGIERS
The comparison between the predicted and actual failure mechanisms is not easy to carry out, since most of the observed damage has been caused by lack of maintenance and high level of water infiltration, as mentioned in the section 2.2. However, in order to consider the decreasing of the mechanical properties in the materials due to these parameters, the actual wall thickness recorded in the survey is reduced when the level of decay is considered particularly relevant. damaged buildings in the block B
% damaged buildings
100 90 80 70 60 50 40 30 20 10 0
median value of λ calculated on the surveyed sample in the block B
0
0,1
0,2 0,3 0,4 λ: load factor multipliers
0,5
deterministic scenario with 100 years return period for the Casbah of Algiers
Figure 9: Load factor multiplier (λ) distribution in the surveyed sample
100
% of damagedes facades
% of damaged facades
The load factor multipliers (λ), Figure 9, calculated on the blocks B of Figure 1 are < 0.1g for 26% of the facades,
