E. Cosenza (ed), Eurocode 8 Perspectives from the Italian Standpoint Workshop, 185-198, 2009 Doppiavoce, Napoli, Italy
EXISTING MASONRY BUILDINGS: GENERAL CODE ISSUES AND METHODS OF ANALYSIS AND ASSESSMENTGuido Magenes a, Andrea Penna ba
University of Pavia and EUCENTRE, Pavia, Italy, [email protected] b EUCENTRE, Pavia, Italy, [email protected]
ABSTRACT The paper presents first an introduction to the main differences between the EN 1998-3 approach to seismic assessment and strengthening of existing masonry buildings and the Italian norms approach. Then, issues related to the definition of material properties, to structural analysis, modelling and performance checks are discussed in more detail, pointing out the difficulties associated with the practical application of the Eurocode and reporting the Italian attempts to overcome such difficulties. The conclusions call for a thorough revision or re-draft of EN 1998-3 to take into account the specific problems of existing masonry buildings. KEYWORDS Existing masonry buildings, seismic assessment, analysis, modeling, codes. 1 INTRODUCTION The problem of the seismic assessment and retrofit of existing buildings, among which a great number are unreinforced masonry, has become by now one of the main topics of interest in the world of constructions, also due to progressive relative reduction of new construction activity with respect to interventions on existing structures. The topic itself is extremely complex, due to the enormous variability of structural forms and materials that can be found in countries with a long history of civilization such as in Europe, ranging from simpler dwellings to spectacular monumental structures. Such variety constitutes a great hindrance to a strict codification of methodologies and approaches, such as it may be possible with new designs. Considering specifically masonry buildings, not only the diversity of structural forms and materials is enormous from country to country, but first of all, such structural forms very often do not lend themselves to be approached with the same engineering criteria used for reinforced concrete or steel construction, even when the simpler and regular residential buildings are considered. The attempt of transposing Eurocode 8 part 3 (CEN-EN 1998-3) to the Italian reality (attempt made in the drafts of OPCM 3274, 2003, and OPCM 3431, 2005) presented, from the Italian viewpoint, a series of novelties that in part were a serious progress towards a safe and rational approach to assessment, in part were not compatible with the reality of the problem due to the impossibility to extend to masonry buildings concepts and procedures which would be appropriate for other types of structures such as r.c. or steel framed buildings.
G. Magenes, A. Penna
An important step forward coming from EN 1998-3 was the introduction of the fundamental problem of the knowledge of the structure, and the conceptual definition of different knowledge levels and consequent confidence factors for assessment. However, in the rational definition of knowledge levels of masonry buildings it should be noted that in most real cases: - no construction drawings neither structural design are available, neither test reports; - the building was built in absence of design regulations, and in the best case conforming to a rule of art, so no simulation of design is thinkable; - often the direct experimental measurement of material parameters is not feasible or, if in principle feasible, completely unreliable. In the new Italian structural norms (now published in the NTC, 2008, document and in the relevant guidelines Circ. NTC08) it was therefore felt essential to define specific criteria for masonry regarding the different knowledge levels. A discussion of the issues pertaining to the knowledge of the building can be found in the paper by Binda and Saisi (2009) in these Workshop proceedings. Another important issue, in which the Italian norms felt the necessity to introduce more articulation with respect to EN 1998-3, was the definition of the type of intervention in relationship with the increase in safety/performance level that is being pursued (local intervention, improvement/strengthening intervention, retrofit), taking into consideration the situations in which the complete retrofit of the building is not possible, as discussed in the paper by Borri and De Maria (2009). At the same time, it was felt that an informative and updated annex on strengthening techniques and strategies would have been an important complement to the norms, in the light of recent post-earthquake experiences in Italy. This issue is treated by Modena et al. (2009) within the present Workshop proceedings. A considerable expansion and partial correction of the criteria for safety/performance assessment presented in chapter 4 of EN 1998-3 was considered necessary for masonry buildings regarding two main aspects: - the need to address local mechanisms, often related to out-of-plane response of walls of or parts of the structure (Figure 1a), which could be approached by ad-hoc methods, and which constitute an essential guidance in the design of the strengthening/retrofit intervention; this issue is not addressed in EN 1998-3; - the impossibility to apply to masonry buildings the schematic ductile mechanism/brittle mechanism conceptual framework which is applied to r.c. or steel structures and the consequent difficulty to apply the methodologies of analysis proposed in EN 1998-3. Regarding the first issue, the paper of Lagomarsino (2009) presents the approach that is presently suggested by the Italian code, and the possible use of limit analysis for the assessment of local mechanisms. The second issue will be discussed in the present work. During an earthquake both out-of-plane and in-plane response are simultaneously mobilized, but it is generally recognized that a satisfactory seismic behaviour is attained only if out-ofplane collapse is prevented and in-plane strength and deformation capacity of walls can be fully exploited (Figure 1b). A global model of the structure is usually needed when the resistance of the building to horizontal actions is provided by the combined effect of floor diaphragms and in-plane response of structural walls (although in some cases simplified partial models could be used). In this paper, attention will be paid to the methods of global analysis of existing masonry buildings.
Existing Masonry Buildings: General Code Issues and Methods of Analysis and Assessment
2 SEISMIC ASSESSMENT OF EXISTING MASONRY BUILDINGS 2.1 Local and global response As demonstrated by the post-earthquake damage surveys carried out after all earthquakes affecting areas where masonry buildings are common, one of the main sources of vulnerability for such structures is associated to local failure modes, mainly due to out-ofplane response of walls. The building seismic response can be governed by such mechanisms when connections between orthogonal walls and between walls and floors are particularly poor. This is often the case in existing stone masonry buildings without tie rods and ring beams, with lack of interlocking at the connection of intersecting walls, presence of simply supported wooden floors and thrusting roofs. Only if connections are improved by proper devices (e.g. tie-rods), local mechanisms can be prevented and a global behavior governed by the wall in-plane response can develop.
(a) (b) Figure 1. Examples of first-mode local damage mechanisms (a, from DAyala & Speranza, 2003) and global response mechanism (b).
2.2 Strength criteria for structural elements Essential elements for global assessment of a building are suitable strength criteria for the structural elements. Eurocode 6 and 8, as well as the Italian norms make reference to walls (or piers) and to beams (or spandrels), for which strength criteria are provided. Strength criteria for in-plane response of masonry structural elements are proposed in EN 1996 and in Annex C of EN 1998-3. They both include a strength criterion with a Coulombtype formulation for the evaluation of shear strength:VR = f v tlc
where: t is the wall thickness, lc is the length of the compressed part of the cross section, and f v = f vo + 0.4 d f v ,lim is the shear strength where: fvo is the initial shear strength in the absence of vertical load, d is the compressive stress along lc, and fv,lim is a limiting value for fv, related to failure of units. The definition of fv,lim in Annex C of EN 1998-3 apparently is consistent with EN 1996, but in reality the latter sets fv,lim equal to 0.065fb, where fb is the compressive strength of the unit, while the former sets it equal to 0.065fm, where fm is the masonry compressive strength. The rationale for this is unclear and the result leads to an evident underestimation of the shear strength in existing buildings.
G. Magenes, A. Penna
A criterion for assessing the capacity of masonry elements subjected to normal force and inplane bending is also reported in section C.4.2.1(3) of EN 1998-3, which is different from the procedure given EN 1996. The approach of EN 1998-3 seems more appropriate for seismic assessment, and is based on the evaluation of the ultimate moment capacity of a wall section subjected to in-plane forces. The same conceptual approach is followed by the Italian norms both for existing building assessment and for new masonry design. In OPCM 3431 and Circ. NTC08, the possibility of applying an alternative shear strength formulation for existing masonry typologies such as undressed stone masonry is foreseen. Such a criterion, which follows the formulation by Turnek and Sheppard (1980) as exemplified by Benedetti and Tomaevic (1984), had been introduced in the Italian standards since almost three decades, and it is rep