PhD Student, Indian Institute of Technology Guwahati
Seismic strengthening of Masonry buildings using steel bands of different configurations
Traditional masonry structures are widely found in various countries, which include important heritage buildings, government offices and buildings for common people. However, at present most of the buildings are reinforced concrete structures but Unreinforced Masonry (URM) Building construction practice are still widely adopted at various parts of the country due to being affordable, materials availability and easy to construct. URM buildings have proven to be good thermal and sound insulator. Being a load bearing structure, the walls are thicker which indirectly made the building fire resistant. Despite of the advantages URM buildings are susceptible to partial or total collapse during earthquakes due to weak in seismic performance. The brick masonry, which constitutes the building, are brittle material with low tensile strength, which exhibits poor ductility during earthquakes. Hence, URM buildings at high seismic areas are highly vulnerable. In addition, the vulnerability analysis of this buildings have been categorised under medium-high vulnerability (Sinha and Brzev , Kumar , Khan and Moin  and Ansary ).The vulnerability of Unreinforced brick masonry building increases due to the introduction of openings in the structure which is largely unavoidable since the openings are doors and windows. But quantity, size and position of the openings do largely influence the overall capacity of the structure(Shariq et al(2007). Due to poor seismic performance, a need for strengthening of existing URBM buildings have become crucial. To counter the horizontal action due to seismic activity, introduction of steel flats can be a suitable strengthening intervention since, they are easily available. A few codes of practice suggest various schemes to strengthen such buildings without providing sufficient details on design and detailing of strengthening schemes. Though using these schemes increases the lateral load carrying capacity of such buildings, the codes do not provide any method to estimate the actual increase in load carrying capacity. Effectiveness of using steel bands over URM buildings with different opening configurations is an essential issue to be studied. The work consist of both numerical and experimental study of the effectiveness of various strengthening schemes to strengthening the Unreinforced Masonry Buildings using the locally available steel bands.
Abstract: Eight half‐scale brick masonry walls were tested to study two important aspects of confined masonry (CM) walls related to its seismic behavior under in‐plane and out‐of‐plane loads. Four solid wall specimens tested to investigate the role of type of interface between the masonry and tie‐columns, such as toothing varying from none to every course. The other four specimens with openings were tested to study the effectiveness of various strengthening options around opening to mitigate their negative influence. In the set of four walls, one wall was infilled frame while the other three were CM walls of different configurations. The experimental results were further used to determine the accuracy of various existing models in predicting the in‐plane response quantities of CM walls.Confined masonry walls maintained structural integrity even when severely damaged and performed much better than infill frames. No significant effect of toothing details was noticed although toothing at every brick course was preferred for better post‐peak response. For perforated walls, provision of vertical elements along with continuous horizontal bands around openings was more effective in improving the overall response. Several empirical and semi‐empirical equations are available to estimate the lateral strength and stiffness of CM walls, but those including the contribution of longitudinal reinforcement in tie‐columns provided better predictions. The available equations along with reduction factors proposed for infills could not provide good estimates of strength and stiffness for perforated CM walls. However, recently proposed relations correlating strength/stiffness with the degree of confinement provided reasonable predictions for all wall specimens. Copyright © 2016 John Wiley & Sons, Ltd.
Pub.: 21 Jul '16, Pinned: 24 Aug '17
Abstract: This work discusses the simplified estimation of earthquake-induced nonlinear displacement demands as required by nonlinear static procedures, with particular attention on short-period masonry structures. The study focuses on systems with fundamental periods between 0.1 and 0.5 s, for which inelastic amplification of the elastic displacement demand is more pronounced; hysteretic force-displacement relationships characteristic of masonry structures are adopted, because these structures are more commonly found within the considered period range. Referring to the results of nonlinear dynamic analyses of single-degree-of-freedom oscillators, some limitations of the Eurocode 8 and Italian Building Code formulations are first discussed, then an improved equation is calibrated that relates inelastic and elastic displacement demands. Numerical values of the equation parameters are obtained, considering the amount of hysteretic energy dissipation associated with various damage mechanisms observed in masonry structures. Safety factors are also calculated to determine several percentiles of the displacement demand. It is shown that the proposed equation can be extended to more dissipative systems. Finally, the same formulation is adapted to the estimation of seismic displacements when elastic analysis procedures are employed. Copyright © 2017 John Wiley & Sons, Ltd.
Pub.: 20 Jan '17, Pinned: 24 Aug '17
Abstract: Historical constructions are part of the world heritage, and their survival is an important priority. Comprising mostly unreinforced, load-bearing masonry, heritage buildings may date anywhere from antiquity to the 19th and early 20th century. Being exposed to the elements over the years, they are in various states of disrepair and material degradation. Based on postearthquake reconnaissance reports, these structures occasionally behave rather poorly, even in moderate seismic events, undergoing catastrophic damage and collapse, whereas retrofitting is governed by international conventions regarding noninvasiveness and reversibility of the intervention. The complexity of their structural systems (continuous structural components, lack of diaphragm action, material brittleness, and variability) challenges the established methods of condition assessment of preretrofitted and postretrofitted heritage constructions. The most advanced state of the art in materials and analysis tools is required, far more complex than with conventional buildings. Thus, an assessment procedure specifically geared to this class of structures is urgently needed, in order to assist engineers in this endeavor. The objective of this paper is the development of a performance-based assessment framework that is palatable to practitioners and quite accurate in seismic assessment of unreinforced masonry buildings with no diaphragm action. The underlying theoretical background of the method is illustrated with reference to first principles: global demand is obtained from the design earthquake scenario for the region, using empirical estimates for the prevailing translational period of the system; deformation demands are localized using an approximation to the translational 3-D shape of lateral response, estimated using a uniform gravitational field in the direction of action of the earthquake; acceptance criteria are specified in terms of relative drift ratios, referring to the in-plane and the out-of-plane action of the masonry piers. The quantitative accuracy of the introduced procedure is evaluated through comparison with detailed time-history dynamic analysis results, using a real life example case study. Qualitative relevance of the results is evaluated through comparison of the location and extent of anticipated damage estimated from the proposed assessment procedure, with reported records of the building damages that occurred during a significant past earthquake event.
Pub.: 28 Jun '17, Pinned: 24 Aug '17
Abstract: An analytical model describing the flexural response of vertically spanning out-of-plane loaded unreinforced masonry walls is presented in this paper. The model is based on the second-order Euler-Bernoulli beam theory and captures important characteristics of the out-of-plane response of masonry walls that have been observed in experimental tests and from numerical studies but for which an analytical solution was still lacking: the onset and the evolution of cracking, the peak strength of the out-of-plane loaded walls, and the softening of the response due to P−Δ effects. The model is validated against experimental results, and the comparison shows that the model captures both the prepeak and postpeak response of the walls. From the analytical model of the force-displacement curve, a formula for the maximum out-of-plane strength of the walls is derived, which can be directly applied in engineering practice.
Pub.: 06 Jul '17, Pinned: 24 Aug '17