The connections between walls of unreinforced masonry (URM) buildings and flexible timber diaphragms are critical building components that must perform adequately before desirable earthquake response of URM buildings may be achieved. Field observations made during the initial reconnaissance and the subsequent damage surveys of clay brick URM buildings following the 2010/2011 Canterbury, New Zealand, earthquakes revealed numerous cases where anchor connections joining masonry walls or parapets with roof or floor diaphragms appeared to have failed prematurely. These observations were more frequent for adhesive anchor connections than for through-bolt connections (i.e., anchorages having plates on the exterior facade of the masonry walls). Subsequently, an in-field test program was undertaken in an attempt to evaluate the performance of adhesive anchor connections between unreinforced clay brick URM walls and roof or floor diaphragm. The study consisted of a total of almost 400 anchor tests conducted in eleven existing URM buildings located in Christchurch, Whanganui and Auckland. Specific objectives of the study included the identification of failure modes of adhesive anchors in existing URM walls and the influence of the following variables on anchor load-displacement response: adhesive type, strength of the masonry materials (brick and mortar), anchor embedment depth, anchor rod diameter, overburden level, anchor rod type, quality of installation, and the use of metal mesh sleeves. In addition, the comparative performance of bent anchors (installed at an angle of minimum 22.5° to the perpendicular projection from the wall surface) and anchors positioned horizontally was investigated. Observations on the performance of wall-to-diaphragm connections in the 2010/2011 Canterbury earthquakes, a summary of the performed experimental program and test results, and a proposed pull-out capacity relationship for adhesive anchors installed into multi-leaf clay brick masonry are presented herein AM - Accepted Manuscript
The full scale, in-situ investigations of instrumented buildings present an excellent opportunity to observe their dynamic response in as-built environment, which includes all the real physical properties of a structure under study and its surroundings. The recorded responses can be used for better understanding of behavior of structures by extracting their dynamic characteristics. It is significantly valuable to examine the behavior of buildings under different excitation scenarios. The trends in dynamic characteristics, such as modal frequencies and damping ratios, thus developed can provide quantitative data for the variations in the behavior of buildings. Moreover, such studies provide invaluable information for the development and calibration of realistic models for the prediction of seismic response of structures in model updating and structural health monitoring studies. This thesis comprises two parts. The first part presents an evaluation of seismic responses of two instrumented three storey RC buildings under a selection of 50 earthquakes and behavioral changes after Ms=7.1 Darfield (2010) and Ms=6.3 Christchurch (2011) earthquakes for an instrumented eight story RC building. The dynamic characteristics of the instrumented buildings were identified using state-of-the-art N4SID system identification technique. Seismic response trends were developed for the three storey instrumented buildings in light of the identified frequencies and the peak response accelerations (PRA). Frequencies were observed to decrease with excitation level while no trends are discernible for the damping ratios. Soil-structure interaction (SSI) effects were also determined to ascertain their contribution in the seismic response. For the eight storey building, it was found through system identification that strong nonlinearities in the structural response occurred and manifested themselves in all identified natural frequencies of the building that exhibited a marked decrease during the strong motion duration compared to the pre-Darfield earthquakes. Evidence of foundation rocking was also found that led to a slight decrease in the identified modal frequencies. Permanent stiffness loss was also observed after the strong motion events. The second part constitutes developing and calibrating finite element model (FEM) of the instrumented three storey RC building with a shear core. A three dimensional FEM of the building is developed in stages to analyze the effect of structural, non-structural components (NSCs) and SSI on the building dynamics. Further to accurately replicate the response of the building following the response trends developed in the first part of the thesis, sensitivity based model updating technique was applied. The FEMs were calibrated by tuning the updating parameters which are stiffnesses of concrete, NSCs and soil. The updating parameters were found to generally follow decreasing trends with the excitation level. Finally, the updated FEM was used in time history analyses to assess the building seismic performance at the serviceability limit state shaking. Overall, this research will contribute towards better understanding and prediction of the behavior of structures subjected to ground motion
The susceptibility of precast hollow-core floors to sustain critical damage during an earthquake is now well-recognized throughout the structural engineering community in New Zealand. The lack of shear reinforcement in these floor units is one of the primary reasons causing issues with the seismic performance of these floors. Recent research has revealed that the unreinforced webs of these floor units can crack at drift demands as low as 0.6%. Such observation indicates that potentially many of the existing building stock incorporating hollow-core flooring systems in cities of relatively high seismic activity (e.g. Wellington and Christchurch) that probably have already experienced a level of shaking higher than 0.6% drift in previous earthquakes might already have their floor units cracked. However, there is little information available to reliably quantify the residual gravity load-carrying capacity of cracked hollow-core floor units, highlighting the need to understand the post-cracking behavior of hollow-core floor units to better quantify the extent of the risk that cracked hollow-core floor units pose
As part of a seismic retrofit scheme, surface bonded glass fiber-reinforced polymer (GFRP) fabric was applied to two unreinforced masonry (URM) buildings located in Christchurch, New Zealand. The unreinforced stone masonry of Christchurch Girls’ High School (GHS) and the unreinforced clay brick masonry Shirley Community Centre were retrofitted using surface bonded GFRP in 2007 and 2009, respectively. Much of the knowledge on the seismic performance of GFRP retrofitted URM was previously assimilated from laboratory-based experimental studies with controlled environments and loading schemes. The 2010/2011 Canterbury earthquake sequence provided a rare opportunity to evaluate the GFRP retrofit applied to two vintage URM buildings and to document its performance when subjected to actual design-level earthquake-induced shaking. Both GFRP retrofits were found to be successful in preserving architectural features within the buildings as well as maintaining the structural integrity of the URM walls. Successful seismic performance was based on comparisons made between the GFRP retrofitted GHS building and the adjacent nonretrofitted Boys’ High School building, as well as on a comparison between the GFRP retrofitted and nonretrofitted walls of the Shirley Community Centre building. Based on detailed postearthquake observations and investigations, the GFRP retrofitted URM walls in the subject buildings exhibited negligible to minor levels of damage without delamination, whereas significant damage was observed in comparable nonretrofitted URM walls AM - Accepted Manuscript
An experimental program was implemented where wall panels were obtained from two existing vintage unreinforced clay brick masonry (URM) buildings and subjected to a diagonal compression loading condition in order to induce a diagonal tension failure mode. The principal aims of the experimental program were to establish the diagonal tension strength of the vintage clay brick URM walls present in the two buildings and to establish a benchmark mortar mix suitable for use when manufacturing replica clay brick test assemblages that adequately represent the strength characteristics present in vintage URM buildings. It was concluded that the use of recycled clay bricks acquired from vintage URM buildings and a mortar mix of 1:2:9 (cement:lime:sand) appropriately replicates the material strength characteristics that are present in existing vintage clay brick URM buildings located in New Zealand AM - Accepted Manuscript
The development of cost effective and minimally-invasive seismic retrofit techniques is required for clay brick unreinforced masonry (URM) buildings because of their poor performance in past earthquakes. A laboratory-based experimental program was implemented to address this need, having well-defined but artificial boundary conditions, and utilising constituent construction materials that replicated the characteristics of masonry found in historic URM buildings. The purpose of this experimental program was to investigate the performance of near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strips as a seismic retrofit solution for improving the out-of-plane performance of walls in URM buildings. In the experimental testing program nine masonry beams and five full-scale masonry walls were tested. Details of the design methodology for flexural retrofit of URM walls using the NSM CFRP retrofit technique and laboratory test results are reported. The results of the experimental tests confirmed that the NSM CFRP retrofit technique is a minimally-invasive option for seismic strengthening of URM walls to resist out-of-plane earthquake induced lateral forces. Two recent case-study projects that implemented the NSM CFRP technique are also briefly presented http://www.journals.elsevier.com/engineering-structures/ AM - Accepted Manuscript
The large number of earthquake-prone vintage unreinforced masonry (URM) buildings in many seismically active parts of the world results in a need for minimally invasive and cost-effective strengthening techniques to enhance the poor earthquake performance of such buildings. The objective of the research reported here was to investigate the applicability of using near-surface-mounted (NSM) carbon fiber–reinforced polymer (CFRP) strips as a retrofitting technique for improving the in-plane shear strength and displacement capacity of multi-leaf URM walls constructed using solid clay brick masonry. The use of this technique for repairing earthquake damaged URM walls was also investigated. Ten multileaf wall panels measuring approximately 1,200×1,200 mm 1,200×1,200 mm were constructed using recycled vintage solid clay bricks and retrofitted using NSM CFRP strips with varying reinforcement ratios. These panels were loaded in diagonal compression, and the results were compared with those obtained from testing of nominally identical unretrofitted wall panels constructed using the same materials. In addition, four wall panels extracted from existing buildings were tested in an as-built condition and then later retested after being repaired using the NSM CFRP strip technique. Based on the experimental results it was established that the NSM CFRP strip technique provides a simple and cost effective method for substantially enhancing the shear strength and displacement capacity of understrength or damaged URM wall panels http://ascelibrary.org/journal/jccof2
