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Seismic Characterisation of Unreinforced Masonry Buildings in Auckland, New Zealand

Seismic Characterisation of Unreinforced Masonry Buildings in Auckland, New Zealand
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    9 th  International Masonry Conference 2014 in Guimarães 9 th  International Masonry Conference, Guimarães 2014 1   Seismic Characterisation of Unreinforced Masonry Buildings in Auckland, New Zealand WALSH, KEVIN 1 ; CUMMUSKEY, PATRICK 2 ; DIZHUR, DMYTRO 3 ; INGHAM, JASON 4   ABSTRACT:  The 2010-2011 Canterbury earthquakes and corresponding Royal Commission reports have resulted in changes to the legislative environment and led to increased public awareness in New Zealand of the earthquake performance of unreinforced masonry (URM) buildings. As a result, building regulators, owners, tenants, users, and heritage advocates will be facing a unique challenge in the near future where improvements and demolitions of URM buildings are expected to occur at an unusually high rate. Auckland is the largest city in New Zealand, and because of the relative prosperity of Auckland during the period 1880-1930 when most URM buildings were being constructed in New Zealand, the city has the greatest stock of URM buildings in the country. Identifying those buildings most at risk in  Auckland’s  large and varied building stock has warranted a rapid field assessment programme supplemented by strategically chosen detailed assessments. Information that can be procured through rapid field inspections includes the building geometric typologies (e.g., heights, building footprint geometry, isolated versus row configuration, and the relationship of these factors to pounding potential), elevation type (e.g., perforated frame versus solid wall), presence of bond beams, wall construction (e.g., solid versus cavity, number of leafs), bond patterns, and basic construction material type (e.g., clay brick versus stone). Furthermore, investigation into the architectural history, heritage status, functional use, and perceived social/community value of Auckland’s URM buildings will affect the direction of retrofit strategies and priorities. As the owner of a large and varied portfolio of URM buildings as well as the local organisation responsible for assessing building safety, Auckland Council is developing exemplar inspection, assessment, and prioritisation strategies that will target the risks associated with URM buildings, in particular, so as to preserve and enhance safety, and the economic and community value of these special buildings. Keywords: Auckland, New Zealand, unreinforced masonry, clay brick, stone masonry, Global Earthquake Model, RiskScape, out-of-plane assessment, cavity wall, built heritage NOTATION  AC  Auckland Council;  MM Modified Mercalli (intensity scale);   ACPD  Auckland Council Property Department ; %NBS percent New Building Standard;  Avg. average;  NZSEE New Zealand Society for Earthquake Engineering;  CDEM Civil Defence and Emergency Management;  URBM unreinforced brick masonry;  CERC Canterbury Earthquakes Royal Commission;  UoA University of Auckland;  Est. estimated;  ULS ultimate limit state (design parameters);  GEM Global Earthquake Model ;   URBM unreinforced (clay) brick masonry; IEP Initial Evaluation Procedure;  URM unreinforced masonry; and LLRS lateral load-resisting system;  URSM unreinforced stone masonry 1)  Doctoral Student, University of Auckland and Strategic Planning Intern, Auckland Council, 2)  Special Projects Policy Advisor, Auckland Council, Building Control, 3)  Research Fellow, University of Auckland, Department of Civil and Environmental Engineering, 4)  Professor, University of Auckland, Department of Civil and Environmental Engineering,  Walsh; Cummuskey; Dizhur; Ingham 9 th  International Masonry Conference, Guimarães 2014   2 1 INTRODUCTION  As previously noted [1], the apparent seismic hazard in Auckland is relatively low, especially in central Auckland where pluralities of population and URM buildings reside. However, the vulnerability of Auckland’s built infrastructure is high. As of 2012, Auckland’s eco nomy accounts for an estimated 37% of New Zealand’s GDP, and the region’s economic growth has outpaced New Zealand’s national economic growth in 7 of the past 11 years [2] . Auckland’s regional population of about 1.3 million in 2006 [3] accounted for 32.4% of the nation’s population. Hence, a major natural disaster in Auckland would be detrimental to much of New Zealand.  Auckland Council released its updated  Earthquake-Prone, Dangerous & Insanitary Buildings Policy   [4] in response to the Building Act [5] requirements that territorial authorities adopt such policies (Section 131) to identify if buildings are considered earthquake-prone (Section 122), dangerous, or otherwise unsuitable for human occupation. The 2011 policy was an update to the 2006 policies of the former  Auckland territorial authorities. Under the current policy, a process has been outlined for identifying earthquake-prone buildings and, where applicable, working with building owners to establish a scope and timetable for any building improvements. Auckland Council regulatory policy mandates that most normal buildings deemed by Building Control to be earthquake-prone be retrofitted within 20 years, and additional allowances are provided for buildings with heritage value. In response to these regulations, Auckland Council has worked with contracted professional engineers and with researchers from the University of Auckland to assist the Auckland Council departments of Property, Building Control, Civil Defence, and Heritage to inspect and document buildings in the Auckland region most likely to be vulnerable to earthquakes. All affiliates follow inspection guidelines to procure data, records, and photographs to aid the council in prioritising buildings for further assessment and, potentially, seismic retrofitting. The inspectors have been utilising a programme based largely on the Initial Evaluation Procedure (IEP) as per NZSEE [6], but they have also been applying more advanced metrics in order to improve both the accuracy and the bandwidth of building assessment scores so as to facilitate efficient retrofit prioritisation. 2 BUILDING TAXONOMIES AND DATA CONSIDERED With the intention of extending previous research [1] and leveraging the data being procured through the aforementioned inspection processes for application to hazard models, the authors considered two specific building taxonomical classification schemes  –  the Global Earthquake Model (GEM) [7] and RiskScape [8,9]. Charleson [10] provides a review of other existing structural taxonomies and explains the suitability of GEM on a global scale. RiskScape is the preferred hazard modelling system of the Institute of Geological and Nuclear Sciences (GNS) in New Zealand, so classifying building data to its taxonomy provides the most regional relevance and correlation with previous work in this area [11,12]. Table 1 lists the general attribute groups considered by each taxonomy, and attribute groups considered as part of this research programme are marked with “X” . Note that the biggest difference between the two classification schemes is that RiskScape considers more attributes related to occupancy and costs, whereas GEM considers structural attributes more strictly and more thoroughly. The term “commercial” buildings is defined as including industria l buildings and multi-unit, rent-tenanted residential buildings in addition to those traditionally considered commercial. However, low-unit residential (e.g., single-family houses and condominiums) are excluded. This definition is consistent with the regulatory authority of Auckland Council Building Control. The current (March 2014) amalgamated database of commercial unreinforced masonry (URM) buildings in Auckland includes 831 documented buildings. The authors expect that this list represents a large majority of commercial URM buildings in Auckland. Estimates of the total number or percentage of URM buildings are considered as if 1026 is the total number of commercial URM buildings in Auckland, based on an estimate by Russell [13,14] and corresponding to a percentage of  Typology, Seismic Assessment and Retrofit Strategies for Auckland, New Zealand’s URM Buildings 9 th  International Masonry Conference, Guimarães 2014 3 the entire commercial building stock proposed by Cousins [15] of about 5% - 6%. However, not all of the documented 829 URM buildings have been inspected, and not all of the estimated 1026 URM buildings have been identified. Where appropriate, a distinction has been made between whether quantitative values represent “documented” or “estimated” buildings as well as whether percentages represent proportions including or excluding unknown building attribute data. Furthermore, a bias exists in the documented data. Investigators have prioritised buildings most likely to be vulnerable to earthquakes, producing a partiality in the data pool to older, taller buildings located close to the city centre. These biases have been accounted for and corrected as much as possible, as noted in the following sections. Finally, non-occupied monuments, kilns, chimneys, and ruins are not considered in this study. Table 1. Summary of building taxonomies and attributes   GEM [7] RiskScape [9]  Attribute group Data availability  Attribute group Data availability Direction * Material of the lateral load-resisting system X Construction type X Lateral load-resisting system X Height (Incl. # of storeys above ground) X Floor height * Storeys X Date of construction or retrofit X Year of construction X Occupancy (Usage type) X Use category X Building position within a block * Shape of the building plan * Structural irregularity * Exterior walls * Wall cladding class * Roof * Roof cladding class * Roof pitch * Floor * Floor type * Foundation system * Condition X Contents value Deprivation Index Employee daily income Floor area ** Footprint area ** Occupancy (# people) ** Parapet * Replacement-cost ** Vehicle value Vehicles Notes: X = attribute information currently considered within existing inspection programmes; * = attribute information that will be considered within planned, future inspection programmes; ** = attribute information available through other data sources such as Quotable Value NZ 3 PRIMARY LATERAL LOAD-RESISTING MATERIAL AND SYSTEM Cousins [15] suggests that URM buildings are 5.4 times more vulnerable (e.g., empirically-based mean ratio of repair cost to replacement cost is 5.4 times higher) than post-1980 reinforced concrete (RC) buildings. By comparison, the second-most vulnerable building type is pre-1980 RC at 2.3 times the vulnerability of post-1980 RC. Hence, as building performance in the Canterbury earthquakes has confirmed [16,17,18], URM buildings warrant special attention in a discussion on building typologies in  Auckland for purposes of seismic hazard modelling.  Walsh; Cummuskey; Dizhur; Ingham 9 th  International Masonry Conference, Guimarães 2014   4  As noted, 831 buildings have been documented in Auckland with the primary lateral load-resisting system (LLRS) documented as either unreinforced brick masonry (URBM) or unreinforced stone masonry (URSM). Auckland Council Building Control has provided the vast majority of this information, so the structure type nomenclature used in the Auckland Council / University of Auckland (AC / UoA) study has been matched to the precedent set by Building Control. These categories are listed in Table 2 along with their presumed equivalents in the GEM [7] and RiskScape [9] taxonomies. The number of documented buildings within each AC / UoA structure type category, as well as the estimated total number and percentage of buildings within each taxonomic construction type category, is also listed in Table 2. Table 2. Summary of primary lateral load-resisting material and structural system attributes for commercial URM buildings in Auckland Auckland Council  / University of Auckland (AC / UoA) GEM [7] RiskScape [9] StructuretypecategoryDocumented#bldgsMaterialoftheLLRSsystem level1MaterialoftheLLRSsystem level2LLRSLevel 1Est.#Est.% of all commercial bldgsConstr. typecategoryEst.#Est.% of all commercial bldgs URBM (unreinforced brick masonry) 812 MUR (Masonry, unreinfor ced) CLBRS (Fired clay solid bricks) or CLBRH (Fired clay hollow bricks) and/or RCB (Reinforced concrete bands) LWAL (Wall) 1006 5.8% Brick masonry (to include stone masonry) 1026 5.9% URSM (unreinforced stone masonry) 19 STRUB (Rubble (field stone) or semi-dressed stone) or STDRE (Dressed stone) LWAL (Wall) 20 0.1% University of Auckland researchers have documented 19 occupiable buildings in Auckland with above-grade, load-bearing unreinforced stone masonry (URSM) walls. While stone masonry represents such a small portion of the Auckland building stock that they could likely be neglected in high-level hazard models, these buildings tend to be associated with significant heritage, community, and religious value. Furthermore, the GEM [7] taxonomy accommodates such specific material characteristics as formation of stone (e.g., rubble, semi-dressed, or dressed), type of stone (e.g., limestone, basalt, etc.) and type of mortar (cement, lime or lime-cement). Basalt is the prominent stone-type amongst these buildings. 4 NUMBER OF STOREYS In the amalgamated database, 809 URM buildings have been documented with a particular number of storeys above grade, and the associated percentage groupings are illustrated in Figure 1.  As discussed later, for typological groupings to include multiple attributes (e.g., structure type, number of storeys, and age of construction), buildings have been grouped into categories of 1-3 storeys and 4-7 storeys, consistent with previous typological groupings used in New Zealand [11]. Two buildings were identified as having eight storeys above grade, but both buildings have frame systems supporting most or all of the gravity loads while the masonry is expected to provide the primary lateral load resistance.   Typology, Seismic Assessment and Retrofit Strategies for Auckland, New Zealand’s URM Buildings 9 th  International Masonry Conference, Guimarães 2014 5 22.0%59.1%12.1%3.6%1.6% 1.0% 0.4% 0.2% 0%10%20%30%40%50%60%70%12345678+    %   o   f   b  u   i   l   d   i  n  g   U   R   M   s   t  o  c   k  r  e  p  r  e  s  e  n   t  e   d   b  y   t  r  a   i   t Number of storeys above gradeDocumented % of URM buildings excluding unknown   Figure 1. Proportions of documented commercial URM buildings in Auckland by number of storeys above grade The bias in the documented data for number of storeys above grade likely over-represents taller buildings given the initial emphasis on inspecting buildings near the city centre as well as the inherent higher profile of taller buildings and the higher risk consequence associated with them. Hence, the total number of estimated buildings with 4 - 8 storeys (55) was capped at the current number of documented buildings with 4 - 8 storeys, and proportional extrapolations were made only for buildings with fewer than 4 storeys. 5 YEAR OF CONSTRUCTION OR RETROFIT In the amalgamated database, 830 URM buildings (all but one) have been documented with a particular or approximate year of construction, reconstruction, or implementation of seismic retrofit to the primary lateral load-resisting system. For typological groupings to include multiple attributes (e.g., structure type, number of storeys, and age of construction), buildings have been grouped into ranges of years consistent with major updates to the loading standard and with previous typological groupings used in New Zealand [6,11,12,19]. The proportions associated with these simplified year groupings are illustrated in Figure 2. For breakdowns of URM building construction by decade, please refer to Walsh and Ingham [1] and Russell and Ingham [14]. Most URM buildings in New Zealand were srcinally constructed before 1940, although a few were constructed in Auckland as late as the 1950s [14]. In the 1965 standard building by-law [20], URM buildings with more than two storeys were prohibited as new construction in Auckland, and no URM buildings of any height have been identified as having been constructed after 1965. Hence, the buildings represented in Figure 2 (and in Table 3 as follows) as being constructed post-1965 denote major reconstruction or retrofit to an existing URM building. Unfortunately, the specific type of reconstruction or seismic retrofit has not yet been documented for most buildings. 6 TYPOLOGICAL GROUPINGS BY STRUCTURE TYPE, NUMBER OF STOREYS, AND  YEAR OF CONSTRUCTION To facilitate typological groupings conducive to assigning accurate building fragility functions within seismic hazard models, the three primary structural attribute categories of structure type, number of storeys, and year of construction were combined in a hierarchical fashion. Note that, while such hierarchies within building taxonomies are common and perhaps most useful regionally, they can be
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