By Jason Ingham, Guido Magenes, Katrin Beyer and Rob Chai
6.1 Introduction 6.1 Introduction
In the early 1970s, masonry was almost completely absent from university structural engineering curricula in most earthquake prone countries worldwide, given its perceived deficiencies when compared to more modern construction materials such as reinforced concrete and structural steel. At that time, the international scientific literature on structural masonry was either dedicated to the study of historical masonry (e.g. Heyman, 1966) or was developed in non-seismic countries where gravity loading and wind loading were the dominating actions in design (e.g. Sahlin, 1971). However, at that time several factors were decisive in drawing new attention to masonry structures in earthquake-prone countries, namely: - Developments in the construction industry prompted advancements towards more robust masonry
structural systems through the use ofconfined and well-detailed reinforced masonry, with equally promising results for earthquake resistance compared to other systems
- The growing interest in assessing the safety of existing buildings and their retrofit options when subjected to earthquakes
- The spread of strength or ultimate limit state design methods as opposed to working stress methods that were shown to be inadequate to assess the real safety against building damage and collapse, especially when structures are subjected to horizontal loading.
Seismic design in masonry was given further impetus in the early 1970s when New Zealand was pioneering the ductile seismic design principles for reinforced concrete structures, mainly through the
work of Professors Bob Park and Tom Paulay (Park and Paulay, 1975).It was in this environment that Nigel’s creative work on masonry began, and his subsequent decades of research produced findings
and results that remain fundamental to this day for engineers and scholars worldwide interested in understanding the seismic behavior of masonry structures. The authors of this paper had, at different times, the privilege of being introduced to the subject and exposed to Nigel’s thoughts and work. This opportunity has been a constant source of inspiration for their own work on structural masonry and this paper serves as their humble attempt to pay homage, not only to Nigel’s contributions to the subject, but also to his mind-opening, profound and at the same time solid and straight-to-the-point approach to engineering problems. His immense contributions brought forth the fascinating albeit complex subject of seismic design and assessment of masonry structures, much to the attention of structural engineers.
6.2 The
6.2 The University of University of Canterbury YCanterbury Yearsears
Prior to the early 1970s, little research attention was given to the appropriate seismic design procedures for reinforced clay brick and concrete block masonry (Holmes, 1968), and that lack of attention became the motivation for a number of published studies on the topic in New Zealand over the next
two decades. These efforts were partly motivated by the conservative assumptions that had traditionally been applied to masonry design prior to this time, and further motivated by the combined development
of a new seismic design code for New Zealand (Glogau, 1972; SANZ, 1973) and observed damage in the 1968 Tokachi-oki earthquake and 1971 San Fernando earthquakes. In that period in New Zealand, greater attention was particularly given to the implementation of the limit state design philosophy, and the associated need to quantify behavioral factors for masonry design such as available levels
of ductility capacity, observed levels of damping, quantification of the extent of strength reduction associated with repeated cyclic loading, and maximum levels of design masonry shear stress. In 1974 Otto Glogau (Chief Structural Engineer at the New Zealand Ministry of Works) reported on the performance of masonry buildings in past earthquakes, including the effects of infilled masonry in framed buildings and the performance of masonry veneer fixed to timber frames (Glogau, 1974). In the same issue of the NZSEE Bulletin, Priestley and Bridgeman (1974) reported on research undertaken on behalf of the New Zealand Pottery and Ceramics Research Association that specifically addressed the cyclic response of reinforced clay brick masonry units using the test setup in Figure 6.1. The study investigated a larger size hollow clay brick unit than had been previously considered, with attentions given to the mechanism and control of shear failure and the extent of load degradation. A total of 18 tests were reported, with 14 test walls constructed from two skins of 230 × 75 × 70 mm clay brick units separated by a 64 mm grout gap, and 4 tests constructed of hollow clay blocks having a nominal width of 140 mm. The main variable in the reported testing program was the quantity of horizontal and vertical reinforcement. Five walls contained horizontal steel confinement plates and two walls contained overburden axial load simulated by a vertical prestressing force. Important conclusions from this research were that distributed horizontal reinforcement was effective in improving the nominal wall shear strength and that cyclic load degradation could be greatly reduced by providing stainless steel confining plates in the bottom few horizontal bed joints at each end of the wall.
Nigel left the Ministry of Works in 1976 and joined the Department of Civil Engineering at the University of Canterbury as a Senior Lecturer. His masonry research interests continued to be motivated by developments in New Zealand standards, and in particular by a proposed draft for a new masonry
code (SANZ, 1980). The specific focus of Nigel’s masonry research efforts at this time were to establish suitable maximum shear stress limits. In 1977 a testing program that investigated six heavily reinforced concrete masonry shear walls was reported (Priestley, 1977). Test parameters included the longitudinal
reinforcement ratio, the applied axial load, and the horizontal bed joint confinement plates, similar to his earlier work investigating clay brick masonry walls (Priestley and Bridgeman, 1974). Particular attention was given to ensure that the walls tested were constructed using realistic levels of construction quality, and construction detailing included the lapping of vertical starter bar reinforcement projecting from the foundation, the use of open ended masonry units, and the inclusion of clean-out ports in the bottom course of masonry. The concrete masonry units had a width of 143 mm and the associated
masonry prism strengths ranged between 18.3-24.5 MPa. The principal finding from this testing was that existing limits for the maximum masonry shear stress were unrealistically low, with all walls exceeding the maximum shear stress value permitted at that time by substantial margins, and with two walls having shear strengths of four times the code level. It was therefore concluded that as long as the provided horizontal reinforcement was adequately anchored, higher shear stresses should be allowed in
future masonry design codes. In particular, it was recommended that a maximum shear stress of 1.25 MPa be allowed for walls expected to sustain displacement ductility factors of up to 4 and that a higher shear stress limit of 2.5 MPa should be allowed when the displacement ductility factor does not exceed 2. In addition, it was concluded that the existing recommendation for a strength reduction factor for masonry of φ=0.65 was unnecessarily low, and that the factor should be increased toφ=0.85.
In 1979 Nigel again collaborated with researchers from the New Zealand Pottery and Ceramics Research Association to investigate the dynamic performance of brick masonry veneer panels (Priestley et al., 1979). This research was motivated by the poor reputation of unreinforced masonry veneers when subjected to earthquakes, with much of this reputation being attributed to the failure of brick masonry facades and walls during the 1931 Napier and the 1968 Inangahua earthquakes. Seven unreinforced and two reinforced clay brick masonry veneer walls tied to conventional timber-frame backings were subjected to out-of-plane sinusoidal accelerations in the appropriate frequency range imitating earthquake loading, see Figure 6.2, where the stud spacing, veneer-tie type and the initial distribution
of pre-formed cracking were the main variables. Out-of-plane face loading was specifically considered because the draft Code of Practice for light timber frame construction required the entire in-plane load
demands to be carried by the timber frame bracing to which the masonry veneer wall is fixed. From this testing, it was concluded that when unreinforced masonry veneers were built to the specifications prescribed in the draft Code of Practice, acceptable response could be expected for earthquake loading
levels in excess of those expected for the highest seismic zone in New Zealand. Furthermore, it was found that pre-formed horizontal or diagonal panel cracking had little or no apparent influence on the ultimate performance of the veneers.
In 1980 Nigel embarked on a significant undertaking and wrote the background to the draft New Zealand Masonry Design Code (SANZ, 1980a) and dedicated it to the memory of Otto Glogau, his former colleague from the Ministry of Works (Priestley, 1980). The basis for the draft code was based, to a significant extent, on Nigel’s previously published masonry research findings (Priestley
and Bridgeman, 1974; Priestley, 1977) that had confirmed the available but limited ductility capacity of reinforced masonry, and on the recognition that reinforced masonry could be designed based on the same principles of reinforced concrete design adapted for low strength materials. Three grades of material properties were prescribed based on the extent of inspection and associated quality of workmanship, and a procedure was presented on how to implement ductile design of reinforced masonry buildings. Criteria were provided for masonry shear wall buildings, in cases of complex geometry, such that individual walls could be classed as being either primary or secondary walls, where secondary walls were assumed to not carry in-plane loads but required detailing to sustain the lateral deformations that they would be expected to be subjected to, see Figure 6.3. In the same year Nigel presented a paper at the 7th World Conference on Earthquake Engineering in Turkey (Priestley, 1980a)
that dealt with many similar topics, and also referred to Nigel’s research on base isolation (Priestley et al., 1977) and the seismic response of structures free to rock on their foundations (Priestley et al., 1978). Nigel also published a book chapter in 1980 that provided an effective summary of his thoughts and research on masonry up to that time and brought this work to the attention of a wide international audience (Priestley, 1980b). In 1985 Nigel published a revised provisional standard (Priestley, 1985), in response to the code committee’s expressions of dissatisfaction regarding the format of the 1980 document, and recommended that the Masonry Design Code more closely follow the format of the Concrete Design Code, (SANZ, 1982). In 1986 Nigel contributed to a discussion paper related to ‘Structures of Limited Ductility’ (NZNSEE, 1986), where despite good results from his earlier research efforts, he acknowledged that the use confinement plates will ‘inevitably be unpopular, and most masonry will continued to be unconfined’.
In 1981 Nigel went on to assess the ductility capacity of masonry wall systems, where the ductile capacity of cantilevered unconfined reinforced masonry walls was computed using an ultimate compressive strainε m = 0.0025 (Priestley, 1981). Figure 6.4 plots the available ductility of a wall with a height-to-length aspect ratio of 3 for various longitudinal reinforcement and axial load ratios. In this chart the masonry compressive strength has been normalised with respect to the value of 8 MPa assumed the construction complies with Grade B inspection. Charts were developed for both low and high strength grades of longitudinal reinforcement, and a relationship was presented for how the ductility capacity for unconfined masonry cantilever wall with aspect ratio µ3 could be modified to estimate the ductility capacity of similar walls having a different aspect ratio. Shortly afterwards a companion article was presented for the corresponding procedure to determine the ductility capacity of confined concrete masonry shear walls, where the confinement was provided by steel plates inserted into the horizontal bed joints within the plastic hinge zone (Priestley, 1981a, 1982). These procedures for establishing the ductility capacity of unconfined and confined reinforced masonry walls were underpinned by a comprehensive grouted concrete masonry prism testing program (Priestley, 1983).
Two series of prism tests were reported, with Series 1 testing considering 140 mm prisms and Series 2 testing considering 190 mm prisms. The influence of loading rate was investigated by considering applied axial compressive strain rates of approximately 0.000005/sec and 0.005/sec. It was concluded that the unconfined prisms developed a peak stress at a compressive strain of approximately 0.0015, that stress-strain response was not significantly influenced by block width or the presence of vertical reinforcement in the grouted flues, that increasing the strain rate by 1000 times resulted in an average 17% increase in compressive strength, and that a modified Kent-Park stress-strain relationship suitably described the observed experimental response. The studies on the performance of unconfined and confined masonry walls were combined and summarized along with a design example in an article published in an early volume of the journal of the US Masonry Society (Priestley, 1981a).
In the early 1980s, Nigel began to conduct research on slender masonry walls. With Don Elder he tested three slender concrete masonry walls (Priestley and Elder, 1982). The aim of these tests was to specifically address the behaviour of slender walls, since previous testing had focused more towards understanding the behaviour of squat walls. These walls were 6.3 m high, which was the limiting height available at the Structures lab. The test units incorporated portions of the first and second floor slabs as well as a bond beam at the wall top, see Figure 6.5. All walls had similar detailing of vertical reinforcement, though Wall 2 had horizontal bed joint confinement plates in the plastic hinge zone, which were absent in wall 1. Wall 3 differed from wall 1 by having a lower level of applied axial load and had a longer lap-splice for the starter reinforcement extending from the foundation. The results of these tests validated the procedures published previously by Nigel (Priestley, 1980, 1981, 1982) but also highlighted a concern about lap-splicing the wall longitudinal reinforcement within the potential plastic hinge region. This work also showed that the capacity of conventional masonry walls may be
suspect due to the lack of confinement at the wall toes, and that confining steel plates substantially improve the response of the walls. This study was the last major experimental study related to reinforced concrete masonry walls that Nigel undertook while at the University of Canterbury. In
1986 Nigel published an overview paper for the international audience highlighting all the research accomplishments in masonry stemming from New Zealand thus far (Priestley, 1986).
Nigel’s research on masonry walls in the 1980s was driven to a large extent by preferences of the design community and construction industry towards cantilever wall systems. Despite considerable success and promising results from analytical and experimental studies of cantilever walls, Nigel was intrigued by the possibility of re-configuring reinforced masonry walls into ductile moment frames for low-rise buildings, which he later dubbed the “masonry moment-resisting wall-frames” shown schematically
in Figure 6.6(a). The beam length in a moment-resisting frame would be longer than the length of the coupling slabs in comparable systems. The premise of his approach to the new system was that, through capacity-design principles, the masonry wall-frame system can be designed and detailed to emulate the strong column-weak beam mechanism, which has been advocated for the design of concrete moment- resisting frames. Plastic hinges, involving yielding of distributed reinforcement, will be forced to form at masonry beam ends at ultimate limit states, ensuring a dependable lateral strength to the system and a stable source of energy dissipation when the structure is subjected to intense earthquake ground motions. Since non-ductile response of elements in the system is to be avoided, a natural concern for the resilience of the new masonry moment-resisting wall-frame system involves the transfer of shear
Figure 6.4 - Available ductility capacity of a vertical cantilever reinforced masonry wall with an aspect ratio of 3 (Priestley, 1981). (a) Low strength 275 MPa reinforcement (b) High strength 380 MPa reinforcement
forces from beam hinging to the connecting wall elements, highlighted for the joint region in Figure 6.6(b). The design of the wall-beam joint must also consider potential strain-hardening of the beam reinforcement due to large ductility demand imposed on the structure. It was also recognized that hysteretic response of the joint may degrade under reversed cyclic loading if beam bar forces cannot be adequately transferred to the joint due to poor bond condition, which is in turn dependent on the size of the joint and bar diameter. To demonstrate the potential ductile response of moment-resisting frames in masonry construction, Nigel conducted the first ever full-size masonry wall-frame joint test at the University of Canterbury in 1983. The lateral force versus lateral displacement response, whichµ∆ = 6
was monitored at the top of the test specimen and shown in Figure 6.6(c), showed remarkable ductility capacity for what was essentially an unconfined masonry system. A displacement ductility factor of was obtained in the specimen before 15% degradation of lateral strength was observed. Importantly, the wall-beam joint region remained elastic and protected from any significant cracking, as can be seen in Figure 6.6(d), while inelastic rotations occurred at beam ends as intended by the capacity design principles. The success of a single test led to the masonry wall-frame system being accepted by the New Zealand Masonry Code in 1985.
During these years, which were nearing the end of his time at the University of Canterbury, Nigel added a new dimension to his research in masonry by investigating the out-of-plane response of unreinforced masonry (URM) walls (Priestley, 1985a). This research was focused on assessing the earthquake characteristics of existing URM walls, rather than the design of new reinforced masonry buildings, and Nigel commented that “the response of unreinforced masonry walls to out-of-plane
(face load) seismic excitation is one of the most complex and ill-understood areas of seismic analysis”. It is noted that the elastic analysis technique that was commonly applied at that time was focused on masonry stress levels that were “rather insignificant for unreinforced masonry”, resulting in excessively conservative results, and that the seismic capacity of URM walls responding out-of-plane is instead governed by stability and energy considerations. Load paths within unreinforced masonry buildings are discussed, as is the influence of flexible diaphragms. The conditions at wall failure are presented in terms of displacements necessary to cause instability, see Figure 6.7, and it was recommended that dynamic testing and corresponding analysis be undertaken to further refine the presented methodology for assessment. It is noted that the walls in the upper levels of unreinforced masonry buildings are likely to be most critical, and that adequately securing the walls to diaphragms is an essential step for