Plan de estaciones

By José I. Restrepo, Christopher Latham, Sri Sritharan and Nihal Vitharana

3.1 Introduction 3.1 Introduction

Nigel was an outstanding engineer and human being with progressive and social views of the world very hiughly esteemed by his peers, friends and his family. Many of his post-graduate students were from various parts of the world and many from countries with developing economies who had a meager income and he never forgot to consider their welfare, well beyond his role as a mentor. Nigel’s such qualities would be eternally carried forward because most of his students are scattered throughout the world contributing in various ways. Nigel was a simple and no show-off man. He could be seen interacting with technical staff day and night doing experiments first at the University of Canterbury then at the University of California San Diego, and lastly at the ROSE School, which he co-founded with Michele Calvi.

Nigel’s clarity of thought, ability to understand very complex problems in very little time and ability to propose simple and practical and economical solutions, all backed up by a sound but simplified theoretical approach, are some of his most fascinating facets. Nigel’s theories created lively discussions and apparent disagreements among peers. Many researchers required “further research” just to find out, that after arduous, long and often sophisticated work, his simple theories were, in fact, quite accurate and well within the acceptedlex artis. One could say that Nigel did ninety percent of the work in ten percent of the time, whereas the more mundane of us spent ninety percent of the time working on the remaining ten percent. He was never afraid to look at a problem in a very different perspective from his younger age; whether it is confined concrete, bridges under thermal gradients, soil-structure interaction or water-retaining structures.

As a structural experimentalist, Nigel almost immediately developed a comprehensive view of an issue in question. This proved useful when one consulted him, as he clearly (and sometimes bluntly) would let anyone know the limitations of a proposed test or the practical value of its expected findings. There is little doubt that Nigel was a pioneer and the best structural system experimentalist of all times. He was incredibly talented, quick-witted, creative and prolific. He mastered not only the field of experimental mechanics, but also the mechanics of concrete, thermal and deformation-induced loadings, masonry and structural dynamics and soil-structure interaction, among a few. In short, Nigel was another giant in the field of Earthquake Engineering and the fourth of a prodigious New Zealand generation of earthquake engineers which also include: Tom Paulay, Bob Park and Ivan Skinner. His work has inspired many academics and professionals. This chapter summarizes the main accomplishments in the field of large-scale experimental structural engineering.

3.2 Nigel’s First Steps as an Experimentalist 3.2 Nigel’s First Steps as an Experimentalist

Nigel’s first exposure to structural concrete experimental mechanics was during his PhD studies at the University of Canterbury, which he carried out with minimal supervision in a startling short period of two years between 1964 and 1966 (Priestley, 1966). Nigel completed his PhD when he was just 23 years old. In this work, he presented a comprehensive analytical and experimental investigation about moment redistribution in prestressed concrete beams, which up until then had been rather controversial. To support this work, Nigel tested seven simply supported and seven continuous beams and in each test he obtained a wealth of data that he shared by appending it in his dissertation.

3.3 Accomplishments at Central Laboratories 3.3 Accomplishments at Central Laboratories

Upon completion of his PhD, Nigel went on to become Head of the Structures Laboratory of the New Zealand Ministry of Works (Central Laboratories). The Structures Laboratory was involved in three different types of testing: small scale model testing in support of design efforts by Head Office, large scale laboratory testing of concrete models, and in-situ testing of structures either built or under construction. Routine inspection of the prestressed concrete Newmarket viaduct in Auckland, see Section 5.2, revealed cracking in the soffit of the bridge, where conventional analysis would have indicated no tension should exist because at that time the temperature distribution was assumed to be uniformly distributed through the section in standards throughout the world. In Nigel’s own fashion of intuition, he knew it could be different and he proved this by carrying out a simple 1-dimensional finite difference analysis considering diurnal temperature and solar radiation. Few decades later, attempts by other researchers with sophisticated 3-dimensional finite element models showed that Nigel was correct with his simple approach. His model is now adopted throughout the world in bridge standards and is known as 5th_-power

parabolic temperature distribution. Not many engineers would, however, know that this was developed by Nigel in his hay days in New Zealand. The cracks were found to be thermally active: they opened

during days of high sunshine, and closed at night. Nigel began to study this problem, combining finite differences and experimental work. He built a ¼ scale simply-supported prestressed box-girder span and comprehensively instrumented it with thermocouples, strain-gauges and displacement transducers. The top surface of the bridge was enclosed within an environmental box which included 100 infrared light bulbs controlled through a variable output transformer to model diurnal variations of solar radiation, and propeller fans to control night-time cooling, see Figure 3.1. The design guidelines that stemmed from this research (Priestley, 1971; 1972; 1976; 1978a) form a major contribution to the design of bridges for thermal loading, bridges in many parts of the world.

In 1968 Nigel took a one year leave from Central Laboratories and traveled to Lisbon, Portugal as a Post-doctoral Fellow at the Laboratorio Nacional de Engenharia Civil (LNEC), whose Structures group was being led by Prof. J. Ferry Borges. This period marked a turning point for Nigel, as it was at LNEC where he acquired a strong background in structural dynamics and earthquake engineering as he himself pointed it out (Priestley, 1996):

The writer had the great good fortune, and considerable pleasure, to spend a year in 1968/69 as a post-doctoral fellow at the Laboratorio Nacional de Engenharia Civil in Lisbon. At that time J. Ferry Borges was the head of the Structures group at LNEC, and it was a result of his personal decision that my application was approved. I came to LNEC with no real background

in Earthquake Engineering, and spent the year attempting to get up to speed in the general areas of structural dynamics and earthquake engineering, while simultaneously trying to hide my ignorance. I do not think that Ferry Borges was fooled, but he was tolerant, and patient, and I ended up learning a great deal, to the extent that my future professional activities were to be

completely dominated by a fascination for the seismic response of structures.

Upon his return, Nigel conducted several in-situ tests on bridges and bridge components. Of these tests, the lateral load test of an extensively instrumented 1.8 m diameter steel-encased pile embedded on soft marine mud is a true landmark (Priestley, 1974). Nigel meticulously designed and conducted this test. Data logged in this test enabled the calculation of the lateral pressure profile acting on the pile and of the lateral subgrade material moduli. This was the world’s first full-scale lateral load test performed on a pile and today it is a point of reference for such types of tests. The calibrated p-y curves

from this experiment form the basis of the recommendations made for analysis of deep foundations in geotechnical engineering.

3.4 The University

3.4 The University of Canterof Canterbury Famous “Ps”: Paulaybury Famous “Ps”: Paulay, Park and Prie, Park and Priestleystley

In 1976 Nigel joined the Department of Civil Engineering at the University of Canterbury after conversations with Professor Bob Park while golfing in Wairakei, and when Professor Harry Hopkins was ending a shining career as Head of Department and was about to be succeeded by Professor Park. Professor Hopkins had a strong vision for the Department. He was also responsible for the hire of Professors Bob Park, Tom Paulay. The hiring of the three “Ps” in the field of structural concrete and the ability of all three to combine strengths, collaborate with industry and help solve complex problems in seismic design practice, propelled the University of Canterbury to world fame in the field.

At Canterbury, Nigel carried out collaborative work mainly with Bob Park, Tom Paulay and also with Professor Athol Carr, who worked in structural dynamics and computational mechanics. In the early days in his new job at Canterbury, Nigel and Athol worked in rocking walls as a potential seismic system for use in buildings (Priestley et al., 1978b). With limited and small-scale testing, they could extend Professor Housner’s theory and propose a simple method for predicting maximum displacements of rocking systems using the displacement response spectra and linearized properties for the rocking system. This, in a way, was a very early precursor of the Direct Displacement Based Design method Nigel would go on to develop in the early 90s, and is also an early precursor of the work on low-

damage walls that he developed further in the PRESSS program, which will be briefly described in the following section and detail in Chapter 7.

Nigel is the person who identified the significance of temperature loading on the behaviour and durability-performance of concrete cylindrical tanks which had shown various cracking patterns despite designed with adequate “factor of safety” with respect to crack width. In mid-late 1970’s, in line with Nigel’s’ simple approach to any given problem, he instrumented wall panels near the Christchurch airport with rudimentary thermocouples. He borrowed a pyranometer to measure solar radiation from

the then Lincoln Agricultural College. This simple test showed various temperature gradients resulting in significant thermal stresses. He showed that on a vertical face, the radiation from other buildings could be up to 20% of direct solar radiation. Nigel went to develop temperature gradients for winter and summer conditions which are believed to be the world’s first which are in AS 3735 (SA, 2001) and NZS 3106 (SNZ, 2009). Ironically, these are used in many designs/investigations throughout the world

today probably without knowing that a young researcher developed these in New Zealand. Nigel worked together with Bob Park in the testing of reinforced concrete columns using a NZD

300,000 servo-controlled hydraulic 10 MN dynamic tension/compression capacity universal testing machine supplied by Dartec in the UK. He was responsible for design of the foundation, set-up and commissioning of this machine, see Figure 3.2. The Dartec machine became a workhorse for testing reasonably large size columns. Data acquired from column tests carried out with this machine was used to validate the concrete confinement models of Scott et al., (1982) and of Mander et al., (1988a, 1988b), and to propose prescriptive code design requirements. The Dartec machine was also used by Bob and Nigel on seismic pile-to-cap connections (Park et al., 1984) and was used by Nigel to conduct seismic tests on encased steel piles (Park et al., 1983). These encased steel piles were the forerunners of the steel jackets he would investigate at the University of California at San Diego and successfully recommend as a retrofit scheme for use in the bridge retrofit program carried out by the State of California after the damaging 1989 Loma Prieta earthquake (Chai et al., 1991).

Shear transfer in reinforced concrete is an aspect of research that Nigel took special interest in at the University of Canterbury. He collaborated with Bob Park and Tom Paulay in relevant aspects of shear transfer mechanisms in beam-column joints, with support from large scale experimental work (Paulay et al., 1978). His work on shear extended to the shear capacity of circular columns, which involved extensive testing and collaboration with Tom (Ang et al., 1989; Wong et al., 1993). Nigel would culminate his work on shear in columns at the University of California at San Diego by presenting the UCSD shear model (Priestley et al., 1993), which could predict shear failure in elements after flexural yielding had occurred. The conceptual clarity and simplicity of this model has resulted in widespread acceptance in the community.

Nigel also had his independent line of research on reinforced and unreinforced masonry as well as thermal and dynamic effects on liquid storage tanks. In reinforced masonry, Nigel moved the design away from Working Stress into Ultimate Strength, making the New Zealand Code for the Seismic Design of Masonry Structures a role model for codes around the world (Priestley, 1985). This work was supported by extensive testing, of which the work leading to the seismic design of masonry walls for ductility (Priestley, 1981, 1982) is worth highlighting. Chapter 6 gives a detailed description on Nigel’s work on masonry. With Stuart Thurston, Nigel developed a methodology to predict the temperature rise and thermal stresses in hardening concrete (Thurston et al., 1980). To prove its accuracy, Nigel monitored the concreting of the foundation of airport hangar at the Christchurch International Airport. It also coupled heat-of-hydration and heat-transfer in a simple but smart way. This is in fact the first such prediction model which set the benchmark for predicting thermal stresses in early-age concrete including creep relaxation. At the University of Canterbury, Nigel maintained his line of research on thermal action on water-retaining structures, and with Nihal Vihtarana (Vitharana et al., 1998) he carried out a set of complex tests and proposed a methodology to incorporate thermal loading in the design of liquid storage tanks. This was

the first reported testing on structural wall elements subjected to thermal gradients in conjunction with applied tensile axial loads. With its findings, they developed useful guidelines for evaluating thermal stresses in concrete structures, which are used in many modern standards on water-retaining structures, and developed thermal stress tables by smartly manipulating beam-on-elastic foundation formulations. On the structural dynamics side, not involving experimental work, Nigel led a group of the New Zealand Society for Earthquake Engineering on the Seismic Design of Liquid Storage Tanks (1986). This work is

a major contribution in the field and is a point of reference for international guidelines.

When the first author was a Senior Lecturer at the University of Canterbury in the late 90s, he was very privileged to work in two bridge seismic retrofit projects with Nigel. The Aotea Quay Overbridge, a 1930s bridge strategically located in Wellington, was retrofitted by enlarging the footings and by wrapping the columns with FRP jackets, a first outside the United States, see Figure 3.3. In this project Nigel provided the design guidelines written with support from testing he had performed at

the University of California at San Diego where he had pioneered such well-known retrofit technique (Priestley et al., 1992). We also worked together in the seismic retrofit of the Thorndon Overbridge, a major lifeline crossed by the Wellington fault. Nigel was the primary external consultant for Beca, the structural engineering company responsible for the design of the retrofit. The Overbridge presented several design deficiencies such as short longitudinal bar cut-offs, small number of columns transverse reinforcement, short seating for the girders, foundation deficiencies and various others. Proof of concept testing was carried out on pile, pile cap column specimens, in the unretrofitted and retrofitted conditions at the University of Canterbury, see Figure 3.4.

In 1986 Nigel looked for new challenges and moved to the University of California at San Diego. Nigel cared for his graduate students whom he would leave behind at Canterbury. Despite all hectic work associated with moving overseas with a young family, Nigel wanted to make sure that the students were coping well in difficult times in various ways. As an example, the fourth author’s wife (Padmini Vitharana, a student at Canterbury as well) was eight months pregnant. Nigel visited them at night with a cot for their expected new arrival. The cot was passed onto another student when Nihal and Padmini were leaving Canterbury hence passing Nigel’s legacy on kindness and care towards others.

3.5 Ad

3.5 Advancing the Statvancing the State-of-the Art in Structe-of-the Art in Structural Tural Testing at esting at the University of Cthe University of California at San Dalifornia at San Diegoiego

The Charles Lee Powell Structures Laboratories of the University of California at San Diego, the largest of its kind in the world, were funded in 1986 under the direction of Professors Gil Hegemier and Frieder Seible. The University of California at San Diego looked at hiring a researcher of significant talent and world-class trajectory to ensure the new labs were successful. Nigel was found to be the most suited candidate for this position. He assumed this new challenge and joined the University of California at San Diego in early 1987 and collaborated with Frieder Seible in several research projects, including the testing of the full-scale 5-story masonry building. This work was part of the Technical Coordinating Committee for Masonry Research (TCCMAR) program for development of a limit- state design standard for masonry buildings in seismic zones. This program culminated in the testing of a full-scale five story reinforced masonry building (Seible et al., 1994). The building consisted of 150 mm wide concrete masonry blocks with concrete topped precast prestressed floor slabs. The building was tested using hybrid simulation, termed pseudo-dynamic testing then. There were several challenges in performing this testing. The building had a C-shape plan configuration. This necessitated two actuators per floor level. To correctly simulate the distributed loading present in an actual structure each actuator was attached to a long beam which had two elastomeric pads attached on the bottom of either end. These beams were then clamped to the floor slabs to transfer the forces by friction. Initial testing started during the day in the summer of 1992. It quickly became apparent that testing would need to be performed at night instead. The overall vertical temperature gradient in the lab during the day in summer was giving poor results. Also, the sun light streaming in through the labs windows was heating up individual transducers and giving false displacement readings. The testing continued over a period of about two months with increasing demand and damage to the structure. This showed the effectiveness of the design methodologies developed in the small-scale component testing. Chapter 6 provides an in-depth discussion about the impact of this test program in industry.

Figure 3.4 - Seismic retrofit of the Thorndon Overbridge, Wellington in 2000. Nigel was an external consultant in this project. Proof of concept testing was carried out at the University of Canterbury (Presland et al., 2001). In the top left photo Bob Park (right above) and Nigel (extreme right) inspect the construction of a test unit.

Figure 3.3 - Seismic retrofit of the Aotea Quay Overbridge, Wellington in 1996. First seismic retrofit application using FRP jackets outside the United States. Nigel provided design guidelines for this project (Gray and White 1997).