Practical Lessons from Nigel

By Joe Maffei, Carlos Blandón, Sri Sritharan and Katrin Beyer

2.1 Introduction 2.1 Introduction

In August of 2008 many of Nigel’s collaborators and former students gathered at Lake Tahoe, California for a symposium in honor of Nigel’s 65th _{birthday. The presentations showed the wide range of Nigel’s}

influence, and the questions and discussion represented the atmosphere of curiosity and collaborative energy present at every meeting and institution that has absorbed Nigel’s inspiration. The proceedings are published by the IUSS Press (Kowalsky and Sritharan, 2008) and include a number of revealing short tributes in addition to the technical papers. It was clear that Nigel’s work had a meaningful objective: to serve the practicing structural engineering community worldwide.

Many noted Nigel’s clear thinking, creativity, and personal connection to his students. The students recalled his ability to deliver lengthy lectures from memory. (Many of us recall the laboratory technicians at the University of Canterbury saying how Nigel could remember long strings of four-digit dial gage readings without ever making a mistake.) We suggest that you find and read this proceedings volume; here are a few excerpts from the tributes:

Mervyn Kowalsky: The reluctant leader is in almost all cases, the most effective …such leaders possess brilliance and talent and are always the most humble and genuine…

John Stanton, referring to the PRESSS project on precast seismic systems: He put together a terrific group of people, fed them a diet of intellectual challenges and culinary delights, and expected them to give their best as he led them into uncharted territory.

Michael Collins: It amazed me how quickly Nigel found and rectified my mistake.

Rob Park: Nigel Priestley was the clearest thinking and most innovative engineering thinker I have ever met.

José Restrepo: Complex work landing on his hands has always been distilled into beautifully simple solutions.

John Mander: …his presented work, in whatever form, is clear, no-nonsense, concise, and revolutionary.

Rob Chai: …a giant whose height can only be dwarfed by his modesty and humility.

Scott Arnold: …he explained ‘I understand what you are referring to, but the structure has not read the code and the hinges will form as I’ve described.’

Greg MacRae: …and I also want to say that I am really sorry about the …shaking table test where we put the wrong scale factor into the controller…

In his two “Myths and Fallacies” publications (Priestley, 1993, 2003), Nigel critically challenged assumptions about stiffness, elastic analysis, and detailing that are still commonly made in seismic design and are prescribed by building codes. He found that such assumptions are not only false, but they can also impede or distract the structural engineer from conceiving an appropriate design. The following section describes some of Nigel’s practical lessons.

2.2 Practical lessons from Nigel 2.2 Practical lessons from Nigel

Here is a sampling of the lessons from Nigel, many arising from projects and problems where the authors have done their best to think like Nigel.

a. Love your work and dedicate to it a. Love your work and dedicate to it

It is clear that Nigel had great passion for his work and followed his passion. His productivity was astounding.

b. Work isn’t everything b. Work isn’t everything

For example, we should all admire Nigel’s expertise in the Italian distilled spirit grappa.

c. Have conferences in beautiful places c. Have conferences in beautiful places

The International Bridge Workshop held in 1994 in Queenstown, New Zealand attracted many of Nigel’s collaborators and friends the world over in a beautiful setting (Figure 2.1). Figure 2.2 shows presentation by Rui Pinho at the Rose School Seminar, May 2006 in Pavia Italy; Rui’s slides of fragility

curves had to compete for the attention with the room’s frescoes.

d. There is a “flexure-then-shear” failure mode in reinforced concrete d. There is a “flexure-then-shear” failure mode in reinforced concrete

We are not sure if it was Tom Paulay or Nigel who first identified a mixed failure mode where nonlinear response first takes place in flexure and then switches to shear, resulting in failure soon after. This mode of failure is evidenced in Figure 2.3 on a wall tested by Mestyanek (1986). Nigel clearly identified it in bridge column failures in Loma Prieta, and subsequently developed the UCSD model for shear strength, applicable to columns, and extended to walls in FEMA 306 (ATC, 1999) and by Krolicki et al. (2011). The UCSD model was a major milestone in understanding and predicting the behavior of reinforced concrete elements in earthquakes. An interesting aspect of the model is that is shows that the expected behavior mode is not sensitive to the level of axial load.

Figure 2.1 - Bridge workshop in Queenstown New Zealand, 1994 with par ticipants from around the world. The program was equal parts skiing and technical discussions. Those pictured include Peter North, Joe Maffei, Des Bull, John Berrill, Athol Carr, Howard Chapman, Barry Davidson, Richard Fenwick, Kazuhiko Kawashima, Donald Kirkcaldie, Bob Park, Toru Terayama, Hajime Ohuchi, Ian Billings, Mick Pender, Dale Turkington, Brian Maroney, Po Lam, Greg Fenves, Eduardo Carvalho, Nigel, Michele Calvi, Frieder Seible, and Camillo Nuti.

e. Here’s how to design or evaluate pier structures e. Here’s how to design or evaluate pier structures

With Rutherford + Chekene, the first author contributed to the retrofit of a pier structure for the Exploratorium, a hands-on science museum in San Francisco (Figure 2.4). Nigel’s methodology for this type of structure is clear and to the point - in refreshing contrast to many other US codes and guidelines.

Port facilities located in the west US coast required improvement in the design methodologies that were used before the Loma Prieta (1989) and Northridge (1994) earthquakes. Nigel was actively involved in providing analytical and design approaches that incorporated key but basic principles of structural design such as limit states, plastic hinge length, and moment curvature of reinforced concrete sections, among others.

A good example of his capacity to simplify complicated problems is the design example that he produced for the design of typical wharves existing in the west coast (Priestley, 2000). In a few pages, the design example addresses the effect of the soil, the nonlinear behavior of the pile and the connection, the axial load variation and even the torsional response to obtain the displacement capacity of the wharf. Nigel was always driven to test that the values he used for design were adequate, and he helped lead

a team that carried out full scale testing of connections and piles of wharf structures (Figures 2.5 and 2.6). The design principles and experimental findings developed and applied by Nigel are now part of design guidelines for such structures (ASCE-COPRI, 2014).

Figure 2.2 - The ROSE School Seminar 2002 at the Salone degli Affreschi of the Collegio Borromeo.

Figure 2.3 - Shear failure of a wall after flexural yielding (Mestyanek, 1986).

Figure 2.4 - The Exploratorium in San Francisco, built on a 1930s wharf with existing non-ductile concrete piers, seismically strengthened with new 6-ft (1.8m) diameter piers. Nigel’s clear methodology for wharf structures was the technical basis for the design.

Figure 2.5 - Photo of the team that developed design regulations for the Port of Los Angeles and that now are the base of recent ASCE-COPRI design guidelines (2014). Participants include Po Lam, Arul Arulmoli, Max Weismair, José Restrepo, Peter Yin, Nigel, Geoff Martin and Omar Jaradat.

f. Here’s how to design unbonded PT for frames f. Here’s how to design unbonded PT for frames

Another unique and influential contribution of Nigel was in convincing structural engineers about the use of unbonded prestressing in seismic-resistant design. Though the concept was previously investigated, it was the PREcast Seismic Structural Systems (PRESSS) program that pushed the boundaries and facilitated the use of unbonded post-tensioning broadly in seismic design practice

(Priestley et al., 1999). This system and its impact in practice is described in more detail in Chapter 7 of this volume.

As the leader of the PRESSS program, Nigel worked with academic and industry partners to design multiple precast frame and wall systems that used unbonded post-tensioning as a means to connect the precast members with each other or with the foundation. The unique benefits of this concept include minimal structural damage and re-centering capability for the seismic force-resisting system that is designed to experience a dependable yield mechanism while providing adequate lateral force resistance.

The ductile response of four precast seismic frame systems and one jointed wall system were demonstrated successfully in the PRESSS five-story building-the largest structure to be tested inside the laboratory at that time in 1999. In addition to the innovative lateral-force-resisting systems, the test building included two realistic types of floor structures: pre-topped double-tee precast floors and hollow core slabs with in situ topping. The value and success of the PRESSS program is evident in that (a) several precast buildings have been built using the unbonded post-tensioning in high seismic

Figure 2.6 - Full-scale tests carried out at the University of California, San Diego. The pictures were taken by Nigel during a short stop at the Engelkirk Research center. The appreciation and respect from the laboratory staff for Nigel was evident as everybody quickly mobilized to excavate the rock fill around the pile, so Nigel could take a photo of the damage.

regions in the U.S., New Zealand and other countries, and (b) the unbonded post-tensioning concept has been used by other researchers and practitioners for other structural systems developed using materials such as steel, masonry, and timber.

g. Here’s how to do it for structural walls g. Here’s how to do it for structural walls

Based on the principles and design approach defined by Nigel and his team, vertical unbonded pre- stress for walls has become increasingly common. In the San Francisco Bay Area, such walls are commonly cast-in-place, with about ten projects having been constructed in the last several years, including new structures and walls used in retrofitting.

h. Walls can rock and that can be good h. Walls can rock and that can be good

Early on Nigel published work related to foundation rocking, following on from the approach by Housner (Priestley et al., 1978). At the time, few engineers in practice thought about such things. Recently, a major retrofit project in San Francisco -The War Memorial Veterans Building, by SGH Structural Engineers- used an innovative system of rocking walls.

i. The C-column bridge bent and the problem of unbalanced moment i. The C-column bridge bent and the problem of unbalanced moment

Nigel’s study of C-shaped bridge bents showed that having a large gravity moment at a column or wall plastic hinge zone can lead to undesirable ratcheting of displacement in one direction only. In a practical application of this concept, the design of the tower at the DeYoung museum in San Francisco

used unbonded post-tensioning in the tilted walls to counteract the gravity moment, so that the plastic hinge region sees balanced cyclic moment demands under earthquakes (Figure 2.8).

Figure 2.7 - Parking garage for Mills Peninsula hospital in Burlingame California, by Culp and Tanner structural engineers, using the PRESSS moment frame system. The owners of the new base-isolated hospital desired operational performance from their parking garage. The PRESSS system provided it at a cost lower than competing systems.

j. Her

j. Here’e’s how s how to retrto retrofit using fiber cofit using fiber compositesomposites

The recommendations from Nigel and his collaborators on how to use fiber for shear strength and (in elliptical configurations) for confinement have provided structural engineers with practical seismic retrofit methods, (Figure 2.9).

k. Slab flanges contribute to coupling beam strength k. Slab flanges contribute to coupling beam strength

Related to the project shown in Figure 2.9, Nigel’s publications provide clear recommendations on practical topics such as “how much slab width contributes to a coupling beams strength?” Nigel’s

answer: ½ of clear span on each side of beam web.

l. Distribute the flexural reinforcement l. Distribute the flexural reinforcement

Tom Paulay and Nigel seemingly were the first to realize that concentrating flexural reinforcement near the extreme fibers of a wall or beam is neither necessary nor useful for seismic-dominated designs. This

Figure 2.8 - The tower of the DeYoung museum in San Francisco, structural design by Rutherford + Chekene.

Figure 2.9 - Twelve-story concrete wall building retrofitted by Rutherford + Chekene, including horizontally oriented carbon fiber to change walls from shear governed to flexure governed. Nigel’s colleague Frieder Sieble acted as a Peer Reviewer.

practice, shown in Figure 2.10a, is efficient for resisting static loads such as those coming from gravity or static earth pressure. This efficiency no longer holds for reversing seismic loads. The arrangement shown if Figure 2.10b provides nearly equivalent flexural strength while reducing reinforcement congestion, improving the performance of beam-column joints, better controlling of shear deformation in beam plastic hinge regions, and reducing the potential for sliding shear failure.

m. Here’s how masonry works m. Here’s how masonry works

While the world was thinking in working stress seismic design for reinforced masonry buildings, Nigel had developed capacity design guidelines (Priestley, 1986) and had introduced the concept of the

confinement plate to increase the compression strain capacity and limit crushing at the toe of masonry walls (Priestley and Bridgeman, 1974). His book with Tom Paulay (Paulay and Priestley, 1992) moved the seismic design of masonry buildings many notches up.

n. Concrete is not as stiff as you think n. Concrete is not as stiff as you think

Nigel’s method for estimating the stiffness of concrete elements, based on simple principles to estimate yield curvature, show how many other methods for estimating stiffness are off the mark (Priestley, 2003; Schotanus et al., 2007; Maffei et al., 2004).

o. Stiffness and strength are not independent o. Stiffness and strength are not independent

This is of course a major contribution from Nigel, and a key underpinning of his work in displacement- based design. It is well highlighted in his Myths and Fallacies paper of 2003. What amazes us is how

quickly Nigel realized all of the consequent implications of this fundamental truth. In contrast, the structural engineering community remains slow to incorporate this concept into design practice.

p. Here’s how to design a coupled wall p. Here’s how to design a coupled wall

In the US, most new tall buildings in high-seismic areas are using concrete core walls, typically with coupling beams. These are designed using non-prescriptive approaches with nonlinear response-history analysis. Despite the design freedom of such an approach, some engineers are uncomfortable deviating from elastic analysis results, even though those results are based on stiffness assumptions shown by Nigel to be false. In the displacement-based design textbook, Nigel emphasizes that the strength

for coupling beams can be assigned almost arbitrarily and he gives useful rules for proportioning, considering the total coupling strength compared to the wall axial load and strength.

q. Stand on the shoulders of others, even as you blaze new paths q. Stand on the shoulders of others, even as you blaze new paths

Refreshingly, we have found that Nigel has always been scrupulous about acknowledging contributions that precede his efforts.

Figure 2.10 - Arrangements of flexural reinforcement in beams (Priestley, 2003). (a) Conventional

reinforcement

(b) Distributed reinforcement

r. Here’s how to know when it really is the vertical earthquake component r. Here’s how to know when it really is the vertical earthquake component

We have seen a number of presentations in which an instance of damage is attributed to the vertical earthquake component. Typically this has seemed speculative, as most types of damage that might be attributed to vertical motion (e.g. buckling of vertical wall reinforcement) could also be caused by lateral motion only. Nigel is the only one we know who found clear evidence of damage that could only have come from vertical motion. This was a case of damage to a column at a basement level where surrounding walls restrict any lateral motion.

s. Cut thru vagaries and conventional wisdom; seek clear evidence s. Cut thru vagaries and conventional wisdom; seek clear evidence

In the professional career of the first author there have been instances where adherence to conventional wisdom has led our profession into trouble. One case is steel moment frames. Their ductility capacity was questioned in the early 1990s after the tests by Mike Engelhardt, but conventional wisdom held that moment frames were an excellent seismic system and that the research must be flawed. The 1994 Northridge earthquake proved the research to be correct and conventional wisdom to be wrong. Similarly, in Chile after the 1985 earthquake, some researchers assured the Chileans (without clear technical evidence we see in retrospect) that since their buildings had numerous walls, tie reinforcement was not needed. The 2010 Maule Chile earthquake disproved this proposition.

Nigel was great at dismantling of “conventional wisdom” because he valued clear truths over vague assurances. This was his calling.

t. Earthquakes have no borders-explore the world t. Earthquakes have no borders-explore the world

Like Bob Park (who took a sabbatical in China back when few westerners had been there) and Tom Paulay, Nigel travelled the world building professional and personal ties. He was a pivotal member of three major institutions on three different continents.

2.3 References 2.3 References

ASCE/COPRI (2014) - 61-14 Seismic Design of Piers and Wharves (ASCE 61-14), American Society of Civil Engineers, Reston, VA. ATC (1999) - Evaluation of Earthquake Damaged Concrete and Masonry Wall Buildings, prepared by the Applied Technology Council

(ATC-43 project) for the Partnership for Response and Recovery, published by the Federal Emergency Management Agency, Report No. FEMA 306, Washington D.C.

Kowalsky M., Sritharan S. editors. (2008) - Proceedings of the M.J. Nigel Priestley Symposium, King’s Beach, CA, August, IUSS Press, Pavia, Italy.

Krolicki J., Maffei J., Calvi G.M. (2011) - Shear strength of reinforced concrete walls subjected to cyclic loading. Journal of Earthquake Engineering, 15(S1), 30-71.

Maffei J., Stanton J., Priestley M.J.N., Park R. (2004) - Design Approaches, in Seismic Design of Precast Building Structures, State of the Art Report [Robert Park editor], Commission 7, Federation International du Beton, Lausanne, Switzerland, Chapter 4, January. Mestyanek J.M. (1986) - The earthquake resistance of reinforced concrete structural walls of limited ductility. M.E. Thesis, Department of

Civil Engineering, University of Canterbur y, Christchurch New Zealand.

Paulay T., Priestley M.J.N. (1992) - Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley & Sons, New York. Priestley M.J.N., Evison R.J., Carr A.J. (1978) - Seismic Response of Structures Free to Rock on their Foundations, Bulletin of the New

Zealand National Society for Eart hquake Engineering, Vol. 11, No. 3, pp. 141-150, September.

Priestley M.J.N. (1986) - Seismic Design of Concrete Masonry Shearwalls. ACI Journal Proceedings 83(1):58-68.

Priestley M.J.N. (1993) - Myths and fallacies i n earthquake engineering-conflicts between design and reality. Bulletin of the New Zealand National Society for Earthquake Engineering, 26(3), 329-341.

Priestley M.J.N., Seible F., Calvi G.M. (1996) - Seismic Design and Retrofit of Bridges, John Wiley & Sons Inc., New York. Priestley M.J.N., Sritharan S., Conley J.R., Pampanin S. (1999) - Preliminary results and conclusions from the PRESSS five-story precast

concrete test building. PCI journal, 44(6), 42-67.

Priestley M.J.N. (2000) - Seismic Criteria for California Marine Oil Terminals, volume 3: Design Example. TR-2103-SHR, Naval Facilities Engineering Service Center, Port Hueneme.

Priestley M.J.N. (2003) - Myths and fallacies in earthquake engineering, revisited. IUSS press, Pavia, Italy.

Priestley M.J.N., Bridgeman D.O. (1974) - Seismic Resistance of Brick Masonry Walls. Bulletin of the New Zealand Society for Earthquake Engineering 7(4):167-87.

Schotanus M., Maffei J. (2007) - Computer modeling and effective stiffness of concrete wall buildings, Proceedings of the International FIB Symposium on Tailor Made Concrete Structures - New Solutions for Our Society, Amsterdam.