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TitleMJN Priestley High
TagsEarthquake Engineering Civil Engineering Nature
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Total Pages156
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John NigelJohn NigelPriestleyPriestley

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Standard (Standards New Zealand, 2004) recommending that the design seismic action for mixed

systems be determined by a rational analysis (without providing details for such a rational analysis).

Such mechanics-based reasoning, as used for mixed structural systems, was ubiquitous in Nigel’s

teaching at the time (and, one imagines, throughout his career). At the ROSE school Nigel would

teach using an overhead projector (commonly referred to as OHP) in which his hand written reasoning

would be clearly projected onto the wall as he talked. An example projection from his 2003 class on the

seismic design of bridge structures is shown in Figure 4.5, where the type of reasoning just described

for mixed systems is clearly presented and easy to follow.

One can also see from Figure 4.5 how Priestley et al. (2007) would recommend the evaluation of equivalent

viscous damping for mixed systems. The case shown above is for the specific case where the displacement

Figure 4.5 - An illustration, from an OHP slide prepared by Nigel during his 2003 ROSE School course on Seismic Design and Retrofit of
Bridges, of the clear reasoning Nigel would use to explain how ductility and equivalent viscous damping could be found in mixed systems

– in this case a bridge with piers of different height.

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demand on all sub-systems is the same and in general, Nigel would argue that it should be found as follows:








whereξi is the damping and∆i the displacement of sub-system i andξsys is the total system damping. This
expression could therefore be used to account for the possibility of different hysteretic characteristics

being present within a mixed system when relating inelastic and elastic spectral displacement demands

(Figure 4.2c and 4.2d). This provides an improved alternative to the use of expressions such as the

equal displacement rule present in current codes.

Early critics of DDBD identified the use of equivalent viscous damping as a limitation of the method.

This was overcome through the use of equivalent viscous damping expressions that are calibrated to

the results of non-linear response history analyses - thus giving them equivalent accuracy to empirical

R-m-T relationships used in FBD. However, whereas in FBD the use of the equal-displacement rule

appears to be prevalent for any type of structural system, in DDBD different EVD expressions were

developed for a wide range of structural systems. This included studies by Blandón (2004) and Grant

et al. (2005), Dwairi et al. (2007), and more recently Pennucci et al. (2011).

By around 2006, guidelines for the DDBD of a number of different structural systems had been

developed and verified, including various MSc and PhD studies at the ROSE School covering DDBD

of bridges (Alvarez, 2004), moment resisting frame structures (Pettinga, 2004), RC wall structures

(Beyer, 2005), and dual frame-wall systems (Sullivan et al. 2006). In addition, some means of dealing

with issues such as torsion, P-delta, soil-structure interaction, and elastic damping had been formulated.

Thus, in 2007, the first text on DDBD was published by IUSS Press (Priestley et al., 2007) representing

a huge milestone for the approach.

4.4 Future of the DDBD4.4 Future of the DDBD

By developing the DDBD method, Nigel has provided the engineering community with an approach

that addresses many of the shortcomings and issues with current code design practice. In addition, the

methodology provides engineers with a better sense of the role that structural proportions, material properties,

member detailing, and capacity design concepts all play in the seismic risk of a building or structure.

Despite the significant developments made to the Direct DBD procedure and the extensive testing it

has undergone, the approach is not yet widely used in practice. The reasons for this may well be that (i)

it is not a codified procedure, (ii) is not implemented in commercial software and (iii) does not appear

worthwhile financially to consultants, principally because it is not a codified procedure. Reflecting on

these points, they are all non-technical issues and can be addressed with time. To this extent, a model

code was included in Chapter 14 of the DDBD book, in an effort to illustrate how the approach might

be transitioned into practice. This model code was developed further in the years that followed, and

Nigel was co-editor of a more detailed version published in 2012 (Sullivan et al., 2012a). This model

code incorporates the results of various developments that were made at the ROSE School and in other
areas of Italy as part of a national effort by the RELUIS consortium ( Software for

DDBD has also undergone development (see, for example, Sullivan et al., 2012) but will require more

development before being widely used in consulting.

Developments of the DDBD method for specific structural and non-structural systems are expected to

continue for years to come, widening its applicability. Of course, none of this would have happened

had it not been for the foresight of Nigel in establishing a design methodology that at the time of its

inception represented a complete inversion of traditional seismic design approaches.

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memoria nostra durabit,

si vita meruimus

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