New Zealand Engineering 1997 October

by Professor Bob Park, deputy vice-chancellor and a professor of civil engineering at the University of Canterbury.

"Structural design for earthquake resistance in New Zealand, as specified by design standards, has advanced significantly during the last two decades. These developments have brought about the realisation that many building and bridge structures designed before the mid-1970s may be deficient according to the requirements of current seismic design standards.

"It is of considerable concern that the structural deficiencies of many of these older structures are not appreciated. The seismic assessment of these structures, and their upgrading where necessary, should be undertaken since damaging earthquakes could occur anywhere in New Zealand at any time."

The major advances made in the seismic design of building and bridge structures through the years have been the outcome of a better understanding of the nonlinear dynamic response of structures, the mechanisms of post-elastic deformation of structures, and the methods for detailing members and joints of structures so as to achieve ductile behaviour. It has become appreciated that the current and past levels of seismic forces used in design will not ensure behaviour in the elastic range of structures during severe earthquakes. Hence the need for ductile behaviour, defined as the ability of a structure to deform beyond the elastic range (after yielding) without a significant reduction in strength.

In New Zealand, a major step forward has been the introduction of seismic design procedure referred to as capacity design. The basis of this procedure was first described in 1969¹ in a paper by Hollings and further developed in 1975 in a book by Park and Paulay². In the capacity design procedure for reinforced concrete structures the designer chooses the most appropriate mechanism for the structure to achieve adequate ductility during a major earthquake, normally by ductile flexural yielding occurring at selected plastic hinge positions. The chosen plastic hinge regions are designed for adequate flexural strength and ductility. All other regions of the structure are then made adequately strong in flexure, and the shear strengths of the whole structure made adequately large, to ensure that the post-elastic deformations occur only at the selected plastic hinge regions. The first New Zealand design standard to require the capacity design procedure was the Code of Practice for General Structural Design and Design Loadings for Buildings NZS 4203 issued by the Standards Association of New Zealand in 1976.

Significant developments in the ductile detailing of structures of reinforced concrete and other materials have also occurred in New Zealand since the 1960s. The Code of Practice for the Design of Concrete Structures NZS 3101, first issued by SANZ in 1982, was a big step forward in this respect.

These developments in seismic design standards have brought the realisation that many structures in New Zealand designed before the mid-1970s may be deficient according to the seismic requirements of current design standards. The need for the seismic assessment of "old" building and bridge structures, and to upgrade (retrofit) if necessary, has been emphasised by the damage caused by many recent major earthquakes overseas. The most recent example was the Hyogo-ken Nanbu earthquake which badly damaged many building and bridges in Kobe, Japan on 17 January 1995³. In that earthquake, damage to reinforced concrete buildings was much more severe for buildings built before the current Japanese seismic code came into effect in 1981. Most buildings built after 1981 suffered only minor damage.

The structural deficiencies of many existing building and bridge structures designed to early standards in New Zealand and in other countries are generally not just a result of inadequate strength. For example, the longitudinal reinforcement present in many existing reinforced concrete building structures results in a lateral load strength which approaches or exceeds that required by current standards for the earthquake resistance of structures. The poor structural response during severe earthquakes is normally due to a lack of a capacity design approach to ensure the formation of an appropriate mechanism of deformation and/or to poor detailing of reinforcement, which means that the available ductility of the structure may be inadequate to withstand the earthquake without collapse.

There has been increased activity in many countries in the seismic assessment of buildings and bridges, and in retrofitting where necessary to improve seismic performance. The decision to retrofit has normally been made by comparing the details of the as-built structure with the requirements of current seismic standards. The emphasis in these retrofit projects has been to bring structures up to near current standard requirements by the provision of additional strength and/or ductility. However, the evidence of tests and analysis of existing structures, and of observed earthquake damage, is that not all structures designed before the current generation of standards will respond poorly to severe earthquakes. For example, many existing structures have a lateral force strength greater than expected by the designer (overstrength) due to a number of reasons.

Overstrength

The most common reasons for structural "overstrength" of reinforced concrete structures are the actual steel and concrete with strengths higher than initially specified, member sizes and quantities of steel reinforcement being larger than necessary, the use of strength reduction or material factors in design, other load combinations requiring great strengths at some sections, and the participation of non-structural elements. The effects of overstrength are not always beneficial to structural behaviour. For example, the effects of possible overstrength of members need to be carefully considered in design to ensure that undesirable failure mechanisms (for example, due to brittle shear failures) do not occur as a result of a change of failure hierarchy. Also, the presence of non-structural elements (for example, brick infill) could lead to short column or soft storey failures.

Taking account of overstrength is a necessary part of the capacity design procedure used in New Zealand to ensure that the preferred mode of post-elastic deformation occurs during a severe earthquake and that the level of seismic design actions used is appropriate. To prevent soft storey failures (pancaking) and shear failures of moment resisting frames the shear forces in beams, and the bending moments, shear forces and axial forces in columns are amplified to take account of flexural overstrength at beam plastic hinges resulting from the effects of steel overstrength and other factors. Confinement of concrete, while desirable for ductility, can also cause a significant increase in the flexural strength of reinforced concrete columns, which needs to be accounted for when calculating the design column shear force, and when determining the length of the region of column to be confined, when plastic hinges occur in columns.

Seismic assessment to determine the earthquake risk associated with the stock of older building and bridge structures in New Zealand (generally pre-1970s) requires an agreed screening procedure, a more detailed assessment procedure for use when necessary, and a catalogue of available retrofit methods, for structures constructed of all materials. Vulnerable older buildings are not simply those constructed of unreinforced masonry. The severe earthquake damage occurring in Kobe, Japan in 1995 again emphasised the deficiencies of some of the pre-1970s buildings and bridges constructed of reinforced concrete or structural steel (see Kobe photos).

A study group of the New Zealand National Society for Earthquake Engineering, chaired by Mr David Brunsdon of Wellington, has been working with the Building Industry Authority to produce a document on seismic assessment. In June 1996 a draft document entitled "The Assessment and Improvement of the Structural Performance of Earthquake Risk Buildings" was produced⁴, which has been circulated for comment. The document gives background technical material for structures incorporating reinforced concrete or structural steel frames. More work is required to extend it to other types of structures and materials. In addition, the study group has been conducting benefit/cost analyses of high risk buildings and considering possible approaches to mitigate the risk. Section 46 of the Building Act currently provides a means for territorial authorities to require significant structural upgrading in situations involving the change of use of buildings. It is the writer's opinion that modifications to the Act are required to give territorial authorities the right to require structural upgrading in other cases when change of use is not involved, if found necessary by seismic assessment.

Responsible approach

Nevertheless, ideally, it should not require regulations to make building owners spend dollars on upgrading the seismic resistance of buildings found deficient. What drives the owner to retrofit should be the "responsible" approach; that is, concern for the safety of staff and clients working in and using the building, the value of the contents of the building, and the considerable disruption to the business and other activities normally conducted in the building as a result of earthquake damage. This "responsible" approach has led to the decision to structurally upgrade the Central Library building at the University of Canterbury in Christchurch this coming summer although no change of use of the building is involved. Other pre-1970s buildings at the University of Canterbury have also been assessed recently and where necessary will be structurally upgraded in the near future. The "responsible approach" is widely accepted and used in California. Can we convince our New Zealand building owners that the financial cost of upgrading when necessary is worthwhile? The cost may be considerable. For example, according to a study commissioned by the Christchurch Heritage Trust it is estimated that strengthening the 155 most important heritage buildings in Christchurch to 66 percent of the level required for new buildings would cost about $74 million. However, ignoring the presence of structurally deficient buildings in our cities is surely a recipe for disaster, as was the case for the unsuspecting city of Kobe, Japan in 1995.

Existing bridge structures also require consideration. It is known that approximately 80 percent of New Zealand's existing state highway bridges were constructed before 1970, and many of those bridges may be vulnerable to earthquake attack due to lack of ductility of bridge piers and/or lack of linkages to hold the bridge spans on to the supports. There is a need to establish a study group in New Zealand to write a manual outlining agreed procedures for bridge seismic assessment and retrofit. A number of issues need resolution. For example, how to score various bridge deficiencies in a preliminary assessment procedure needs further discussion. Transit New Zealand has recently published two research reports on the seismic evaluation and retrofit of bridges, one written by Works Consultancy Services Ltd⁵ and the other written by Dr J Maffei as part of his PhD thesis work at the University of Canterbury⁶. Some difference in viewpoints exist. The information in these reports, and the great deal of experience of bridge seismic assessment and retrofit now available in California and Japan, could be drawn on by a study group and a bridge seismic assessment and retrofit manual written. Transit New Zealand needs to give urgent priority to the seismic assessment of the whole of New Zealand's pre-1970s bridge stock. The retrofit task ahead may be large. In 1986, a survey of Japan's bridges found that 30 percent required some form of retrofitting; in California it was found that as at May 1994 24 percent of the state highway bridges were vulnerable to collapse⁶. The current seismic upgrading of the Thorndon Overbridge in Wellington is welcome recognition of the deficiencies of older design standards but more work remains to be done in the future.

References

1. J P Hollings, "Reinforced Concrete Seismic Design", Bulletin of New Zealand National Society for Earthquake Engineering, Vol. 2, No. 3, 1969, pp 217-250.

2. R Park and T Paulay, "Reinforced Concrete Structures", John Wiley and Sons, New York, 1975, pp 769.

3. R Park, I J Billings, G C Clifton, J Cousins, A Filiatrault, D N Jennings, L C P Jones, N D Perin, S L Rooney, J Sinclair, D D Spurr, H Tanaka and G Walker, "The Hyogo-ken Nanbu Earthquake (The Great Hanshin Earthquake) of 17 January 1995. Report of the NZNSEE Reconnaissance Team", Bulletin of the New Zealand National Society for Earthquake Engineering, Vol. 28, No. 1, 1995, pp 1-98.

4. New Zealand National Society for Earthquake Engineering, "The Assessment and Improvement of the Structural Performance of Earthquake Risk Buildings", Draft prepared for the Building Industry Authority, Wellington, June 1996.

5. Works Consultancy Services Ltd, "Seismic Assessment of New Zealand Highway Bridges: Development and Testing of Preliminary Screening Procedures", Transit New Zealand Research Report No. 58, 1996.

6. J Maffei, "Seismic Evaluation and Retrofit Technology for Bridges", Transfund New Zealand Research Report No. 77, 1997.