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Resilient buildingsFEATURESECTION

TWO OF THE MOST vital components of a resilient community are functional buildings and a functional infrastructure. With the application of good seismic design practice, resilient buildings that continue to function as intended after an earthquake can be achieved.

While most buildings in Christchurch performed up to Building Code expectations – especially for life safety – the empty areas of the CBD and eastern suburbs are a stark reminder that resilience was not achieved to the extent that most of us expected.

Lessons from CanterburyThe New Zealand Building Code is a world leader and a very good tool to use as a starting point. However, its sheer scope and complexity means that it is very easy to get bogged down in the detail and lose sight of the bigger picture.

Following extensive experience gained after the Canterbury earthquakes and

preparation of national seminars on the findings and lessons learnt, we have distilled the lessons into six main themes.

Site location and conditions crucialAs building industry practitioners, we may not have much influence over building siting, but sometimes there is an oppor-tunity at the prepurchase stage. There are several things to look out for:

● Low-lying coastal sites have potential for tsunami damage, particularly in vulner-able Pacific Ocean-facing areas and at the head of long tapering inlets. There are not many opportunities within the building sector for mitigation of this sort of hazard.

● Low-lying riverside sites, particularly adjacent to estuaries, can be vulnerable to liquefaction. The key drivers are a high water table and loose fine-grained soils. Unfortunately, many New Zealand settle-ments sprung up in these areas because of the historical reliance on sea transport.

● Hillside sites require extra care to achieve seismic resilience. Unless the land is poten-tially unstable, the engineering problems can usually be solved. However, usually the added complexity brings increased design fees and building costs.

● The LIM for the property may alert you to location-related issues, but it is usually worthwhile checking the territorial author-ity’s hazard maps.

Reduce complexityComplexity may be caused by plan and vertical irregularity – often unavoidable on hillside sites – with split floor levels and differing structural systems. During the design process, some of these features arise as the designers try hard to accommodate the client’s wishes.

What started out as a simple concept ends up as a complex structure, rapidly consuming design fees. While complex structures can be modelled relatively easily

Resilientbuilding design

The Canterbury earthquakes have firmly placed building resilience in the spotlight. As an industry, we did not perform as well as the public

expected – how can we do better in the future?

BY ROGER SHELTON, BRANZ SENIOR STRUCTURAL ENGINEER

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Resilient buildings FEATURESECTION

using today’s computer analysis software, two detrimental effects come into play:

● The structural designer loses the intuitive feel for the structure that will so often act as a warning that something is not right.

● The dynamic behaviour of the structure under the earthquake ground motion is different to what was assumed in the loading standard.

The result is that the performance of the structure under earthquake action is more difficult to predict with certainty than a simpler structure.

It is worth adding that complex buildings are also more likely to have a higher risk of weathertightness problems.

Reduce weightThe desirability of reducing weight is pretty well accepted by most in the building industry. Besides seismic vulnerability, other downsides include bigger foundations and higher construction costs. Seismic weight can

be compensated for by more structure, but as we have seen, this usually adds building costs for equivalent building resilience.

Another angle is to plan the building with heavy items as low as possible. For example, it may be possible to locate plant rooms low in the building, where they will also be easier to service.

Structural stiffness is goodWhile there is a perception that the struc-ture needs to flex in an earthquake to survive, this is a concept focusing solely on the prevention of collapse. Today, and more than ever after the Christchurch experience, there is an expectation that minimisation of damage to buildings is essential for seismic resilience.

The primary way to achieve this is to provide a structure with sufficient lateral stiffness that its deflections under seismic action will be low enough to avoid damage to non-structural components and finishes.

The key measure here is the relative horizontal deflection between storeys – engineers call this drift. If this is too great, brittle elements such as windows will shatter, service pipes will leak at joints, claddings may fall and, in the worst case, heavy elements such as stairs will slide off their supports with disastrous consequences.

There are clever ways to reduce structural flexibility, but usually the most effective way is more structure – more bracing walls, bigger columns and beams.

The alternative is to provide separation between structural and non-structural components, but this comes with the price of more costly detailing and can create problems in achieving weathertightness.

Care with connectionsConnections between structural elements and between structure and non-structural components are vital in achieving

Hazard map for Porirua highlights the local issues.

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Resilient buildingsFEATURESECTION

building resilience. Connections are clearly the province of the structural engineer, but other members of the team also need to be aware of the importance of robust connections.

Examples of connection failures are ceiling collapses, cladding panels falling onto pedes-trian ways and, in the extreme situation, floor slabs parting from shear walls. The latter is also an example of faulty diaphragm design, which is an area receiving much attention from the structural engineering community at present.

Providing connections with sufficient structural ductility is essential. The whole load path from member to member must be considered. Frequently, the steel connection itself is quite robust, but the connection to the substrate can be friable or brittle. Examples are splitting of timber members

at fasteners such as bolts and pull-out or break-out of bolts and fixings cast into concrete members. Allowing for movement caused by the effects of temperature and shrinkage make connection detailing all the more important.

Connections are a classic case where it is very true to say that the devil is in the details.

Robust building servicesFailure of non-structural components including building services is typically the primary reason for non-occupation of a building. As we have seen recently in Christchurch and Wellington, this may lead to costly business interruption.

Failures can range from the movement of a domestic hot water cylinder up to leakage of a water pipe in a multi-storey commercial building. Most failures of non-structural

components in earthquakes are a result of excessive building deflection and the lack of structural stiffness, but toppling or falling due to lack of anchorage is also common.

One of the practical difficulties is the sheer number of items to be considered and the complexities of the arrangements often needed to fit systems into leftover spaces, for example, ceiling spaces.

A well developed plan for coordinating the activities of the various parties involved – the architect, structural engineer, services engineer, HVAC contractor, ceiling contractor and sprinkler contractor at the very least – is the only way to ensure that each party’s requirements fit into the whole. An essential ingredient in this is ensuring that building models are shared at the design stage and that each team member is fully briefed before committing to pricing the job.

With planning, building services can survive earthquakes. Complex buildings present challenges.