Introduction

Imagine a reinforced concrete or steel building under construction. Visualize its structural framework, its beams, columns and floor slabs and its openness and emptiness. That is the building structure. Everything else yet to be constructed, all the remaining elements of construction and occupancy yet to be provided, fall into the category of non-structural elements.

Non-structural elements are, by definition, not intended to resist any seismic forces other than those resulting from their own mass. They are also, in the main, elements that structural engineers do not design and for which architects, and mechanical or electrical engineers take primary responsibility. The diverse types of non-structural elements can be divided into three groups:

• Architectural elements such as cladding panels, ceilings, glazing and partition walls

• Mechanical and electrical components like elevators, air conditioning equipment, boilers and plumbing, and

• Building contents, including bookcases, office equipment, refrigerators and everything else a building contains.

Non-structural elements, therefore, transform a structure into a habitable and functional building. The occupants, the fabric and contents of buildings and the activities undertaken in them are the life-blood of society. So it should come as no surprise that architects need to ensure, on behalf of their clients, that non-structural elements perform adequately during earthquakes. The two compelling reasons for taking the seismic performance of non-structural elements seriously are: firstly, the danger these elements pose to people both within and adjacent to the perimeter of a building, and secondly, the economic investment in buildings and enterprises occurring within them.

The value of non-structural elements expressed as a percentage of the total cost of a building, excluding the price of the land, depends upon the type of building considered. In an industrial or storage building with few mechanical services and architecturally designed elements, nonstructural costs can be in the 20 to 30 per cent range. In more heavily serviced and complex buildings non-structural elements can comprise up to 85 per cent of the total cost. In some cases, such as an art museum or a high-tech research or computer centre, the value of non-structural elements, especially the building contents, may well exceed the sum of all other costs associated with the building. It makes sense to protect this investment from earthquake damage. Non-structural elements have proven to be very vulnerable to seismic damage as evidenced by the 1994 Northridge, California earthquake. If a quarter of the most severely damaged of the 66,000 buildings surveyed are excluded, most of the remaining damage occurred to non-structural elements.1

In a seismic study of a 27-storey building in Los Angeles subject to the Maximum Credible Event, direct economic loss of non-structural elements exceeded by six times the cost of structural damage.2 This analysis excluded the cost of indirect losses of revenue and building use. For some buildings these less tangible but nonetheless real costs will greatly exceed the value of direct losses. While emergency services, hospitals and similar facilities need to be operational immediately after a damaging quake, more and more businesses are becoming aware of how important business continuance is to their financial viability in a post-earthquake environment.

During earthquake shaking, non-structural elements represent a significant hazard to people. Injuries are caused by building elements such as glazing or suspended ceilings shattering or collapsing, or by building contents being flung around. Filing cabinets and equipment overturn, containers of hazardous materials break open or gas from ruptured pipes ignite. Damage scenarios vary from building to building and room to room. Try to image what damage might occur if the room you are in now is suddenly shaken and its fabric such as partitions, windows and ceiling are damaged, and contents, including yourself, are flung about. However, it is very likely your risk of injury is far less than if you were in other more hazardous locations; like walking down a supermarket

Acceleration

Acceleration

Acceleration (ground level)

Acceleration (ground level)

Shaking

▲ 10.1 Shaking at ground level is amplified up the height of a building.

Deflected

Deflected

Shaking

▲ 10.2 Interstorey drifts from earthquake shaking.

Shaking

▲ 10.2 Interstorey drifts from earthquake shaking.

aisle between high storage racks or against a building with weakly attached cladding panels.

So what is the root cause of non-structural damage? Although the obvious reason is that a building is shaken by an earthquake, it is worth singling out two specific aspects of seismic shaking that explain observed damage; acceleration and interstorey drift. As explained in Chapter 2 and illustrated in Fig. 10.1, a building amplifies ground accelerations as it resonates in response to the dynamic movement at its base; the higher-up a building the greater the level of acceleration amplification. Horizontal accelerations induce inertia forces in non-structural elements that cause them to slide, overturn, break loose from their attachments to the main structure and both suffer and cause further damage. The two strategies for preventing such damage is: firstly, to ensure that non-structural elements themselves have enough strength to resist their own inertia forces; and secondly, to physically restrain the elements by attaching them to structural members. Chapter 11 considers these requirements in detail.

The second type of non-structural damage is caused by intersto-rey drift. As a building sways to-and-fro during a quake each floor drifts or deflects horizontally further than the floor below (Fig. 10.2). Interstorey drift or relative horizontal movement between floors can damage non-structural elements that connect to both floors. As discussed below, careful separation of non-structural elements from the structure avoids this damage.

While the damage scenarios above identify some of the safety hazards people face in the immediate presence of non-structural elements, this chapter also explores a less obvious but more serious situation. That is, the likelihood of non-structural elements damaging the primary structure of a building to such an extent as to cause partial collapse with ensuing risk of injury and huge economic losses. The two types of nonstructural elements most capable of causing global rather than local building damage, and may also be hazardous for people in their vicinity, are infill walls and staircases.

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