Design

The extremes found in all sectors of developing countries are manifest in the quality of the engineering design and the construction of buildings. Particularly in major cities, some engineering consultants practice high levels of seismic expertise. But overall, the standard is low and in most projects seismic engineering input is non-existent. And as noted previously, engineering input is no guarantee of satisfactory seismic performance. Apart from a structural engineer's personal competence in seismic design, which may be dubious given a lack of seismic design content in schools of engineering curricula, other issues that can reduce the seismic resilience of buildings include code requirements, configuration irregularities, design detailing and quality control of construction.

In many developing countries, code design force levels are low by international standards. After investigating the 2003 Boumerdes, Algeria earthquake, Fouad Bendimerad comments, 'While the Algerian earthquake code prescribed design values for buildings in the order of 15 per cent gravity, there is evidence that in the epicentral region both the horizontal and vertical accelerations from the earthquake exceeded 100 per cent of gravity' , 5 That some affected interest groups in the building industry, such as building developers and others concerned about building affordability, resist seismic design forces being increased beyond those specified in existing out-dated codes, is understandable. But the consequences of low design forces are weak buildings. To survive a damaging quake, such buildings require an unreal-istically high level of ductility. Designers should be aware of the return period of the design-level earthquake of their own code. Only then can they advise their clients of the seismic risk to a building during its anticipated design life. As discussed in Chapter I3, some clients are willing to pay extra for improved seismic protection.

A review of recent earthquake damage to buildings in developing countries has found that the majority of seismic vulnerability of engineered buildings arise from configuration irregulari-ties.6 This finding aligns with that of a Venezuelan study which, while acknowledging the existence of a modern seismic code, concludes that 'significant conceptual errors in the design of the lateral force-resisting systems of new buildings are recurring on a near-universal level, often as a result of ignoring the potential adverse effects of nonstructural elements on the structural sys-tem'.7 The study's list of primary deficiencies include those discussed in Chapters 8 and 9; namely, soft and weak storeys, short columns, strong beam-weak columns and torsion (Fig. 16.4).

Masonry infill walls that are not separated from moment frames are usually the main cause of each of these configuration problems. The only way to overcome them is by introducing the techniques of Chapter I0 which, for most countries, represent new building practices. The two most practical solutions for developing countries to improve the seismic performance of their new buildings are to use reinforced concrete shear walls to resist seismic forces and to adopt confined masonry construction. Unfortunately, both solutions reduce ground floor openness and transparency and inevitably entail greater construction cost when designed and built properly. Several leading structural engineers with developing country experience are of the view that reinforced concrete shear walls possess significant advantages over moment frames from a seismic perspective.5, 8 The historic seismic performance record of shear walls is far better and, due to their less sophisticated design, detailing and construction, they are more dependable. Alternatively, a confined masonry structural system, as outlined in Chapter 5, can be adopted. But its structural capabilities restrict it to low- to medium-rise construction only and necessitate rather rigorous limitations on layout, openings and structural footprint.

The provision of more rational seismic structural systems is another area where developing countries need to consider a new approach to how they build. More regular column orientation in-plan (see Fig. 5.34) and realistic column dimensions are required. Moment frames

▲ 16.4 Soft storey building, Venezuela.

(Reproduced with permission from Wiss, Janney, Elstner Associates, Inc.).

▲ 16.4 Soft storey building, Venezuela.

(Reproduced with permission from Wiss, Janney, Elstner Associates, Inc.).

▲ 16.5 Slender columns can not provide adequate seismic resistance. Mumbai.

(Reproduced with permission from R. Sinha).

▲ 16.5 Slender columns can not provide adequate seismic resistance. Mumbai.

(Reproduced with permission from R. Sinha).

▲ 16.6 Inadequate welds caused three brace failures. Bam, 2003 Bam, Iran earthquake.

(Reproduced with permission from Jitendra Bothara).

▲ 16.6 Inadequate welds caused three brace failures. Bam, 2003 Bam, Iran earthquake.

(Reproduced with permission from Jitendra Bothara).

incorporating 'stick' columns, commonly around 230 mm square are too weak and flexible to function as seismic resisting elements. Such small cross-sectioned members should have no more expected of them than to resist gravity forces (Fig. 16.5). As illustrated in Fig. 5.44, moment frames need to be designed using the Capacity Design approach that results in columns stronger than beams - the so called 'weak beam-strong column' approach.

Designers need to be prepared for initial negative reactions to the introduction of sound earthquake-resistant practices. Clients who are used to columns fitting within the thickness of partition walls may be mildly shocked at the greater sizes of Capacity Designed columns required to ensure adequate seismic performance. Changes to traditional approaches, which are strongly embedded in a country's construction culture, require considerable justification and professional insistence.

An architect needs to choose a structural engineer for his or her project carefully. Both need to present a united front when challenging and changing traditional ways of building. The engineer must be amenable to discarding traditions that have proven seismically inadequate and to adopt new approaches. He or she should be able to demonstrate their personal technical competence, not only in conceptual seismic design, but also by how they deal with detailed design. Unlike many other structural design situations, structural performance under seismic forces is very sensitive to the quality of detailing.9 In reinforced concrete design, reinforcing detailing is extremely important, just as welding details are crucial for steel construction (Fig. 16.6). As illustrated

▲ 16.7 Incorrectly bent column ties led to this column almost collapsing. 1987 Tarutung, Indonesia earthquake.

in Figs 12.5 and 16.7, such an apparently small detail as incorrectly bent column ties can lead to the collapse of an entire building. Too often, this and other reinforcing factors are detailed by draughtsmen not trained in seismic detailing and who do not appreciate the critical importance of their work.

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