Approaches

Introduction

Having considered the basic principles of seismic resistance in Chapter

2, we now step back and take a wider perspective to examine the current philosophy of seismic design. This chapter begins with a brief historical overview of earthquake resistant design, outlining some of the key developments directly relevant to the seismic design of buildings. This is followed by a review of the philosophy of seismic design as generally adopted internationally. Several important architectural implications are briefly noted before a concluding discussion on ductility.

Ductility, one of the principal concepts of contemporary seismic resistant design practice, was introduced in Chapter 2. Its application for each of the main structural seismic resisting systems - structural walls, cross-braced frames and moment frames - is described in Chapter 4. However, before that detailed examination of ductility this chapter explains in general terms how architects and structural engineers achieve ductile structures; that is, structures that endure earthquake shaking strong enough to exceed their strength without collapsing. A final example illustrates the steps involved in designing a simple ductile reinforced concrete structure.

Historical overview

Claims of earthquake awareness informing the design and construction of vernacular architecture in long-established communities are not uncommon. But it is unusual for a specific example from such a tradition to stimulate contemporary seismic innovation. A reviewer of the Renzo Piano Building Workshop's new Hermès building in the seismically-prone city of Tokyo explains: 'At 50 m tall and with a main structural span of only 3.8 m, the unusual slenderness of the structure results in high overturning moments during an earthquake and high levels of tension in the columns. The engineer, Ove Arup & Partners, found inspiration in the tall, thin wooden Buddhist pagodas of Japan.

Records show that in the past 1400 years, only two have collapsed -believed to be because the columns are discontinuous from floor to floor' .' In the Hermès building a similar principle was adopted by allowing columns to lift off the foundations during a quake and activate energy-absorbing dampers in the process.

In spite of examples of admirable seismic performance being cited from antiquity, especially where wood construction was employed, the history of earthquake resistant design based upon modern scientific methods is extremely short. While the wonders of Egyptian architecture date from 3000 BC, five thousand years ago, the beginnings of modern earthquake engineering practice emerged from the earthquake devastation of two cities at the turn of the twentieth century, only one hundred years ago. The 1906 San Francisco, California earthquake and the 1908 earthquake in Reggio and Messina, Italy in particular were pivotal in stimulating scientific enquiries that have led to what has now become modern earthquake engineering.2

Aboriginal Shelters Australia

▲ 3.1 Damage to the city of Messina from the 1908 earthquake.

(Reproduced with permission from Arturo Tocchetti; copyright Russell Klghtley).

Some earthquake resistant features had been introduced into buildings prior to the 1906 San Francisco event,3 but the main advances immediately following it were in the field of seismology including the development of instruments to record ground shaking. The 100,000 plus fatalities in southern Italy (Fig. 3.1) and the fact that some buildings survived the quake spurred on engineers who for the first time developed a method of designing buildings based upon Newton's Second Law of Motion described in Chapter 2.

After the 1923 Kanto earthquake in Japan and its estimated 140,000 fatalities most of whom were killed by post-earthquake fires, some successful applications of earthquake engineering focused the international spotlight on Japanese developments. These influenced researchers and designers in the USA whose efforts were further stimulated by the 1925 Santa Barbara, California earthquake. The iterative yet progressive nature of earthquake design developments continued. Field measurement of earthquake shaking and damage observation and its analysis was followed by laboratory testing. Understanding subsequently deepened by mathematical (now computer) modelling led to code modifications which were reviewed in the light of more recent earthquake data, and so the step-by-step refinement of seismic design practice continued.

Bertero and Bozorgnia note that as a consequence of the Santa Barbara event 'with the cooperation of many engineers and architects, the Pacific Coast Building Officials Conference adopted the Uniform Building Code (UBC) ' .4 This 1927 Code included the first seismic design guidelines in the USA, but the first enforced seismic code was the 1933 Los Angeles City Code. Some ten years later its provisions acknowledged the importance of building flexibility, but it was not until 1952 that the natural period of vibration, the primary dynamic characteristic of a building that is taken for granted today, was explicitly incorporated into a code.

Up until the mid-1950s, structural engineers focused their attention upon providing buildings with sufficient strength and stiffness to meet code defined force levels. Their understanding and application of relatively new earthquake engineering technology had yet to grapple with what is now considered to be a crucial issue; namely, what happens to a structure if its inertia forces exceed those for which it has been designed? The concept of ductility was first codified in a general form by the Structural Engineers Association of California in 1959, but it wasn't until approximately ten years later that several New Zealand structural engineers proposed a method whereby ductile structures of any material could be reliably designed and built.5

This method, known as Capacity Design is a design approach that is explained fully in the following section. Briefly, it imposes a hierarchy of damage upon a structure so even when inertia forces exceed design values, damage is concentrated in less vital sacrificial members. Other members more critical to the survival of a building - like columns -suffer little or no damage. Once columns are damaged they may not be able to support the weight of construction above and, as so often occurs during damaging quakes, buildings collapse.

Capacity Design to a greater or lesser extent is now well established in the world's leading seismic design codes. It is an important component of state-of-the-art seismic design practice. Although rigorously developed through extensive laboratory tests and computer simulations, it is sobering to reflect that at the time of writing Capacity Designed buildings have yet to be put to the ultimate test - an earthquake at least as strong if not stronger than a design-level earthquake. While Capacity Design offers a design methodology capable of preventing building collapse, an awareness of the need to protect the fabric and contents of buildings from earthquake damage is growing. Whereas former codes concentrated on saving lives, now a more holistic appreciation of the seismic performance of buildings - including reducing non-structural damage and post-earthquake disruption -is gaining greater emphasis in codes.

Unfortunately, Capacity Design was not incorporated into building designs until the early 1970s in New Zealand and later in other countries. Therefore, the vast majority of buildings in all cities pre-date the current codes that attempt to achieve buildings with dependable structural ductility. This large proportion of the building stock is therefore vulnerable to serious brittle damage, as observed in the 1994 Northridge, California and the 1995 Kobe, Japan earthquakes and more recent earthquakes elsewhere (Figs. 3.2 and 3.3). This explains why the subject of retrofitting or improving the earthquake performance of existing buildings, as discussed in Chapter 12, is so relevant.

Some readers may have noticed that there is almost no mention of architects cited throughout the short history above. Did architects contribute to those developments? Apparently not, according to the histories that focused upon the seismological and engineering advances of those times. Certainly, architects were involved to some degree; for example, members of code review committees. But it was probably the 1971 San Fernando, California earthquake that highlighted, albeit rather negatively, the importance of the architect in achieving good seismic performance. In the quake's aftermath the infamous failures at the newly commissioned Olive View Hospital attracted widespread attention (Fig. 3.4). Strong non-structural elements in the form of masonry infill walls precipitated the collapse of several elevator towers and were responsible for serious structural damage to the main block, necessitating its demolition. Now the

▲ 3.2 Partial collapse of a car parking garage, Los Angeles. 1994 Northridge, California earthquake.

(Reproduced with permission from A.B. King)

▲ 3.3 Brittle structural damage, Kobe. 1996 Kobe, Japan earthquake.

(Reproduced with permission from Adam Crewe).

▲ 3.2 Partial collapse of a car parking garage, Los Angeles. 1994 Northridge, California earthquake.

(Reproduced with permission from A.B. King)

▲ 3.3 Brittle structural damage, Kobe. 1996 Kobe, Japan earthquake.

(Reproduced with permission from Adam Crewe).

▲ 3.4 Damage to the Olive View hospital. 1971 San Fernando, California earthquake.

(Reproduced with permission from Bertero, V. V. Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

▲ 3.4 Damage to the Olive View hospital. 1971 San Fernando, California earthquake.

(Reproduced with permission from Bertero, V. V. Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

architect's role in contributing to sound seismic building configuration is widely recognized. This crucial aspect of seismic resistant design receives detailed treatment by Christopher Arnold and Robert Reitherman in their classic book,6 in other publications authored by Christopher Arnold and others, and is a re-occurring theme of this book.

But let us return to the previous question regarding architects' roles in advancing the practice of earthquake-resistant design. It appears that their input is not to be found in contributions to particular technical developments but rather in their eagerness to adopt new structural forms, especially moment frames. Architects from the Chicago School at the end of the nineteenth century were quick to escape the architectural restrictions imposed by load-bearing masonry walls. They embraced iron and then steel rigid framing. Frames not only offered greater planning freedom epitomized by the ' free-plan' concept, but also extensive fenestration and its accompanying ingress of natural light. Although rigid frames have functioned as primary structural systems in buildings for over a hundred years, as mentioned previously, only those built after the mid-1970s comply with strict ductility provisions and, therefore, can be expected to survive strong shaking without severe structural damage.

Structural engineers sometimes struggle to keep pace with architects' structural expectations. Consider Le Corbusier's influential 1915 sketch of the Domino House, a model of simplicity and openness (Fig. 3.5). While structurally adequate in seismically benign

▲ 3.5 Reinforced concrete structure of the Dom-Ino House, Le Corbusier, 1915.

environments, its structural configuration is inappropriate for seismic resistance. Its columns are too weak to cantilever two storeys high from the foundations, an absence of beams prevents reliable moment frame action, and the reinforced concrete stair induces in-plan torsion. Further seismic weaknesses are introduced by masonry infill walls. Sadly, these inherent seismic deficiencies have been unwittingly ignored by structural engineers who have assisted architects to achieve Le Corbusier-inspired design concepts in seismi-cally active regions. Reflecting upon the destruction of the El Asnam modern concrete buildings during the 1980 Algerian earthquake, architect Marcy Li Wang points out that every one of Le Corbusier's 'five points of a new architecture' that have been widely embraced by architects worldwide, leads to seismic deficiencies (Fig. 3.6).7 'While "the five points" have set generations of architectural hearts beating faster, they have more sinister overtones for structural engineers and other seismic specialists who would recognize pilotis as ' soft stories ' that have been the failing point of dozens of modern buildings in earthquakes all over the world.'8 Architects need to understand how building configuration affects seismic performance. It is unrealistic to expect that engineers can somehow design poorly architecturally configured buildings to perform well in moderate to severe earthquakes.

Current seismic design philosophy

One of the themes emerging from a history of seismic resistant design is that of international collaboration. The following review of the current philosophy of seismic design acknowledges that theme as it draws upon the earthquake provisions of each of the four countries or regions that at various times have provided leadership in the development of modern codes. Relevant aspects of codes from Japan,9 Europe,10 USA11 and New Zealand12 are referenced to offer an international perspective.

Although there are many points of detail on which the codes differ, taken together they present a reasonably united philosophy to outwit quakes. Readers are encouraged to check how the following points align with those of their own earthquake code. Since structural engineers are the intended readership of codes, these codes are not particularly

Villa Savoye Perspective
▲ 3.6 Villa Savoye, Poissy, Paris, Le Corbusier, 1929. This villa incorporates all of Le Corbusier's 'five points' of architecture. A combination of piloti and irregularly placed concrete block walls resist horizontal forces.

accessible to other professionals. Hence, readers will appreciate structural engineering assistance with code way-finding, interpretation and the answering of questions related to their local situations.

One general comment at this point about all codes is that they proscribe minimum standards. As discussed later, particularly in Chapter 13, there are some projects where architects recommend to clients that higher than the minimum standards of seismic resistance be adopted.

Design-level earthquakes

Seismic resistant design is intended to achieve two objectives:

• Protect human lives, and

• Limit building damage.

The first objective is achieved primarily by the provision of adequate strength and ductility. This ensures that a building is protected from full or partial collapse during large earthquakes that occur infrequently. The second objective limits building damage during lesser, more frequently occurring earthquakes, in order to minimize economic losses including loss of building functionality.

A code design-level earthquake is defined as one with an average reoccurrence interval of approximately 500 years. A building with a design life of 50 years has therefore approximately a 10 per cent chance of experiencing a design-level earthquake with that return period (refer to Poisson's equation in Chapter 1). Codes specify the intensity of design accelerations appropriate for that magnitude of earthquake through their response spectra (see Fig. 2.9(b)). Collapse is to be avoided during the design-level earthquake but considerable structural and non-structural damage is usually considered acceptable. Those lengths of structural members that have functioned as ductile structural fuses may be badly damaged but due to careful detailing they are expected to maintain most of their original strength and be repairable. The maximum permissible horizontal deflections of buildings during the duration of strong shaking are limited by codes to control damage and prevent overall instability. But there is still a risk that an especially strong pulse might cause permanent deformations that are so large in some buildings as to require their demolition.

The situation described above can be restated as follows: over a fifty-year period that could approximate its design life, a building has a 10 per cent chance of experiencing the design-level or a larger earthquake. The intensity of shaking depends upon both the regional seismic zone in which the building is located and the underlying ground conditions. During the design-level earthquake the structure and entire fabric of the building, including its contents, will almost certainly be seriously damaged. Lives will not be lost but post-earthquake entry may be prohibited by civil defence personnel and the building may require demolition. This is the rather depressing reality of the scenario where a building designed to code requirements has ' survived' the design-level earthquake. Current design approaches aim to prevent collapse but not damage during such a large event. No wonder some clients request enhanced performance (Chapter 13) and researchers are busy investigating ductile but damage-free structural systems (Chapter 14).

Although structural and building fabric damage is not required to be prevented during a large earthquake, designers are obliged to avoid damage during small earthquakes that occur relatively frequently (Fig. 3.7). A second and smaller design earthquake with a return period in the range of 25 to 50 years is the maximum event for which damage is to be avoided. These short return period events represent an 86 per cent and 63 per cent likelihood of occurrence, respectively, during an assumed fifty-year design life of a building. Since building damage correlates strongly with the amount of horizontal deflection in any storey, or interstorey drift, codes limit the maximum deflections during frequently occurring earthquakes. Apart from unrestrained building contents that may be damaged no structural or non-structural building elements are expected to require repair.

The previous two paragraphs apply to typical buildings in a community, like those accommodating apartments, shops or offices. But what about especially important buildings like schools, hospitals, and fire stations? Since societies expect them to perform better, the strengths of these facilities are increased beyond that of less critical buildings by factors of up to 1.8. This strength enhancement enables them to survive the code design-level earthquake with significantly less damage than that incurred by a typical building. Damage limitation requirements for these important or critical facilities are also more rigorous. Full building functionality is expected immediately following an earthquake with a return period far in excess of 50 years.

▲ 3.7 Plywood sheets cover broken windows after the small 2001 Nisqually Earthquake, near Seattle.

(Reproduced with permission from Graeme Beattie).

▲ 3.7 Plywood sheets cover broken windows after the small 2001 Nisqually Earthquake, near Seattle.

(Reproduced with permission from Graeme Beattie).

Ductility

During the initial seconds of a design-level earthquake, when a structure vibrates elastically with normal amounts of damping, the consequent inertia forces become large. As mentioned in Chapter 2, because earthquake loading is cyclic and a structure usually possesses some ductility, structural engineers reduce the design-level seismic forces to well below those that would occur if the structure were to continue to remain elastic. Ductility is a measure of how far a structure can safely displace horizontally after its first element has been overstressed to the extent its steel yields or fibres, in the case of wood structures, begin to rupture (Fig. 3.8). The degree of ductility indicates the extent to which earthquake energy is absorbed by the structure that would otherwise cause it to continue to resonate. Those areas of structures designed to absorb or dissipate energy by steel yielding are called structural fuses or plastic hinges.

Gravity force

Gravity force

Deflection

Plastic hinge region enlarged below

Model of column

Deflection

Plastic hinge region enlarged below

Force

Yield strength

Column loses strength at its maximum deflection or failure point

Ductility

Model of column

Ay Au Deflection

Idealised graph of force against deflection

1 Fine bending moment cracking. Vertical tension reinforcement is still in the elastic range.

Vertical tension steel

Cracks widen, tension steel starts to yield.

Cracks even wider, steel yielding and cover concrete spalled off. Typical damage in a plastic hinge.

1 Fine bending moment cracking. Vertical tension reinforcement is still in the elastic range.

Cracks widen, tension steel starts to yield.

Cracks even wider, steel yielding and cover concrete spalled off. Typical damage in a plastic hinge.

▲ 3.8 A vertical reinforced concrete cantilever column subject to a horizontal force. The force-deflection graph and the damage states reflect the ductility of the structural fuse or plastic hinge region.

Codes allow designers to reduce the inertia forces likely to occur in a design-level earthquake in proportion to the ductility a given structural system might possess. A huge reduction in design force is allowed for high-ductility structures. Due to such low design forces that are as little as one-sixth of the design force for a brittle structure and the consequent low structural member strengths required, relatively shallow beams and slender columns are achievable. But the disadvantages of a high-ductility design include the creation of a more flexible structure that will sustain more structural and non-structural damage than a stronger and stiffer alternative. Smaller members usually result in lower construction costs. But those savings are somewhat offset by the special detailing required in the structural fuse regions of ductile structures to allow yielding and plastic action without excessive damage or loss of strength.

Fire protection with separated infill wall

Frame resists x direction forces

Boundary wall acts as a structural wall to resist y direction forces

Frame resists x direction forces

I

Boundary line

Street frontage

Boundary line

Plan

▲ 3.9 Plan of a building whose structural walls on the boundary provide excess seismic strength in the y direction. Frames resist x direction forces.

Alternatively, designers can opt for a low-ductility structure. Due to its lesser ductility its design forces are higher. Consequently, structural members have to be stronger and larger, with obvious architectural implications. Its increased structural footprint, with perhaps deeper columns or longer walls, may now not integrate well with the desired internal layout.13 While structural dimensions are larger than those of a high ductility structure, the advantages to a client are a stiffer and stronger building with less damage expected to non-structural and structural elements. Also less sophisticated and costly structural detailing is necessary at locations likely to suffer damage. They are expected to be far less severely damaged in the design-level earthquake compared with similar regions within a high-ductility structure.

Some codes permit a structure to remain elastic through the design-level earthquake. This is a sound option if, other than for structural reasons, a larger than the required amount of structure is readily available to resist seismic forces. This strategy is often employed where two boundary walls provide fire resistance (Fig. 3.9). The walls are probably far stronger than needed for forces acting parallel to their lengths and, therefore, can resist elastic response force levels that are not reduced by ductility considerations. In this situation, structural detailing can be kept at its most simple and cost-effective, although structural engineers must consider the consequences of an earthquake whose intensity exceeds that of the design-level earthquake.

Seismic Forces Architecture
▲ 3.10 Single-storey building with x direction horizontal forces resisted by cantilever columns. Shear walls resist y direction forces.

Maximum bending moment

Maximum bending moment

Inertia force H Column Column acting at the shear force bending top of a column diagram moment diagram

▲ 3.11 The inertia force deflects the column and produces column shear force and bending moment diagrams (The bending moment is drawn on the compression-side of the column rather than the tensionside as is customary in some countries).

Capacity Design

It is relatively easy for structural engineers to increase the structural strength of a member, or even that of a complete structural system. Additional reinforcing bars can usually be added to a reinforced concrete member. In steel or timber construction, substitution of a larger cross-sectional area increases member strength. Unfortunately, it is nowhere near as easy to increase ductility even in a steel structure. So, how are ductile structures designed? How can structural collapse be prevented if earthquake shaking exceeds the strength of the design-level earthquake?

A ductile structure is designed using the Capacity Design approach. This involves the following three steps:

• Choose how the structure is to deflect in a seismic overload situation so that the structure is able to absorb sufficient earthquake energy before it deflects to its limit.

• Provide a hierarchy of strength between and within structural members to allow structural fuses or plastic action only in non-critical members and to prevent brittle failure occurring anywhere, and finally,

• Detail structural areas that are intended to act as fuses so they avoid severe damage and excessive loss of stiffness and strength.

To understand the application of Capacity Design, consider a simple single-storey building (Fig. 3.10). Assume simply-supported steel or timber trusses span between the pairs of reinforced concrete columns. The x direction seismic forces on the building are resisted by columns functioning as vertical cantilevers. Two concrete shear walls, not considered further in this example, resist y direction seismic forces. The seismic force acting on a column and its internal structural actions such as bending moments and shear forces are shown in Fig. 3.11.

Before applying the Capacity Design approach, we pause to explore the following question: If the earthquake force exceeds the strength of the column how will the column be damaged? The correct answer depends on how the column is reinforced and the strength of its foundations (Fig. 3.12).

Gravity load

Gravity load

Shear Failure Tower

(a) Shear failure

(b) Over-reinforced failure

(c) Foundation failure

(d) Ductile yielding of reinforcement and spalling of cover concrete in plastic hinge region

(a) Shear failure

(b) Over-reinforced failure

(c) Foundation failure

(d) Ductile yielding of reinforcement and spalling of cover concrete in plastic hinge region

▲ 3.12 Four potential types of failure for a cantilever column. The only ductile and desirable overload mechanism (d) occurs if the other three are suppressed by making them stronger than the force to cause (d).

The first and most likely possibility is that the column will suffer shear failure (Fig. 3.12(a)). If the shear strength of the column is less than the strength at which any of the other damage modes occur, a sudden brittle diagonal shear crack forms. The column almost snaps in two. The crack creates an inclined sliding plane which greatly reduces the ability of the column to support vertical loads. Shear failures are often observed in quake-damaged columns not designed in accordance with the Capacity Design approach (Fig. 3.13).

A second and less likely scenario is where excessive vertical reinforcing steel is placed in the column. It is 'over reinforced ' . Provided no other type of damage occurs beforehand, as the bending moment at the base of the column increases due to increasing seismic force and thus stretches the left-hand side vertical reinforcing steel in tension, compression stress builds up within the concrete on the other side. Because of the large amount of vertical steel acting in tension the concrete under compression finds itself the weaker element. As often demonstrated to classes of civil engineering students the over-stressed concrete suddenly and explosively bursts and the column falls over (Fig. 3.12(b)).

Beam Concrete Steel

▲ 3.13 Brittle reinforced concrete column shear failure. Los Angeles, 1994 Northridge earthquake.

(Reproduced with permission from A.B. King).

▲ 3.13 Brittle reinforced concrete column shear failure. Los Angeles, 1994 Northridge earthquake.

(Reproduced with permission from A.B. King).

The foundation system may also sustain possible damage or collapse. Either the foundation soil might be too weak to support the combined vertical and horizontal stress under the column footing or the footing might overturn because it is undersized (Fig. 3.12(c)) . Both of these mechanisms lead to severe building damage.

The final type of damage is shown in Fig. 3.12(d). Once again, assume no other prior damage has occurred to the column. As the inertia force at its top increases, the bending moment at the column base causes cracks in the concrete and increases the tension stress in the vertical reinforcing steel until it begins to yield, or in other words enters the plastic range. The maximum horizontal force the column can sustain has been reached. No additional force can be resisted so if the inertia force continues to act on the column, cracks grow wider as the reinforcing steel yields plastically. The area at the base of the column with its wide cracks and where the steel has been strained plastically is a structural fuse region, often called a plastic hinge zone. Even though the vertical steel has stretched plastically in tension it is still strong. Due to the high compression stress from the bending moment, as well as the gravity load on the column, the cover concrete spalls from the compression side. This damage is not serious. The column is only slightly weaker and damage to cover concrete can be repaired quite easily. Of the four types of damage this is the only one that can be described as ductile.

We now return to the three steps of Capacity Design. Since we have identified a ductile overload damage mechanism, namely ductile bending deformation at the column base, we select it as the desired mode of damage. Then by applying the remaining two design steps described below, we ensure that that type of damage and that alone occurs in the design-level event. Tensile yielding of the column vertical steel absorbing earthquake energy is the source of ductility.

The next Capacity Design step involves providing a hierarchy of strength. All undesirable types of damage such as shear and foundation failure must be prevented. So, disregarding the value of design force acting on the column, we calculate the bending strength of the column using the actual area of vertical steel provided. This bending strength is usually greater than the design bending moment because of the need to round-up the number of reinforcing bars to a whole number during the design process. Then the actual bending strength is increased by a small factor of safety to determine the maximum possible bending strength. This acknowledges that reinforcing steel and concrete are usually stronger than their minimum specified strengths.

Once the maximum possible bending strength at the column base is calculated, the damage modes of Fig. 3.12(a) and (c) can be prevented by ensuring they occur at a higher level of seismic force. Column ties of sufficient diameter, and close enough vertical spacing between ties, completely prevent shear failure prior to the maximum possible bending strength occurring. The column footing is also dimensioned using the maximum possible bending strength of the column. Overturning and foundation soil failure is therefore prevented. Finally, the structural engineer checks if the column section is 'over reinforced ' . If so, the column depth is increased and the amount of reinforcing steel reduced.

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Additional ties in plastic hinge region

Cross-section near column base

Additional ties in plastic hinge region

Cross-section near column base

Longitudinal reinforcement

Reduced tie spacing in plastic hinge region

Elevation showing reinforcing steel

▲ 3.14 Additional reinforcing steel to confine the concrete in the fuse or plastic hinge region of a column.

The final step of the design process involves detailing the structural area designated as the fuse region. In this example it means (1) adding extra horizontal reinforcing ties to stop the main vertical bars from buckling in compression after the cover concrete has spalled and (2) decreasing the vertical spacing of the ties so as to confine the concrete in the fuse or plastic hinge region more firmly. It is like applying bandages to stabilize the reinforcing bars and confine the cracked concrete so it doesn't fall out from the column core (Fig. 3.14). Figure 3.15 shows an example of a plastic hinge in a column where spiral reinforcement has been effective in confining the concrete.

Once all of the Capacity Design steps are completed construction commences and the structure is ready to be put to the test. If, for architectural or other reasons, high ductility was assumed in order to achieve slender columns, inertia forces from a moderate to severe earthquake will definitely exceed the design bending strength of the columns. Plastic hinges will form, accompanied by concrete cracking and spalling of cover concrete. Because all undesirable brittle types of failure are prevented by virtue of being stronger than the ductile mechanism, ductile behaviour is ensured. The column definitely suffers damage but is repairable. Any loose concrete is removed and fresh concrete cast. Careful inspection will reveal whether or not cracks need epoxy grouting. Fallen cover concrete is reinstated with cement plaster. The column is ready for the next quake!

The preferred ductile mechanisms and the architectural implications of structural systems other than the vertical cantilever columns considered above such as structural walls, cross-braced frames and moment frames are discussed in the following chapter.

▲ 3.15 A column plastic hinge. Closely-spaced spiral ties confine the column core. Cover concrete has spalled off. Mexico City, 1985 Mexico earthquake.

(Reproduced with permission from R.B. Shephard).

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