THE STRUCTURAL DESIGN OF TALL AND SPECIAL BUILDINGS Struct. Design Tall Spec. Build. 19, 853–865 (2010) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/tal.676
AN OVERVIEW OF BUILDING CODES AND STANDARDS IN CHILE AT THE TIME OF THE 27 FEBRUARY 2010 OFFSHORE MAULE, CHILE EARTHQUAKE FABIAN ROJAS1, MARSHALL LEW2,* AND FARZAD NAEIM3 1
University of Southern California, Los Angeles, California USA and University of Chile, Santiago, Chile 2 MACTEC Engineering and Consulting Inc., Los Angeles, California USA 3 John A. Martin & Associates, Los Angeles, California USA
SUMMARY The 27 February 2010 offshore Maule, Chile earthquake is a significant event in which modern tall buildings were subjected to strong ground motions having long duration of strong shaking. Although there were few total collapses of buildings in Chile, there were some buildings with serious damage. An understanding of the building codes and standards in force in Chile at the time of the earthquake are a key in understanding the behaviour of tall buildings in this event and future events that are coming. Copyright © 2010 John Wiley & Sons, Ltd.
1.
INTRODUCTION
Chile is an advanced and progressive nation. It has a high literacy rate and education is valued. Chilean engineers have a high degree of education and many of the foremost educators are graduates of prestigious engineering programs in the USA and Europe. Therefore, Chilean engineers generally have a very good understanding of building materials and structural behaviour. Unlike Haiti, Chile has building codes and standards that are widely followed and enforced by national law. These building codes have been based on international standards and local experience and include seismic provisions. 2.
CHILEAN CODES AND STANDARDS
Building codes in Chile are issued by the Instituto Nacional de Normalización (National Institute of Normalization; INN). The INN is the organization in Chile in charge of studying and preparing national technical standards. The INN represents Chile as a member of the International Organization for Standardization and the Comision Panamericana de Normas Tecnicas. For structural design, the Chilean codes can be grouped into three main areas (Cruz, 2009): • Definitions of Loads and Actions • Material behaviour and strength, and detailing requirements • Seismic loading Table 1 provides a list of codes and standards currently adopted in Chile. The process by which the INN creates building codes is shown in Figure 1. * Correspondence to: Marshall Lew, MACTEC Engineering and Consulting, 5628 E. Slauson Ave., Los Angeles, California 90040, USA. E-mail:
[email protected]
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Table 1. Chile codes and standards (Cruz, 2009) Area Actions Materials and design
Earthquake
Number
Topic
Date
NCh431 NCh432 NCh1537 NCh427 NCh430 NCh1198 NCh1928 NCh2123 NCh433 NCh2369 NCh2745
Snow Loading Wind Loading Dead/Live Loads Design of Steel Design of Reinforced Concrete (ACI 318) Design of Wood Design of Reinforced Masonry Design of Confined Masonry Earthquake Resistant Design of Buildings Earthquake Resistant Design of Industrial Structures and Facilities Earthquake Resistant Design of Base Isolated Buildings
1977 1971 1986 1977 2008 2006 2003 2003 1996 2003 2003
Figure 1. Code adoption process in Chile
3. BASIS OF THE CHILEAN NCH433 SEISMIC PROVISIONS The seismic code provisions are contained in the Official Chilean Code NCh433.Of 96 on Earthquake Resistant Design of Buildings (INN, 1996). This code was approved by the INN and superseded the 1993 edition. The new code was developed and written by the working groups of the Asociación Chilena de Sismología e Ingeniería Antisísmica. This code was declared official by the Ministerio de Vivienda y Urbaniso and made effective by Presidential Decree no. 172 on 5 December 1996. This is a brief overview of the general seismic design provisions of Chile. Section 5 of NCh433 states the basic principles and objectives of the building code provisions. Section 5.1.1 states ‘. . . this code applied together with the specific design code for each of the materials named in paragraph 5.3, aims to produce structures that: (a) Resist moderate intensity seismic actions without damages; (b) Limit damage to non-structural elements during earthquakes of regular intensity; (c) Prevent collapse during earthquakes of exceptionally severe intensity, even though they show some damage.’ Copyright © 2010 John Wiley & Sons, Ltd.
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NCh433 also states that ‘. . . the compliance with the provisions of this code does not guarantee, in every case, that the aforementioned objectives will be achieved.’ It is also important to note that the following statement is also included in this section: ‘In particular, the provisions for reinforced concrete wall buildings are based on their satisfactory behaviour during the earthquake of March, 1985. The design of those buildings was performed in accordance with the NCh433.Of72 code.’ Thus, the basis of the Chilean seismic provisions are very similar to the design philosophy that is found in the Recommended Lateral Force Requirements and Commentary of the Structural Engineers Association of California (2009), which is also known as the ‘SEAOC Blue Book’. 4.
SEISMIC ZONATION OF CHILE
Chile has three seismic zones, as shown in Figure 2. The highest seismic zone, Zone 3, is generally along the coastal region of the country. Zone 2 is generally more inland and Zone 1 is further inland. The capital city of Santiago is within Zone 2. The seismic zonation is used to determine the effective acceleration value, Ao, which is used in determining the design seismic coefficient and design spectrum as defined in NCh433. The values of the effective acceleration value are given in Table 2. Note that the highest zone (Zone 3) corresponding to the coast areas of the country that are closest to the shallowest portions of the South American subduction zone. As the subduction zone deepens
Figure 2. Seismic zonation maps of Chile (NCh433.Of 96) Copyright © 2010 John Wiley & Sons, Ltd.
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Table 2. Effective acceleration value, Ao (NCh433.Of 96) Seismic Zone
Ao
1 2 3
0·2 g 0·3 g 0·4 g
Table 3. Types of foundation soils (NCh433.Of 96) Soil type
Generalized description
I II III IV
Rock Dense sand and gravel or stiff cohesive soils Medium dense sand and gravel or medium stiff cohesive soils Soft cohesive soils
to the east, the seismic zonation decreases to Zone 2 in the central portion of the country (west to east) and then to Zone 1 (generally along the border with Argentina). 5.
SITE SOIL CONDITIONS
The Chilean code accounts for site soil amplification effects. NCh433 defines four site soil profile types. Table 3 presents an abbreviated description of the four foundation soil types. Potentially liquefiable soils (i.e. saturated sands, silty sands and silts) and soils prone to densification during vibration require special study according to NCh433. These soil profile types are used to define parameters in the code that determine the design base shear. The characterization of the soil type is based on the properties of soil in the upper 10 m below the foundation level. 6.
‘IMPORTANCE’ FACTOR
Although the word ‘importance’ is not used in the code, there are ‘importance’ factors assigned to four building categories defined in the code; the base shear and design spectrum values are directly proportional to these factors. The ‘importance factors’ are given in Table 4. 7.
RESPONSE MODIFICATION FACTORS
The NCh433 code defines values for the response modification factor, Ro (or R). This factor is intended to reflect the energy absorption and dissipation characteristics of the resisting structure, as well as the practical experience on the seismic behaviour of the different types of structures and materials used. The response modification factors are shown in Table 5. In buildings that utilize different systems structural materials along the height, the seismic analysis is carried out with the lowest Ro (or R) value. The R values are used for equivalent static analysis and the Ro values are used for spectral modal analysis. The Ro values are used to calculate the R*, which is dependent on the period of the first mode of vibration of the structures, as is shown in the equation below and Figure 3. Copyright © 2010 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 853–865 (2010) DOI: 10.1002/tal
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BUILDING CODES AND STANDARDS IN CHILE
Table 4. ‘Importance’ factors (NCh433.Of 96) Building category A
B
C D
Description
I
Governmental, municipal, public service or public use (such as police stations, power plants and telephone exchanges, post offices and telegraphs, broadcasting stations, television channels, waterworks and pumping stations, etc.), and those whose use is of special importance in the event of a catastrophe (such as hospitals, first aid units, fire stations, garages for emergency vehicles, terminal stations, etc. Buildings whose content is of great value (such as libraries, museums, etc.) and those which frequently receive a great number of people: • Assembly rooms (100 persons or more); • Stadiums and bleachers (2000 persons or more); • Schools, nursery schools and university buildings; • Prisons and detention precincts; • Commercial stores (500 m2 floor area or more than 12 m in height); • Shopping malls (total area greater than 3000 m2) exclusive of parking areas. Buildings intended for private or public use that do not belong to category A or B, and construction of any type, who failure may jeopardize other constructions classified as A, B or C. Isolated or provisional structures not intended for living and cannot be classified in any of the other aforementioned categories.
1⭈2
1⭈2
1⭈0 0⭈6
Table 5. Response modification factors (NCh433.Of 96) Structural system Space moment-resisting frames Shear walls and braced systems
Structural material
Structural Steel Reinforced Concrete Structural Steel Reinforced Concrete Reinforced Concrete and Confined Masonry If reinforced concrete walls take at least 50% of the storey shear in any storey If reinforced concrete walls take less than 50% of the storey shear in any storey Wood Confined Masonry Reinforced Masonry Of concrete blocks or units of similar geometry with full grouting and double-wythe masonry Of clay bricks with partial or full grouting and concrete blocks or units similar geometry which have partial grouting Any type of structures or material that cannot be classified in one of the above categories; spectral modal analysis shall not be used.
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R
Ro
7 7 7 7
11 11 11 11
6
9
4
4
5·5 4
7 4
4
4
3
3
2
–
Struct. Design Tall Spec. Build. 19, 853–865 (2010) DOI: 10.1002/tal
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Figure 3. NCh433 Of 96-R* Response spectra modification factor
R* = 1 +
T* 0, 10T0 +
T* R0
where T* is the period of mode with the highest translational equivalent mass in the direction of analysis; To is the parameter relative to the soil foundation type and Ro is the value for the structure, in accordance with Table 5 8.
SEISMIC DESIGN
NCh433 gives the following guidance for the combination of seismic loadings with dead and live loads according to the following: • For allowable stress design 䊊 Permanent Loads + Live Loads ± Earthquake 䊊 Permanent Loads ± Earthquake • For design with the method of load and resistance factors (LRFD) 䊊 1·4 (Permanent Loads + Live Loads ± Earthquake) 䊊 0·9 (Permanent Loads) ± 1·4 Earthquake Wind loads are to be dealt with in accordance with NCh432. Only two methods of analysis are allowed by the code: (a) the static method and (b) the spectral modal method. In either case, the structural model should have a minimum of three degrees of freedom per floor, two horizontal displacements and the floor rotation with respect to a vertical axis. There are provisions for the analysis of accidental torsion. However, there are no provisions or restrictions for irregularities or irregular buildings. Copyright © 2010 John Wiley & Sons, Ltd.
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8.1
Static analysis
Static analysis is only to be used for structures that satisfy the following requirements: (a) All structures of categories C and D located in seismic zone 1; (b) All structures that do not exceed more than 5 storeys nor 20 meters in height; (c) 6- to 15-storey structures, provided that they comply with the following conditions for each direction of analysis: (i) the ratios between the total building height H, and the modal periods with the highest translational equivalent mass in the ‘x’ and ‘y’ directions, Tx and Ty, respectively, must be equal to or greater than 40 m per second; (ii) the distribution of the horizontal seismic forces of the static method must be such that shears and overturning moments at each level, shall not differ more than 10% with respect to those obtained through a spectral modal analysis with the same base shear. For the 6- to 15-storey structures, the base shear obtained from the application of horizontal static seismic forces must be at least equal to the value as determined from the equation for base shear in Section 6.2.3 of the NCh433 code. For all practical purposes in Chile, the static method is generally applicable only to structures 5 storeys or less in height. The base shear is determined by the following equation: Qo = CIP where C is the seismic coefficient defined below; I is the ‘importance’ factor; and P is the total weight of the building above the base level. The seismic coefficient, C, is obtained from the expression: C=
2 ⋅ 75 A0 T ′ T* gR
(
)
n
where n, T′ are the parameters relative to the foundation soil type; Ao is the effective acceleration value (see Table 2); R is the response modification factor (defined in Table 5); T* is the period of mode with the highest translational equivalent mass in the direction of analysis; and g is the acceleration of gravity In no case should the value of C be less than Ao/6 g. There are also maximum values of the seismic coefficient, C, depending on the values of R as given in Table 6. In general, the value of the seismic coefficient, C, will increase as the soil type ranges from Type I (rock) to Type IV (soft soil).
Table 6. Maximum values of the seismic coefficient (NCh433.Of 96) R 2 3 4 5⋅5 6 7
Copyright © 2010 John Wiley & Sons, Ltd.
Cmax (Sao/g) 0⋅90 0⋅60 0⋅55 0·40 0·35 0·35
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Figure 4. Diagram of the structure (Guendelman and Santos, 2004)
The horizontal seismic forces on structures no more than five storeys (as illustrated in Figure 4) may be calculated by the following expression: Fk =
Ak Pk Q0 ∑ Aj Pj
j =1, n
where:
8.2
Z Z Ak = ⎛ 1 − k −1 ⎞ − ⎛ 1 − k ⎞ ⎝ ⎠ ⎝ H H⎠
Spectral modal analysis
This is a standard response spectrum method, with at least three degrees of freedom per floor and the maximum response of the structure is calculated through a combination of modes using the complete quadratic combination (CQC) method of modal combination. About 5% of critical damping is generally assumed. The response spectra used is calculated with the following equation: Sa =
( ) ; p, T ( )
1 + 4 ⋅ 5 Tn T 0 where α = 3 T 1+ n T 0
IA0α R*
p
o
are the parameters relative to the foundation soil type; Ao is the
effective acceleration value (see Table 2); R* is the response modification factor; and Tn is the vibration period of mode n. Copyright © 2010 John Wiley & Sons, Ltd.
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Figure 5. NCh433 Code Spectra
Figure 5 shows the elastic spectra for sites in seismic zone II for the four soil types (I through IV) (Sa R*/Ao); these spectra would be typical for tall building construction of either steel or reinforced concrete in Santiago. The base shear must not be less than IAoP/6 g and not need to be taken larger than ICmaxP, where Cmax is taken from Table 6. 8.3
Deformation control
The maximum relative displacement between adjacent floor levels is limited to 0·002 times the storey height at the centre of masses and 0·003 times the storey height at the extreme points. There is also a minimum separation between buildings. The distance of a building to the dividing plane at any level shall not be less than R*/3 times the displacement at that level and neither 0·002 times the height of the level with a minimum of 1·5 cm (0·6 in.) according to NCh433. 8.4
Earth pressure on underground walls
NCh433 has provisions for the evaluation of soil thrusts considering soils with a horizontal surface, acting on vertical perimeter walls braced by floor slabs. The static component of the soil thrust must be evaluated for a geostatic state of stress (i.e. at-rest earth pressures). The seismic component of the soil thrust is to be evaluated by the following expression:
σ s = CR γ H Ao g where σs is the seismic pressure evenly distributed along height H of the wall; H is the wall height in contact with the soil; γ is the total (wet) unit weight of the soil or fill placed against the wall; Ao is the maximum effective soil acceleration according to the seismic zoning; CR is the coefficient equal to 0·45 for hard, dense or compacted soils; equal to 0·70 for loose or soft soils and equal to 0·58 for Copyright © 2010 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 853–865 (2010) DOI: 10.1002/tal
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Table 7. Seismic earth pressures based on NCh433.Of 96 Seismic pressure (uniform distribution) in pounds per square foot Seismic zone III II I
Firm soils (H)
Loose soils (H)
Loose fill in sloped excavation (H)
21·6 16·2 10·8
33·6 25·2 16·8
27·8 20·9 13·9
loose fills deposited between the wall and the slope of an excavation carried out in dense or compacted soils; and g is the acceleration of gravity. Table 7 provides a tabulation of the seismic earth pressures that would be required by the Chilean NCh433 code for the different seismic zones and soil backfill conditions assuming a wet unit weight of 120 lbs per cubic foot (1900 kg per cubic meter) for the soil and the height of the wall is given in units of feet. Thus in Chile’s largest city of Santiago, building basement walls with firm soil backfill would need to be designed for a uniform seismic earth pressure equal to 16·2 H. 8.5
Basis for NCh433 design of steel and reinforced concrete structures
Annex B of NCh433 provides ‘transitory references’ for the design provisions in Chile with regard to steel and concrete construction. Section B.1 stipulates that the provisions for steel by the American Institute of Steel Construction (1989, 1992, 1993) be used. Section B.2 stipulates that for reinforced concrete, the provisions of Building Code Requirements for Reinforced Concrete, ACI 318-95 (ACI Committee 318, 1995) be used. However, NCh433 Section B.2.2 states an exception to the ACI 318-95 requirements where it states that ‘. . . when designing reinforced concrete walls it is not necessary to meet the provisions of paragraphs 21.6.6.1 through 21.6.6.4 of the ACI 318-95 code. This exception permits the designers to not satisfy ACI’s requirements for boundary elements in structural walls. ACI 318-95 required that boundary elements be provided at boundaries and edges around openings of structural walls when the maximum extreme fibre stress, corresponding to factored forces including earthquake effect, exceeds 0·2fc’ unless the entire wall is reinforced to satisfy the provisions in Section 21.4.4. ACI 318-95 also required that boundary elements, where required, should have transverse reinforcement as specified in Section 21.4.4. The boundary elements are to be proportioned to resist all factored gravity loads on the wall, including tributary loads and self-weight, as well as the vertical force required to resist overturning moment calculated from factored forces related to earthquake effect. Also, transverse reinforcement in walls with boundary elements is to be anchored within the confined core of the boundary element to develop the specified yield strength of the transverse reinforcement. The basis for this exception to ACI 318-95 was the observations that there was ‘satisfactory behavior’ of reinforced concrete buildings in the magnitude (MS) 7·8 3 March 1985 earthquake that occurred offshore of San Antonio (USGS, 2010). The Earthquake Engineering Research Institute organized a reconnaissance team to go to Chile for this event and a report was published in Earthquake Spectra (Wyllie et al., 1986). It is interesting to note that one of the ‘structural lessons learned’ in the Wyllie et al. report is: ‘Reinforced concrete shear walls with significantly stressed chords require confined boundary members for desirable performance in strong ground shaking. This lesson was illustrated by the boundary member failures at the Acapulco and Canal Beagle, and good performance of other Viña del Mar buildings.’ Copyright © 2010 John Wiley & Sons, Ltd.
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Although catastrophic failures of modern reinforced concrete buildings built under these code provisions were not very common in the 27 February earthquake, this exception to ACI boundary element requirements may have resulted in damage to numerous reinforced concrete walls causing significant dislocation of occupants and loss of use in those buildings affected. 9. STRUCTURAL AND SEISMIC PEER REVIEW Gradual implementation of a peer review program began in 2003 by national building authorities that require that structural and seismic design of all building projects be reviewed by an independent reviewer. The reviewers must be chosen from a roster of authorized professionals maintained under the authority of the Chilean Ministry of Housing. The peer reviewers must also have the skill and experience commensurate with the complexity and importance of the structure. The peer reviewers are classified based on their qualifications into three different levels of skill (Cruz, 2009). 10.
BUILDING PERMITS
A building permit is required before construction can begin. The building permit requires that the independent structural and seismic reviewer must approve the plans. In addition, the set of drawings must also be submitted to the building department of the municipality (or township) where the site is located. The documents become public record. 11. CONSTRUCTION QUALITY AND INSPECTION Quality construction is regulated by four factors: township regulations, construction specifications set by structural design team in charge of the project, internal standard of the construction companies and external construction inspection. For public projects, the law and the specifications require that public projects have external independent construction inspection. For a private project, construction inspection is optional and is dependent on the desires of the owner of the project. However, construction inspection is becoming a standard for medium to large projects or when the owner of the project is different from the construction companies in charge of the project. The purpose of external inspection is to assure the quality of the construction and that the specifications set by the township, structural engineers and the law are being followed. Structural engineers provide the drawings and technical specifications of the projects; the drawings and specifications define and specify the construction materials to be used, the physical properties of the materials and the geometry of placement. These documents also specify the control that needs to be applied to ensure compliance. Also, the structural engineers are legally responsible to visit the construction site on a regular basis to approve the construction by confirming that the plans and specifications given by them are being followed. Townships regulate the quality of the construction through specifications of construction set by them; these must be followed by the construction companies to be able to obtain the township approval of the building. In addition, each township has officials and inspectors that have the charge of visiting the construction site typically every 15 days to assure that the specifications of the township and the law are being followed. For example, to control the concrete quality and its resistance, there are specifications that require that samples of the concrete be obtained every 50 m3 with at least one sample of the material used in the slabs, wall, columns and beams at each floor. The sampling and testing of the concrete has to be Copyright © 2010 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 853–865 (2010) DOI: 10.1002/tal
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F. ROJAS, M. LEW AND F. NAEIM
done by an external independent laboratory that must be accredited by the government. The steel reinforcement also must be certified to meet the specifications of the project. After the construction is finished, the township has the responsibility to do a final inspection of the building and provide final approval. During this process, the building owner will need to provided to the township with an as-built set of architectural and structural drawings signed by the various design professionals involved in the project; these documents should show all the modifications that where made during the construction. In addition, they also need to provide all the material quality control results obtained during the construction as well as the book of the construction which documents all of the names of the people in charge of the construction project that have the legal and civil responsibility for the construction. The book of construction includes all of the comments made by the structural engineers, independent construction inspectors, and the township inspectors. The book documents the time of their different visits to the construction site. The importance of this document is that it becomes public record. 12.
CIVIL RESPONSIBILITY FOR DAMAGE IN BUILDINGS DURING EARTHQUAKES
In Chile, buildings are sold as being ‘Earthquake Proof’. Building developers have 10 years of responsibility for any damage in the structural resisting elements of the buildings. For non-structural elements, the developers have five years of responsibility. Thus, in Chile, there is a high expectation on the part of the general public that buildings that experience earthquakes are more than just ‘life safe,’ but are not to be damaged. This expectation on the part of the Chilean public is counter to the stated principles and hypotheses of the NCh433 seismic code. 13.
CONCLUSIONS
Building codes and standards in Chile are very similar to those used in the United States since the basic provisions for steel and reinforced concrete are those developed in the USA by AISC and ACI. The system of peer review for structural and seismic compliance provides a level of quality control and quality assurance. The level of civil responsibility in Chile far exceeds the level of civil responsibility in the USA with the Chilean expectation of earthquake proof buildings with explicit warranties for set time periods; in the USA, the level of responsibility is more negligence based. The relatively good reported performance of modern structures in Chile can be attributed to the building codes and standards adopted by the country and the level of expertise of the design professionals. We are unable to provide any opinions about the quality of construction and conformance to plans and specifications not having enough experience to render any judgments. However, the exceptions to the ACI confinement requirements in the NCh433 code may have caused a weakness in some buildings that suffered significant damage to wall elements. These weaknesses may also present some vulnerability to future seismic events in many Chilean buildings with similar structural detailing. REFERENCES
ACI Committee 318. 1995. Building Code Requirements for Structural Concrete (ACI 318-95) and Commentary (318R-95). American Concrete Institute: Farmington Hills, MI pp. 369. AISC. 1989. Specification for Structural Steel Buildings-Allowable Stress and Plastic Design. American Institute of Steel Construction: Chicago, IL. AISC. 1992. Seismic Provisions for Structural Steel Buildings. American Institute of Steel Construction: Chicago, IL. AISC. 1993. Load and Resistance Factor Design Specification for Structural Steel. American Institute of Steel Construction: Chicago, IL. Copyright © 2010 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 853–865 (2010) DOI: 10.1002/tal
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Cruz E. 2009. UNESCO–Ipred–Itu Workshop on ‘Make the people a part of the Solution’ Presentation: ‘Status of Earthquake preparedness activities in Chile’. Guendelman T, Santos E. 2004. Chapter 9: ‘General Aspect of the NCh433 Of96’. Static Analysis and Dynamics of Structures. Class Notes: Analisis Estructural Avanzado, University of Chile, Santiago, Chile. Instituto Nacional de Normalización. 1996. Earthquake resistant design of buildings. Official Chilean Code NCh433.Of96. Structural Engineers Association of California. 2009. SEAOC Blue Book—Seismic Design Recommendations. SEAOC: Sacramento, CA. USGS. 2010. Historic Earthquakes-Offshore Valpariso. Chile, 1985 March 03 22:47:07 UTC Magnitude 7.8. http://earthquake.usgs.gov/earthquakes/world/events/1985_03_03.php [20 July 2010]. Wyllie LA, Abrahamson N, Bolt B, Castro G, Durkin ME, Escalante L, Gates JH, Luft R, McCormick D, Olson RS, Smith PD, Vallenas J. 1986. The Chile Earthquake of March 3, 1985. Earthquake Spectra 2: 2.
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Struct. Design Tall Spec. Build. 19, 853–865 (2010) DOI: 10.1002/tal
