Vibration Isolation and Seismic Restraint for Mechanical Equipment in Chile

Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century…

PROCEEDINGS of the 22nd International Congress on Acoustics

Structural Acoustics and Vibration (others): Paper ICA2016-78

Vibration isolation and seismic restraint for mechanical equipment in Chile

Nicolás A. Bastián-Monarca(a), Ian Azrak(b)

(a) Silentium, Chile, [email protected] (b) Mason Industries, United States, [email protected]

Abstract

Vibration control of mechanical equipment in seismic countries is a subject that should be addressed with some caution. One of the most utilized elements in controlling vibration is a spring isolator, one that has high deflection (relative to neoprene isolators). The problem is that the resonant frequency can coincide with the disturbing frequency of an earthquake, which means that in an earthquake the spring will begin to ‘jump’, causing the displacement of the mechanical equipment. The displacement of the equipment, beyond generating high costs (damage, loss of use, flooding, etc.), also creates the risk of personal injury and even death. Due to the aforementioned risks, it is necessary to consider the seismic ‘variable’ when doing a proper vibration control design in a seismic country or region. Criteria, designs and recommendations are presented in this report in order to perform proper vibration isolation in conjunction with seismic restraint for mechanical equipment and systems in Chile. In addition, you will be shown some failures from the M8.8 earthquake on February 27, 2010 in Chile.

Keywords: Vibration isolation, seismic restraint

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

1 Introduction The control of vibration transmission has received much attention in recent years due to its effect on the functionality of systems involved [1] and health. Vibration transmission can cause instability or even failure, as in the case in buildings subject to earthquakes [2]. Thus, vibration isolation is a vital requirement throughout much of engineering [3], particularly when there is a strong source of vibration such as a motor [4]. In this sense, technical or mechanical floors in any building project are a strong source of vibration due to the high density of equipment. In these scenarios, the direct radiation of a vibrating machine is usually not as serious as the radiation from the floor structure as a result of the structure-borne sound transmission from the machine to the floor structure [5]. In the past decade the use of active or semi-active vibration isolators had been a new trend for the designs of anti-vibration systems [6-11]. However, the advantage of the passive anti-vibration method are that they are stable and easy to set up [12]. Moreover, passive vibration isolators are the most simple, inexpensive and reliable means of protecting sensitive equipment from environmental shock and vibration [13]. Therefore, vibration isolation using passive isolators is very cost effective and hence is widely used in the engineering industry [14]. Thus, the passive isolation and seismic restraint of a structure from mechanical equipment is studied in this paper. The use of vibration isolators (simply isolators from this point forward) in order to isolate mechanical equipment is a typical solution that engineers or acousticians always recommend. In the large range of isolators, neoprene and spring isolators are the most utilized, the reason being the static deflections that they offer. Spring isolators normally are offered between one inch and five inches of static deflection, which neoprene isolators are normally offered in a range between 0.15 inches and 0.55 inches [15]. Based on that, it is clear that when high deflection is required, springs be offered as the best solution. The problem with spring isolators is that they have a natural frequency that can potentially coincide with the frequency of an earthquake, which means that during an earthquake, the spring isolator can enter into resonance and being to jump violently. If the spring isolator does not have a seismic housing, the excitation of the spring will cause the equipment that it is supporting to being to move and fall over. If the machine is a fan or other airside type of HVAC equipment the danger is that it could fall over and pull down the ductwork, but if the equipment is wetside HVAC equipment it could fall over and bring down the piping with it, causing flooding within the building. Furthermore, if the equipment is located outdoors or on a roof, the possibility exists that the equipment could fall off the building. In this work, the general concepts of the control of vibrations are presented along with a practical method of controlling vibrations of mechanical equipment and the design requirements per ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) [16] in order to achieve an effective vibration control system for particular pieces of equipment. Furthermore, the Chilean Seismic Building Code, NCh-3357/2015 [17] will be described in detail with regard to non-structural components, which applies to all types of mechanical equipment in buildings. After that, some real life applications in certain projects will be shown and the conclusions drawn.

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

2 Vibration isolation

2.1 Overview Vibration isolation is commonly adopted by engineers to reduce the vibratory effect caused by machines in buildings [18]. The mechanical vibration process can be subdivided into four main stages, generation-transmission-propagation-radiation. The transmission stage is often the best compromise for noise and vibration control activities in view of cost and practicability [19]. Under that premise, the most effective way to eliminate vibrations between the equipment and supporting structure is a vibration isolator [20].

2.2 Practical approach The design of a complete vibration isolation system can be divided up into [21]: 1) The selection of the required transmissibility or static deflection for the vibration isolators. 2) Selection of the appropriate form of mounting. 3) Selection of the location for the vibration isolators and determination of the loads of each one. 4) The selection of suitable isolators to correspond with the previous three points. 5) The treatment of the various service connections to ensure that these do not cancel out the effect of the vibration isolators. The static deflection needed to isolate a particular piece of equipment can be obtained by referring to ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) [16]. In general a higher deflection is required to isolate lower equipment motor frequencies [22]. There are normally three possible ways of mounting equipment. The first is to attach vibration isolators directly to the existing mounting feet of the equipment. The second method is to mount the equipment on a steel frame and to attach the vibration isolators to this, while the third possibility is to mount the equipment on a concrete ‘inertia’ base [21]. In figure 1, the three cases are shown.

Figure 1: Left: Chiller mounted directly on isolators. Center: Fan mounted on a metal base and isolators. Right: Pump mounted on concrete inertia base with isolators.

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

The manufacturer of the equipment generally gives the location of the isolators since the equipment usually has mounting hole locations (typically in the corners) where the equipment can be anchored [20]. For equipment that is installed on steel bases or concrete inertia bases where the base is too long with respect to the width, one ought to install intermediate isolators so that that base itself does not have excessive bending or collapse upon its length. This means that upon selecting isolators, the intermediate supports will typically have to support a greater load than that of the corner isolators. In Figure 2, an example of this case is shown.

Figure 2: Cooling tower mounted on 6 isolators, 4 in the corners and 2 intermediate.

Normally when dealing with the connections between equipment and piping, the focus is on spherical connectors or flexible hoses when dealing with wetside equipment such as pumps, chillers, cooling towers, etc. To avoid vibration the typical choice is a long flexible stainless steel hose, usually longer than stock connectors. To avoid blade passage frequency, spherical connectors are used which accept both minimal axial and longitudinal movement.

2.3 Criteria of design As mentioned in section 2.2, the criteria used to select isolators, the type of base and the minimum static deflection is ASHRAE. The selection is made as a function of the following variables:  Type of Equipment  Horsepower  Revolutions per minute  Distance between supporting columns It is important to mention that one of the most important variables with regard to minimum static deflection required to isolate equipment is the distance between support columns and where the equipment is located with respect to that gap. This is due to the fact that ASHRAE considers what the deflection will be in the structural floor below because the isolators should have more static deflection than the floor below. In Table 1, the type of isolator, type of base and the minimum static deflection for different HVAC equipment are shown, depending on the previously mentioned variables.

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

Table 1: Type of Isolator Recommended for Different HVAC Equipment. Source: [16]

Equipment Location Up to 6 m 6- to 9 m 9- to 12 m Slab on Grade Floor Span Floor Span Floor Span Iso- Min Iso- Min Iso- Min Iso- Min Shaft Power, kW Base lator Defl., Base lator Defl., Base lator Defl., Base lator Defl., Equipment Type and Other Rpm Type Type in. Type Type in. Type Type in. Type Type in. Refrigeration Machines and Chillers Reciprocating All All A 2 0,25 A 4 0,75 A 4 1,5 A 4 2,5 Centrifugal, scroll All All A 1 0,25 A 4 0,75 A 4 1,5 A 4 1,5 Screw All All A 1 1 A 4 1,5 A 4 2,5 A 4 2,5 Absorption All All A 1 0,25 A 4 0,75 A 4 1,5 A 4 1,5 Air-cooled recip., scroll All All A 1 0,25 A 4 1,5 A 4 1,5 A 4 2,5 Air-cooled screw All All A 4 1 A 4 1,5 B 4 2,5 B 4 2,5 Air Compressors and Vacuum Pumps Tank-mounted horiz. ≤7.5 All A 3 0,75 A 3 0,75 A 3 1,5 A 3 1,5 ≥11 All C 3 0,75 C 3 0,75 C 3 1,5 C 3 1,5 Tank-mounted vert. All All C 3 0,75 C 3 0,75 C 3 1,5 C 3 1,5 Base-mounted All All C 3 0,75 C 3 0,75 C 3 1,5 C 3 1,5 Large reciprocating All All C 3 0,75 C 3 0,75 C 3 1,5 C 3 1,5 Pumps Close coupled ≤5.6 All B 2 0,25 C 3 0,75 C 3 0,75 C 3 0,75 ≥7.5 All C 3 0,75 C 3 0,75 C 3 1,5 C 3 1,5 Large inline 3.7 to 19 All A 3 0,75 A 3 1,5 A 3 1,5 A 3 1,5 ≥22 All A 3 1,5 A 3 1,5 A 3 1,5 A 3 2,5 End suction and split case ≤30 All C 3 0,75 C 3 0,75 C 3 1,5 C 3 1,5 37 to 93 All C 3 0,75 C 3 0,75 C 3 1,5 C 3 2,5 ≥110 All C 3 0,75 C 3 1,5 C 3 2,5 C 3 3,5 Packaged pump systems All All A 3 0,75 A 3 0,75 A 3 1,5 C 3 2,5 Cooling Towers All Up to 300 A 1 0,25 A 4 3,5 A 4 3,5 A 4 3,5 301 to 500 A 1 0,25 A 4 2,5 A 4 2,5 A 4 2,5 501 and up A 1 0,25 A 4 0,75 A 4 0,75 A 4 1,5 Boilers Fire-tube All All A 1 0,25 B 4 0,75 B 4 1,5 B 4 2,5 Waer-tube, copper fin All All A 1 0,12 A 1 0,12 A 1 0,12 B 4 0,25 Axial Fans, Plenum Fans, Cabinet Fans, Fan Sections, Centrifugal Inline Fans Up to 560 mm diameter All All A 2 0,25 A 3 0,75 A 3 0,75 C 3 0,75 610 mm diameter and up ≤500 Pa SP Up to 300 B 3 2,5 C 3 3,5 C 3 3,5 C 3 3,5 300 to 500 B 3 0,75 B 3 1,5 C 3 2,5 C 3 2,5 501 and up B 3 0,75 B 3 1,5 B 3 1,5 B 3 1,5 ≥501 Pa SP Up to 300 C 3 2,5 C 3 3,5 C 3 3,5 C 3 3,5 300 to 500 C 3 1,5 C 3 1,5 C 3 2,5 C 3 2,5 501 and up C 3 0,75 C 3 1,5 C 3 1,5 C 3 2,5 Centrifugal Fans Up to 560 mm diameter All All B 2 0,25 B 3 0,75 B 3 0,75 B 3 1,5 610 mm diameter and up ≤30 Up to 300 B 3 2,5 B 3 3,5 B 3 3,5 B 3 3,5 300 to 500 B 3 1,5 B 3 1,5 B 3 2,5 B 3 2,5 501 and up B 3 0,75 B 3 0,75 B 3 0,75 B 3 1,5 ≥37 Up to 300 C 3 2,5 C 3 3,5 C 3 3,5 C 3 3,5 300 to 500 C 3 1,5 C 3 1,5 C 3 2,5 C 3 2,5 501 and up C 3 1 C 3 1,5 C 3 1,5 C 3 2,5 Propeller Fans Wall-mounted All All A 1 0,25 A 1 0,25 A 1 0,25 A 1 0,25 Roof-mounted All All A 1 0,25 A 1 0,25 B 4 1,5 D 4 0,25 Heat Pumps, Fan-Coils, Computer Room All All A 3 0,75 A 3 0,75 A 3 0,75 A/D 3 1,5 Units Condensing Units All All A 1 0,25 A 4 0,75 A 4 1,5 A/D 4 1,5 Packaged AH, AC, H and V Unites All ≤7.5 All A 3 0,75 A 3 0,75 A 3 0,75 A 3 0,75 ≤11 Up to 300 A 3 0,75 A 3 3,5 A 3 3,5 C 3 3,5 ≤1 kPa SP 301 to 500 A 3 0,75 A 3 2,5 A 3 2,5 A 3 2,5 501 and over A 3 0,75 A 3 1,5 A 3 2,5 A 3 1,5 >11 Up to 300 B 3 0,75 C 3 3,5 C 3 3,5 C 3 3,5 > 1 kPa SP 301 to 500 B 3 0,75 C 3 1,5 C 3 2,5 C 3 2,5 501 and over B 3 0,75 C 3 1,5 C 3 1,5 C 3 2,5 Packaged Rooftop Equipment All All A/D 1 0,25 D 3 0,75 ------See Note 17 ------Ducted Rotating Equipment ≤300 L/s All A 3 0,5 A 3 0,5 A 3 0,5 A 3 0,5 Small fans, fan-powered boxes ≥300 L/s All A 3 0,75 A 3 0,75 A 3 0,75 A 3 0,75 Engine-Driven Generators All All A 3 0,75 C 3 1,5 C 3 2,5 C 3 3,5 Base Types: Isolator Type: A. No base, isolators attached directly to eqopment 1. Pad, rubber or glass fiber B. Structural steel rails or base 2. Rubber floor isolator or hanger C. Concrete inertia base 3. Spring floor isolator or hanger D. Curb-mounted base 4. Restrained spring isolator 5. Thrust restraint

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

3 Seismic restraint

3.1 Overview Even though seismic restraint is not related to acoustics or noise, new technological advances in isolation have been able to create a product that not only isolates vibrations but also can provide seismic restraint. As mentioned previously, it is a big problem for acoustical consultants that work in seismic countries when having to deal with vibration isolation while keeping in mind seismic restraint. When taking into account the seismic restraint, the tension and shear of the mount and their connecting anchor points should be calculated along with the equipment to isolator connections. If there is a failure amongst the three above points, the entire system can fail during an earthquake. In order to calculate the tension and shear forces, the Steel Construction Manual [23] or the most current concrete anchor codes should be consulted.

3.2 Earthquakes and codes in Chile Chile is one of the most seismically active countries in the world. Various studies exist that show the long history of the great quantity of earthquakes in the history of the country in the last century [24-27]. The three earthquakes that affected the country and helped to develop the seismic codes are the following: 1) 1960 Earthquake that occurred in the city of Valdivia, Magnitude 9.5 on the Richter Scale [28]. After that earthquake, building code NCh433-1996 was developed [29]. 2) 1985 Earthquake that occurred in the city of Valparaíso, Magnitude 7.8 on the Richter Scale [26]. After the occurrence of that earthquake, building codes NCh2369-2003 [30] and Nch2745- 2003 [31] were developed. 3) 2010 Earthquake that occurred in the Bío Bío region, Magnitude 8.8 on the Richter Scale [30]. After that earthquake, building codes NTM-001 from MINVU [33], that is based on the American code ASCE-07 [34], and in 2015, NCh-3357/2015 [17].

3.3 Code NCh-3357/2015 [17] This code establishes the minimum criteria of seismic design for non-structural components and systems that are permanently fixed in buildings. This applies to mechanical equipment, such as generators, fans, cooling towers, etc. Below the definitions and equations used per the codes in order to calculate the seismic requirements in non-structural components will be presented:

1) Seismic design force: The horizontal seismic design force Fp, is applied to the center of gravity of the component or distributed about the mass of the component and determined by Equation 1.

0.4 푎푝훼퐴퐴푊푝 푧 퐹 = (1 + 2 ) 푝 푅 (1) 𝑔 ( 푝) ℎ 퐼푝

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

The code specifies that he value of Fp should be in the following range:

0.3 훼퐴퐴퐼푝푊푝 1.6 훼퐴퐴퐼푝푊푝 < 퐹 < (2) 𝑔 푝 𝑔

where Fp is the seismic force of the non-structural component, 훼퐴퐴 is the design spectral acceleration, classified by D.S.61/2011 of MINVU [35]. In table 2 the possible values are shown. ap is the dynamic amplification factor, which varies between 1.0 and 2.5. Ip is the importance factor, which varies between 1.0 and 1.5 Wp is the operating weight of the component. Rp is the response modification factor, which varies between 1 and 8. z is the height of the component with respect to the ground and building height. For components located below ground the value should be 푧 = 0, h is the height of the building with respect to the ground, and g is the gravitational acceleration, in cm/s2. The horizontal seismic design force, Fp, ought to be applied in in two orthogonal directions in combination against the load of the component. In addition, there will be a simultaneous vertical seismic design force, Fpv, equal to ±(0.24훼퐴퐴푊푝)/𝑔.

Table 2: Parameter 휶푨푨 of design acceleration. Source: [35].

Type of ground 휶푨푨 A 977 Z B 1.101 Z C 1.144 Z D 1.455 Z E 1.576 Z Where the parameter Z, which depends on the seismic zone, is defined in Table 3. For ground type F, according to code NCH433-1996 [29], a special study shall determine seismic forces. Table 3: Modification factor Z of spectral acceleration according to seismic zone. Source: [29].

Seismic zone Z 1 0.5 2 0.75 3 1

4 Failures during earthquakes In this section, mention will be made of the failures that occurred during the earthquake of February 27, 2010 in Chile (referred to as 27F going forward). The pictures were taken the 28th of February, 2010 in different work sites during a technical inspection that was undertaken in Santiago, Chile by the company Silentium, Ingeniería del Silencio. The principal failures that were seen on 27F were with regard to the selection of isolators, installation of isolators, and the selection of the anchor bolts. Each one of the aforementioned failures will be detailed below.

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

4.1 Selection of isolators One of the principle problems is the selection of isolators that do not also provide seismic restraint. It is necessary to utilize isolators that have a rigid housing that restricts excessive movement of the isolator and the equipment once an earthquake occurs.

Figure 3: Left: Isolators to the side of collapsed chiller to which they belonged. Right: Failure of isolation system of pump inertia base due to lack of snubbers.

4.2 Installation of isolators Another problem that was seen on 27F was that the correct isolators with seismic restraint were selected but installed with concrete anchors too close to the border of the housekeeping pad, causing a failure of the anchor bolt system. Figure 4 below shows this example.

Figure 4: Isolator in tack but concrete failure below, installed too close to the edge of housekeeping pad.

4.3 Anchor bolt selection Important variables that should not be overlooked are the connection between the equipment and isolator and the connection between the isolator and structure. On 27F, there were failures seen due to the shear forces acting on insufficiently selected anchors.

Figure 5: Failure of bolt due to poor selection, acted on by shear forces of earthquake.

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

5 Conclusions Keeping in mind what has been explained in this report, it can be concluded that the design of vibration control systems for mechanical equipment in a seismic country must consider the seismic variable in its design. The design recommendations are based in the international criteria in ASHRAE for vibration control and the Chilean seismic code NCh-3357/2015 for seismic restraint. Upon unifying these two criteria, an effective and integral design of vibration control can be achieved and furthermore guaranteeing the continued operation of isolated equipment during and after an earthquake. The damages that a vibration control system that is not equipped with seismic restraint can cause aside from being very costly can be lethal. Finally, is can be concluded that engineers or acoustical consultants that work in seismic countries and develop vibration control projects have the responsibility to include seismic restraint considerations, since the lack of doing so could put the lives of people in great risk.

References [1] El-Sinawi AH. Active vibration isolation of a flexible structure mounted on a vibrating elastic base. Journal of Sound and Vibration 271 (2004) 323-337. [2] Mead DJ. Passive Vibration Control. John Wiley and Sons, 1999. [3] Dynamic analysis of high static low dynamic stiffness vibration isolation mounts. Journal of Sound and Vibration 332 (2013) 1437-1455. [4] Elliot SJ, Serrand M, Gardonio P. Feedback stability limits for active isolation systems with reactive and inertial actuators. Journal of Vibration and Acoustics 123 (2001) 250-261. [5] Mak CM, Jianxin S. A power transmissibility method for assessing the performance of vibration isolation of building services equipment. Applied Acoustics 63 (2002) 1281-1299. [6] Fuller CR, Elliott SJ, Nelson. Active control of vibration. UK: Academic; 1996. [7] Snyder SD, Tanaka N. Active control of vibration using a neural network. IEEE Transactions Neural Networks 6 (4) (1995) 819-828. [8] Shaw J. Active vibration isolation by adaptive control. Journal of Vibration and Control 7 (1) (2001) 19- 31. [9] Benassi L, Elliott SJ. Active vibration isolation using an inertial actuator with local displacement feedback control. Journal of Sound and Vibration 278 (4-5) (2004) 705-724. [10] Zhang CL, Mei DQ, Chen ZC. Active vibration isolation of a micromanufacturing platform based on a neural network. Journal of Materials Processing Technology 129 (1-3) (2002) 634-639. [11] Kang YK, Park HC, Kim J, Choi S-B. Interaction of active and passive vibration control of laminated composite beams with piezoceramic sensors/actuators. Materials & Design 23 (3) (2002) 277-286. [12] Chen KT, Chou CH, Chang SH, Liu YH. Intelligent active vibration control in an isolation platform. Applied Acoustics 69 (2008) 1063-1084. [13] Chatterjee S, Singha TK, Karmakar SK. Effect of high-frequency low-amplitude vibration on the performance of a class of semi-active base isolation systems with on-off damping. Journal of Sound and Vibration 274 (2004) 893-914. [14] Huang X, Liu X, Sun J, Zhang Z, Hua H. Vibration isolation characteristics of a nonlinear isolator using Euler buckled beam as negative stiffness corrector: A theoretical and experimental study. Journal of Sound and Vibration 333 (2014) 1132-1148.

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

[15] Bastián N, Galleguillos C. Control de Vibraciones y Restricción Sísmica de Componentes y Sistemas no Estructurales en Chile. IX Congreso Iberoamericano de Acústica FIA 2014. Valdivia, Chile (2014). [16] American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Ashrae handbook: HVAC applications. SI edition (2011). [17] Norma Chilena (2015). NCh3357/2015: Diseño sísmico de componentes y sistemas no estructurales. [18] Mak CM, Yun Y. A study of power transmissibility for the vibration isolation of coherent vibratory machines on the floor of a building. Applied Acoustics 71 (2010) 368-372. [19] Malcom J. Crocker. Handbook of noise and vibration control. John Wiler & Sons, Inc. Hoboken, New Jersey, United States of America (2007). [20] Bastián N. Control de Vibraciones de Equipamiento Mecánico en Recintos Críticos. 46º Congreso Español de Acústica, Encuentro Ibérico de Acústica y Simposio Europeo sobre Acústica Virtual y Ambisonics TECNIACUSTICA 2015. Valencia, España (2015). [21] Sound Research Laboratories Ltd. Noise Control in Industry. CRC Press; 3 edition (December 31, 1990). Beranek and I.L. Ver, Eds, Wiley, New York. [22] Crocker M.J, Price A.J. Vibration Isolation of a Book Trimming Machine. Applied Acoustics 3 (1970) 207-216. [23] AISC. Manual of Steel Construction: Allowable Strees Design. 9th edition (July, 1989). [24] Lomnitz C. Grandes terremotos y tsunamis en Chile durante el períoso 1535-1955. Geofísica Panamericana 1 (1) (1971) 151-178. [25] Astiz L, Kanamori H. Interplate coupling and temporal variation of mechanisms of intermediate depth earthquakes in Chile. Bulletin of the Seismological Society of America 76 (1986) 1614-1622. [26] Campos J, Hatzfeld D, Madariaga R, Lopez G, Kausel E, Zollo A, Iannacone G, Fromm R, Barrientos S, Lyon-Caen H. A seismological study of the 1835 seismic gap in south-central Chile. Physics of the Earth and Planetary Interiors. 132 (2002) 177-195. [27] Udías, A.; Madariaga, R.; Buforn, E.; Muñoz, D. & Ros. M. The Large Chilean Historical Erathquakes of 1647, 1657, 1730, and 1751 from Contemporary Documents. Bulletin of the Seismological Society of America 102 (4) (2012) 1639-1653. [28] Cisternas M. Atwater B.F. Torrejón F. Sawai Y. Machuca G. Lagos M. Eipert A. Youlton C. Salgado I. Kamataki T. Shishikura M. Rajendran C.P. Malik. J.K. Rizal Y. Husni M. Predecessors of the gian 1960 Chile earthquake. Nature Letters 437 (2005) 404-407. [29] Norma Chilena (1996). NCh433-1996: Diseño sísmico de edificios. [30] Norma Chilena (2003). NCh2369-2003: Diseño sísmico de estructuras e instalaciones industriales. [31] Norma Chilena (2003). NCh2745-2003: Diseño sísmico de edificios con aislación sísmica. [32] AON Benfield. Event Recap Report: 02/27/10 Chile Earthquake. Chigago, IL 60601. [33] Ministerio de Vivienda y Urbanismo (2013). Norma técnica minvi ntm 001: Diseño sísmico de componentes y sistemas no estructurales. [34] American Society of Civil Engineers (2002). Minimum design loads for buildings and other structures. SEI/ASCE 7-02 Second Edition. [35] Ministerio de Vivienda y Urbanismo (2011). Decreto supremo 61: Diseño sísmico de edificios.