Seismic qualification of passive mitigation devices
This document is produced as a Report for Industry within Task 6: "Enhancement of Test Standards for the Qualification of Products" of the European Union funded project: "Cooperative Advancements in Seismic and Dynamic Experiments" (CASCADE, project reference: HPRI-1999-CT-40006), carried out in the framework of the specific research and technological development programme "Improving the Human Research Potential and the Socio-Economic Knowledge Base". The main purpose of this document is to act as a guide to those who are involved in the application of passive earthquake mitigation devices: Device manufactures and developers, structural engineers that choose to use them in their seismic designs, construction companies that intend to implement them and those building owners who, as final customers, often require an independent assessment when a new technology is introduced. It focuses on the qualification of such devices that is: The process of ensuring the desired behaviour in a particular application. In this document, a passive seismic mitigation device is understood as a self-contained unit connected to a building structure that does not require any external energy supply or external control to mitigate the seismic actions induced by an earthquake. In addition, a device should be removable from the structure without excessive effort and thus is replaceable. Passive devices that mitigate the effects of earthquakes have many applications today, ranging from typical office buildings to bridges, power plants and even works of art. They are based on a variety of mechanisms ranging from the action of viscous fluids to friction and metal yielding. Passive mitigation devices can be as simple as a round steel bar or as complicated as a sophisticated shock absorber originating from supersonic jet planes' based technology. In view of the large variety of applications, available technologies and historical development, application standards or guides for qualifying a passive device are in general not comprehensive and often limited to a particular type of device. This makes it very difficult, if not impossible, to generalize qualification procedures but it is shown, in this document, that a set of general criteria exists and, based on those, general procedures may be suggested for certain classes of devices. In its Part I, this document intends to provide a brief insight into the various types of devices in use today and how they are currently qualified. It discusses (without any claim to completeness) different device mechanisms and concepts. Among those are devices filled with various fluids such as viscous, visco-elastic or magneto-rheological fluids as well as metal-yielding and friction devices. Their action can be in one, two or three directions. In Annex A, a brief description is added on passive structural control concepts where many of these devices find applications and which are a new structural concept for buildings and bridges. Where qualification of devices is covered in codes, Part I discusses their provisions in the light of the following qualification criteria: (1) a verified hysteresis law, (2) repeatability of intended behaviour, (3) proven life cycle durability, (4) insensitivity to environmental conditions and (5) acceptable quality control procedures. These criteria form a general framework for device qualification and are discussed in detail in the first chapter of Part II. Except for applications in base isolation systems, code provisions for device qualification are not very developed. Part I therefore concludes with a discussion on current seismic qualification practice which usually takes place outside code regulations. The discussion is based on selected examples, among them a stadium structure and a high-rise building in the US. The relevant passages of the contract documents for these structures are given in abbreviated form in Annex B for various types of devices. After a detailed discussion on the general qualification criteria mentioned above, Part II of this document then continues with the selection of simple, yet versatile hysteresis laws that are able to cover a broad array of devices. These laws are grouped into (1) visco-elastic, (2) elasto-plastic and (3) viscous elasto-plastic, the latter being a combination of (1) and (2). They have been selected on the basis of simplicity, versatility and widespread application in numerical structural analyses. It then proposes a simple procedure to test them for criterion (1): a verified hysteresis law. The suggested procedure to test criterion (1) may be used for other types of hysteresis laws, if it is shown that it can verify their parameters. This is hardly possible for so-called evolutionary hysteresis laws. These laws change with the history of loading and to verify their parameters is much more involved. An example here is the natural rubber bearing used in base isolation. Its hysteresis stabilizes only after several cycles. In a similarly concise way, procedures are developed to test for the other qualification criteria mentioned above and thus, a proposal for a qualification procedure applicable to a large number of different devices utilized in various applications is outlined. Passive seismic mitigation devices are developed with a specific behaviour in mind and often for a specific application. Therefore, the device manufacturer or designer must provide an intended hysteresis law with the device, and the application side must provide the application conditions and limits, like maximum required device displacements, vertical loading, temperature loading, moisture ranges, fatigue load collectives, etc. The suggested procedures are fairly general in these respects and must be tailored to a specific device and application. They also allow for the use of "numerical qualification models", if the model components are sufficiently validated. This may involve even stochastic simulations. Numerical qualification models are of particular interest in the qualification of large devices that are often found in bridges and where appropriate test rigs are rare or non-existent. In the end, it must be agreed between device provider and customer which procedures are to be applied in a qualification covering what range of parameters. Part II is intended to give a helping hand to all participants involved in this process based on a set of fundamental qualification criteria.