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Chapter 1: Buildings

B.  SEISMIC REHABILITATION PROCESS

Introduction to Seismic Rehabilitation Process

Seismic risk evaluation and seismic upgrading is a complex and often intimidating task.  Planning for a risk that can come at any time, but happens infrequently is difficult.  Earthquakes are unique in natural disasters in that they can occur without warning, and thus can place people at risk in vulnerable structures who in other natural hazards would be able to retreat to a safe location.  This places an extra burden on building owners, and on local government, to ensure that occupants and other users of facilities are not unduly exposed.  Thus, the principle purpose of hazard mitigation is the protection of life, even when the risk to a single individual at any one time is comparatively small.

To gain an understanding of the "seismic rehabilitation process," it is best to subdivide the endeavor into a series of steps, and identify the disciplines required for each step - from that of the building ownership and management, to that of the architect and structural engineer.  The seismic rehabilitation process is not entirely concentrated in the discipline of structural engineering.  It calls on the disciplines of Seismology, Public Administration, Risk Analysis, Statistics, Economics, Building Conservation Technology, and Materials Testing and Inspection, to name just a few.  The decision for an owner may have to do with an evaluation of the risk and economic viability of an asset.  However, for a whole community, it is difficult decisions over where to place public investment when limited resources are available.

One of the most important things to consider is that what may seem obvious at first glance may become less obvious on more detailed examination.  Buildings that, because of their age and the nature of their structural system, may have been identified in overall surveys as hazardous (say, because they are constructed of unreinforced masonry,) may on closer inspection, be found to be comparatively less risky than nearby newer structures of non-ductile concrete frame.  These newer buildings may be used by many more people in any given day, and thus placing a far greater risk on the community than the older masonry building.  If this is found to be the case, then which do you upgrade?  A prudent administrator in such a situation may find that the route to take is to fix the one which combines high occupancy with collapse potential instead of the one with lower occupancy which none-the-less may also be easily damaged, but may not be likely to collapse.  Another factor may be the need to upgrade facilities that are critically important use in a post-earthquake environment, such as fire stations and hospitals.  All such factors must be weighed when making the kinds of decisions that require the investment of comparatively large sums of money to carry out tasks whose value will only become apparent when and if an earthquake indeed does happen.

It is also important to understand one of the lesser known facts about the structural engineering design process that can have a significant impact on the costs, whether they be with public funds or private investment.  In many cases involving the larger more complex structures, it has proven to be the case that any funds beyond the minimum that is invested in engineering analysis up front, may have a large payoff in the end in terms of money saved on construction costs.  There are even examples of projects that cost one fifth or less of the originally estimated costs as a result of in depth engineering analysis leading to a more creative design solution.  The codes often prescribe a set procedure, but these same codes allow other more time consuming and sophisticated analysis techniques to be used in lieu of that procedure.  It is important for the decision makers to understand that the difference does not mean that such a design would fail to meet code, but that the standard code procedures tend to be over conservative by comparison because, absent the more detailed analysis, one would have no scientifically derived basis for the design decisions that are specific to the subject building.

In light of this, the decision maker on a given project is on the horns of a dilemma.  He or she must decide if the further investment in engineering analysis will bear fruit before it can be known that it will.  This decision comes at a time when access to funding for the project may be extremely difficult.  Once construction is underway, the funding sources are, of course, already identified, and the schedule set, but getting funds in place at the design stage can be more difficult.  The point is made only to emphasize the importance of careful and complete analysis in the Seismic Rehabilitation Process if scarce resources are to be husbanded for the most cost effective results, either on an individual project, or for a whole district of community.

Seismic Review Process Steps

The Seismic Rehabilitation process can be subdivided into a series of steps.  The first step is to determine the rehabilitation objective.  In order to do this, the subject building(s) need to be evaluated not only in terms of engineering, but also in terms of the societal issues, from which one can determine the value of the rehabilitated structure in both monetary and non-monetary terms.  Considerations such as whether or not the building is historic, is architecturally significant (which can be affected by the rehabilitation design), is used by many people all the time, or for a short period of time (such as a church on Sundays, which is empty during the week), etc.

This evaluation also must cover the technical issues - both the safety of the structure and the safety of the building's non-structural features and contents.  Following the evaluation of all of these elements, a "Rehabilitation Objective" can be defined.  This Rehabilitation Objective is an expectation of the performance of a building after a seismic event of a particular magnitude.  Building performance can be described qualitatively in terms of the safety afforded building occupants during and after the event; the cost and feasibility of restoring the building to pre-earthquake condition; the length of time the building is removed from service to effect repairs; and economic, architectural, or historic impacts on the larger community. These performance characteristics are directly related to the extent of damage that would be sustained by the building.  A more detailed discussion of the determination of a Rehabilitation Objective can be found in FEMA 356

In summary, the objective can be to achieve a performance level after rehabilitation that lies somewhere on a continuum from "Collapse Prevention," through "Life Safety" and "Damage Control" to "Immediate Occupancy."  Above that - reserved for the most critical or hazardous facilities, would be "Operational" level.  For almost all buildings in ordinary public and private use, a performance level that presumes no damage is economically impractical, and not necessary to meet public responsibilities and balance the risks of damage with the costs of construction or upgrading.  In reference to this, it is important to understand that the current codes for new construction are based on an acceptance of damage from earthquake forces in a moderate to large earthquake.  In a large earthquake, that damage could be extensive.  The code objective is to prevent collapse and minimize falling debris, not to prevent damage. 

On the issue of codes, it is also important to understand that most local codes are written to deal with new construction.  As such, their provisions may not be appropriate for the seismic rehabilitation of existing buildings.  Existing buildings, particularly those constructed using technologies and construction methods which have since gone out of use, are not suitable for redesign following the detailed proscriptive provisions of new building codes.  This does not mean that they cannot be made to meet the objectives of the current code in terms of safety and performance as described in the previous paragraph, only that they cannot be assumed to be able to do it in the same way as is done for new construction.  In recent years, there have been a number of important codes, and guidance documents prepared specifically to aid in the rehabilitation and seismic upgrading of existing buildings of different types, including those prepared and published by FEMA used as sources for this Handbook.  In California, and other states, the Uniform Code for Building Conservation has been incorporated into the state's model code.  All of these sources are likely to be more appropriate for rehabilitation design than are the standard codes for new construction.

This Handbook

The balance of this section on buildings in this Handbook is divided into three parts: (1) an introduction to structural concepts, intended to help the non-structural engineer understand how the structural engineering design process can impact a project overall, (2) the assessment step, in which a facility is evaluated according to a series of major seismic risk variables, and (3) a brief description of different seismic upgrade solutions.  These are divided into categories under the different structural types that are likely to be encountered in most projects.  These illustrated solutions are not intended to be comprehensive or detailed, but rather to provide a simple identification of the most basic different solutions that are often carried out for the representative structural deficiencies.

This handbook has condensed and excerpted materials from the FEMA publications listed above,  as well as other sources.  It is intended to provide a basic introduction to what is involved in making a decision to undertake seismic upgrading of buildings of differing structural types and sizes.  The following section provides a description of the different technical items to consider in evaluating different facilities, and then provides a Microsoft Excel Spreadsheet that can be used to develop a comparative assessment based on the "Rapid Visual Screening" methodology developed by FEMA, and published in FEMA 154.  This was first published in 1988, and has recently been modified and updated, and is currently in-press.  The numbers used in this spreadsheet are the ones from the updated 2002 version.

Proceed to C. STRUCTURAL CONCEPTS

 

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