PROJECT OVERVIEW

Title: Identification and Mitigation of Nonductile Concrete Buildings

Project Status: Completed

Client: Federal Emergency Management Agency (FEMA)

PROJECT SUMMARY

This project seeks to provide more robust information about the specific characteristics of older nonductile concrete buildings by identifying certain subclasses with inherent structural deficiencies suspected to be the most significant, and that can be more readily identified, and that represent a more consistent level of risk.  Identification of the most vulnerable features of these subclasses will allow prioritization of research and more rapid implementation of current and emerging performance-based seismic design technologies for mitigation of hazardous nonductile concrete buildings.

Work on this project is a continuation of efforts undertaken on the FEMA-funded ATC 78 Project, under which ATC prepared the interim 50% complete draft report, Identification and Mitigation of Non-Ductile Concrete Buildings.

Task 1: Continue Pilot Study for Evaluation of Method for Selection of Collapse Indicators.

This task involves reviewing and advancing work performed in the ATC 78 Project (the first phase of this project) to develop a method of identifying, characterizing, and ranking the significance of deficient characteristics of older nonductile concrete buildings. The work-product of this task is intended to serve as a pilot study for evaluation of a methodology to select configurational and/or structural characteristics that are significant collapse indicators in older concrete buildings.  An outline for this pilot study is described as part of Task 1.4 in the NIST/ATC 76-5 Program Plan, but progress in the first phase will focus additional studies in more detail than the original outline.

Subtask 1.1 – Review Concrete Building Subclass Information from Related Projects and Define Basic Nonductile Concrete Building Prototypes.

This task requires coordination with ongoing research in this area, including: (1) studies undertaken as part of the NSF/NEES Grand Challenge research project; (2) efforts of the Earthquake Engineering Research Institute (EERI) Concrete Coalition; (3) the NIST/BSSC study to identify concrete building subtypes; and (4) efforts undertaken to implement the NIST/ATC 76-5 Program Plan.  Work in Phase 1 (ATC-78 Project) concluded that a major issue pertaining to determination of the importance of collapse indicators is development of methods to combine the effects of multiple collapse indicators in one building.  It was recommended to study whether initial separation of buildings into structural types would simplify the need to consider all combinations of all collapse indicators.  It was also suggested to study sequencing of inclusion of collapse indicators in such a way that all combinations were not necessary.

Subtask 1.2 – Conduct Problem-Focused Analytical Studies to improve and refine the methodology proposed to identify and prioritize collapse indicators.

Three major issues were identified in Phase 1 that need resolution:

• It is well known that many collapses of concrete buildings initiate in the gravity load carrying system.  The various seismic weaknesses in such systems must be incorporated into the overall methodology.  Stiffness and/or strength of such systems can be easily included in model studies, but P-delta effects cannot be included without creating another variable that will affect the significance of each collapse indicator.  The methodology must be generalized, either within the evaluation procedure or for study of collapse indicators.
•  Many, if not most, older concrete buildings contain walls that will affect response—either significant shear walls, or incidental walls such as fire stops or perimeter closer walls.  Concrete wall structures have been studied in previous projects associated with FEMA P695, Quantification of Building Seismic Performance Factors, but definition of collapse in such buildings is poorly identified.  Walls in combination with frames have not been studied.  A complete collapse indicator methodology must include walls.
• Models used in Phase 1 of this project were adequate to measure flexural response, or adequate to measure shear/compression failure, but not both.  The crossover effect on prioritizing collapse indicators must be studied and incorporated into the methodology.

Subtask 1.3 – Incorporate the results of Subtasks 1.1 and 1.2 into advancing Building Evaluation Rules or other evaluation methodology to identify buildings with high probability of collapse.

This task utilizes the trends from focused analytical studies to assist in developing an enhanced screening procedure that could be used to progressively prioritize (screen) an inventory of older nonductile concrete buildings.  As currently envisioned, this procedure would involve more detail and effort (with greater reliability) than the FEMA 154 procedure for rapid visual screening of buildings (FEMA 2002), but less effort than the ASCE 31 procedure for seismic evaluation of existing buildings (ASCE 2003). 

Task 2: Preparation of a Report Documenting Analytical Studies of Subtask 1.1 and the Draft Building Evaluation Procedures developed in Subtasks 1.2 and 1.3.

Work will include preparationand publication of a final technical report (April 2013)