Prevention of Disproportionate Structural Collapse

NIST Modeling

Many U.S. buildings are vulnerable to extreme loads that may cause partial or total collapse. Modern structures have a limited reserve capacity to accommodate abnormal loads. There is no accepted science-base or design practice to maintain overall structural robustness within a multi-hazard context that considers both design loads and abnormal loads. The National Institute of Standards and Technology (NIST) is an agency of the U.S. Commerce Department, has initiated a new project will address the development of procedures and computational methodologies for assessment of overall structural robustness and will provide the measurement science needs for the development of performance-based provisions in U.S. codes and standards for disproportionate collapse resistance that will ensure improved robustness of building structures.  

The term “disproportionate collapse” is defined as the spread of an initial local failure from element to element resulting in the collapse of an entire structure or a disproportionately large part of it.

Since the terrorist attacks on the WTC towers and the Pentagon, owners of major U.S. buildings including the federal government (the largest single owner of buildings in the U.S.)  have emphasized disproportionate collapse resistance as one of the design requirements (see, e.g., Senate Committee Report 107-57). Currently, GSA, DoD, and DoS require that their buildings be designed and evaluated for disproportionate collapse potential. The NIST WTC Towers Investigation (NIST NCSTAR 1, Recommendation 1) calls for the development of consensus-based codes and standards for disproportionate collapse mitigation, and was reiterated in the investigation of the fire-induced disproportionate collapse of WTC 7 (NIST NCSTAR 1A).

There are no metrics to compare the overall system-level performance of structures, making it impossible to compare and quantify the safety performance of different types of structural systems. There is no accepted science-base or design practice to maintain overall structural robustness within a multi-hazard context that considers both design loads and abnormal loads. Critical measurement science capabilities that are lacking include;

 (1) system-level structural models capable of estimating the reserve capacity of building structures and assessing disproportionate collapse resistance,

(2) measures of structural robustness, and

(3) proven and cost-effective methods to mitigate disproportionate collapse.

The behavior of structural systems near their ultimate limit states is not well understood, and simulating this behavior depends on the availability of accurate structural models. Development of accurate models to predict reserve capacity of structural systems and allow the quantification of robustness, in turn, depends on the availability of validated models of members and connections. At the present time, experimental data on the behavior of connections undergoing disproportionate collapse are lacking. In addition, detailed modeling of a complete structural system to failure is often beyond the capability of existing tools. Reduced models of connections that capture the predominant behaviors and failure modes are needed for cost effective assessment of structural robustness and disproportionate collapse potential.  

While the measurement science needs identified above have not been solved, organizations in the U.S. such as NIST, ERDC, DTRA, and GSA as well as few universities in Belgium, England, and Japan have conducted limited testing of full and scaled models of steel subassemblies. No concrete subassemblies have been tested anywhere. Over the past few years, organizations such as NIST and a few private sector companies under contract to federal agencies have made some progress in the modeling of subassemblies; both at the detailed and reduced models levels. However, measurement science needs such as metrics for structural system robustness, system-level computational tools, and agreed-upon methods for disproportionate collapse mitigation have not been developed.  

This project will provide the measurement science needs for the development of performance-based provisions in U.S. codes and standards for disproportionate collapse resistance that will ensure resilience of building structures and as a result improve life safety and thereby improving the quality of life. Furthermore, the project is part of the “Measurement Science for Structural Performance under Multi-Hazards” program within EL’s strategic goal on “Measurement Science for Disaster Resilient Structures and Communities”. EL is well positioned to address the needs outlined in this project because of its long history in investigating structural failures and the capabilities built over the years in analyzing the failures of complex multi-story structures using state of the art computational tools.  

What is the new technical idea? A key focus of the project is to develop system-level performance metrics to quantify the robustness of building structures. Robustness is a key structural property that is related to disproportionate collapse resistance. Both structural redundancy and integrity are key factors that influence the robustness of the structure. These factors must be quantified to express the robustness in a meaningful and measurable manner. The assessment of the degree of structural redundancy for redistribution of loads and structural integrity for system continuity requires simulation of structural behavior under various local failure scenarios. Realistic and efficient simulations require the development and use of advanced and experimentally validated modeling methodologies to examine the structural system performance. Both traditional and new design concepts will be evaluated to determine the relative merits of various structural systems in resisting disproportionate collapse. The project will examine collapse limit states of structural systems to quantify the reserve capacity of various structural systems, through a combination of push-down and push-over analyses. The project will also develop design and retrofit methodologies that take explicit advantage of the synergies associated with mitigating disproportionate collapse under multiple hazards to enhance overall efficiency and cost-effectiveness.  The required work depends heavily on the use of advanced and detailed structural models to evaluate the nonlinear behavior of structural systems in post ultimate capacity limit states. With today’s high-performance computational tools, it is feasible to predict structural response due to abnormal dynamic loads. Complementary to the analytical evaluation of structures, the project will review and make use of the knowledge gained from controlled demolition technology. Through decades of experience, the demolition community has developed detailed knowledge about the collapse behavior of structural systems.  

The recommendations from a national workshop formed the basis for a coordinated national plan for problem-focused research on mitigation of disproportionate collapse of buildings. The project proposes to develop metrics to quantify the robustness of various structural systems to assess their disproportionate collapse potential. These metrics will be based on experimentally validated computational models of structural systems incorporating the predominant behaviors and failure modes of components and connections. Such models can also be used by design professionals in design for disproportionate collapse resistance. A key component in the development and evaluation of robustness metrics will be a series of push-down and push-over analyses to assess the reserve capacity of a variety of structures with different systems and materials. The project will develop performance objectives, acceptance criteria, and evaluation methods for both new and existing structures, which will be used to develop guidance documents and pre-standards for design and rehabilitation of structures to mitigate disproportionate collapse.  

The NIST  project will produce the following outcomes:

1. Best Practices Guide for design of new buildings and rehabilitation of existing buildings (Complete).

2. Computational methodologies to evaluate the disproportionate collapse potential of building structures for practicing engineers based on the following work:

Experimental:

(a) testing of full-scale subsystems to validate detailed computer models.

(b) testing of 3-D multi-story frames to validate reduced 3-D computer models.

Computational:

(a) development of reduced 3-D models of various structural systems.

(b) comparative assessment of reserve capacities of various structural systems.

(c) evaluation of structural systems capable of resisting disproportionate collapse.

3. Guidelines for assessing disproportionate collapse vulnerability, including both rapid and comprehensive evaluation guides.

4. Comprehensive guidelines for design of new buildings to resist disproportionate collapse.

5. Comprehensive guidelines for retrofit of existing buildings to resist disproportionate collapse.

6. Pre-standards for design of new buildings and retrofit of existing buildings to resist disproportionate collapse. 

FY 2010 the projects overview: 

1. Development of 3-D structural models of 10-story reinforced concrete shear wall and precast concrete buildings.
2. Evaluation of reserve capacity and development of structural robustness metrics for various structural systems.
3. Evaluation and comparison of disproportionate collapse vulnerability of various steel and reinforced concrete structural systems.
4. Design and testing of precast concrete beam-column assemblies.

Major Accomplishments:

Recent Results: 

Impact                 

Best Practices Guide (NISTIR 7396) adopted by ASCE 7-10 Standard as part of the commentary section on General Structural Integrity. 

Outcomes

• Developed experimentally validated 3D models of steel frame buildings for assessment of reserve capacity and vulnerability to disproportionate collapse (Journal publication being developed).
• Developed experimentally validated 3D models of reinforced concrete frame buildings for assessment of reserve capacity and vulnerability to disproportionate collapse (Journal publication being developed). 
• Published “Best Practices for Reducing the Potential for Progressive Collapse in Buildings-NISTIR 7396.”    

The National Institute of Standards and Technology (NIST) http://www.nist.gov/index.html