Acceptance Criteria and Modeling Parameters for Structural Components under Multiple Hazards

Summary

In performance-based design, structures are designed with a specific performance objective in mind. An example of a performance objective would be ensuring little to no structural damage and a generally functional building after an earthquake, major windstorm, or other major hazard event. How can we determine whether a structure can meet its performance objective? First, a computational model of the structure is created and analyzed to determine the required force capacity (ability to resist load) and deformation/rotation capacity (ability to deform/rotate without rupture) of each structural component (e.g., beam, column, beam-to-column connection). These capacities determined by analysis are then compared to acceptance criteria (i.e., limiting values of strength or deformation/rotation) to determine whether the performance objective was met. This project fills existing gaps by developing acceptance criteria that currently do not exist for structures subjected to extreme winds and loads that can cause disproportionate collapse, a phenomenon whereby a local failure leads to collapse of a substantial portion or all of a structure. New acceptance criteria will be determined through large-scale laboratory experiments and further enhanced by computational analysis.

Description

Final damage to a precast concrete beam-to-column moment connection subassembly after displacing the central column downward during testing to simulate a column removal scenario.

Credit: M. Ammons/NIST

Objective
Develop acceptance criteria and modeling parameters of structural components for incorporation in performance-based methodologies and standards/codes for design of structures under multiple hazards.

Technical Idea
In performance-based engineering and design approaches, acceptance criteria and modeling parameters are critical in demonstrating that a structure can meet its performance objectives; they provide the basis by which structural components are modeled in design and the limiting rotation at which they fail. Structural engineers design structures for wind, earthquakes, blasts, and other extreme structural loads using nonlinear static and nonlinear dynamic finite element analysis approaches that rely on acceptance criteria to assess performance. While published values of acceptance criteria in ASCE/SEI 41 are consensus-based, they were developed based on limited data from cyclic tests considering only fully reversed cyclic flexural loading applicable only to seismic design. NIST researchers have identified major limitations in their applicability for wind and disproportionate collapse.

Based on the above discussion, a new framework is needed to allow the development of design criteria for connections that has the capability to consider span length, axial restraint, load- or deformation history (accounting for the influences of low- and high-cycle fatigue) and connection geometry so that it can be used for multiple hazards. 

Research Plan
To achieve the stated objective, the research plan has the following primary tasks:

1. Development of loading protocols:  The primary focus of this task will be on development of loading protocols for determination of acceptance criteria and modeling parameters for steel and reinforced concrete connections subjected to extreme winds and column loss scenarios.

2. Development of an experimental database of acceptance criteria and modeling parameters:  The loading protocols developed in Task 1 will be used to conduct laboratory testing on various steel and reinforced concrete connections to characterize their performance under extreme wind loads and column loss scenarios. The outcome of this task will be a comprehensive database of acceptance criteria and modeling parameters corresponding to different performance objective levels ranging from immediate occupancy to collapse prevention.

3. Validation of computational models and enhancement of database of acceptance criteria and modeling parameters:  The focus of this task is to develop and validate computational models of the tested connections, to provide tools for engineers to assess the system-level performance of structures under extreme winds and disproportionate collapse. Parametric studies using the validated modeling approaches will be conducted to (1) examine the connection performance with geometries or other design parameters not included in the experimental program and (2) identify critical parameters that influence the acceptance criteria and modeling parameters of the connections.

4. Enabling improvements to performance-based design standards for extreme winds and disproportionate collapse:  In this task, the NIST team will work closely with standards developing organizations to implement in their respective performance-based design standards the acceptance criteria and modeling parameters corresponding to different performance objectives developed in this project.

Major Accomplishments

• Developed, designed, detailed, and tested novel precast concrete beam-to-column moment connections for enhanced collapse resistance
• Project team members led the writing and editing of Ch. 1 and Ch. 5 of the Alternative Load Path Analysis (ALPA) Guidelines, which were established by the ASCE/SEI Disproportionate Collapse Technical Committee and will be publicly available this year