Collapse Performance Modeling of Ordinary Reinforced Concrete Columns under Combined Axial and Seismic Loading

Summary

This project studies the collapse performance of concrete columns that do not meet the stringent reinforcement requirements for special columns defined by ACI 318, referred to hereafter as “ordinary” columns. The main characteristic of this type of column is relaxed detailing requirements for reinforcing steel, such as wider transverse reinforcement spacing. Ordinary reinforced concrete columns are found in regions of low to moderate seismicity nationwide and also are typical of columns in older structures, designed under less robust design rules. Observations from past earthquakes have shown that these columns can exhibit nonductile behavior and can be susceptible to shear and axial load failures under strong shaking, which can lead to building collapse. This project will collect and assess available experimental data as well as numerical models on the shear and axial load failure response of reinforced concrete columns, and utilize this information to develop a new numerical model for ordinary reinforced concrete columns to enable their inclusion in the Performance Based Seismic Engineering (PBSE) framework. The outcome of this study will be used by practicing engineers and researchers to assess the performance of existing buildings or to design new buildings with ordinary reinforced concrete columns.

Description

Objective - The objective of this study is to improve the simulation and prediction of the shear and axial load-deformation response of ordinary reinforced concrete columns in the context of PBSE. This study will be conducted at (1) the element-level, where a new shear and axial load failure model will be developed for individual columns; (2) the system-level, where a pilot study will be conducted to examine the performance of the proposed shear/axial failure model in the collapse prediction of single- or multi-story buildings comprised of ordinary columns. The expected delivery date for this project will be Q2 of 2021.

What is the Technical Idea? Collapse assessment of reinforced concrete buildings has been conducted by academic researchers and engineering practitioners for a number of years. The focus of much of this work has been on special reinforced concrete columns, those with improved reinforcement detailing appropriate for regions of moderate to high seismicity. Ordinary reinforced concrete columns represent a less ductile class of column where behavior under large demands can lead to abrupt loss of capacity. The collapse mechanics of ordinary reinforced concrete columns/frames is usually governed by shear failure of the columns followed by loss of their axial load capacity. Experimental and analytical research has been performed in recent decades to develop numerical models for predicting shear failure (e.g. [Vecchio and Collins (1986); Lehman and Moehle (1998); Sezen (2002); Elwood (2004); Ghannoum and Moehle (2012); Baradaran Shoraka and Elwood (2013)]), as well as the axial load failure (e.g. [Elwood (2004), 2004; Baradaran Shoraka and Elwood (2013)]) of concrete columns. Among these available models, the challenging question that still remains is how well each model can predict the response of ordinary concrete columns in an extreme event, and which model is suitable for implementation in the PBSE framework. Moreover, it must be noted that in collapse assessment procedures, shear/axial load failure models are based on the response of single columns, i.e. the element-level, yet are implemented to predict the collapse of a story comprised of multiple columns, i.e. the system-level. However, the collapse assessment of a system may require further improvements and calibration of the available element-based shear/axial load failure models.

Sattar (2013) showed the need for improvements in the shear failure models for nonductile concrete columns to capture the shear failure at small drift ratios (<0.01), which is typical in this type of column. In addition, it is believed that improved models to simulate the axial load failure of concrete columns and the interaction between the shear and axial failure also are needed. In the element-level phase of this project, this study will identify experimental data from available resources worldwide, and then will develop accurate yet simple tools for predicting the shear and axial load-deformation response of reinforced concrete columns under strong shaking. Moreover, during this phase, an inventory of shear/axial tests on concrete columns will be developed, particularly data from tests of ordinary reinforced concrete columns.

The findings of the first phase of this project, i.e. the element-level, will be carried to the second phase, to assess the collapse performance at the system-level, that of a story. A large portion of the research conducted by the earthquake engineering community in recent years has focused on collapse assessment. However, the definition of collapse still remains challenging. This project will conduct a study on the collapse assessment of a system (a building) composed of ordinary columns, in order to (1) evaluate the performance of the shear/axial failure model developed in this study when employed in the response prediction of a building system of columns, and (2) identify the research needs for predicting the system-level collapse.

What is the Research Plan? This project will be a 42-month effort to be initiated in Q1 of FY2017 involving an extramural team working collaboratively with NIST EEG engineers. This extramural project will be awarded under Earthquake Engineering Initiative funds through the NIST FFO grant process to a university research team selected by competitive process. The awardee’s team will collaborate with an internal NIST team conducting complementary work on this topic. NIST team will prepare the solicitation and review the submitted proposals, and select the awardee, in Q2 of FY 17. In Q3, the NIST internal research group will collaborate with the awardee to develop the detailed framework for conducting the multi-organization team effort including the portion to be conducted by EEG. The tasks expected to be conducted by the future post-doctoral researcher include exploring the available numerical models and collecting the available experimental data. Due to the cooperative nature of this project the NIST internal tasks can be modified during the project development phase.