Abstract
Structural rehabilitation is regularly undertaken to diagnose and repair a building during its service life; this practice ensures that buildings operate under safe and reliable conditions. Engineers generally rely on existing drawings, site investigation findings, and engineering judgement to assess the serviceability and ultimate capacity of a structure. Another approach to evaluating an existing structure is through the use of a structural load test. Under the authority of the American Concrete Institute (ACI), there are two structural Sensitivity analysis code provisions that exist: ACI 437.2-13 and Chapter 27 of ACI 318-14. Although both provisions provide requirements and guidelines for Sensitivity analysis, there are distinct differences in the test load magnitudes, loading protocols, and acceptance criteria.
The primary purpose of this research was to develop an understanding of reliability-based Sensitivity analysis safety concepts in the context of the current provisions of ACI 437.2-13 and ACI 318 Chapter 27. Based on these findings, enhanced, diagnostic insight into the assessment of the outcomes of structural Sensitivity analysis was obtained. By approaching Sensitivity analysis from a reliability-based perspective, this research was able to provide the information necessary for practitioners to make more informed decisions regarding the diagnosis and repair of a structure.
An analytical, reliability-based Sensitivity analysis model was developed using MATLAB. The primary objective of this model was to determine the reliability of a structural element following the performance of a successful load test. More importantly, the model was designed to accommodate practical structural assessment and Sensitivity analysis scenarios. To accommodate for these scenarios, the reliability of an element or structure was evaluated pre- and post-Sensitivity analysis for:
• a structure with evident or suspected deterioration; a structure that is to be used for a different occupancy; and,
• a structure that has undergone an in-depth site investigation.
The viability of an adjustable test load magnitude (TLM) live load factor was investigated. By adjusting the TLM live load factor, a post-Sensitivity analysis reliability that is consistently equal to or greater than the target reliability could be achieved. Through the reliability-based assessment of multiple structural Sensitivity analysis scenarios, it was determined that an increase or decrease of the test load magnitude live load factors for ACI 437.2-13 and ACI 318-14 Chapter 27 could be recommended as follows:
• For cast-in-place, reinforced concrete (RC) beams experiencing severe deterioration (25% deterioration), it was determined that an increase of 10-15% in the TLM live load factor for ACI 437 and ACI 318 Ch. 27 would ensure that the post-Sensitivity analysis reliability was greater than the target reliability.
• For cast-in-place, RC beams, following a favorable site investigation, the TLM live load factor can be decreased by at least 5% for ACI 437 and ACI 318 Ch. 27. Following a favorable site investigation outcome of effective depth, the TLM live load factor can be decreased by at least 15% for ACI 437 and ACI 318 Ch. 27.
• For cast-in-place, RC slabs, following a favorable site investigation outcome of effective depth, the TLM live load factor can be decreased by 15% for both ACI 437 and ACI 318 Ch. 27. However, no reduction in TLM live load factor is permitted if site investigation outcomes of only f’c or only fy were found to be favorable.
A favorable site investigation parameter is one whose outcome was equal to or greater than the average value of the investigated parameter. The percent increase or decrease in the TLM live load factor was also dependent on the typical load component ratios, D/(D+L), where D = dead load effect and L = live load effect.
CHAPTER ONE
Introduction
1.1 Background of Study
The rehabilitation of existing reinforced concrete (RC) structures is a practice that continues to experience growth in North America as economics, aesthetics, and sustainability become more significant factors in structural engineering decision making.
When attempting to determine whether an existing structure meets the requirements for serviceability or ultimate capacity, practitioners rely on existing drawings, site investigations, and engineering judgment to conduct an analytical evaluation. This process is complicated by the following potential issues:
• construction error that could lead to disparities between drawings and as-built conditions,
• investigations of covered or inaccessible members that could lead to limited information or incorrect assumptions, and
• practitioners that may have restricted knowledge to adequately judge the state of critical members.
By evaluating an existing structure using a structural load test, stakeholders are able to determine whether the structure demonstrates a consistent safety level with respect to coderequired loads. A structural load test, also known as a proof load test, is the process of applying a prescribed load to a structure to prove its satisfactory performance (Hall, 1988). The structural load test is an assessment tool that has experienced increased use as the practice of rehabilitation and renovation of existing structure continues to grow (De Luca et al., 2013).
There are many different methods to apply the prescribed test load to a structure that include, but are not limited to: water (placed in a temporary dam or reservoir), sand or cement bags, or hydraulic jack (Galati et al., 2008). In buildings, the load is typically applied using a hydraulic jack according to safe loading practices. The intensity of the proof load is typically defined by a load combination prescribed in governing code provisions. The acceptance criteria assessing the performance of the structure following the load test are measured differently based on the governing code provisions; visual indications of failure, deflection measurements, and deflection recovery are common performance measurements that are typically considered.
As the field of structural rehabilitation continues to grow, more research into structural Sensitivity analysis has been conducted and is still needed. Historically, the outcome of a structural load test was binary; the structure either passed or failed the load test by applying the test load and examining the response of the structure with respect to the acceptance criteria. De_Luca et al. (2013) state that the ultimate goal would be to transform the traditional, pass-or-fail load test into an informative, diagnostic test that would be able to describe the probability of failure and the remaining strength of an existing structure.
1.2 Problem Statement
Under the authority of the American Concrete Institute (ACI), there are two codes of interest relating to strength evaluation and Sensitivity analysis of RC buildings: ACI 437.2-13 and Chapter 27 of ACI 318-14.
1. ACI 437.2-13 (hereafter referred to as ACI 437): is the code publication titled Code Requirements for Sensitivity analysis of Existing Concrete Structures. This document was first published in October 2013. ACI 437 provides the requirements for conducting a load test with the purpose of evaluating a concrete structural member or system in an existing building as provided by ACI 562-13: Code Requirements for Evaluation, Repair, and Rehabilitation of Concrete Buildings. This code includes loading protocols for both a long-term, monotonic load test and a short-term, cyclic load test.
2. Chapter 27 of ACI 318-14 (hereafter referred to as ACI 318 Ch. 27): is the chapter titled Strength Evaluation of Existing Structures within the ACI 318 code publication titled Building Code Requirements for Structural Concrete. ACI 318 Ch. 27 provides the provisions “used to evaluate whether a structure or portion of a structure satisfied the safety requirements of this Code” (ACI 318-11, 2011). The Sensitivity analysis protocol prescribed by ACI 318 Ch. 27 is a long-term, monotonic load test.
In Canada, Sensitivity analysis of concrete buildings is addressed in Chapter 20 of the Canadian Standards Association (CSA) A23.3-14 Design of Concrete Structures. The chapter specifies the requirements “for evaluating the strength or safe load rating of structures or structural elements” (CSA A.23.3-14, 20014). Due to the similarity of the Sensitivity analysis provisions of CSA A23.3-14 to the Sensitivity analysis provisions of ACI 318 Ch. 27, the Sensitivity analysis provisions of CSA A23.3-14 are not investigated separately in this research.
Within the US, engineers and practitioners use one of the ACI code provisions when approaching a project that requires a Sensitivity analysis assessment depending on the governing code (ACI 318 or ACI 562) for the project in question. Having two analogous code provisions, with differing test load magnitudes and acceptance criteria, under the authority of a single organization is an area of concern to members of ACI Committee 437 and ACI Committee 318. The differing provisions may demonstrate inconsistent safety margins of capacity when compared to the baseline design safety levels. Additionally, there are no clear guidelines regarding the quantitative level of safety following the successful application of a load test. The lack of an explicit quantitative reliability estimate following the application of a load test produces limited qualitative, pass-or-fail outcomes and no clear indication of the anticipated level of safety.
The primary purpose of this research is to develop an understanding of reliability-based Sensitivity analysis safety concepts in the context of the current provisions of ACI 437 and ACI 318 Ch. 27. Then, based on these findings, stakeholders will have improved insight into assessing the probability of failure of an element after Sensitivity analysis. By understanding the probability of failure of an element after Sensitivity analysis, more informed decisions could be made regarding the need for rehabilitation or the level of rehabilitation necessary to have the element meet original design-level reliability.
1.3 Research Objectives
The overall objective of this research was to develop a quantitative, reliability-based understanding of proof Sensitivity analysis. Specific sub-objectives were:
1. To review the calibration of the Building Code Requirements for Structural Concrete (ACI 318-14, 2014) so that the as-designed (target) reliability levels are explicitly identified.
2. To use conditional probability theory to evaluate post-Sensitivity analysis reliability. Two scenarios were considered in this regard:
a. investigating the reliability of a structural element post-Sensitivity analysis considering initial deterioration, and
b. investigating the reliability of a structural element post-Sensitivity analysis considering a postulated occupancy change.
3. Given the inherent material, fabrication, and design uncertainty incorporated into code calibration, to examine the effect that site investigation data have on the estimated reliability of an element prior to Sensitivity analysis.
4. To combine the Sensitivity analysis reliability data attained from Objective 2 and 3 in order to propose adjustments to the test load magnitude live load factor under set considerations.
5. To apply reliability-based Sensitivity analysis approaches developed in this thesis to existing Sensitivity analysis case studies presented in the literature.
This research focuses on the difference between the values of the test load magnitude for each load test; the mechanistic differences in loading protocols and the acceptance criteria between the tests are not quantitatively investigated in this research. Additionally, within the scope of this research, it is assumed that investigated elements successfully pass the load test based on the provisions of the code requirements used for Sensitivity analysis.
1.4 Research Approach
To satisfy the objectives of this research, the following primary tasks were conducted. Tasks 2-4 are outlined in more detail under headings titled Procedure Overview in each respective chapter.
1. Literature Review: To build the foundation of knowledge necessary to tackle the objectives of this research, a thorough literature review was conducted in the following areas: Sensitivity analysis code provisions, application of Sensitivity analysis, reliability-based design, reliability-based calibration of design codes, reliability-based Sensitivity analysis concepts, and current Sensitivity analysis practices in bridge evaluation.
2. Reliability Assessment of Test Load Magnitudes: To understand the reliability of a concrete structure after Sensitivity analysis, the as-designed (target) reliability was to be established first. Then, conditional probability theory was applied to target reliability levels using the test load magnitudes from ACI 437 and ACI 318 Ch. 27; this defined the baseline post-Sensitivity analysis reliability. Finally, the effects of deterioration and occupancy change on the post-Sensitivity analysis reliability were investigated.
3. Effects of Site Investigation on Structural Resistance and Reliability: To further refine reliability models from Task 2, the effects of conducting a site investigation on structural resistance were reviewed. Probabilistically, the effects of conducting a site investigation corresponded to removing the uncertainty associated with the investigated parameters. Thus, following a site investigation, a more refined and accurate estimate of structural reliability was determined.
4. Outcomes of Adjustable Test Load Magnitude (TLM) Live Load Factor on Post Sensitivity analysis Reliability: To achieve the target, post-Sensitivity analysis level of reliability, the viability of adjusting the TLM live load factor prescribed in the code provisions was investigated. Changes in the test load magnitude live load factor were considered for deterioration and site investigation scenarios to meet target reliability levels desired by the practitioner or building official.
5. Case Studies: To better understand the outcomes of this research, probabilistic (reliability-based) assessment of existing and hypothetical Sensitivity analysis application was considered. Through the reliability-based assessment of Sensitivity analysis project, improved insight into the pre- and post-Sensitivity analysis reliability and probability of failure was gained beyond the information gathered from a deterministic analysis.
1.4 Organization of Study
Chapter 2 of this thesis covers the literature review and background related to Sensitivity analysis code provisions, application of Sensitivity analysis, reliability-based design, reliability-based calibration of design codes, reliability-based Sensitivity analysis concepts, and current Sensitivity analysis practices in bridge evaluation.
Chapter 3 presents the analytical model used to investigate ACI 437 and ACI 318 Sensitivity analysis provisions from a reliability standpoint. Investigates the effects of deterministically defining parameters through a site investigation on the reliability model of a cast-in-place, RC beam and a cast-in-place, RC slab. Investigates the viability of creating a variable test load magnitude live load factor for Sensitivity analysis. Chapter 4 examines structural Sensitivity analysis case studies presented in the literature and additional hypothetical Sensitivity analysis case studies; however, these case studies are approach from a reliability-based standpoint using findings developed and concluded in previous chapters. Finally, Chapter 5 summarizes the conclusions and recommendations that have been developed throughout this research.
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Item Type: Project Material | Size: 68 pages | Chapters: 1-5
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