Structural Education Module 1 (SEM1): Planning SHM Projects

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1 Structural Education Module 1 (SEM1): Planning SHM Projects Summary: The purpose of this education module is to expand your knowledge base on the topic of SHM. You will learn how to apply the SHM knowledge gained in FEMs 0 through 4 to plan for a specific SHM project. At the end of the module, you will be able to discuss the different SHM system alternatives that can achieve the project s specific goals. EXPECTED LEARNING OUTCOMES At the completion of this module you will be able to describe (K) the goals of a specific SHM project. identify (K) the critical sections on which sensors will be installed. discuss (C) the different alternatives and choices for SHM system selection. YOUR ASSIGNMENT Listed above are the expected learning outcomes for this module. After you have read and reviewed the content herein provided, you will be required to take an online examination to determine if you have achieved a satisfactory mastery of these learning outcomes. Finally, you will submit an on-line response to several questions related to the content of this education module as well as list any questions you might have concerning anything you might not understand about the material. The on-line responses will be discussed in an interactive manner in a classroom setting on the date indicated below. Deadline for completion of assignment: see SHM Education Unit Opportunity on Moodle Date of classroom discussion of on-line responses: Tuesday April 25 th, 2017

2 BEFORE YOU EMBARK ON AN SHM PROJECT As you have learned in the fundamental modules (FEM0-FEM4) that you covered last semester, SHM can be used for multiple purposes. Actually, it is very hard to find a universal definition for the term Structural Health Monitoring. It is so broad that it can mean different things at different stages of a structure s life. Ghandi and Thompson (1992) provided an analogy between SHM in humans and structures in which they categorized SHM activities based on three different life stages: Analogy between medical activities and SHM (from Ghandi and Thompson 1992) Phase of the life Human Skin Structural Health Health Birth Birth Monitoring Process Monitoring Sound Life Health Check-up Health and Usage Monitoring Illness and Death Clinical Monitoring Health (Damage) Monitoring Unlike human skin and nerve system, which is equipped with millions of sensors that send signals to let patients and doctors know what is going on, we do not have the luxury to install that many sensors in infrastructure projects. In addition to the cost being prohibitive, installing millions, or even thousands, of sensors would take huge resources, time and labor. Furthermore, interpreting data from millions of sensors can be overwhelming. Therefore, we have to be selective and optimize our choices of sensor numbers and types. This choice will be greatly affected by which phase of life is your structure in; i.e., birth, sound life, and illness. FACTORS AFFECTING SHM PROJECT PLANNING So, why does the phase of life affect SHM project planning? There are many reasons for that. For example, while it may be possible to embed sensors in a structure that is being built, it is not possible to do that for a structure that has already been in service. The figures below show both embedded and surface-mounted types of sensors.

3 Embedded sensors installed on reinforcement cage prior to pouring concrete Surface-mounted sensors installed on cured concrete surface Accessibility can also limit your choices. A heavily traveled bridge can impose limitations on when and how many sensors can be installed without causing major disruptions to traffic. Availability of a power source is another question that you should ask to determine how the SHM system will be powered; e.g., using a power outlet, solar panel, or one of the new energy harvesting systems.

4 Solar panel powering a SHM system in J.J. Audubon Bridge Project, New Roads, Louisiana The scope of the project will dictate whether a long-term (weeks, months or even years) or a short-term (collect a certain measurement or conduct a live load test only) SHM system will be needed. While these are two main categories of SHM activities, ISIS and SAMCO (2006) provides a more detailed categorization of SHM with some additional sub-categories that can be seen in the figure below. More details of these SHM project categories will be discussed later. Furthermore, SHM systems for damage-detection can be designed to provide information at a level of detail from I to IV (see FEM2) depending on the goals of the SHM project. Categories and sub-categories of SHM systems (ISIS and SAMCO 2006) Finally, of course budgets can be a huge constraint on the scope of a project. Funding is not endless, and one of the first things that you should ask is how much funding is available for a certain SHM project. Additional features, increasing the scope, modifying the specifications are

5 all possible, but it is only through negotiations with the bridge owner who will ultimately control the budget, and hence the delivered SHM system. Therefore, before you sit down and start planning for an SHM project and designing the SHM system, you will need to ask these questions SHM TESTING CATEGORIES Now let s cover SHM testing categories and subcategories in more detail. We will go over the four main categories discussed earlier and some of the subcategories under each one. More details can be found in ISIS and SAMCO (2006). Static Field Testing It is stated in ISIS and SAMCO (2006) that static field testing is the most commonly used method to determine the load carrying capacity of structure, and provides data about a structure s behavior and ability to sustain live loads. Static field load testing is not new and has been used for almost a century before the advent of new SHM technologies. It provided a proof load confirmation to existing structures and focused on measuring deflection for which many methods had been in existence for a long time. During a static field load test, known loads (e.g. weighed trucks) are slowly placed on the bridge at critical loading positions that cause the maximum desired effect. Alternatively, trucks can crawl on the bridge at a slow speed while reading measured data. In both cases, there should not be any dynamic amplification effects; i.e., a pure static test. ISIS and SAMCO (2006) lists three basic types of static load tests: behavior, diagnostic, and proof. - Behavior Tests In this type of test, the objective of the project is to understand the structural behavior of the project and/or to verify the methods of analysis/design. - Diagnostic Tests While the method used for diagnostic tests is the same for Behavior Tests, the objective of the project is different. - Proof Load Tests Proof tests are used to study the load carrying capacity of a structure by inducing proof loads on the structure. Proof loads can be greater than the maximum service load for a given configuration without causing any damage. Dynamic Field Testing Dynamic field testing assess the behavior of structures subject to moving loads. They are more applicable to bridge structures that are subjected to moving vehicular load. However, some of these tests are also used to determine the dynamic characteristics of buildings, which is important for designing these building to resist transient loads such as wind and earthquake loads. - Stress History Tests The purpose of a stress history test is to determine the range of stresses experienced by parts of a structure that are prone to failure by fatigue.

6 - Dynamic Load Allowance (DLA) Tests Design codes provide nominal design loads that engineers have to amplify to account for dynamic effects. The magnitude of the dynamic amplification varies and can be determined using DLA tests. - Ambient Vibration Tests The purpose of an ambient vibration test is to identify the vibration characteristics of a bridge by measuring its response to ambient dynamic forces. - Pull-Back Tests In pull-back tests, the exciting forces is typically a lateral (sideways) dynamic excitation. This can be achieved by cables that are anchored to a stiff object that are then released suddenly. Periodic Monitoring The objective of periodic SHM is to investigate any detrimental changes that might occur in a structure between monitoring periods. It can be conducted using several of the aforementioned test types. The following are only a few of these - Bridge monitoring through testing under moving traffic For bridges, moving traffic rather than especially loaded trucks can be used to conduct this type of test. Periodic changes in the readings from installed sensors under the same traffic conditions can be used to provide an indication of any damage or deterioration. - Monitoring through Static Field Testing Not a very common periodic monitoring test because of the associated costs (e.g. trucks, traffic control, ). Nevertheless, it is useful in certain situations such as validating new design methods or use of innovative materials. - Monitoring Crack Growth Cracking in concrete structures is an expected outcome under service conditions, however, properly design structures should not have excessive cracks in term of numbers, widths, or extents. A structure that exhibits uncommon cracking behavior can be monitoring to see if the cracks grow further or are they stabilized. Continuous Monitoring Recently, continuous monitoring has become a feasible SHM alternative, which was not the case a few decades ago prior to the recent technological advancements in sensor and DAQ technologies and also because of the costs. This type of SHM activity is usually reserved to important structures or research activities. Because of the nature of continuous monitoring projects, data is often retrieved remotely to minimize the need to visit the structure s location to retrieve the data manually.

7 Remote data retrieval system using a cellular modem connection SHM SYSTEM DESIGN METHODOLOGY A general methodology for designing SHM systems that are developed to assess health via detection of damage has been summarized in steps by ISIS AND SAMCO (2006). These recommended steps in the evolution of the SHM system from the structural engineer s perspective are as follows: 1. Identify the damage or deterioration mechanisms that are of concern for the structure, and the critical sections at which they may occur. 2. Categorize the influence of this deterioration on the mechanical response of the structure or its key components under service loads; this includes the development of appropriate theoretical and numerical models of the structure. 3. Establish the characteristic response of key parameters, experimentally and/or theoretically, such as strain, vibration, or tilt and establish the sensitivity of each to an appropriate level of deterioration. 4. Select the most sensitive parameters and define a damage or performance index which relates the change in response under services loads to the level of deterioration. 5. Design the monitoring system, including the selection of sensors, data acquisition and management and data interpretation; this will include a determination of which type of monitoring should be conducted such as static or dynamic, continuous or periodic, controlled loading or ambient loading. 6. Install the system and calibrate with baseline readings. 7. Assess field data and adapt the system as necessary. Keep in mind that even though these steps are based on the assumption that the SHM system is developed to assess health via detection of damage. If the SHM is designed for an objective

8 other than damage detection, e.g. monitoring, the outlined steps can still provide good guidance. REFERENCES Gandhi, M.V., Thompson, B.S. Smart Materials and Structures, Chapman et Hall, ISIS and SAMCO. Educational Module 5: An Introduction to Structural Health Monitoring. (retrieved: December 18, 2016)