Structures may experience unforeseen operating environments or site-specific hazards leading to changes in the structure’s performance, safety, and longevity. These changes often prompt asset owners to undertake analysis efforts to ensure satisfactory structural performance for the updated conditions.
However, conventional analyses that fail to capture the true behavior of a structure can lead to inaccurate analysis results, causing owners to make less than ideal asset management decisions. Structural Integrity (SI) is uniquely positioned to pair our dynamic characterization and advanced structural analysis capabilities to generate a better structural model. SI vibration experts use impact testing, forced vibration, or ambient excitation sources, along with proprietary signal processing software, to non-destructively characterize the dynamic behavior of structural systems. This characterization is used to inform advanced structural analyses by SI analysis experts to provide more accurate results related to operational improvements, damage location, and retrofits.
A conventional analysis approach is to use available plan sets and construction documents to generate a structural model of an asset. This model is then subjected to various loads or operating environments to predict the current structure’s behavior. This approach is summarized in Figure 1.
Unfortunately, this approach gives no assurances that the developed analysis model appropriately represents the actual structure. Variances in material properties, mass distribution, time-dependent properties, accumulated degradation, and changes not represented in the as-built plan sets may lead to an analysis model that fails to identify deficiencies in the real-world structure. This may lead to less than ideal asset management decisions as owners may pursue unnecessary repairs or be unaware of current deficiencies.
To improve asset management decisions, analyses should be informed by real-world testing. Testing can be either destructive or non-destructive, with information collected from either static or dynamic tests. To minimize costs and invasiveness, SI uses non-destructive dynamic testing to identify key characteristics of a structure and uses the test results to improve the structural analysis models. For
instance, accelerometers can record the response of a structure to ambient excitations and provide insights to the structure’s natural frequencies and mode shapes. Knowing the key characteristics about the realworld structure allows SI to update the “conventional” analysis model to ensure that the “benchmarked” analysis model accurately represents the real-world structure, allowing for improved asset management decisions. This approach is summarized in Figure 2.
Analysis models based on plan sets, construction documents or other limited information may lead to models that do not represent the true insitustate of a structure. To improve asset management decisions, analysesshould be informed by real-world testing. Structural Integrity (SI) has the capabilities and experience to both dynamically characterize a structural asset and build a test-informed analysis model. Test-informed analysis models lead to more confidence in the analysis results and subsequent asset management decisions. With both characterization and analysis competencies in-house, SI offers a one-stop shop to help owners better manage their high-value assets.
The following SI projects have used dynamic characterization to lower client costs, improve analysis results, and/or aid owners in asset management decisions.
STRUCTURE: Hydroelectric Dam
OWNER: Water and Power Utility
ANALYSIS TYPE: Nonlinear Time History Analysis
OBJECTIVE: Seismic Vulnerability and Retrofit Assessment
The owner of an unreinforced concrete arch dam was required to address design and safety issues due to increased seismic hazard classification and probable maximum flood levels prior to relicensing by FERC. Built in the 1920s, this dam has significant concrete degradation due to seepage and many freeze thaw cycles. SI performed nonlinear dynamic time history analyses evaluating the as-built vulnerabilities of the structure and assessing the efficacy of several retrofit modifications. Previously, another consultant instrumented the dam to determine the dynamic characteristics, natural frequencies and mode shapes. As part of SI’s scope of work eigenmode analyses were performed with the finite element model. The calculated natural frequencies were within a percent of the on-site measured frequencies. This close agreement gave confidence in SI’s nonlinear analysis methodology to the owner and FERC regulators.
STRUCTURE: Nuclear Fuel Packages on a Vehicular Trailer
OWNER: Nuclear Fuel Manufacturer
EXCITATION: Highway Transportation Vibration
ANALYSIS TYPE: Spectral Analysis and Operational Deflected Shapes
OBJECTIVE: Vibration Study
A fuel manufacturer ships nuclear fuel across the country via trailer transport. SI was contracted to develop a test procedure and a mobile data acquisition system to measure the vibration acceleration experienced by the trailer and fuel packages during shipments across the United States. The system measured accelerations over the course of the multi-day shipment. SI analyzed the data to better characterize the vibration acceleration amplitudes as well as frequency content of the trailer-package-fuel assembly system as vibration is transmitted from the road through the trailer suspension, packages, and into the fuel assemblies. The improved understanding of the vibration signature for the transport system assisted the manufacturer in making decisions on how to best mitigate vibration levels during future transportation.
STRUCTURE: High-Rise Concrete Hotel
OWNER: Private Owner
EXCITATION: Ambient (Wind) Vibration
ANALYSIS TYPE: Nonlinear Time History & Nonlinear Pushover
OBJECTIVE: Support New Design
Tobolski Watkins Engineering (TWE), acquired by SI in 2017, provided structural and seismic consulting related to the nonlinear analysis of a 30-story hotel to determine acceptable performance of the building during large earthquakes and tropical storms. TWE developed a 3D finite element model of the building to perform nonlinear time history analyses, modal analyses, and nonlinear pushover analyses. Additional work included in-situ testing of the main tower to determine the damped natural frequencies of vibration of the bare frame structure, prior to the construction of building cladding and nonstructural components. The purpose of the testing was to provide input for calibrating the finite element model of the building and for use as input to extreme wind load calculations.
STRUCTURE: Induced Draft Fan
OWNER: Power Utility
EXCITATION: Impact Hammer, Externally Driven & Operationally Driven
ANALYSIS TYPE: Foundation Investigation
OBJECTIVE: Root Cause Analysis
A power utility noted excessive operational vibrations in an induced draft fan forcing the fan to be pulled from service, lowering the plant’s power output and daily revenue. SI was brought in to investigate the fan’s concrete foundation as a potential cause. SI performed a variety of in-situ dynamic tests to characterize the system including impact hammer tests of the stationary fan blades and concrete foundation, a frequency sweep of the system using a linear mass shaker and recorded 96 channels of data at various points in the system during quasi-operational runs. Interpretation of the recorded data led to a shift in focus from the foundation to the fan’s bearing. Disassembly of the fan bearing revealed several issues that were quickly resolved. Quasi-operational runs after reassembly showed a marked reduction in the operational vibrations, leading the fan to be put back into service.
STRUCTURE: Remote Shutdown Console
OWNER: Nuclear Utility
EXCITATION: Shake Table
ANALYSIS TYPE: Nonlinear Time History Analysis
OBJECTIVE: Seismic and Shock Base Isolation
Due to an increase in the required seismic ground motion and the inclusion of an aircraft impact load case, a nuclear utility sought to base isolate a remote shutdown console. Since the console needed to remain operational after the dynamic load cases, a fragility analysis was also needed. SI developed a finite element model of the console and used shake table test data, provided by the equipment manufacturer, to validate the modeling approach. SI then performed nonlinear time history analyses to investigate the feasibility of a shock mount system for the console. The inconsole response was used to determine the fragility demands imposed on the console, which were compared to client-defined thresholds.