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Using CFD visualization to improve the integrity and safety of structures

In a post-9/11 world, integrity of structures is of paramount importance. Weidlinger Associates Inc., a structural engineering and applied mechanics firm based in New York City, augments advanced analysis with CFD visualization tools to solve research and design problems related to the safety of civilian and military buildings and infrastructure.

Between 2000 and 2007, Weidlinger Associates designed a replacement for the original poured-concrete steel grid deck of the Bronx-Whitestone Bridge in New York City (see Figure 1). As part of the effort, the firm performed a seismic retrofit of the bridge to provide assurance that the revised bridge maintained a level of performance in earthquakes consistent with the previous design and in conformance with New York City Department of Transportation (NYCDOT) Seismic Design Guidelines.

Figure 1: Bronx-Whitestone Bridge – Deck Replacement Design

In the 1990s, Weidlinger began using an advanced numerical simulation and computational fluid dynamics (CFD) visualization tool to create state-of-the-art visual simulations. Called Tecplot 360, this tool enabled them to quickly make sense of vast amounts of complex information. Since then, Tecplot 360 has been used by select Weidlinger engineers on a variety of projects, most of which entail assessment of structural vulnerability from a variety of natural and manmade hazards. Any tool that can assist in better understanding the scope of the problem and points to areas for investigation and the most efficient design enhancements is beneficial. After an engineer conducts an analysis, he or she has enormous quantities of data, which have to be harnessed somehow to understand what has actually been computed. Tecplot 360 offers many different ways to look at the data. And because a lot of data can be seen at the same time, the biggest possible picture can be developed.

The Bronx-Whitestone seismic analysis was one of many successful collaborations among Weidlinger’s transportation and applied science engineers. As a leading structural engineering and applied mechanics consultant, the firm has designed and rehabilitated many high-profile buildings and bridges, and has been awarded many Small Business Innovative Research grants for generating advanced technology and software. The firm also offers special services in vulnerability assessment; risk analysis; forensic, earthquake, wind, and blast engineering; soil/structure interaction; and sustainability. The synergy of applied science research and structural engineering practice sets Weidlinger apart and leads to participation in projects of national and international significance.

When long-span structures on multiple supports, such as the Bronx Whitestone Bridge, are subjected to seismic motions, the structure, the foundations for each support, and the soil on which it is founded form a complex dynamic response system. With the onset of an earthquake, the soil deforms as part of the wave propagation phenomenon in what is termed “free-field” motion (i.e., soil in the absence of the structure). When the seismic waves impinge on a foundation, the embedded structure causes both scattering and reflection of the seismic waves. Additionally, the presence of the foundation in the soil affects the behavior of the soil itself; the mass and stiffness of the foundation changes the effective resistance of the immediately adjacent soil, thereby attracting seismic load to the structure. The subsequent dynamic movement of the foundation will affect the superstructure;this motion both feeds back to the foundation and further radiates back out into the adjacent soil.

Weidlinger used SASSI2000 to assess the dynamic interaction of the soil-foundation system. This ensured a more accurate numerical simulation of the seismic soil-structure interaction, which increased confidence in the results. To visualize the results of the dynamic analysis of the soil-structure interaction and to better understand the patterns of response at different depths and within different layers of soil, Weidlinger used Tecplot 360.

Figure 2: Horizontal Acceleration Spectra in Soil

Figure 2 shows horizontal acceleration spectra for a design seismic event in a column of soil near one of the bridge foundations. The image on the left portrays the “free-field” acceleration spectra (response in the absence of the foundation), whereas the image on the right portrays the spectra in the soil immediately adjacent to the foundation. The presence of the foundation changes the horizontal motion in the soil at different depths and at different spectral periods. The ability to visualize these patterns in soil motion improves an engineer’s ability to make physical sense of what would happen in a seismic event. Furthermore, identifying how the soil motion affects the foundation allows an engineer to focus his or her time and resources on response periods that have the greatest impact on the performance of the bridge and on whether a retrofit is required.

Before software similar to Tecplot 360 became available, hundreds of spectra would have to be inspected to arrive at the same level of understanding. Tecplot’s data structure format allows the engineer to prepare the raw data for viewing in short order. It is invaluable where analysis software does not have advanced post-processing capabilities. Identifying hot spots is not sufficient to draw conclusions, but it is very helpful in focusing attention on the raw data that requires further study. Previously, an engineer actually sliced through the data outside of the software and brought it into the application in pieces; now these two steps can be performed simultaneously. Another way to look at it is that Tecplot 360’s visual screening methods give engineers the quality control they require. The more they see in a single frame, the better overall picture they get. The end results are better and faster recommendations to clients and safer structures.

Adam Hapij is a senior associate at Weidlinger Associates Inc.

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