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Sensitivity of a wind vulnerability model to wind driven rain deposition estimates Timothy Johnson1, Jean-Paul Pinelli2, Thomas Baheru3, Arindam Chowdhury 4, Johann Weekes5 , Kurt Gurley6, 1 Department of Civil Eng., PhD Candidate, Florida Institute of Technology, Melbourne, Florida, USA 2 Department of Civil Eng., Professor, Florida Institute of Technology, Melbourne, Florida, USA 3 Department of Civil and Environmental Eng. PhD Graduate, Florida International University, Miami, FL, USA 4 Department of Civil and Environmental Eng. Associate Professor, Florida International University, Miami, FL, USA 5 Department of Civil and Coastal Eng. PhD Graduate, University of Florida, Gainesville, FL, USA 6 Department of Civil and Coastal Eng. Associate Professor, University of Florida, Gainesville, FL, USA email:
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ABSTRACT: Recent large-scale tests at the Wall of Wind (WoW) hurricane research lab provide some valuable insight into the wind-driven rain deposition distribution on low rise buildings in hurricane conditions and the subsequent surface run-off water. The tests lead to a better understanding of the volume of water that impinges on a building surface and the resulting amount of water that can penetrate through the building envelope, which is one of the predominant factors that contribute to interior damage. An engineering model developed for the Florida Public Hurricane Loss Model incorporates the results of these tests to quantify the volume of water that ingresses through the building envelope due to a building component failure or defect. The goal of this paper is to present this model and these test results in a manner that maintains the building vs. wind-driven rain relationship yet is practical in terms of vulnerability modeling. The results indicate these latest test results have a significant influence on the overall building vulnerability due in particular to the assessment of surface runoff rain. KEY WORDS: Wind-Driven Rain; Hurricane Vulnerability Assessment; Interior Damage; Model Sensitivity. 1
INTRODUCTION
Hurricane catastrophe models such as the Florida Public Hurricane Loss Model (FPHLM) are constantly evolving by incorporating knowledge from both field and laboratory test results [1]. This is necessary for validation and calibration of the models. In particular, post hurricane damage surveys highlighted significant water damage to the building interior even with relatively little exterior damage [2],[3]. Subsequently, in recent years, there has been a greater focus on interior damage estimation and its causes [4],[5]. The primary cause of interior damage was attributed to wind driven rain (WDR) that impinges on the building envelope and penetrates the façade through deficiencies or “defects” in the building envelope, as well as pressure or debris induced breaches. The issue of impinging rain and the building vs. driving rain interaction has been studied by Straube and Burnett [6],[7], Blocken et al. [8] and others [9], although not under tropical storm conditions. Recent efforts by Baheru et al. [10],[11],[12] are the first to quantify the fraction of direct impinging rain and accumulated surface runoff on a building system under simulated hurricane conditions. The focus of this paper is the combination of these test results with the exterior and interior damage model developed for the FPHLM [13],[14], to quantify the volume of water penetrating into a building and the subsequent interior damage. The tests specifically focused on the two main sources of water intrusion: direct impinging rain and surface runoff rain. The metrics used to quantify these sources were defined as the Rain Admittance Factor (RAF) and the Surface Runoff Coefficient (SRC) respectively. The challenge the modelers face lies in the utilization of limited available test data into a damage model with limited damage resolution to capture the variation of deposition characteristics along the building surface. Consequently, the model may be overly sensitive to variations in the deposition characteristics, which may warrant further refinement. A one-story frame construction gable roof model will serve as the test case to incorporate the test results. 1.1
RAF and SRC Test Results
The RAF is generally referred to as the ratio of free stream horizontal rain that deposits on a building façade. It is the fraction of the approaching horizontal rain that strikes the building. It accounts for the effect of a large portion of the rain moving around the structure with the wind rather than striking the building surface and is dependent on the building shape. RAF is independent of the wind speed, but is a function of the wind direction with respect to the building and is defined by the equation (1):
14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015
2 𝑅𝑅𝑏,𝐷𝐼 (1) 𝑅𝑅𝑣 where RRb,DI is the rate of WDR deposition at a given location on the building façade due to direct impinging raindrops. RRv is the rain rate in an unobstructed free stream wind profile at the mean roof height. For a full description on the tests carried out to measure RAF under hurricane rain and wind conditions, see Baheru et al. [12] Ultimately, contour lines of RAF on the walls and roof of the test models were produced for 3 wind directions (0, 45 and 90º). One example is reproduced in Figure 1. The figure shows the deposition characteristics of the RAF values on the building surface for a zero degree wind direction (i.e. normal to the ridge). As expected, rain deposition is only experienced on the windward faces of the building and no direct impinging rain is present on other building faces. (RAF values = 0 are shown by white surfaces). It should be noted that the roof values of RAF implicitly include the roof slope. The contour lines were further simplified into zones of constant mean values of RAF with standard deviations. 𝑅𝐴𝐹 =
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Figure 1: RAF contours (a) and simplified (mean and stdev) zones (b) for a gable model with wind direction of 0º Alternatively known as the rain deposition factor (RDF) the RAF values reported by other authors provides some indication into the differences between the tropical and non-tropical conditions. Table 1 shows a comparison between RAF values modeled by Staube et al. in non-tropical WDR conditions vs. those of Baheru given tropical WDR conditions at various heights on the building surface. Straube did not specifically model the wind direction but did suggest the use of a cos(θ) factor to modify the RAF values for any wind angle (θ) degrees from the normal to face wind direction. Table 1: Comparison of the RAF values by Author Wind Direction
Normal to face 45 deg
Location on surface
bottom 3rd midpoint rd top 3 rd bottom 3 midpoint rd top 3
RAF model with overhang (Straube)
