Fred L. Haan

Tornado-Induced Wind Loads on a Low-Rise Building

Current design wind loads for buildings and other structures are based upon model tests in low-speed boundary-layer wind tunnels that generate straight-line winds. Winds resulting from tornadoes that can occur during extreme weather events such as thunderstorms or hurricanes differ greatly from conventionally conceived atmospheric boundary-layer winds. This paper presents transient wind loads on a one-story, gable-roofed building in a laboratory-simulated tornado and compares them with the provisions of building standards. Tornadoes were simulated in smooth, open terrain with vortex core diameters from roughly five to twelve times the plan dimension of the building model (0.46 to 1.06 m). A 1:100 scale model of a building with dimensions of 9.1 m×9.1 m×6.6 m and gable roof angle of 35° was used for this study. Comparisons of peak loads measured in this study showed that tornado-like vortices can generate load coefficients greater than those prescribed by ASCE 7-05 for straight-line wind over open terrain....

Laboratory Simulation of Tornado and Microburst to assess Wind Loads on Buildings

The current design wind loads for buildings and other structures are based upon model tests in low-speed boundary-layer wind tunnels that generate straight-line winds. However, winds resulting from tornadoes and microbursts that could occur during storm events such as thunderstorms or hurricanes are far from being regular atmospheric boundary-layer type winds. In recent full-scale measurements using Doppler radar, it has been observed that tornadoes could produce intense winds in the region below 20 m from the ground. Microbursts are characterized by a strong localized down-flow and an outburst of strong winds near the surface. Thus, microburst winds have significant vertical velocity components and mean horizontal velocity distributions that are different from usual boundary-layer winds. This paper presents quasi-steady and transient wind load effects on a tall building in a laboratory-simulated tornado and microburst. Experiments were conducted in the Tornado/Microburst Simulator at Iowa State University. The microburst is simulated as a round jet, 1.83 m or 6 ft in diameter, impinging onto a flat ground plane. The tornado was simulated with a maximum vortex core diameter of 1.12 m or 3.7 ft. A 1/500 geometrically-scaled model of a tall building, 216 m (708 ft) in height with a square cross section of 54 m (177 ft), is used for this study. Comparisons of peak loads measured in this study with loads specified in ASCE 702 showed that tornados of F2 strength or stronger would exceed the minimum design wind load provisions by a factor of 1.8 or greater.

Failure Progression Analysis of Observed Residential Structural Damage within a Tornado Wind Field

Data collected from recent tornadoes in Tuscaloosa, Joplin, and Moore shows a consistent pattern of damage to residential structures. For an EF-4 or EF-5 tornado, damage levels increase from the outer edges toward to the center line of a tornado track. This is not just because of higher wind speeds at the center of a tornado vortex; the wind velocity fields around structures are also different at the tornado center. Analysis from the damage pattern from the tornado showed that the failure progression of residential structures within a tornado wind field depends on the relative location and direction of the house to the tornado track. With the same wind speed, different damage levels can be observed if structures located in different relative distances from the center-line of a tornado damage track. This should be considered when predicting tornado wind speed based on residential structural damage.

An Experimental Investigation on the Flow Separation on a Low-Reynolds-Number Airfoil

An experimental investigation was conducted to study the transient behavior of the flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds numbers of 68,000. A high-resolution PIV system was used to make detailed flow field measurements in addition to the surface static pressure distribution mapping around the airfoil. The measurement results visualized clearly that a separation bubble would be generated on the airfoil upper surface if the adverse pressure gradient is adequate. The length of the separation bubble could be up to 20% of airfoil chord length and its height only about 1% of the cord length. The transient behavior of the flow separation on the airfoil, which includes the “taking-off” of the laminar boundary layer from the airfoil surface at the separation point, the generation of unsteady Kelvin-Helmholtz vortex in the separated boundary layer, the rapid transition of the separated laminar boundary layer to turbulent flow, the reattachment of the turbulent flow to the airfoil surface to form separation bubble, and the burst of the separation bubble to cause airfoil stall, were elucidated clearly and quantitatively from the detailed flow field measurements.

Using Tornado Damage Surveys to Improve Laboratory Tornado Simulations

Laboratory tornado simulators, intended for estimating tornado-induced wind loading on structures, have been in development for a little more than ten years with the first such facility being developed at Iowa State University. The ideal validation of such facilities would compare pressures on buildings in the lab to pressures on buildings in the field. Since this is nearly impossible from a practical standpoint, other validations must be sought. In the past, the Iowa State facilitys performance has been compared with Doppler radar velocity profiles, with surface pressure profiles and with damage patterns for individual structures. This paper summarizes how the simulators results compare with the newly acquired damage data from the Moore, Oklahoma tornado of May 21, 2013. The data from this tornado consists of geo-located, structural damage data. The structural damage was plotted as a function of radial distance from the center of the damage path. This function was then compared with lab simulator estimates of how tornado-induced forces vary with distance from the center of the vortex. The lab simulator over-predicted the damage for most radial positions, but the analysis highlights the promise of the lab simulation. The shape of the curves was reasonably consistent and could be improved with a more rational connection between predicted forces and resulting damage.

Unsteady Flow Structure Interactions of a Gable-Roofed Building Model in Tornado-Like Winds

An experimental study was conducted to quantify the characteristics of wake vortex and flow structures around a gable-Roof building model as well as the resultant pressure distribution and wind loads (both forces and moments) acting on the test model in tornadolike winds. In addition to measuring the pressure acting on the tested high-rise building model using pressure transducers and wind loads using a high-sensitivity load cell, a digital Particle Image Velocimetry (PIV) system was used to conduct detailed flow field measurements to quantify the evolution of the unsteady vortex and turbulent flow structures around the test model in tornado-like winds. The measurement results revealed clearly that the evolution of the wake vortex and turbulent flow structures around the test model as well as the resultant wind loads induced by tornado-like winds were significantly different from those in conventional straight-line winds. The detailed flow field measurements were correlated with the pressure and wind load measurement data to elucidate the underlying physics to gain further insight into the flow-structure interactions between the tested gableroof building model and tornado-like winds.