Static structures (e.g. – buildings) – Cover with projecting restrainer; e.g. – snow stop – Rod-type with plural supports
Reexamination Certificate
2001-08-10
2003-08-05
Mai, Lanna (Department: 3637)
Static structures (e.g., buildings)
Cover with projecting restrainer; e.g., snow stop
Rod-type with plural supports
C052S096000, C052S084000, C244S199100
Reexamination Certificate
active
06601348
ABSTRACT:
BACKGROUND OF THE INVENTION
In general, the present invention relates to the prevention or reduction of wind suction forces induced on the roof of a flat top building, or only slightly inclined roof, generally less than a 40% grade, due to incident high winds. More particularly, the invention relates to unique rooftop structures and method for mitigating wind suction using an associated novel structural protuberance that extends at least partially into the shear layer/transition layer of the flow, whether permanently fixed to the roof top by suitable means, partially or fully embedding or otherwise integrating within the roof by molding, forming, setting, etc. These novel structures reduce or may eliminate the amplification of pressure drops caused by wind gusts flowing over the rooftop.
Windstorm related losses average several billion dollars annually. Roof covering failure, in particular, is a widespread type of damage observed after hurricanes. Once an area of the roof is damaged, building and home interiors are exposed to further damage from inclement weather. The focus of concern, here, is the damage caused to flat top or shallow pitched roofs of buildings due to high winds associated with a storm regardless of the particular meteorological designation of the storm. High winds cause unwanted roof suctions that can severely damage or completely destroy the roof as well as the building structure. More recent studies indicate that the worst mean and peak suctions on flat building roofs occur for ‘cornering’ or ‘oblique’ wind angles which are those wind components directed toward any corner of the building where roof-wall junction is ‘sharp’, i.e., incident winds directed over a range, in
FIG. 1A
labeled
14
A,
14
B with a representative angle, &lgr;
1
, from approx. 25° on either side of the central direction represented by arrow
12
—for further reference see Attachment A, Banks, D. (spring 2000), as well as
FIG. 4B
illustrating incident cornering wind characteristics. As one can see, for cornering or oblique wind angles, conical-shaped vortices, also called delta wing vortices, form along the roof edges. For incident winds directed generally normal, or perpendicular, to a wall of the building with no significant cornering component, i.e., those incident winds directed over a range, in
FIG. 2A
labeled
34
A,
34
B with a representative angle, &lgr;
2
, from approx. 20° on either side of the direction represented by the central arrow
32
—see also
FIGS. 2B
,
3
A-
3
B,
4
A,
5
A-
5
B, and
6
—the vortex induced suction is generally not as destructive as vortices formed during cornering winds. While previous studies have attempted to progress toward linking wind flow characteristics to surface pressures, prior to the instant analysis, the mechanism linking vortex structure and roof surface pressure has been little understood. The rigorous analysis performed and resulting dynamic link between vortex behavior, surface pressures, and wind flow characteristics as identified herein, have led to the ingenious structures of the invention. Based upon the work of the applicants, comparison of simultaneously recorded image data of a rooftop corner vortex and pressures therebeneath indicate and confirm that the peak suction lies beneath the vortex core, and moves with the vortex. An explanation of the analysis and experimentation performed by applicants is found in Attachment A, Banks, D. (spring 2000), and Attachment B, Wu, F., excerpts from dissertation, Chapter 8—both of which are also identified below and are incorporated herein by reference.
The greatest force on the building is known to be the uplift on the roof, and this is a very common failure mode. The worst suction on both gabled and flat roofs are known to occur beneath the vortices that form in the separated flow along the roof edges. For the flow considered generally normal to a wall,
FIG. 2A
within the range defined by &lgr;
2
, a condition known as “bubble separation” predominates within which the vortices are formed, see also
FIGS. 1C
,
3
A-
4
A, and more-particularly, FIG. 2.12 on page 67 of Attachment A. In this situation, the position of the reattachment varies considerably, and is considered unstable, and the vortices which form along the roof edge in the separated flow form and are convected away from the edge at irregular intervals. For reference, see
FIG. 1C
where reattachment at the rooftop
10
occurs at
29
. In contrast, for cornering flow, the flow separation on flat or gabled roofs takes the form of stable dual conical vortices. Thus, it remains to more closely examine the flow mechanism by which a vortex instantaneously controls rooftop suctions. By focusing on understanding the vortex behavior as it is connected to rooftop suctions, the unusual corresponding pressure characteristics may be more fully examined. To do this, a novel analytical model for vortex pressure field was developed and assessed experimentally. This new model quantifies how two parameters, streamline curvature and flow speed above the vortex, control surface pressure. Experimental data confirms that the model accounts for changes in surface pressure with wind direction and proximity to the roof corner. Finally, the model suggests that by inhibiting the flow reattachment, the effect of the vortices on the roof can be by and large, eliminated.
Turning to the two-dimensional schematic ‘snap-shots’ of FIGS.
1
C and
3
A-
3
B, one can better appreciate the dynamics vortex flow model of the invention: Within the “transition region/layer” (TR) the velocity of the fluid (for example, air) is higher than that of the fluid on the same streamline, upstream, due to the well known fluid mechanics concept of the “continuity equation”. The continuity equation embodies the concept of conservation of mass, and as applied to the situation here, one can appreciate that air. behaving essentially as an incompressible fluid, speeds up as it passes over the roof-edge of a building. Boundary layer theory dictates that the flow speed right at the roof surface must be zero so that the flow speed in the transition region decreases rapidly toward the roof. This results in shear stress and vorticity within the fluid flow so that one can make the correlation that the transition region/layer roughly corresponds to a ‘shear layer’.
In the normal wind condition, the region of slow or re-circulating flow under the transition or shear layer is called the separation region, or, separation bubble. “Reattachment” of the flow is defined to occur at the ‘end’ of the separation region and is the point/area at which the flow returns to traveling generally parallel to the roof surface, once again, easier seen in
FIG. 1C
at
29
. As one can see, in the normal incident wind case, the separation region encompasses the vortices as well as an area surrounding the vortices. Ideally, the preferred structures of an apparatus of the invention are positioned and affixed to disrupt, or, ‘catch’/separate, the flow within the on coming flow's transition region (TR) such that this point of reattachment (e.g.,
29
) is moved further out and away from the roof-wall edge, toward the right in FIG.
1
C.
In wind engineering research, interest in understanding roof corner vortices is high not only because of the direct correlation to high roof suction, but because of several peculiarities observed during pressure measurement:
1) The discrepancy between full-scale and model-scale peak pressures—while the results of scaled model studies and full-scale test provide matching mean pressure coefficients over the whole building, the peak and root mean square (rms) pressure coefficients do not match under the separated flow, where the vortices are located. There, the full-scale rms and peak suctions are higher for the full-scale tests. This is a concern, since the building codes of many countries are based upon scaled-model tests in boundary layer wind tunnels.
2) The quasi-stead theory is often used in building codes to predict peak pressures based upon knowledge of mean pressure
Banks David
Sarkar Partha P.
Wu Fuqiang
Macheledt Bales LLP
Mai Lanna
University of Colorado Research Foundation
Yip Winnie
LandOfFree
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