FIELD OF APPLICATION
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Buildings and Civil Engineering.
TECHNICAL FIELD
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Energy conservation and control.
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Protection of roofs of buildings against stormy winds.
TECHNICAL PROBLEM
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The roofs exposed to strong insolations store energy in the form of heat and by radiating large quantity towards the interiors of the buildings.
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The roofs exposed to stormy winds are subjected to high mechanical forces, which may be lead to their detachment.
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Proposed Solution:
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Covering of roofs and walls by perforated sheets. Different types of perforated steel sheets are arranged with holes of different diameters, may be used according to the results desired (see paragraph 2-1 “Physical Properties Used”).
ADVANTAGES
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protective shield improves the comfort of habitation in a highly sunny area,
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allows an energy conservation in using air conditioners and this too with a less costly investment because of lower costs of materials used,
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makes the habitation safer in case of exposure to stormy winds.
SUMMARY OF THE DESCRIPTION
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1. Presentation
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2. Performances
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2-1 Physical Properties Used
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2-2 Some readings of the measurements recorded on a model and an experimental house.
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3. Examples of Spacers and assembly of the protective shield
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3-1 Spacers
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3-2 Assembly of the protective shield
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4. Action of air on the protective shield
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4-1 Windward side
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4-2 Leeward side
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5. Conclusion
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1—Presentation
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The protective shield is formed by doubling of the existing roof by painted and perforated galvanized steel sheets (FIG. 1. see abstract). Not long before laying, these steel sheets are subjected to a forming by cold rolling similar to full (non-perforated) steel sheets meant for rooting and cladding panels.
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Forming aims to provide an adequate rigidity to metal sheet to withstand different mechanical stresses, such as its own weight, weight of the roof fitter, wind forces and finally, the weight of snow etc.
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Particularly, the assembly is well suited for roofs already covered with metallic sheets, because in this case, with the help of spacers, they have to be fixed at the level of fixtures of the old root. In this way, no additional preparation is necessary.
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Such perforated steel sheets are already in the market, which are presently meant for cladding panels.
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In the following, a comparison between the performances of two types of perforated steel sheets will be found as an illustration, but any type of perforated steel sheets may be suitable for the manufacture of a protective shield against heat and wind.
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In the following, those elements are also detailed which serve to describe the protective shield for root; but these elements relate to to the protective shield for wall as well, which is much easier to assemble (see details at the end of paragraph 3-2).
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2—Performances
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2-1 Physical Properties Used:
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During calm weather or on the leeward side of a roof as soon as a perforated steel sheet is illuminated by the sunlight (FIG. 2), a temperature difference appears between the air layer in contact with steel sheet on the illuminated side, and the adjacent air layer. The heated air being less heavy than the cold air, rises as per Archimedes principle and gets dissolved in the ambient atmosphere away from the steel sheet.
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This mechanism produces a continuous suction of air present below the steel sheet and which passes through the perforations and gets heated thereby. Thus, air acts as a coolant fluid which exchanges the heat energy with the steel sheet by circulation on coming in its contact.
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It results in maintaining the steel sheet temperature close to the value of the ambient air temperature. However, it has been recorded that the temperature of the perforated steel sheet may go up to 8° C. higher than the ambient temperature on leeward side of the experimental house.
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The original roof placed below the perforated steel sheet, receives very less energy in the form of infra-red radiations coming from the perforated steel sheet. Particularly, it receives energy coming from the solar radiations passing through the holes. The heat transmitted by these radiations spreads in the entire old roofing which leads to a very moderate rise in its temperature. When the old roof is dark colored, its temperature remains close to the perforated steel sheets. When it is bright colored its temperature may be up to 2° C. lower.
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During moderate winds (wind speeds lower than 15 m/s), a wind velocity gradient is observed in air-flow. This wind velocity gradient develops from the lower values to much higher values in proportion to its distance from the roof Therefore, the wind speed is much higher on upper surface of the perforated sheets than on its lower surface. From this wind speed gradient, a natural suction of the air present between the roof and the perforated steel sheet results through perforations. This phenomenon is better explained by Bernoulli's effect: in a fluid flowing at subsonic speeds, an increase in the speed is accompanied by a reduction in pressure.
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This phenomenon makes cooling of the perforated steel sheets even more effective. Two types of steel sheets have been tested and are represented in FIG. 3 on scale of 1:1.
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The choice of the type of steel sheet must be guided by the preferred desired protection:
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If the protection against the wind is less urgent, as in Guyana, the steel sheets with smaller perforations are selected, because they cover more and provide a better performance to protective shield for protection against heat, e.g. in steel sheets of Type A (FIG. 3 a). the surface covered is 85% of the total surface. i.e. only 14.5% of the old roof surface still receives solar radiations.
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With steel sheets of Type B (FIG. 3 b), only 77.3% of the roof which is in shadow, i.e. 22.7% of the total surface of the roofing remains illuminated by the Sun. On the hand, the old roof receives more heat during strong winds due to the larger holes; the loss of wind energy is larger, which reduces the risk of detachment of the entire roof
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2-2 Measurements recorded on a model and an experimental house
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Two types of structures had been used for testing the performances of the “protective shield against heat and wind for the buildings”, i.e. an experimental house, which is seen in picture of FIG. 4 and a model shown in detail with the help of FIGS. 5, 6 and 7. The object of the model is to verily the usefulness of putting a device such as “protective shield against heat and wind for the buildings”. In fact, it has displayed interesting performances as regards the protection against heat. But it has also exhibited its limitation due to its small size. As for the experimental house, it has allowed to verify the results obtained with the model. Moreover, it has also allowed to disclose in a more defined manner, at least visible on the model, the phenomenon, such as temperature gradients long the length of steel sheets.
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FIG. 5: Descriptive view of the model on a scale of 3.5 cm: 1 m.
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FIG. 6: Positions of the points of recording and the legend of measured temperatures.
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FIG. 7: picture of the model, with and without protective shield.
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The model had been made of wood core plywood of 12 mm thickness. Then wood has been tar-coated in order to make it resistant to moisture and insects. The base roofing had been made by a steel sheet painted in marine blue color.
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This has exposed this model to the worst possible conditions as regards radiations.
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The gap between the lower and the upper steel sheets may vary from 80 to 300 mm because of the device visible in the picture of FIG. 7 a. FIG. 6 allows in schematically indicating the positions, where the temperatures had been increased and this was observed in three configurations.
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Without protection (FIG. 6 a).
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With a protection by a full (non-perforated) steel sheet of sky-blue color (FIG. 6 b).
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With a protection by perforated steel sheet of type A (FIG. 6 c).
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Table 1 shows a summary of the recording campaign during the “short dry summer of March 2006”. It has really increased temperatures and not average ones.
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The rows of the table correspond to an increase in temperature caused during the same day between 11 h in the morning to 12.30 h. In each row, the condition of insolations and winds are approximately the same. Average wind speed was between 5-6 m/s, with some gusts reaching 10 m/s for about 5-10 seconds, however occurring at intervals of 2-10 minutes.
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The faces on which the temperatures had been raised are turned towards the prevailing air direction: exactly Eastwards for the model and in East. North-East direction for the experimental house.
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In column “Increased”, M refers to the model, and EH refers to the experimental house.
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Column “State” may be read as the state of protection:
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NR=naked (unprotected) rooting
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PSNP=protective shield with full (non-perforated) steel sheet
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PSP=protective shield with perforated steel sheet of the Type A for the model and Type B for the experimental house.
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In column “d mm” the gaps between the protective shield and the old roofing are recorded in millimeters.
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Note: For linking
FIG. 6 and the Table 1, letter d in the table should be replaced by Greek letter delta (δ). Similarly, q must be read as theta (θ).
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Table 1: Temperature Increases in March 2006.
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The temperatures in degrees Celsius are indicated by letter q.
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In column qa increased ambient temperatures readings recorded by a mercury thermometer (accuracy 0.1° C.). The measurement qa had been recorded in a wind sealed area without sunshine.
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Index (i) indicates that the temperature had been recorded by a mercury thermometer inside model (at about 200 mm from wall) or experimental house (at the center of the room).
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Index (s) indicates a surface temperature recorded by an infra-red thermometer with an accuracy of 0.5° C., this by taking into account of dispersion of recordings close to a point.
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Index (b) indicates the base of the model or a room (non air-conditioned) located at first floor of the experimental house and,
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Index (c) indicates the roof of the model or the experimental house (i.e. the volume located directly below the steel sheet).
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Table 1 allows comparing different configurations of the protective shields with the roofing without any protective shield. The readings for the model show the effectiveness of the ventilation on a surface of smaller size (even without a protective shield).
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For the experimental house, the peak temperatures of the naked (unprotected) steel sheet recorded: 62° C. on the side exposed to wind and up to 75° C. on the wind sealed (West) side.
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The gap between the old roofing and the steel sheet of the protective shield play an equally important role, but interesting performances were determined from the lowest gaps (80 mm). On the other hand, it is useless to indefinitely increase this gap, because beyond 200 mm, no improvement is observed in thermal performances. On the other hand, more important gaps may be envisaged for protection against stormy winds.
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For the experimental house, a gap d (delta) of 175 mm had been selected by taking into account of the results obtained in the model. The low difference between the ambient temperature and the temperature of the perforated steel sheet (last row of Table) demonstrates the effectiveness of the protective shield in this configuration.
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3—Examples of Spacers and Assembly of Protective Shield
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3-1 Spacers
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Representation of FIG. 8 defines a spacer of the type used for the experimental house. For convenience in assembly. it is recommended to place spacer with its opening turned towards the base.
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The spacers must be adopted according to the type of roof steel sheet to be covered. Therefore, common characteristics (on fixed side in FIG. 8), common properties (described below), and variable sides of one type of steel sheet for covering the other (mainly side a, b and c in FIG. 8) were noticed.
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Common properties for all the spacers:
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For resistance purposes, flap 2-flap 3 together should be located as close as possible, to the peak of steel sheet undulation to he covered.
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In case of steel sheets with undulations on flat peak (FIGS. 8 and 9 a). this condition is easy to make. It is sufficient to ensure that side (a) is equal to or greater by max. 10 mm than the distance between the external folds of the two consecutive undulations (10 mm to be distributed between the two extremities).
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For steel sheets which have undulations at rounded peak (FIGS. 9 b and 9 c), side (a) must exceed by 15 to 20 mm (maximum and to be distributed between two extremities of the spacer) than the distance between the axis of two consecutive peaks.
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If this condition is not fulfilled, under weight of the base of the spacer, there is a risk of sagging. The reason is that clamping bolts must necessarily pass through these peaks of undulations. and that a space must be maintained within the spacer for introducing the key for clamping of these bolts.
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However, it was noticed that for the steel sheets called as “undulated steel sheets”, it is possible to superimpose the peaks of undulation on the edges of the spacer (with possible overshooting by 5 mm, FIG. 9 c).
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Side (a) was thus selected in such a manner that the spacer covers a number of undulations, while having a length equal to or greater than 300 mm. For each extremity of the spacer, a clamping bolt was placed at the level of the undulation peak closest to this extremity.
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side (b) is the second variable side. This depends on the type of protection envisaged. The readings of Table 1 have shown that the protective shield is already effective with a gap of 80 mm between steel sheets. The corresponding spacer will have a much greater mechanical strength.
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For the experimental house, the gap of 175 mm had been obtained with a side b=150 mm, at which a height of undulation of 25 mm.
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A completed detailed calculation has allowed to establish that the spacer for which b=150 mm is resistant to bending and buckling, if force F does not exceed in the configuration of FIG. 10.
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The calculations indicate that the limited value of this force, when it acted parallel to the roof and was oriented towards the base (FIG. 10), is of 1800 Newton. The area most stressed is that of flaps 2 and 3.
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A load may also be increased by the weight of the fitter climbing on the roof for assembly. Moreover, it is advisable to instruct fitters not to remain on the peak of a spacer during assembly operations.
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Obviously, more side (b) is increased and more is the risk of sagging of spacer under load. For values of (b) greater than 150 mm. it is recommended to increase the side (c) from the base of the spacer and to add a second rivet on the first one (FIG. 11).
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The series of pictures of FIG. 12 show the different steps of manufacturing a spacer on site by the skilled fitter. The example shown here leads to manufacture of a spacer of the model house:
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(a) Cut a U shaped profile (60 mm; 150 mm; 35 mm) of 430 mm length.
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(b) Cut flaps 1, 2 and 3,
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(c) Folding of flap 1 outwardly from U profile.
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(d) Folding of flap 2 inwardly from U profile,
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(e) Folding of flap 3 such that it covers flap 2,
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(f) Place rivet for assembly of flaps 2 and 3, (rivet head must be within the spacer for allowing assembly of the clamping bolt),
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(g) Placing the spacer on the roof. Spacer must be placed exactly above the beam where the steel sheet of roof was fixed. If possible, the existing holes must be used and do not forget to place a a rubber washer between the steel sheet and the spacer in order to ensure sealing.
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(h) Place the clamping bolt. Using a tar-coated paperboard placed between spacer and steel sheet, allow solidifying the seat of the spacer, but it is not compulsory.
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(i) Spacer is ready to receive the ledge of rectangular section 30×50 mm, on which the perforated steel sheet of the actual protective shield will be fixed.
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3-2 Assembly of Protective Shield
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The distance separating two consecutive spacers may be equal to about 1.5 times side (a) of FIG. 8. The length of 300 mm may be increased and may even be increased up to double, without causing the resistance of the assembly in the geographic areas less exposed to stormy winds. It is, therefore, watched that the distance between two spacers should be less than or equal to side (a). Once the spacer is placed on the old roof, it receives the ledge, which will be embedded in its upper part (FIG. 13 a). First, the ledge has received a fungicidal treatment and had been completely covered by a tar-coated aluminium film (impervious type of film for repairing the root). This film is meant for protecting the ledge from the attacks of insects and moisture.
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A simple overfill of the spacer allows to hold the ledge during the assembly, but also reinforces its holding by means of an auxiliary adhesive film seen in FIG. 13 b.
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It is worth avoiding to fix the ledge on the spacer by bolts placed on its peak, because bolt heads cause a risk of scratching the organic coating under the perforated steel sheet during its placement.
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After its placement, perforated steel sheet is fixed by ordinary short heads for roofing (galvanized anchor bolt type, diameter 6 mm and 40 mm below head). These are the bolts which ensure a connection between the ledge and spacer & a connection between perforates steel sheet and ledge.
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A correctly placed bolt must pass through perforated steel sheet and the peak of spacer before penetrating the ledge.
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For restricting the bolt surface to come in contact with rain water, it is preferable to place these bolts in the bottom of profiled undulations of the perforated steel sheet (FIG. 13 b).
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For facilitating assembly, it is preferable to use lubricated bolts (for example-by automobile grease.
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For a protective shield for walls, the manufacture and assembly of spacers is similar to that shown in FIGS. 12 and 13. The spacers and their ledges are simply placed on a horizontal line spaced at 1.2 m. These spacers may have a side a (FIG. 8) fixed at 300 mm and a horizontal space of 600 mm may be maintained between the two consecutive spacers. The perforated steel sheets will be fixed in such a manner that a gap of less than 300 mm exists between the base of steel sheet and the ground. For fixing a limited size of perforated steel sheets for protective shields for walls, the height reached by the steel sheets will be limited in such a manner that their peak in the assembled condition penetrate inside the dark area projected by roof at 9 hours in the morning on the East side and at 17 hours on the west side. In this manner, the effectiveness of the device will not be affected, but for aesthetic reasons, larger dimensions may also be adopted. However, a gap of at least 300 mm is maintained between the peak of steel sheet and the roof bottom for allowing a good air circulation in the space between wall and steel sheet.
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4—Effect of Wind on Protective Shield:
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The most unfavorable winds are those which have a strong component in a direction perpendicular to the lower stopper of the roof. The roofs most exposed, in this case, are the roof with slopes on two sides. When the wind is directed parallel to the roof, it generates relatively uniform pressures all over the roof The loss of (wind) load because of perforated steel sheets causes a reduction in flow speed in comparison to ordinary roofs and leads to a reduction in the pressure difference between the building interiors and the roof exteriors. Therefore, two following paragraphs are devoted to roofs with slopes on two sides exposed to winds perpendicular to the stoppers of the roof ridge.
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4—Windward Side
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It is the face of the roof mechanically most strong. Close to the roof, on the windward side, lines of wind current are inclined in the direction of the slope (FIG. 14). It follows a pressure gradient rising in proportion that it moves away from the center of curvature (relation deduced from Bernoulli's Theorem), i.e. in a proportion that it is close to the roof. At the same time, the speeds are distributed according to the gradient which varies in the opposite direction.
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From this fact, both the phenomena coexist close to the protective shield:
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at its base, air rushes in under perforated steel sheets by passing through the holes and between the spacers, which causes large loss of (wind) loads, and a large reduction in air speed under perforated steel sheets. For information only, with winds at 10 m/s, air currents of 3 to 4 m/s under perforated steel sheet of the model. It had been measured by a hot-wire anemometer.
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Higher on the structure, the speeds are higher on the upper face of perforated steel sheets than on their lower face. From this follows a suction of air jets circulating under the lower face and presence of depression zones located just above each perforation of steel sheet (FIG. 15).
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These mechanisms have the effect, which have a tendency to be compensated: there is an effect of coating exercised by the wind on the base of protective shield and a suction effect, which reduces the effect of coating in the upper part.
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Overall, presence of protective shield reduces air speeds. which flows over the roof.
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In comparison to a structure without a protective shield, the result is that a protective shield causes a reduction in pressure difference between the interiors and exteriors of the house.
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4-2 Leeward Side
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The leeward side area, relatively calm in comparison to the earlier one, is an area of almost uniform lower pressures. The rolling generated on passing the roof peak may even have the effect of a coating by turning air down on the roof. This effect is all the more marked with stronger wind.
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The lowest pressures are present close to the roof ridge because of air circulating under perforated steel sheets.
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To summarize, in case of exposure to stormy winds, the areas most exposed to base pressures are located close to roof ridges.
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Therefore, for reinforcing protection of the protective shield in these areas, it is essential to reduce, as much as possible, the spacing between spacers located on the beam closest to roof ridge.
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Moreover, perforated steel sheet acts as a ventilating ridge tile and other steel sheets which make a connection between two roof slopes of the protective shield, must have free edges (FIG. 16). i.e. without folds.
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The connection between these ventilating ridge tiles with the steel sheets of the protective shield body must be made by rivets (5 mm diameter for steel sheets of type A, 6 mm diameter for steel sheets of type B), and this at the rate of at least one rivet per edge, as is shown in the distribution of rivets on the protective shield of experimental house seen in picture of FIG. 16.
5—CONCLUSION
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The “protective shield against heat and wind for buildings” improves the comfort inside the buildings covered by it, in reducing the roof temperature and by allowing to obtain a most uniform temperature in different rooms. Naturally. this leads to energy conservation for air-conditioning.
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By reducing wind speed coming directly in contact with it, the protective shield reinforces the resistance of the buildings in case of stormy winds. It was noticed that the ventilating ridge tiles also play an essential role in reducing the risk of detachment of the roof by the wind at the peak level of the roof.
