MAPPING RAINFALL DISTRIBUTION CHARACTERISTICS ON FACADES USING SURFACE DEPOSIT GEOMETRY Rainfall distribution on facades

B. ATKINSON and P. SNAPE UNITEC Centre for Materials Performance, School of Construction, UNITEC Institute of Technology, Auckland, New Zealand.

Durability of Building Materials and Components 8. (1999) Edited by M.A. Lacasse and D.J. Vanier. Institute for Research in Construction, Ottawa ON, K1A 0R6, Canada, pp. 943-955. Ó National Research Council Canada 1999

Abstract

The three most relevant kinds of precipitation that effect the performance of a building facade are described as ‘direct’, ‘splashback’, and ‘runoff’. The determination of the intensity of direct rainfall on a facade is seen as the first step in understanding the characteristics of durability and performance of an external envelope. Away from the facade other indicators have been devised such as the ‘driving rain indices’ by the NBRS, BRE and others. These can be used as a measure of severity at regional or local scales. In the field, and at the surface, vertical plate gauges can be placed at strategic locations and long term measurements estimated. There is generally a good correlation between the surface distribution of direct rain on a facade and the extent of maintenance problems, including material degradation, failure and appearance, as well as internal damage by water penetration. The long term nature of vertical plate gauge measurements detracts from its usefulness. This paper reviews methods of measurement of direct rainfall distribution at various scales. It introduces a method based on the geometry of surface deposits which allows a rapid comparison of facade performance, and indicates likely trouble spots.

Keywords: Facade performance, rainfall distribution, surface deposit mapping. 1 Introduction

Studies of rainfall distribution on building facades forms part of the general research work related to the ‘performance equation’ that is carried out at UNITEC. (Atkinson 1997, 1998) has outlined the philosophy that is followed, leading up to this and other studies of material performance. In this paper one facet of the causal factor ‘environment’, that of direct rainfall, is of special significance. For most external materials the rainfall absorption curves follow an inverse exponential function with respect to time. If the intensity of rainfall exceeds the gradient of this curve a rainfall ‘run-off’ will occur. For a given constant intensity (mm/hr.) it is only a matter of time before a run-off occurs at the surface. The period which elapses between the rain first reaching a surface, and the occurrence of a run-off, we refer to as the ‘interception period’. The interception period is important because of its influence on the probability of frost or moisture attack, and the incidence of a run-off, which apart from cleaning a surface, results in local concentrations of water at joints and junctions. Investigations into these effects are legend. (Parker 1966) gave typical mass saturation curves for rendered and unrendered brickwork. These follow a recognisable saw-tooth pattern; the peaks representing the intervals of time between maximum saturation. The main difference is that the input is at indeterminate intervals rather than at regular intervals, and a slow build-up of moisture within the wall, or a slow evaporation can occur responding to the statistical chatter of the weather and other causal factors. In connection with the wetting of external surfaces there is a close relationship between rainfall and wind independent of their separate effects. Except for a splashback at ground level, and on the surface of projections, no rain reaches a vertical surface in still air. On an open site for a given gradient height Hellman discovered the variation in wind speed with height according to a power law. The preceding example, for instance, presupposes the existence of a wind; and whilst this is generally true for most of the time, the directional frequency of wind at times of rain may not be the same as the directional frequency of wind at all times. Mean wind speeds during rain are greater than mean wind speeds taken over all times. (Prior 1985) has verified earlier workers’ results which give UK factors varying from 0.9 to 1.5. More important therefore than either wind or rain is the combination of both which is generally known as driving-rain.

2 Driving-rain

History has shown that the results of general field surveys, or specific surveys such as those of paint failures on timber substrates, emphasize the importance of moisture in the deterioration of external materials. The amount of driving-rain reaching a surface is nearly always the main criterion governing the rate of deterioration of materials and design elements. It also governs the amount and distribution of foreign particles, such as street dust, pollutants, and organic matter that adhere and accumulate on external surfaces; and thereby to a large extent is an important factor effecting external changes in appearance as well as composition. (Hoppestad 1955) originally defined an index of driving-rain as the product of the mean wind speed (m/sec) and rainfall (mm). He drew a contour map for Norway which showed the relative severity of driving-rain in different parts of the country. Several maps have been devised for Denmark, Canada, and the UK (BRE and Met. Office). In calculating the driving-rain in this way an assumption is made that the quantities of rain reaching a surface are proportional to the index value. The index, currently modified in various compensatory ways to take into account terrain, building shape, and aspect, is essentially a relative geographical measure of severity rather than an exact measure. It can therefore be omni- directional or directional, regional or microclimatic.

2.1 Vertical gauges More exact measures of driving rain using vertical raingauges (Rv mm), have been made by various researchers in the past working in a number of fields. (Pers 1932) designed a four-way ‘vectopluviometer’, which is essentially the same type of gauge used later in Norway, known as the NBRS gauge. (Fourcade 1942) used an elaborate four-way vertical gauge for estimating driving-rain and condensates at ground level. The gauge was designed for agricultural and forestry use. An octagonal or eight-way vertical gauge is favoured in the UK. The first of these was built in 1936 at the Building Research Station by Beckett (Type DR1), and has been used by (Lacy 1951,1964). (Hamilton 1954), in Candada, used a variety of tilting and directional vertical gauges in an attempt to simplify catchment measurements in forest areas. In all these cases the gauges, set with the vertical apertures facing the cardinal points, are placed away from buildings on open ground. In 1959 the first measurements of driving-rain were made in Denmark using an omni-directional ‘dish’ gauge. (Korsgaard and Madsen 1962) have described this gauge, which does not measure the quantities of rain falling on the external walls, but allows closer approximations to be made. Later computations by (Prior and Caton 1985) concluded that the earlier DRI’s were marginally too low. The correlation between computed driving-rain indices and the vertical catches of most of these gauges is fairly good on open sites, and has been found to be linear. Being a product of variables, DRI’s unfortunately are not additive. The sum of the directional DRI rose indices does not equal the omnidirectional index. Nor does the highest DRI relate to the prevailing direction of the wind-with-rain. Most gauges include a horizontal aperture (RH mm) which allows estimates of the rainfall angle of incidence (arctan(Rv /RH )) to be calculated near ground level. A map showing these roses was incorporated into B.R.S. Digest 127 (1971). British Standard 8104 : (1992) builds on much of the above early pioneering work. It is clear that a directional driving-rain rose is necessary at the correct scale and microclimatic location, if a reasonable correlation with general field survey results of external deterioration is to be made. Unfortunately these are not available at microclimatic levels, but indicators can be deduced from urban area DRI values, where meteorological measurements quite often exist outside Prior’s Master Met. stations. If these include rainfall intensity measurements then computer processing can yield satisfactory DRI working tools. A ‘walkabout’ around a locality and site often reveals as much, if not more microclimate information. The total or omni-directional driving-rain index (DRI) for the Manchester area UK, where most of the research work for this paper was originally carried out, is based on a mean annual rainfall of 805 mm, and a mean wind speed of 5.20 m/sec, giving a DRI of 4.18 m²/sec. yr. This value, with a suitable K value (Prior 1985), corresponds to the value given on the meteorological office DRI contour map. The driving-rain directional roses shown in Digest 127 (BRE 1971) were based on three categories of precipitation and Beaufort wind speeds: for this reason weighting factors had to be used which were roughly proportional to the intensity of rainfall and mean wind speed. Thus estimates of actual quantities of rain reaching a vertical surface were not made. An alternative method was adopted for the Manchester area surveys based on a Monte Carlo sampling of wind-with-rain data.

2.2 Rainfall intensity, droplet size spectrum, and directions If the mean wind speed for any direction was constant; and providing rain fell for the same proportion of time from each direction; and with equal intensity; then the driving-rain rose could be represented simply by the normal directional frequency wind rose. The quantities (Rv) and mean incident angles of rain (i) can be computed by using a 50%tile volume median raindrop terminal velocity (vt) based on the mean intensity of rainfall (Rh), and using equations given by many authors dating back to (Laws and Parson 1943), (Gunn and Kinzer 1949), (Best 1950), and (Berry and Pranger 1974). It is important to note that the estimates of driving-rain derived from regional data considerably over-estimate quantities actually falling on a wall because of momentum and aerodynamic flows. (Lacy 1964) found, for example, that only 10%-25% of computed driving-rain reached a test wall because of the effect the wall had on wind flow, and because of the sheltered site. The computed figures based on synoptic station data refer to wind speeds 10 m. above ground level on an open site, and to rainfall catches on the unobstructed ground. The estimated amounts using this approach represent the amounts that would pass through an imaginary mesh surface 10 m. above ground level. For Garston, Lacy found that the most exposed walls of a two-storey house received about 25% of the computed driving-rain, and the most sheltered walls about 6%. The quantities of rain falling on a flat roof are about the same as on the ground nearby. The windward slope of a pitched roof receives proportionally more rain per m2 than the lee slope, with some adjustment for wind flow effects. The total quantity of driving rain that falls on a building is equal to what we call the ‘rainfall sink’ about a building shown as isohyet contours (Fig. 1). Fig. 1: The rainfall sink

2.3 Rainfall source and vector diagrams A useful tool was invented to assist in our research. As an additional aid to site investigations of material deterioration, a rainfall 'vector ' diagram was devised which indicates the proportion of rain falling on a given wall between two specified directions or poles. The directional diagram is illustrated in (Fig.2).

Fig. 2: DRI Vector Diagram The normal component of driving rain falling on a surface is equal to RH tan i sin ø, where RH is the normal horizontal catch, i is the incident angle of the rain to the vertical and ø is the horizontal angle of the rain to the wall. By summing the normal components of the driving rain over 180º the total D.R.I. for any wall facing a given compass point can be estimated. For the vector diagram, five percentage vector values for each of the eight compass directions, averaged over equal segments of 180º exposure, were plotted from an arbitrary origin; and an approximate curve was sketched in. The resulting closed area bounded by two vectors was called a rainfall vector diagram. The proportion of the total possible rain falling on a surface between two specified directions is thus equal to the area of the vector diagram enclosed by the two poles. Each diagram, for each direction, has an area proportional to the total vertical direct rain driving onto its surface. The diagram may be used to estimate the effects of curtailment or obstructions on the quantities of rain reaching a given point on a surface. For example, from any given point in a reveal, two curtailment lines can usually be drawn on plan from the point itself to the external corners of the reveal. By placing the origin of the appropriate surface diagram on the point, and correctly aligning the base line of the diagram with the wall, the area enclosed by the two lines can be estimated as a ratio of the total area of the diagram. The product of this ratio and the total estimated driving-rain for that orientation, yields the total amount of rain reaching the chosen part of the wall. The method is illustrated in (Fig. 3).

Fig. 3: Curtailment lines and vector surface diagrams

The vector diagram is most advantageously used with each directional diagram drawn separately on squared transparent plastic. In this way two diagrams can be overlaid, and placed say in the corner of a reveal: the differences which occur in the quantities of driving-rain reaching either bounding surface can be estimated and compared with site observations. The southern reveal of a window which faces south-east, for example, may receive almost negligible amounts of rain compared with the opposite reveal. The diagrams shown in this work are based on estimates of relative quantities of driving-rain only, as an aid to assessing external performance. Current work has been directed towards more accurate computer estimates. 3 Rainfall distribution on buildings

Perhaps the most unusual aspect of building design is that there is a dearth of information on the distribution of rain over or around a building (Fig. 1). External design detailing is not usually based on fundamental knowledge of surface water flow, because such a body of knowledge is hard to find. This is probably because the principles of design detailing are basically traditional; represented by a few simple economic design rules handed down to successive generations. Unfortunately whilst these details may protect the interior from moderate extremes of suburban climate, trouble arises when they are used indiscriminately above two-storey level, or in places of severe exposure at lower levels. There is also a dearth of measurements of driving-rain taken on actual buildings, but from as early as 1937, various attempts have been made to devise suitable wall gauges in order to judge distributions. (Sandberg 1968) estimated building centreline wall distributions based on wind vectors derived from a wind tunnel, and computer calculations of droplet flight paths. (Rogers 1974) repeated the exercise, but in both methods the results do not fit our field observations of surface deposits - the problem probably deriving from the adopted raindrop spectrum and the wind vector fields. Flat rectangular plate gauges with a raised rim and collecting channel have been used by the Building Research Station for some years. This type, originally designed by (Holmgren 1937) (Trondheim) has been used by the Building Research Station on buildings at Garston, Thorntonhall and Glasgow. Other types of gauge have been designed, such as the circular pot type due to (Croiset 1959), and various recessed gauges. BRE have undertaken work with flat copper plate gauges with raised rims and collecting bottles. If sufficient evidence were available the results of measurements made with wall gauges would show means and extremes of driving-rain; the distribution of rain over various elevations; and allow correlations to be made with computed driving-rain, and with, for example, free standing octagonal gauge measurements. Lacy correlated the results of all three methods of estimating driving-rain using gauges, fixed to a 2.8 m high testing wall on the B.R.S. exposure site. The results of five years exposure indicated that the total driving-rain on the wall varied between 10% and 25% of that derived from nearby standard meteorological measurements. Theoretically the greatest quantities of rain should fall on the corners and edges of buildings where the streamlines are known to close, and larger raindrops (spectrum domain approx. 0.1>dr>6.0) probably shatter before impact because of inertia and centrifugal forces. It is evident that measurements made so far using wall gauges are of limited use to the designer in respect of a particular building because of the long term nature of the measurements; but the general principles of driving rain have been established. Minimum exposure times of several years are generally considered necessary in meteorological work of this kind if reliable averages are to be obtained, and low probability events recorded. 3.1 Surface deposits and their geometry General observation of many buildings will reveal striking differences in external appearance due to pollution deposits. It was noted that the staining and suface deposit ‘fringes’ (unwashed areas) below projections varied in depth from place-to-place on the same elevations, as well as on different elevations. It was conjectured that a useful relative measure of averaged rainfall distribution over such a surface might be found by comparing the depth or width of deposit fringes on different parts of an elevation providing estimates of ø (angle of wind to the wall) could be made. It was also necessary to examine the relationship between the incident angle i of the average median diameter droplet and the depth of the cut-off line below projections. The cut-off line, or bottom edge of most deposit fringes is not a definite line. There is usually a gradual change from black or brown to the self colour of the material. Below the eaves of several survey buildings on west elevations in our survey area, and on low absorptivity surfaces, the gradation occurred over 10 to 20 mms of wall surface. On the same type of surface, facing south and east an increase in the width of the cut-off band was observed, which indicated a decrease in the mean incident angle. The cut-off band width will vary according to the solubility of deposits; absorptivity and texture of the material; the normal incident angle i of the median droplet; and the frequency of rain from different directions (variations in ø). If an ogive rainfall distribution is assumed over the band width, the incident angle of the median diam. droplet can be assumed to lie close to the bottom of the band limits. The amount of rain falling on a wall for low values of ø (< 30º) is also small compared with the total amounts of driving-rain which strike at greater angles. This is particularly true for west elevations in the Manchester survey area, but probably not for north-west or south-east elevations (Fig. 2). The amount of rain falling on a wall varies according to droplet interaction with wind vectors upstream, and to streamline forcing flows leading the droplet either around or against the surface. An easily recognised overall pattern of surface deposits can be found on flat elevations which have regular relief. Even so, microclimatic variations in driving-rain effect these patterns close to mullions, projecting structural members, canopies, and around the exposed edges of an elevation. It was concluded therefore, that the most suitable building for initial examination would be one having a fairly flat major elevation facing north-east or east. About equal amounts of rain fall from all directions for these orientations. Ideally, the deposit fringes should be measured away from nearby surface geometry projections, preferably on grid points which correspond with perhaps a repeated infill bay or window sill. The wall surfaces should be slightly absorbent. . A building was found in Stockport, a little to the south of the survey area, which was suitable for study. It was about four years old at the time of inspection. It had an exposed, fairly flat, east north east elevation, and was clad in precast concrete units below sill level as shown in (Fig. 4). Measurements of the depth of the deposit fringes below each sill were made at a point to the left of the centre of each bay of the elevation. The nearest building, situated adjacent and to the north, was about 11m. away. It was about half the height of the one chosen for study, and probably affected the air flow around it. A photographic technique was used to measure the depth of the sill fringes, using 35 mm projector slides. Both coloured and black and white slides were used.. Deposit fringe depths were measured on the screen image and adjusted for photo-perspective distortion. The results were entered in a matrix, each element of which corresponded to a measuring point. Empty elements were left around the edges, and were later filled with extrapolated values after the matrix was adjusted for errors along rows and columns. An assumption was made during iterative smoothing that the fringe depths formed a continuous three dimensional plane. In other words, the variations in rainfall distribution were smooth and continuous across the elevation. This is probably true on average, for a parallel plane 500mms or more off the elevation, but streamline distortion occurs around some vertical projections and other obstructions where these are large enough to cause surface turbulence or curtailment.

Fig. 4: The typical ground level precast concrete panel

At this stage an interesting discovery was made. It was found that the depth of the deposit fringes (d) varied logarithmically with the height (h) above ground level, and differently along each grid line, according to a law of the form :

h0 - h = a.loge (w/d) - C (1) where h0 = the roofline height (m), w = sill projection (tan i = w/d ) (m), C = effective height (m) for d = 0 , and a = surface location variable (m). This remarkably consistent effect may be related to Prandtl’s logarithmic law for neutral or adiabatic equilibrium in the boundary layer wind flow. It shows the importance of flow in the far field as well as the near or surface flows. It was also found that the incident angle of the driving rain corresponded to the regionally computed angle (mesh surface) shown on the vector diagram at a point in the centre of the maximum windward pressure zone, about 0.6 of the ho for this building. Estimates of the distribution of the driving-rain (Rv ) are mapped in contour form in (Fig. 4). The isohyets are contoured by using the relationship Rv = RH tan i where RH is taken as 81 mm and tan i is equal to the ratio of sill projection w to fringe depth d . The contours (Imp.) show an increase in driving-rain towards the roof line with maximum values towards the centre of the block. The influence of the penthouse flat can also be seen. There is some doubt about the validity of the contour values close to the edge (Fig.4 ), but the general configuration appears satisfactory. A large lower central area receives virtually no rainfall. In this zone the rain is probably carried away from the building; an effect suggested by examination of the streamlines. Lacy suggested that walls facing north in the London area, an area having the lowest DRI in the UK, received about 1/16th of the normal rainfall. For the chosen building a computational check on the mean value of the rainfall surface contour height showed that the elevation received 29 mms on average, about 1/28th of the normal rainfall or 38% of the computed value using meteorological data. On the chosen elevation most of the rainfall run- off is cast away from the wall by the sills and sloping slabs; the remaining areas receive rain according to the above estimates. For many buildings the rainfall run-off characteristics are equally important as those of driving-rain; for it is by this next process that driving-rain is redistributed over an entire facade as it flows (eventually) downwards creating splashback and super-drops. (Ishikawa 1949) adopted some aspects of the method described above, using Tokyo medium rise buildings as case studies. Some aspects of his findings confirm those found by us. He was at some disadvantage because of lack of computing support. In 1985 our method was repeated on two CBD buildings in the city of Adelaide, South Australia, and yielded similar results. These correlated well with the observed material degradation, appearance and reports of rainfall leaks in the facades.

4 Conclusions

By measuring the staining patterns on external facades some indication of long term rainfall distribution has been obtained in a fairly simple practical manner. It utilises the visible effects produced by historical rainfall. The characteristics of failure patterns can be detected in this way, as well as changes in appearance. It is important to note that the method is indirect because it does not use actual surface measurements of vertical rainfall. These generally take too long to gather Some of the background research to this work over the last three decades has been outlined. A tool described as the rainfall vector diagram has been introduced. 5 References

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The North-West main frontage The North corner

The West corner Typical ground level precast concrete panel