Using 3D laser scanning to improve the reliability of stairway geometry measurements

Transcription

1 Using 3D laser scanning to improve the reliability of stairway geometry measurements Richard Bowman a, Mike Roys b and Nick Davies c a Intertile Research, Brighton East, VIC, AUSTRALIA; b Building Research Establishment, Watford, UK; c Lester Franks, Port Melbourne, VIC, AUSTRALIA Abstract: Falls on stairways can have many contributing causal biomedical and environmental factors, as well as devastating consequences. However, the risks of falls occurring in frail cohorts will be exacerbated where the stairway has inconsistent geometry. In most developed countries building codes require that stairways are constructed to close tolerances, although conventions and practices may vary, as is shown for Australia, the United Kingdom and Singapore. It is generally very difficult to simultaneously make accurate three dimensional measurements of stairways using conventional tools, but a simple visual test might determine whether any physical measurement is required. While a simple and a complex method of measured are described, various types of step inconsistencies can still create challenges. 3D laser scanning of the complete stairway allows every variance in the steps to be observed three dimensionally, where accurate measurements between any two points in the three-dimensional space can be made by placing the 3D cloud of data in a CAD environment where the rise and going can be measured in the appropriate 2D plane. While variances in tread, including slopes, cross slopes and cupping, can quickly be detected and determined as required, some conventions need to be established in order to standardise the procedure. 3D laser scanning also enables other simultaneous simple measurements such as the headroom, the handrail height above the pitch line, distance between handrails, size of openings in balustrading, the perimeter of the handrail, step overlap, etc. There are different 3D laser scanning techniques with varying levels of convenience and accuracy. A sister presentation (Bowman et al, 2015) will consider issues associated with the use and application of such data, and identifies the research that is required to best enable the development of improved stairway regulations. Practitioner Summary: This paper highlights the various methods available for measuring rise and going on stairs and who is likely to require these measurements. It provides an introduction to the risk associated with step dimensions, emphasising the need to ensure steps are built right. The techniques increase in complexity and cost as the importance of the finding become more critical. It also highlights how 3D scanning can be used to provide the ultimate measure of step dimensions and the surrounding environment. Keywords: Stairs, Rise, Going, Measurement, 3D Laser Scanning. 1. Why bother measuring in the first place? Stairs are a very efficient and economic method of transferring between two levels. However, they do also present a high level of risk, especially if they are not designed correctly. For this reason safe stairs are usually included as an aspect of the building which should be controlled using building codes or regulations. In England, guidance over the minimum requirement for acceptable stairs is covered in Approved Document K, associated with Part K of the building regulations, whereas more recent guidance is provided through the British Standard BS :2010. In Australia, the National Construction Code, also known as the Building Code of Australia (BCA), establishes the requirements. There are a number of aspects of stairs that are controlled within these guides; two of the dimensions controlled are rise and going. These are defined below, and definitions of walking line and nosing are also included for completeness. Table 1 highlights the expected values under each of the codes for Australia and the UK. 1

2 Rise - vertical distance between the horizontal upper surfaces of two consecutive treads, or between a tread and a floor, or a tread and a landing Going - horizontal distance between two consecutive nosings, measured along the walking line Walking line - notional line that indicates the average path of users of a stair Nosing - front edge portion of a tread or landing, usually projecting beyond the riser Table 1. Rise and going dimensions (mm), Australia (BCA), England (AD K and BS ) and Singapore Building and Construction Authority Approved Document Acceptable Solutions V6.0. Building Code Rise Going Tolerance Min Max Min Max Rise Going BCA public Constant^ Constant^ BCA private Constant^ Constant^ AD K general access AD K private* BS normal use % 1.5% BS private % 1.0% Singapore Public building mm ~ 5 mm ~ Singapore Residential unit mm ~ 5 mm ~ Singapore Industrial building mm ~ 5 mm ~ The normal relationship between the dimensions the rise and going (twice the rise plus the going) should be between 550 mm and 700 mm. * The maximum pitch for is 42 (hence maximum rise cannot be used with minimum going) ^ This is often interpreted as ±5 mm, the rise and going tolerance that is used in the New Zealand Building Code and AS 1657, Fixed platforms, walkways, stairways and ladders Design, construction and installation. The tolerance in the Guide to the BCA was also 5 mm until it was withdrawn without explanation at the end of ~ A tolerance of 5mm between two consecutive steps in any flight of staircase is acceptable. The values provided in the documents are expected to be achieved for every stair built under the respective codes. In order to comply with these guidance documents, the stairs must therefore be measured to determine whether they meet the criteria or not. Anyone measuring these dimensions would need to consider both whether they fall within the acceptable values described in each document and whether they vary from step to step. Inappropriate dimensions and variance between steps have been shown to be leading causes of non-compliance, and lead to the greatest risk of harm. Some degree of variation is expected, any consequent risks have only been partially determined. Roys and Wright (2003) developed an algorithm for assessing the risk of a large overstep, 50% of an average male shod foot, occurring in descent. It is based on experimental variance of foot placement on an adjustable stair rig. The assumption, again seen in experimental research, is that the risk of this overstep leading to a slipping event is directly linked to the size of the overstep. The difference in relative risk is shown in Figure 1, where the risk of a large overstep on a 14 step stair having a going of 300 mm and zero variance between steps is given a value of 1. Decreasing the going dimension by 25 mm has two orders of magnitude effect on the risk, and this risk is increased by another order of magnitude for a decrease of 50 mm. Similarly, by decreasing the length of one going by a small amount (5 mm, 10 mm or 15 mm) the risk is again significantly increased. The values in the bubbles are based on the same comparison, but this time it is considered in terms of time between events occurring. If a typical family were to use the stairs at home (estimated to be equivalent to 18 descents a day) with a 300 mm going and no variation between steps, such an event would be once in 8000 years, longer than the life expectancy of most buildings. Reducing the going to 225 mm, similar to current private stair minimums, and with no variation between steps, results in an overstepping event occurring every year. A slight variation in one step could cause an overstep event every 3 months, whereas if the going was set to 275 mm the risk would be once every 40 years. It is clear from this analysis that the dimensions, in particular the going dimension, and variance in this dimension, are critical risk factors relating to injury on stairs. 2

3 Figure 1. Increased risk of a large overhang (50% of shoe) relative to 300 mm going with no variation, which is about once in 8000 years for typical family domestic stairs. 2. Who needs to make these measurements? While it is clear that measurement is required, it is rare that such measurements are taken. This is partly because the dimensions are very difficult to measure with any accuracy, and also because many do not consider the stairs to be contributing to the potential risk of harm. In our opinion there are potentially four sets of people who, as part of their job, should be considering measuring stairs: building control, environmental health, occupational health and forensic examiners. Each group has different requirements from these stair measurements, and hence require different levels of accuracy. 2.1 Building control Building control officers are responsible for confirming that a building is built according to the requirements laid out in the various guidance documents. The first part of this would be to ensure the plans are within specification. Stair measurements on plan are easily resolved through the CAD software, and hence it will be obvious quite quickly if the stair does not fall within the guideline recommendations. Building control officers will also make one or more inspections of the property as it is being built to ensure elements are being introduced as specified on the plans. One of these inspections is likely to include a site assessment of the stairs. 2.2 Environmental Health Environmental health officers are responsible for ensuring the buildings are fit for purpose once in use. This assessment can take many forms, but in England this includes and assessment of the potential for risk from a number of hazards determined under the Housing Health and Safety Rating Scheme (HHSRS). One of the more serious hazards, present in 9% of the housing stock, is the risk of falls on or from stairs. The risk assessment will include any flights of steps, whether internal to the building, part of the circulation route to access the dwelling, or external steps associated with the dwelling. They will be aware of the risks associated with inadequate step dimensions and inter step variability, and will want to include an assessment of these factors when making an assessment. 3

4 2.3 Occupational Health Occupational health officers are, amongst other things, responsible for ensuring a home is suitable for people to return to after injury or a stay in hospital. Often their clients will be elderly or children with disabilities. Stairs can be a limiting factor for many of these clients, restricting access to just the entrance floor of the property. In some cases the stairs require a handrail or better lighting to make them usable, but in other cases the stairs are so restrictive that alternative sleeping and hygiene requirements are needed on the entrance level if the home is to remain usable. They may be aware of the risks involved with the stair, and would benefit from being able to make quick and simple measurements to assess the potential risk from the stairs for some cases. 2.4 Forensic examiners Forensic examiners are usually only called in to examine a stair after a fall has occurred. In some of these cases the fall could be fatal. The examiner needs to consider every aspect of the stair in order to determine where the risk of harm exists, what part of this risk is inherent in the stair, and whether the stair is built to the specifications expected, or perhaps altered in some way which could potentially increase the risk of harm. As with most investigations, other aspects of the environment, as well as human factors, could be involved in the hazardous event occurring. So while a very important part of the investigation, the stair dimensions are just one aspect to be considered. 3. How should these measurements be taken? Listed below are a number of measurement techniques, with increasing complexity. 3.1 Visual inspection A lot can be assessed by a simple inspection of the stairs. When approaching the flight consider whether the passage is clear both on the steps and at the landings. Pay special attention to anything that a user could land upon if a fall were to occur, or things that might restrict movement, increasing the potential for a fall to initiate. Consider the step finish; is it bare timber, concrete, tiles, a resilient covering or carpet? Check also for the presence and suitability of handrails, whether there is sufficient headroom over the whole flight and whether the sides of the steps and landing are protected to stop people falling from height. Requirements in both England and Australia highlight how gaps in between open steps and barrier components although the maximum size in Australia is 125 mm and in England it is 100 mm. Consider the lighting on the stair, both the ability to artificially light the steps and any lighting from natural sources. Check for glare and potential shadows that might hide where the step edges are. This is often difficult to assess at a single point in time, and hence some judgment of how the various lighting options might affect the appearance of the steps in use is needed. If there is any doubt, it may be worth checking the stair at a different time of day, or under different weather conditions. For some steps, especially those in public buildings and on access routes for transport or streetscapes, the nosings should be clearly marked by the addition of contrasting strips. The final visual check will be to consider the steps themselves for and movement or imperfections, which could be a hazard. Where the step covering is worn or damaged, or where the steps themselves are damaged, this should be noted. By walking the flight any movement in the steps may be noticeable as might any strange noises, which might suggest movement. 3.2 Crouch and sight test This 10 second test is a very valuable quick assessment for between step variations. If all the step rise and going dimensions are equal throughout the flight, then it should be possible to visually line up all the nosings by simply crouching down at the top landing and eyeing up the nosings. Where there is variation between steps that are big enough to be a problem it will be impossible to line up all the nosings visually. Instead one or more steps will appear to hide behind other steps as you visually move your line of sight up and down. Other steps may stand out from their neighbours, making it impossible to line them up with higher steps. Often in England the rise on the top step is higher than others making it impossible to see if the other steps line up. The bottom step may also have a lower rise than the others, again this would not be visible using the 4

5 crouch and sight test. For any other variation the method gives a very quick check to see large differences. Figure 2 illustrates how this might occur. Figure 2. Lining up nosings using a crouch and sight test. 3.3 Simple measurement Measuring the rise and going directly in-situ is very difficult. Measurements are between two similar points on consecutive steps, which is usually the front face of the nosing for going and the top face of the nosing for rise. If the stair is well built with level equal steps then a quick measurement can be meaningful, however, often the step nosings are not square with each other, the treads slope, or for existing stairs the treads may be bowed or worn. This makes it difficult to know where the measurement should be taken. Measuring going can be even harder, since the nosing often overlaps the tread below. A measurement on the tread of the tread depth (front of nosing to the back of the tread or to the riser) is larger than the going by the size of this overlap. All measurements should also be taken along the walking line, and perpendicular to the surfaces. With all this in mind a simple measurement can be made to give an indication of whether the steps are the right size and whether they vary step to step. Measurements can be taken with a marked stick, a piece of card, a metal rule or even a user s foot. Measurements taken at the edge of the step are easier to make consistent step to step, at the side string, or edge of the step, can be used to square up the measurements, however in many cases a value either in the centre of the step (if the stair is less than 1 m wide) or 270 mm in from the edge of the step would be the most useful, as these are the expected walking lines. Using a card like the one shown in Figure 3 enables a quick comparison against expected values, and with a marker the variance between steps can quickly be recorded. If using a ruler or stick then again recording the position of the nosing along the stick when pushed up against the riser, for going, or on the tread for the rise should show adequacy and variance. Remember to consider the overlap in going measurements. If you don t have anything to make measurements with, it is possible to get a very rough estimate using your own leg. Place your foot onto the tread with your heel as far back as possible (you will possibly feel the nosing of the step above pressing against your calf) and try to make your foot point directly down the stair. The point where the nosing crosses the foot (or the length between the foot and the nosing if your foot is shorter than the going) should be consistent as you go down the flight. Similarly the position of the nosing on your calf should be fairly consistent. While this is not an accurate measure of these dimensions it can be a quick way to get a good impression of whether steps are adequate and consistent. 5

6 Figure 3. An example measuring card that can be folded from an A4 sheet (fold the foot shape through the slot to provide a rigid base. 3.4 Complex measurement For a more accurate measure of rise and going it is better to measure the distance between nosings, and the angle formed between them. These values can be converted using trigonometry to determine rise and going, see Table 2 for some examples. Since the measurement avoids any problems with sloping treads, and also avoids problems making measurements with an overlapping nosing, this makes the measurements much more accurate. Table 2. A selection of rise and going calculations using the complex measurement method. 6

7 In order to take a measurement a long rule or a board can be placed between the steps. Figure 4 illustrates how such a board can be used. In this instance the board is 270 mm wide enabling it to be lined up with the edge of the step and a measurement to be taken at the walking line. By marking the centre of each step a similar measurement can be made for the central walking line. Figure 4. Using a graduated board and inclinometer to determine rise and going. While this technique is more accurate than the simple measurement, it does still suffer with some problems. The main problem is placing the board to ensure that the measurement is between the same points on both steps. While it might be easy with a timber step like the one in the diagram or where sharp metal proprietary nosings are present, things get really difficult when the covering is a thick carpet. Furthermore if the steps are bowed or angled it becomes even harder to place the board at the right point. However, using this method it should be possible to determine the dimensions with some degree of accuracy, and this method is very good at determining variance D laser scanning 3D laser scanning is used to make measurements in the defence, aerospace, automotive and other industries, where precision and accuracy are critical to the outcome. There is a wide range of scanning equipment with varying capabilities and costs. The method generates a 3D cloud of millions of discrete data points that indicate where the surface of each component lies in three dimensions. This point cloud data can be extracted to a CAD 3D model for further analysis where accurate measurements can be made between points (3DRiskMapping, 2008). Where data alignment (or referencing of the individual scans to each other) is performed using the cloud to cloud point cloud alignment method, the method can be sufficiently accurate to yield sub one millimetre results. The field method may involve setting up the instrument at several scan locations around the stairs to maximise data coverage and to maintain sufficient overlap. The instrument should not be positioned on the stairs in situations where this might affect the results. Every nick and blemish on the steps is visible, every bow and bend is laid bare. The variation across the width of the stair on any step to step combination can be determined quickly and accurately. The ability to measure each step with the same accuracy and repeatability ensures measurement consistency. The benefits of using laser scanning include being able to immerse a viewer in the virtual scene without the need for a site visit. High resolution point cloud data can allow measurements of all parts of the stairway, including the whole step surface, balustrade, handrails and adjacent floor and wall surfaces. If anything the technique 7

8 might be considered to provide too much data, making it difficult to determine where and what to assess. While aspects of this are considered in a sister publication (Bowman et al, 2015) one could simply choose to adopt three paths of travel (for instance the centre of the step and/or 270 mm in from each edge of the step, depending on step width). The 270 mm distance has long been used when measuring on spiral winders. The technique is not without problems; for a start the equipment is very expensive and in itself may be cost prohibitive. The manipulation of the cloud data to provide dimensional values also requires specific software. The technique also suffers for resolution difficulties over the distances involved and to some extent points on the stair can be difficult to scan from fixed points. It is possible that handheld scanners can eliminate some of these problems. Although high definition survey equipment market should become more prevalent as its cost decreases, the data acquisition and analysis phases involve many factors where attention to detail is essential. The point data accuracy and the inherent instrumental data noise largely determine the accuracy with which measurements can be made. The reflectivity of the laser from the measured surface and the choice of exposure can degrade the accuracy of the results. Furthermore, the results can also be influenced during data analysis, depending on the adopted point cloud alignment strategy, which depends on the size of the area being measured, as well as the incorporation of independent observation data. Such items need to be carefully planned and if possible tested by a-priori statistical models to show that the required data accuracies can be achieved. Uninitiated novice users of laser scanners are likely to obtain incorrect observations and inaccurate results. In situations where high accuracy is necessary, the required tolerances and data models can best be achieved by consulting with an experienced user of both the equipment and the point cloud data analysis software. 4. Who should use what method? It is clear that as the complexity of the measurement techniques increases there is an investment needed in time and in some cases the cost of the tools. It is therefore sensible to consider each technique as to where it is best placed to serve all the potential measurers. The visual inspection and the crouch and sight test are free to use. They are also very quick to do. As a simple first examination all users should apply these techniques. In many cases what is found by these techniques can direct the interpretation of the risks associated with these stairs. For building control officers a simple measurement technique could be used to provide a quick assessment as to whether the stair is within the specification expected from the technical drawings. Where there is a difference this can be easily determined without having to know what the actual dimension values are. The technique is quick and simple and therefore ideal for such an assessment. Environmental health and Occupational health officers are also able to benefit from a simple measurement technique, but where the risk to health may be critical for the users it may be sensible to consider the complex measurement technique to provide more accurate data. Such measurements should not only help to determine risk, but also highlight what adaptions are required to reduce this risk. The final group, forensic examiners, require better precision. Their starting point for measurement should be the complex method, and care should be taken with this technique to reduce the errors that might occur through applying the technique. Where inaccuracies are forced, by the surface finish or by step irregularities, these too should be recorded and used in the assessment. However, we propose that expert 3D scanning should be used to provide a data rich assessment of stairs, with precise repeatable stair measurements. This technique also provides measurements of handrail height, handrail shape, gaps in balustrading, headroom and measurements of winders, landings and nosings. References 3DRiskMapping, (2008). Theory and practice on Terrestrial Laser Scanning: Training material based on practical applications. Available at: Bowman, R., M.S. Roys, and N. Davies. (2015). Using 3D laser scanning to improve the reliability of stairway geometry measurements. Proceedings 19 th Triennial Congress of the IEA, Melbourne 9-14 August Roys, M.S., and M. Wright. (2003). Proprietary nosings for non-domestic stairs. Information Paper IP15/03. Building Research Establishment, UK. 8