Steep Grade Descent Calculator User Guide

Transcription

1 Steep Grade Descent Calculator User Guide March 2016 Technical Report No. 16 By: Séamus Parker, Principal Researcher, Transport and Energy Non-Restricted to Members and Partners of FPInnovations fpinnovations.ca

2 FPInnovations is a not-for-profit worldleading R&D institute that specializes in the creation of scientific solutions in support of the Canadian forest sector s global competitiveness and responds to the priority needs of its industry members and government partners. It is ideally positioned to perform research, innovate, and deliver state-of-the-art solutions for every area of the sector s value chain, from forest operations to consumer and industrial products. FPInnovations staff numbers more than 525. Its R&D laboratories are located in Québec City, Montréal and Vancouver, and it has technology transfer offices across Canada. For more information about FPInnovations, visit: Follow us on: : 4RT-RR-FO- RoadConstrctn&Maint Technical Report T16 ABSTRACT The FPInnovations steep grade descent calculators were updated to include the influence of curves and switchbacks. A user guide was developed, incorporating the recent changes to assist users of these tools when conducting risk assessments for descending steep grades with loaded log-hauling configurations. ACKNOWLEDGEMENTS This project was financially supported by FPInnovations CORE membership funding. The author would also like to thank all the engineering staff from Western Forest Products for their review of the draft user guide and application examples. REVIEWERS Alex Forrester, Researcher, Resource Roads Dave Mogensen, Western Forest Products Ltd. CONTACT Séamus Parker Principal Researcher Transport and Energy seamus.parker@fpinnovations.ca 2016 FPInnovations. All rights reserved. Unauthorized copying or redistribution prohibited. Disclosure for Commercial Application: If you require assistance to implement these research findings, please contact FPInnovations at info@fpinnovations.ca.

3 Table of contents Background... 4 Tool overview... 5 Parameter description... 6 Inputs... 7 Outputs... 9 Tool application Risk assessment Road design Example Conclusions References List of figures Figure 1. Typical steep grade descent Figure 2. The user interface of the fat truck SGD calculator Figure 3. The user interface of the highway truck SGD calculator Figure 4. Estimating traction using the stopping-distance test Figure 5. Plan view of steep road descent Figure 6. Profile view of steep road descent Figure 7. SGD calculator recommendations for a highway 7-axle truck at sample critical location Figure 8. SGD calculator recommendations for a highway 7-axle truck at sample critical location Figure 9. SGD calculator recommendations for a highway 7-axle truck on steep switchback and with low traction Figure 10. SGD calculator recommendations for a highway 7-axle truck on steep switchback and with low traction, but a reduced pitch grade List of tables Table 1. Traction levels in the SGD calculator... 8 Table 2. Transition points used in the highway SGD calculator Table 3. Summary of road parameters at critical locations for sample descent Table 4. Summary of recommended descent parameters for sample descent FPInnovations T16 Page 3

4 BACKGROUND In the early 2000s, forest road grades along the coast of British Columbia were getting noticeably steeper. The increased prevalence of steep road sections raised safety concerns for WorkSafeBC, resulting in many instances of hauling suspension, road reconstruction, and drafting of specific safe operating procedures. In 2003, the Forest Engineering Research Institute of Canada (FERIC), 1 WorkSafeBC, and several major forest companies initiated a study to assess the safety implications of descending steep roads and to propose safe operating guidelines. In late 2005, WorkSafeBC issued an official guideline that separated hauling operations into two categories: grades less than 18% and grades greater than 18%. 2 This categorization was based on the assumption that reasonably maintained equipment is designed for grades up to 18%, which is the maximum allowable grade specified in the B.C. Ministry of Forests Forest Road Engineering Guidebook. For grades above 18%, a risk assessment must be conducted before hauling and a safe operating procedure with specific conditions for haul suspension must be developed. The WorkSafeBC guideline focused FERIC s research on developing guidelines that could be used to assist the forest industry in conducting risk assessments on grades greater than 18%. In 2006, a comprehensive set of guidelines was published for use with coastal fat trucks 3 (Parker, 2006). A steep grade descent calculator (SGD calculator) was developed to assist planners with the risk assessment process. The SGD calculator is a spreadsheet tool that allows the input of various road parameters (such as grade, length, and traction level) to determine appropriate payloads and descent speeds. Between 2007 and 2009, FPInnovations conducted further research on descending steep forest roads with highway-legal truck configurations (Parker, 2010). While these trucks are designed to meet the dimensional and loading constraints of the public road infrastructure, they are commonly used in offhighway applications, where the axle loads exceed the legal allowances. Highway-legal truck payload capacity is considerably less than that of the coastal off-highway truck applications previously evaluated, even when hauling off-highway loads. In addition, the braking capacity of the service brakes and engine retarders also differs between these truck types, making specific descent guidelines for highway-legal configurations necessary. A second SGD calculator was developed specifically for highway-legal truck configurations. At the request of industry co-operators, this tool (the highway version only) was also converted into a mobile phone application ( app ) for improved portability (Parker, 2012). Both the off-highway fat truck and highway versions of the SGD calculator have been widely adopted by coastal forest companies. Users of the calculator have noted that the tool does not specifically deal with switchbacks and often requested assistance from FPInnovations when assessing steep grade descents of switchbacks. The effect of switchbacks and grades on stopping performance was evaluated in the highway-legal truck study (Parker 2010), but the information was not incorporated into the SGD calculators until recently. As well, the definitions and interpretation of the various road parameters were unclear, indicating the need for a SGD user guide. So in 2015, FPInnovations revised both SGD calculators to account for switchbacks and produced the following user guide. 1 FERIC was the predecessor of FPInnovations Forest Operations division. 2 WorkSafeBC guideline G from OHS Guidelines Part 26 Forestry Operations, WorkSafeBC 3 Fat trucks carry payloads in excess of two times greater than highway trucks. FPInnovations T16 Page 4

5 TOOL OVERVIEW Both the off-highway fat truck and highway versions of the SGD calculator were developed to assist forestry operations planners in conducting risk assessments for descending steep grades with loaded log hauling truck configurations. The tool estimates the braking system temperature based on the road parameters and recommends the maximum payload and descent speed for the specified descent. The stopping capability of a truck configuration is strongly influenced by brake temperature due to the braking fade that occurs at elevated brake temperatures. The truck may stop very easily on grades as high as 30% provided the brake temperatures are low, but it may be challenged to stop on lower grades (<18%) at high brake temperatures. A steep grade descent is typically assessed by determining critical locations on the descent. These locations are determined based on grade, traction level, or the presence of a switchback, that may make an emergency stop challenging. For a truck configuration to be considered acceptable, the truck must be able to stop at this critical location using only its service brakes in the event of a driveline failure. A descent is best described using two sections, as shown in Figure 1, with the descent before the critical location followed by the critical location itself. It is likely that several critical locations will need to be evaluated for each descent. Descent prior to critical location Critical location (steep pitch) Figure 1. Typical steep grade descent. The fat truck SGD calculator was developed for purpose-built off-highway trucks with 51cm (20-inch) diameter drum brakes with water cooling and a payload capacity of 120 tonnes. The highway-legal truck SGD calculator was developed for typical 42-cm (16.5-inch) diameter brake drums without water cooling and a payload capacity of up to 60 tonnes (7-axle). Therefore, care should be taken to ensure that the most suitable calculator is applied for the appropriate truck type. For alternative off-highway FPInnovations T16 Page 5

6 trucks with intermediate-sized braking components (e.g., 18-inch diameter), the fat truck SGD calculator should be used and a conservative factor should be applied to the load output. 4 Parameter description The user interface for the fat truck SGD calculator is shown in Figure 2, while the highway truck version is shown in Figure 3. The fat truck SGD calculator has two different inputs from the highway truck version, and they deal with the options of using water cooling or the steering axle. On highway trucks, water cooling is generally not available and steering axle brakes are always present. The fat truck SGD calculator is only available for 5-axle configurations, while the highway truck version can accommodate 5-, 6-, and 7-axle configurations. Figure 2. The user interface of the fat truck SGD calculator. 4 In a previous example, the fat truck SGD calculator estimated a payload of 120 tonnes. To estimate payload for an intermediate truck, multiply this value by 0.75 to equal 90 tonnes, or its maximum equipment payload limit (whichever is less). If the maximum equipment limit is 100 tonnes and the tare weight is 30 tonnes, then the maximum equipment payload limit is 70 tonnes. FPInnovations T16 Page 6

7 Figure 3. The user interface of the highway truck SGD calculator. Inputs Traction level Traction is one of the most critical parameters affecting steep grade descents, as it determines the maximum retardation forces that can be generated at the tire road interface, which ultimately determines the maximum grade that can be negotiated safely. If sufficient retardation cannot be developed, then the truck will accelerate and lose control on a steep grade. Four levels of traction can be selected, as shown in Table 1. When selecting traction, always select the worst-case traction level for the road section under review. A well-drained gravel surface can be expected to maintain its moderate traction even when wet. However, the presence of silt or clay in the gravel will lower the traction level when wet. If traction aids, such as sand or chains, are used, then traction levels may be increased by one category (e.g., from low to moderate). The characterization of traction can best be FPInnovations T16 Page 7

8 estimated with a stopping-distance test on a level grade of a representative road surface, as shown on the SGD spreadsheet tab Traction estimation (Figure 4). Table 1. Traction levels in the SGD calculator Traction level Description Traction coefficient Very low Ice/snow 0.20 Low Loose gravel/wet hardpan 0.30 Moderate Compact gravel/shot-rock 0.45 High Dry, smooth, compact gravel 0.60 Figure 4. Estimating traction using the stopping-distance test. Steering axle (fat truck SGD calculator) Many coastal off-highway fat trucks are not equipped with functioning steering axle brakes. These brakes should be plumbed through the treadle valve and activate when the service brakes are applied. Select Yes for this option only if the brakes are functioning correctly and are well-maintained. Otherwise, select No. Water cooling (fat truck SGD calculator) Most coastal off-highway trucks utilize water sprayed on the brake drums to prevent the brakes from overheating. However, during cold temperatures, the use of water is dangerous because it causes icing of the road and reduces traction. Select Yes if water cooling is available and will be used (i.e., in nonfreezing conditions). Otherwise, select No. Number of axles (highway truck SGD calculator) The highway truck SGD calculator can accommodate configurations with 5, 6, and 7 axles. Select the number of axles for the truck configuration being evaluated. Cumulative distance prior to pitch This is the distance from the start of the haul (where logs were loaded) to the critical pitch being evaluated. This distance (and grade) determines the resulting brake temperature and, consequently, brake performance on the critical grade. If the start of the haul has an average grade of less than 8% with water cooling (fat trucks) or 5% without water cooling, deduct the distance of that road section from FPInnovations T16 Page 8

9 the total cumulative distance. At such relatively low grades, the engine retarder accounts for most of the retardation. Therefore, the service brakes do not heat up significantly and their performance will not be degraded. Average grade prior to pitch This is the average grade for the cumulative distance before the pitch. The average grade is most easily determined by the difference in elevation divided by the cumulative distance before the pitch. If part of the distance is deducted as described above, then determine the average grade over the remaining distance. Critical pitch grade This is the grade of the critical pitch under evaluation. The grade selected should be the maximum over one configuration length (20 m). For the fat truck SGD calculator, a maximum grade of up to 28% may be used (at high traction). The highway SGD calculator can accommodate grades of up to 30% at the high-traction setting. Critical pitch length This is the length of the critical pitch under evaluation. The pitch length should range between one configuration length (20 m) and 300 m. For longer lengths, add the start of the pitch to the cumulative distance and adjust the average grade prior to pitch accordingly. Critical pitch curve radius Tight curves, particularly switchbacks, influence stopping performance and make steering challenging on low-traction surfaces. The inclusion of this input essentially limits the maximum pitch grade on a tight curve. Enter the minimum curve radius for the pitch under evaluation for the critical sections where the turn angle exceeds 90. This radius is measured at the centreline of the path taken by the tractor s steering axle. Payload limit Determine the maximum equipment load rating of the log truck trailer combination, and estimate the maximum payload capacity (gross combination weight less the tare weight). This value will override the descent capability of the truck trailer combination. Minimum turning radius This is the minimum curve radius that the tractor can turn. It is best to specify a slightly higher radius to allow for slippage. The minimum curve radius is generally a function of tractor wheelbase. Typical tridem drive tractors (6.6 m wheelbase) have a minimum turning radius of 12.5 m (at the centre of the steering axle). Outputs Recommended maximum speed This speed is determined separately for the section before the pitch and for the pitch itself. It is important that the speed is reduced to the recommended speed on the critical pitch before descending. Note that vehicle speed is a function of many factors, including engine speed, transmission/rear-end gear ratios, and tire size. Consequently, the recommended speed will vary between different trucks for FPInnovations T16 Page 9

10 the same gear selection. Therefore, the hauling supervisor is responsible for determining the relationship between speed and gear selection, so that the correct maximum descent speed is achieved. For best results, it is recommended that hauling supervisors consult with their senior drivers to determine the recommended gear selection for particularly steep road sections. Recommended maximum payload The maximum payload in tonnes is assumed to be equally distributed between the truck and the trailer. For off-highway fat trucks, which are not typically equipped with on-board scales and whose load factor is unknown, smaller loads should be hauled until the load factor can be established. The maximum load height should not exceed 6.4 m and 4.8 m for fat trucks and highway trucks, respectively. Safety status The safety status of the haul is displayed at the bottom of the user interface. If the conditions are considered safe, then ACCEPTABLE HAULING LIMITS is displayed in green text. If the conditions are unsafe, then UNSAFE TO HAUL UNDER THESE CONDITIONS is displayed in red text. If the conditions are unsafe, the recommended speeds and payload will be shown as NA. When an unsafe status is indicated, investigate what can be done to allow hauling, such as changing road parameters, traction level, or the number of axles. An Unsafe flag does not necessarily mean that the road section cannot be hauled, but rather that due diligence is required to determine how the haul can be conducted safely. In cases where road construction has been completed, the road section should be reviewed with a senior driver and/or hauling supervisor to determine safe operating procedures (e.g., a guidance mark to assist the driver on switchbacks) or traction modification. In situations where road surface traction is known to deteriorate with changes in weather (e.g., wet silt or clay surface), haul suspension criteria should be determined. TOOL APPLICATION Risk assessment It is essential that users of the SGD tool select accurate road input data when conducting a risk assessment. The parameters that require the most consideration are traction and the cumulative distance prior to critical pitch combined with the average grade (i.e., net elevation). Relatively long steep descents are possible under moderate traction conditions (such as compact gravel). However, reduced traction on a steep section of road will severely degrade the stopping capability of the truck configuration. So, the road needs to be carefully assessed to locate any lowtraction sections, which will need to be taken into account in the risk assessment. Selecting the cumulative distance prior to critical pitch combined with the average grade also requires consideration. These two parameters influence the brake temperature, and thus the brake performance. Excessively hot brakes (>350 C) will result in brake fade, as the brake drums expand, resulting in reduced contact pressure between the drum and brake lining, and degrade stopping capability. The SGD calculator outputs are based on conservative temperature estimates at various critical cumulative distances and average grades. The critical cumulative distances and average grades for the highway SGD calculator are shown in Table 2. There are fewer transitions in the fat truck SGD calculator. The calculator outputs the worst-case load and speed recommendations based on these FPInnovations T16 Page 10

11 numbers. For example, if the cumulative distance is 1.6 km and the average grade is 16% (a net elevation of 256 m), the SGD calculator will display recommendations for a cumulative distance of 3.0 km and grade of 18% (a net elevation of 540 m), which is a significantly worse case condition, at two times the elevation loss. In this case, it would be more appropriate to reduce the cumulative distance to 1.5 km, which would result in a recommendation for 1.5 km at 18% average grade (a net elevation of 270 m); an elevation loss that just exceeds the actual. Therefore users should investigate a range of road inputs (distances, grades) to better understand the sensitivity and implications of their selections. Table 2. Transition points used in the highway SGD calculator Cumulative distance prior to pitch (km) Average grade (%) 0.5 a a a 15 a a 20 a 22 a 24 a a Transitions in the fat truck SGD calculator. The selection of the critical pitch grade and length is also important. Determine the locations where the maximum grade pitches occur over a distance of one and a half configuration lengths (i.e., 30 m). These locations will likely occur most often on switchbacks. There will often be shorter sections (<10 m) where even higher pitches may occur, but what is important is the average over one to two configuration lengths, which affects the stopping capability. Road design The SGD calculators were originally developed as risk assessment tools to assist in operational planning. However, in recent years it has become apparent that forest road engineers are using the tool as a design aid. FPInnovations recommends that the following precautions be taken when using the tool for road design: Consider the ability of other vehicles (e.g., shop trucks, pickup trucks, harvesting equipment) to ascend the prescribed steep grades. Allow for variances of critical pitch grades above the design, since roads are often built steeper than they are designed. Prescribe appropriate construction materials to ensure adequate traction on steep grades. FPInnovations T16 Page 11

12 Trigger increased scrutiny 5 (alternative location, increased construction supervision) when design grades exceed 24% for more than 30 m. After construction, review the design and compare it with operational experience on selected sections of steep grade, which would serve as feedback to the construction team on the tool s application. Example A sample descent is shown in Figure 5 (plan view) and Figure 6 (profile view). Four critical pitches are examined where the pitch grades exceed 18% and the road parameters are summarized in Table 3. For each pitch location, the SGD calculator is used to evaluate the maximum payloads and recommended speeds for the truck configuration planned for hauling this road. Figure 7 shows the recommended speeds and maximum payloads for a highway 7-axle truck at critical location 4. The descent analysis for both a highway 7-axle and an off-highway fat truck configuration is summarized in Table 4. The off-highway fat truck has a payload capacity of 120 tonnes, while the highway 7-axle configuration is limited to 56 tonnes (based on the recommendation for critical location 4) Figure 5. Plan view of steep road descent. 5 Increased scrutiny could involve the road construction team (planner, construction supervisor, hauling supervisor or contractor) viewing the road location in the field well in advance of construction. FPInnovations T16 Page 12

13 Elevation (m) Station (m) Figure 6. Profile view of steep road descent. Table 3. Summary of road parameters at critical locations for sample descent. Location Distance (km) Average grade (%) Critical pitch grade (%) Critical pitch length (m) Critical curve radius (m) Traction Moderate Low Moderate Moderate FPInnovations T16 Page 13

14 Figure 7. SGD calculator recommendations for a highway 7-axle truck at sample critical location 4. FPInnovations T16 Page 14

15 Table 4. Summary of recommended descent parameters for sample descent Location Speed before pitch (km/h) 7-axle highway fat truck Speed on pitch (km/h) 7-axle highway Maximum payload (tonnes) fat truck 7-axle highway fat truck The example above is of a relatively benign and short descent, allowing both configurations to essentially achieve their maximum payload capability. The most critical location determining load capacity typically occurs on steep pitches near the end of the descent, where the brakes will have become less effective due to overheating. In this particular descent, the low traction level at location 2 had no effect on payload capacity since the pitch was only 20% and the pitch occurred only 0.54 km from the start of the descent. As well, all the curve radii at location 4 were more than 25 m, which meant that the configuration s braking and steering capabilities were not impacted. If we consider a steeper and longer grade (18% for 3 km) with a steep pitch of 26%, the recommended speed and payload are much reduced (Figure 8). Under low-traction conditions, the same descent would not be possible (Figure 9). To haul on the same descent under low-traction conditions, the pitch grade must be reduced to 22% and the payload must also be reduced (Figure 10). This example illustrates the importance of knowing the traction conditions and ensuring that switchback grades are minimized as much as possible. For the road designer, it is important to specify where traction enhancements are required, allowing the road builders and supervisors to take appropriate measures. The SGD calculators may be used to determine these critical locations and modify the road design when necessary. FPInnovations T16 Page 15

16 Figure 8. SGD calculator recommendations for a highway 7-axle truck at sample critical location 4. FPInnovations T16 Page 16

17 Figure 9. SGD calculator recommendations for a highway 7-axle truck on steep switchback and with low traction. FPInnovations T16 Page 17

18 Figure 10. SGD calculator recommendations for a highway 7-axle truck on steep switchback and with low traction, but a reduced pitch grade. FPInnovations T16 Page 18

19 CONCLUSIONS The SGD calculators assist planners and hauling supervisors in identifying critical descent locations. Where an unsafe haul is identified, members of the road design/construction and hauling team need to consult with each other to develop safe operating procedures for the hauling location. Traction is one of the most critical parameters in assessing steep grade descents, so the road design/construction and hauling team need to be able to accurately assess traction level and adjust hauling practices when traction levels change. Switchback radius influences the stopping capability of logging truck configurations. Where possible, the switchback radius should exceed 25 m. REFERENCES Parker, S. P. S. (2006). Development of guidelines for descending steep grades: Coastal British Columbia off-highway applications (Internal Report IR ). Vancouver, B.C.: Forest Engineering Research Institute of Canada (FERIC). Parker, S.P.S. (2010). Development of guidelines for descending steep grades: Highway legal log hauling configurations (Internal Report IR ). Vancouver, B.C.: FPInnovations. Parker, S.P.S. (2012). Development of steep grade descent guidelines (Phone application) (Internal Report IR b). Vancouver, B.C.: FPInnovations. FPInnovations T16 Page 19

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