A suspended walkway consists of a treadway of either native or finished material suspended by four wire ropes (Figure 2-10). The two lower ropes support the treadway. The two ropes at the higher level provide handlines and are reconnected to the lower ropes by vertical stanchions. The wire ropes are rigged and anchored so that the center of the ropes sag below a straight line between the ends. This sag prevents the tension in the rope from becoming great enough to snap the rope under normal loads and must be computed in advance.
• Rigging Lines. Four solid anchors and two vertical stanchions are required at each end of both terminals before rigging the wire ropes. Each of the lower wire ropes are fastened to its anchor at one terminal with wire rope clips. A round turn is made at the bottom of the end stanchion, and the end of the rope is passed across the gap to the other terrninal. A round turn is made in the wire rope at the end stanchion of the new terminal. It is fastened to the anchor with wire rope clips. The wire ropes are adjusted to the desired sag allowance, and the wire rope clips are tightened securely at both ends. The upper wire ropes are placed in the same manner, including sag allowances, except that each rope is fastened to its end stanchions with the clips provided on the stanchions.
Anchoring. The holes are drilled and the anchors are placed. Wherever possible, the anchors are placed so that the direction of pull exerted on them is in line with the opposite end anchor. The angle made by the horizontal portion of the upper wire rope (Figure 2-10) and the portion on the rope leaving the anchor should not be greater than 34 degrees.
Sag Allowance. Sag is the vertical distance from the mid point of the cable to the chord (Figure 2-11).
The chord span must be measured before the sag allowance is determined. Sag ratio is the ratio of the sag to the span. For spans up to 12 meters (40 feet), a sag ratio of 1 percent may be used. For spans from 12 to 20 meters (40 to 60 feet), the correct sag can be determined by the formula: Percent sag = 1 + 4/3 ((span - 12) / 3 )
With a maximum cable stress of 2,060 pounds for a .9-cm (3/8-inch) wire rope, this formula allows a safety factor of 4, which is the minimum consistent with safety of personnel using the walkway. Where the span of the gap is greater than 20 meters (66 feet ) or the chord slope is more than 20 degrees from the horizontal, a suspended walkway should not be used. Assuming a maximum 20-meter (66 feet) span, the formula above would apply as follows: Percent = 1 + 4/3 ( (20 - 12) / 3 ) = 1 + 32/9
4.5 percent x 20 meters (66 feet) = .9 meter (3 feet) of vertical sag allowance in cables. NOTE: Above computations are approximate and have been rounded off to the nearest whole number.
To install the cable with the sag allowance computed, the amount of sag allowance is measured vertically below the wire rope at each end stanchion and is clearly marked. The wire rope is adjusted until the line of sight between the two marks on opposite sides of the gap coincides with the lowest point on the wire rope. Sag is allowed in both upper and lower wire ropes in the same manner.
Stanchions. The two vertical stanchions are made and set at each end of the walkway. The lower clips are removed or left off these stanchions when they are placed. After the wire ropes are rigged and the correct sag allowance is set, the remaining stanchions are loosely placed on the wire ropes at one end of the walkway by placing the U-bolts over the wire rope and through the stanchions. The nuts are placed on the U-bolts, and each nut is turned enough to hold it until the treadway is set; nuts are not tightened.
Native Material. Logs about 9 cm (3 1/2 inches) in diameter and 1 meter (39 inches) long are used.
The logs are carefully smoothed to prevent injury to personnel using the walkway. The supporting wire ropes are 65 cm (26 inches) apart. A slot is cut about 2.5 cm (1 inch) deep (Figure 2-12) in each end of the logs on the bottom to fit over the supporting wires. The logs are placed progressively from one side to the other, pushing the vertical stanchions ahead of the work. As the logs are placed, No. 9 galvanized wire or any similar wire is weaved around them and the wire rope.
When the position for the first pair of vertical stanchions is reached (1 meter [3.2 feet] from the end stanchion), an additional vertical notch is placed in the logs to go on each side of the stanchions (Figure 2-12). At this point, one pair of stanchions is securely fixed in place by firmly tightening the nuts on the U-bolts. Then one notched log is placed on the near side of the stanchion and one on the far side, and the wire is weaved around them. The logs for the treadway are laid as before until the position for the second pair of stanchions is reached. This procedure is repeated as needed.
• Native and Finished Material. Finished lumber 5 cm by 30 cm by 2.5 meters (2 inches by 12 inches by 1 inch) long makes a satisfactory flooring for a suspended walkway used in combination with native material.
Logs about 12 cm (5 inches) in diameter and 1 meter (3.2 feet) long are cut and smoothed. They are squared on top for use as stringers (Figure 2-13). A slot in each log is cut at both ends to fit over the wire rope cables. One log is placed over the lower wire ropes at a convenient position near one end of the walkway. A second log is placed over the lower ropes with its center 1.5 meters (5 feet) from the center of the first log.
One 5- by 30-cm (2- by 12-inch) board is sawed in half, and one 1.25 meter (49 inches) long board is nailed and placed between the centers of these two logs, covering half the width of the walkway (Figure 2-13). A 2.5-meter (8.2-foot) length of board is nailed in place beside it. Another log is placed under the free end of the 2.5-meter (8.2-foot) board and nailed into place. Additional 2.5-meter (8.2-foot) lengths and logs can now be added to cross the walkway. The joints in the boards are staggered for added strength, and an end of board projects each time to provide support for placing each new log stringer. The log stringers are tied at both ends with a wire to hold them firmly to the supporting wire ropes and stanchions.
2. Three-Rope Suspended Walkway.
A three-rope suspended walkway is made of a tread rope and two handlines that are connected by stanchions (Figure 2-14). It is used as an expedient walkway and provides only one-way traffic. Because of the flexibility of fiber rope, wire rope is preferred for these walkways. Considerable motion of the ropes is possible in either case; personnel using these walkways must proceed with care.
• When wire ropes are used, the stanchions may be made of wire rope fastened to the main ropes with wire rope clips. This walkway is much stiffer and easier to traverse if pipe stanchions are used and securely clipped to the main ropes.
• In construction, the three main ropes (one tread rope and two handlines) are laid parallel on level ground. The stanchions are secured to the ropes at intervals of about 45 cm (18 inches). The entire assembly is then dragged across the gap and fastened in place. Trees may be used as end stanchions at either end, or pipe stanchions may be set as supports with rock anchors to secure the ends of the lines. One rock anchor is set behind each stanchion for the handlines, and one is set in the center for the tread rope. Fasten the wire ropes to the anchors securely with wire rope clips at one terminal and adjust the tension to provide the correct sag allowance before clipping the ropes to the anchors at the other terminal.
Conclusion: You have now completed the instructional material for Lesson 2. Before you complete the practice exercise for this lesson, you should review the material presented in this lesson. Answers and feedback for the questions in the practice exercise are provided to show you where further study is required.
r gure 2-14 Three-Rope Suspended Walkway
r gure 2-14 Three-Rope Suspended Walkway
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