CoatingsPro Magazine

MAY 2017

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32 MAY 2017 COATINGSPROMAG.COM Safety Watch C able with deck and cable with fence systems are popular among contractors in the bridge painting indus- try as they can create a safe, walkable platform under the bridge during blast- ing and painting operations. However, over-tensioning cables is easy to do and can be very dangerous. W hether you're a coatings appli- cator installing your own cable system or a specialty firm installing one for someone else, understanding the science behind the system will provide a working knowledge that, when applied, can help to keep workers safe and cables within Occupational Safety and Health Administration (OSHA)- regulated design requirements. Design Loading Indicators of over-tensioning are a platform that seems "tight," spreader bars that are bending, or cables that tend to "pop" in and up when installing along a curve. If any of these things are happening, it's best to back off on the tensioning. Tension in cables at mid-span (t) and at tie-off points (T) are based on several variables: load (W), deflection (yc), and span (S). See Diagram 1. For a given cable size, load, and span, the equations can be modified to determine the minimum required deflection to keep cable forces within OSHA require- ments under design loading. Simply stated, increases in load and span will increase cable tension; however, decreases in cable deflection will also increase cable tensions. at nice, tight platform should really be more like a bouncy trampoline. e problem with cable systems is that design loading occurs quite some time after rigging and tensioning of the cables, which takes place when only the weight of the deck or fencing is in place when loads are typically around 3–4 lb./ft.² (14.7–19.5 kg/ m²). Until design loads are applied (typically 25 lb./ft.², or 122.1 kg/m²), cable tension should be a fraction of the maximum allowable design forces per OSHA, and deflections should be only slightly smaller than anticipated full load deflections. However, what often happens, which is a very scary scenario, is that tensioning of cables is done initially to get that nice, tight platform, then as full design loads are approached during abrasive blasting operations, cable tensions increase and tend to exceed OSHA's design criteria. Unless a stress gage is utilized and monitored regularly under varying load conditions, maximum tension values in the cables during work cycles are never known and, again, most likely exceed OSHA regulations. To give an example of how the equations work under design loading, let's look at a ⅝-inch-diameter (1.6 cm) cable w ith deck system, installed w ith cables spaced at 5 feet (1.5 m) on center, hangers placed at a ma ximum of 20 -foot (6.1 m) inter vals due to diaphragm locations, and ty pical design loading of 25 lb./ft.² (122.1 kg /m²). Based on OSH A regulations, the allowable ma ximum tension for a ⅝–inch-diameter (1.6 cm) 6x19 independent w ire rope core (IWRC) w ire cable is not to exceed 6.87 kips. Utilizing the equations and designing for a uniform load (W ) of 25 lb./ft.² (122.1 kg /m²) x 5 feet (1.5 m) = 125 lb./ft. (610.3 kg /m), w ith a minimum total def lection (yc) of 12 inches (30.5 cm), the tension force (T) in the cables is 6.4 kips, which is less than the 6.87 kip ma ximum allowable load per OSH A . In this scenario, all is good. However, it is often difficult to predict initial cable def lections during rig ging and tensioning, which correspond to specific design def lections under full loading, so over-tensioning cables is easy to do. Let's now take a look at what an overzealous worker can do to our numbers above when he or she tensions the cables such that final deflections become half of what was presumed in the design. Using a revised deflection of 6 inches (15.2 cm) in the equations with full design loads and spans unchanged, cable tension increases from 6.4 kips to 12.6 kips, which is well over the 6.87 kip OSHA-regulated design limit. Reducing the deflection to a nice, tight 3 inches (7.6 cm) gives us a cable tension under design loading of even more — 25 kips! OSHA has some pretty big factors for safety requirements pertaining to cable platform systems, including all Images courtesy of Providence Engineering Diagram 1. By Patti L. Seitz, P.E., Senior Structural Engineer for Providence Engineering Corp. Setting Up Safe Cable Platform Systems

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