Do Falls Weaken Bolts?
Gear Guy digs into the strength of bolts, how to tighten them correctly and relates a couple of close-calls from his past many moons ago.
Will falling on a bolt weaken it?
—James Nixon, Portland, OR
If dripping water can over time reduce granite to sand, you can imagine that falling on a bolt weakens it. But, realistically, you cannot weaken a 3/8- or 1/2-inch bolt that was properly placed in solid rock merely by falling on it, any more than you can bend a spoon just by thinking about it.
For a bolt to weaken materially from a fall you would have to exceed its fatigue strength and you would have to do so thousands if not tens of thousands of times. Fatigue failure is caused by repeated changes in tension in the bolt, such as you would have with frequent weighting and unweighting. You prevent fatigue failure simply by tightening down the bolt. Torquing down a bolt stretches it slightly, creating tension. A properly tightened bolt acts as a rigid spring pulling the hanger and the rock together. As long as this tension is greater than the load caused by a fall, the bolt won’t feel any tension change and won’t fail in fatigue.
The trick is to torque down the bolt properly—a loose bolt could fail! Proper torque depends on the bolt diameter, grade, the co-efficient of friction between the bolt and rock and bolt material (zinc-plated or stainless). Most manufacturers provide recommended torques, but these are impractical because we don’t use torque wrenches, and even if we did, rock variables would make the specifications little better than guesses. Not to worry, because as long as you wrench a bolt down tight, you are in the ball game. Power Fasteners, for example, recommends torquing its Power/Rawl 5-piece ½-inch bolt to 35 to 45 foot-pounds. But they also say you can tighten it “three to five turns past finger tight.” To me, their recommendation is the maximum I can exert on a bolt before the wrench or socket skips off and I bark my knuckles. In short, this is a very tight bolt.
You didn’t ask about other bolt-failure modes, but while we’re at it I have had two bolts break and heard of others. The first, in 1982, was a ¼-inch by 1¼-inch split-shank, threaded Rawl Drive at a two-bolt hanging belay on El Cap’s Iron Hawk. This bolt, probably set in 1977 during the first ascent, sheared under body weight. It broke despite being less than five years old and likely never having held a fall. The culprit? Corrosion. The bolt, placed in what I would consider an arid environment, was rusted nearly all the way through. Sure, we don’t use 1/4-inch bolts anymore, but corrosion can cause any diameter bolt to fail. You can’t completely prevent corrosion, but you can slow it to a point where it becomes negligible by using corrosion-resistant stainless steel.
The second failure was in a 3/8-inch by 3-inch zinc-plated Rawl 5-piece that broke in the early 1990s while I was tightening it down with a short crescent wrench. The hex head on this bolt twisted off under light pressure, so overtorquing wasn’t the problem. I called Rawl and spoke with their engineers, who said that the likely culprit was an air bubble that developed in the metal during the manufacturing process. The bolt was probably defective.
Because bolts can and do fail, and for a variety of reasons, never trust a lone bolt at a rappel, belay or lowering station—that seemingly defective bolt I was placing was going to be a one-bolt rap anchor 200 feet off the deck. If I had tightened it just a little bit less, it wouldn’t have broken in my hands, but while I was on rappel. OMG!
This Gear Guy question appeared in Rock and Ice issue 165 (January 2008).
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