Webbing on bolt hangers
In American canyoneering, common rigging practice for bolts is for webbing to directly contact standard bolt hangers. Because of the relatively sharp edges and/or thickness of the bolt hanger, people in other disciplines and locations have questioned this practice since these factors may lead to a reduction in breaking strength of the webbing at those contact points.
- This rigging practice is essentially ubiquitous in southwest American canyoneering and there have been no incidents known to this wiki involving this failure mechanism (edit this page to update this information -- doing so requires free registration to prevent spam).
- In a quick backyard test, Paul Stovall measured the breaking strength of a loop of webbing directly around a standard bolt hanger at 9.85kN and 9.95kN.
- While not directly applicable, DMM climbing found that breaking tensions for slings over a thin wire were 6.6-9 kN with a Lark's Foot hitch and 9.6-13.9 kN with a basket hitch
- Spencer Christian has seen standard bolt hangers cut webbing first hand with only three rappels and ascents using new webbing.
- Webbing in hangers-Grant Prattley https://overtheedgerescue.com/canyoning/webbing-in-hangers/
- Kevin Daniels of Fixe Hardware is quoted by Paul Stovall as saying, "Don’t do this. Tell them to stop."
- American Mountain Guide Association curriculum approves of the practice.
- Fred Bowers reports that Steve House says he does it.
- The relatively sharp edges of the standard bolt hanger may cut through the webbing, especially with cyclic dynamic forces such as may occur when ascending
- The somewhat-rounded but still-sharp edges of the standard bolt hanger have a smaller radius than the minimum pin size specified for slings of similar material, so these sharp bends will cause fatigue due to differing inner and outer radii of the webbing
- The thickness of a standard bolt hanger is smaller than the minimum pin diameter that one should use with webbing
In favor of
- Other pieces of the system are likely to have lower breaking strengths so this is unlikely to be the weakest link in the system making it irrelevant for actually causing accidents
- The anchor will usually be weakened by other factors (such as sunlight, abrasion against rock, animal chewing) before it is weakened by rubbing against the hanger
- Because webbing is thin (much more so than rope or cord), the minimum bend radius is much smaller since the inner and outer radii of the webbing are closer together, and this makes attaching webbing directly to standard hangers acceptable
- Canyoneers rarely ascend ropes, so the cyclic dynamic loads most likely to cause a problem with webbing directly on a standard bolt hanger rarely apply
- A two-bolt anchor with a loop of webbing around each bolt results in four strands of webbing supporting the load. This means any force is already reduced by a factor of 4, so the anchor breaking strength is only somewhat lower than four times the breaking strength of a single piece of webbing over a sharp edge.
One suggestion to mitigate this issue is to attach quick links (mallions/rapides) directly to the hangers and then attach the webbing to the quick links. If the quick links are made of a different metal than the hanger, galvanic corrosion may be an issue. Attaching the webbing to a quick link may also load the outside of the webbing more than the inside of the webbing because the quick link can be more rounded than the bolt hanger.
Another alternative is to use bolts with round heads or chain attached directly to the anchor so that no webbing is used in the system.
A number of factors may influence the decision whether to use this practice or not:
- Modern hangers; old hangers had sharper edges
- Professional or recreational settings
- First descents; it may be prohibitive to carry a sufficient number of quick links relative to the additional safety provided
- Rope Test Lab post prompted by a picture of a bolt in Rubio Canyon
- Canyon Rigging post in response to observing this practice being taught in a training course
- More details: https://www.facebook.com/groups/canyonrigging/permalink/1024530160937521/?comment_id=1024789887578215&comment_tracking=%7B%22tn%22%3A%22R4%22%7D
- Instructor places webbing directly on hanger: https://amga.com/part-9-in-the-oramga-climbing-fundamentals-video-series-anchors-replacing-old-webbing/