Theft-Proof Glue-In Anchor Assembly

Lower-Offs & Bolt Belays

Lower-offs and bolt belays are a hotly debated subject amongst route developers.

Whilst the same type of bolt may be used for either a lower-off or a bolt belay, the intended purpose of a lower-off differs from that of a multi-pitch bolt belay and regarding them separately helps to bring structure to this topic.

Spacing Between Anchors

A consideration for lower-offs and bolt belays is the spacing between a pair of bolts. Design principles used by the construction industry apply to a wide range of anchor diameters and embedment depths installed into uniform concrete of consistent compressive strength. Relatively shallow embedment anchoring in natural stone is entirely different, so do industrial recommendations even reflect what climbers are doing?

The axial spacing between two anchors has a tremendous influence on the load-bearing capacity. The maximum failure load a pair of anchors can be subjected to is only achieved when both breakout cones can develop unrestricted (stress cones not overlapping). With concrete failure and rock, depending on the type, the fixing produces a conical breakout body or stress cone that begins in the area of expansion or from the base of the shaft for a glue-in anchor. There are several factors that influence the profile of the cone; however, for rock climbing anchors installed into concrete with embedment depths between 50mm to 100mm, the angle is about 45°.

Applicability To Natural Stone

When applied to natural stone the key differences are:

a) the spacing rules are for concrete that normally fails differently to rock.
b) the way construction bolts generally fail is different to the way most climbing bolts fail.

Whilst recognising there is a very wide range of rock types and corresponding properties, civil construction design principles do provide developers with relevant anchor performance behaviour and principles that can be adopted.

Expansion Anchors

For competent rock and a sufficient tensile load, expansion anchors will either pull out or fracture somewhere along the shaft, typically where the hanger is connected. Shear loads cause rock crushing and bolt bending that can be resisted by using larger diameter bolts e.g. 12mm (1/2 inch).

Glue-In Anchors

The tensile failure load of individual glue-in anchors is usually calculated as a function of the depth of the anchor and there are many reports that investigate the subject. For example, Cook et al (Cook, R.A., Konz, R.C., Factors Influencing Bond Strength of Adhesive Anchors, ACI Structural Journal, 98 (2001) 76-86) conducted a comprehensive investigation of more than 1000 tests with twenty types of adhesive products. For climbing bolts with thinner anchor diameters and embedment depths, the failure mechanisms can be summarised in three scenarios:
  1. 1
    If the embedment depth is very small, a cone failure occurs.
  2. 2
    If the depth of the anchor is greater: a combined mechanism of cone failure with sliding at the adhesive/rock or anchor/adhesive interface below the cone either towards the base of the cone or base of the anchor.
  3. 3
    If the embedment depth is very high, the anchor is so resistant that the failure occurs for breakage of the steel bar.

There are around nine models that address relationships between the principle variables in anchor design: concrete, adhesive and anchor rod. The most common of which is the uniform bond stress model. Some of these equations become meaningless as the predicted values exceed the theoretical yield force for the diameters of steel and titanium grades from which bolts are fabricated. Simply put, the material breaks before the more complex failure modes involving the rock type and adhesive exert greater influence.

From specific testing performed by L. Contrafatto and R. Cosenza (University of Catania – Italy. Department of Civil Engineering and Architecture) conducted in basalt, limestone and sandstone with Hilti RE500 epoxy, the only suitable model for basalt combined an elastic bond-stress model with a cone failure model. In practical terms, it is not possible to use a single model to predict behaviour for rock climbing bolts given the wide range of base materials. Similarities only occur when the mechanical properties of the stone are similar to concrete, which is logical, if not perhaps obvious.

For harder rock (e.g. basalt or granite), there is a combination of small cone failure and sliding at the adhesive / rock, or anchor / adhesive interface below the cone. This interface failure is either towards the base of the cone or the base of the anchor. For softer rocks such as sandstone, for each diameter and anchor length failure is accompanied by the formation of a stone cone that remains perfectly bonded to the rod for all its embedded length or only in the upper portion of it. Ultimate strength is only achievable with sufficient embedment depth when the failure depends on the yield stress of the rod. This underlines the value of conducting pull-out tests before bolting a new area with soft rock.

The engineering concepts involved are clearly complex and would require Finite Element modelling and sophisticated computer code to model the crack growth and material properties of different rock types. For climbers, the principles governing anchors installed in concrete are transferable to natural stone in that they provide a reasonable guide when determining bolt spacing in rock climbing applications.

Tensile Test In Basalt
a) Tensile Test In Basalt
Tensile Test In Limestone
b) Tensile Test In Limestone
Tensile Test In Sandstone
c) Tensile Test In Sandstone

Tensile tests in a) basalt b) limestone and c) sandstone using threaded rod and Hilti RE500 epoxy. Note the different failure characteristics relative to material strength.

The recommended spacing of any two bolts is generally a function of embedment depth based on the theory that all bolts have a perfect 45° stress cone emanating from the base of the bolt. However, this is not truly reflective, as failure behaviour depends on the nature and quality of the stone type as seen above.

Current 'Rules of Thumb'

There are several recommendations (below); however, which one is best adopted?

  • UIAA recommend a minimum spacing of 200mm.
  • Ramset 1.5 times the embedment depth.
  • Hilti specifies their spacing as twice the embedment depth.

The majority of anchors do not exceed embedment depths of 100mm (3.94 inches) so if we adopt the most conservative rule (Hilti at two times embedment depth), a breakout angle at 35° and embedment depth of 100mm, we calculate a surface spacing of 70mm (2.75 inches). Multiplied by two anchors and this is clearly within the 200mm (7.87 inches) spacing recommended by the UIAA.

For extended shaft glue-ins of 150mm (5.90 inches) embedment depth, cones theoretically overlap assuming the UIAA blanket separation recommendation regardless of whether 35° or 45° breakout angles are adopted. In this example, rules based on the actual embedment depth are required to achieve design separation.

In practice, however, the UIAA rule is sufficient for all anchors and ensures that other complications do not arise by developers overcompensating and installing anchors too far apart. For the case of extended shaft (150mm / six inches) glue-ins, only 50mm (two inches) of space would be affected where the stress cones overlap and at a relatively shallow depth unlikely to reduce performance to that which could be considered dangerously weak.


Installed at the top of a single-pitch climb (traditional or sport), solely for a climber to regain the ground and recover any gear placed during the ascent.

The lower-off bolt(s) will be placed to optimise gear recovery since this is primarily a ‘clip and go’ exercise without a number of climbers taking a belay off the anchor.

Configuring a lower-off is subject to significant variability and is certainly not without endless debate and discussion. This is a difficult topic and it is fair to say that a universal lower-off configuration / solution does not exist simply because the optimum solution for one route / crag / area / user demographic may not be so for somewhere else. Safety is paramount whatever configuration a developer chooses.

Key point: lower-offs are anchors solely used for lowering or secondary top roping and their configuration reflects those requirements. Not, for example, other teams abseiling down to share the same anchor or as a team on the ascent.

Lower-Off Position

A lower off should be located where:
  1. There is a natural stance from which to clip.
  2. Avoids or minimises the rope from rubbing against the rock.
  3. Facilitates recovery of quick draws.
  4. Positions the belayer’s end of the rope away to one side of the climber when top roping.
Placing a lower-off higher above a natural stance or forcing climbers to tackle hard moves that require clipping the lower-off from poor holds is counterproductive and will not result in a pleasant experience.

Lower-Off Strength

Lower-offs need to be constructed using certified climbing bolts to address strength requirements, and be safe to use, durable and easy to maintain.

Since bolts are rated by strength, not application, what is used for runners on the route can equally be used at the lower-off. Some developers choose to use deeper bolts for lower-offs (and belays) for increased margin against lower-off misuse (e.g. falling from above the anchor or resisting wear).

Lower-Off Redundancy

Simply defined as the inclusion of extra components in case of failure in other components and for installing rock anchors, this equates to placing pairs of anchors at a lower-off or multi-pitch belay.

Redundancy and equalisation are entirely separate concepts and commonly misunderstood: a lower-off can have redundancy (two anchors) with only one anchor loaded (no equalisation).

Since we are installing fixed rock anchors certified for rock climbing use, there is NO requirement to 'load share' between both bolts.

Key point: A popular misconception is that a lower-off or belay must share the load equally between each bolt however no such requirement exists at all. This is reflected by many of Europe’s climbing organisations that also accept single bolt lower offs.

The 'need' for equalisation often reflects a traditionally constructed belay 'way of thinking' that is, of course, entirely relevant where natural forms of protection (cams, nuts) are used. Fixed rock anchors for climbing are permanent, capable of multi-directional loading and correspondingly certified to sustain much higher loads.

Key point: bolts are to be used in conjunction with an energy absorber (a climbing rope); therefore, ‘shock loading’ a redundant bolt is irrelevant, provided the fixing is correctly installed and certified for the intended purpose (climbing).

Redundancy can also apply to components (e.g. maillons [quick-links]) used to build lower-offs and belays, considering that rated strength is often less than the bolts to which they are attached. It is prudent to consider component failure in any anchor configuration and its consequences should failure occur. For example, the use of rope or nylon tape to link wear components e.g. rappel rings back to bolts, is subject to misuse and potentially fatal consequences if, by inexperience or mistake, the rope / nylon tape is threaded by a climbing rope and a climber subsequentially lowered off. This arrangement has been the cause of several accidents that easily could have been prevented by adopting a different anchor configuration.

Lower-Off Metal Grade

Lower-offs and belays introduce more components and an increased likelihood of corrosion occurring should inappropriate material combinations be adopted.

Key point: Ensure that all materials are matched in grade and ideally from the same supplier.

Lower-Off Durability

A key requirement for any lower-off configuration is that wear components are incorporated to ensure the durability of the anchors themselves. This is particularly relevant to glue-in anchors where lowering directly from the eye is possible but doing so over time results in the anchors becoming worn dangerously thin. Replaceable components include quick links, chains, mussy hooks and gym-style lowering karabiners however anything fixed will concentrate wear in the same location unlike rappel rings that can rotate and distribute the wear more evenly.

Key point: Fitting replaceable components such as rappel rings safeguards the anchors against wear while reducing the cost of maintenance. 

Safe Use of Lower-Offs

Safety considerations for any potential configuration are:
  1. Safe operation.
  2. Rope retention.
  3. Potential for misuse.
  4. Likely end use.

Ideally, all lower-offs would be a simple clip and lower-off configuration, for this eliminates the need to untie (a principal cause of accidents) and can be achieved one-handed. Many developers choose to install quick clips or mussy hooks (commonly) in North America for this reason; however, this is likely to involve increased component costs for the developer and possibly from ongoing maintenance, given there will not be rotating wear points involved.

For any configuration that requires untying, it should enable the climber to thread components with a loop of rope and attach this to the belay loop with a locking karabiner such that the climber is never detached from the rope. Therefore, if chain is used, the link size should be large enough to thread rope as well as being able to clip into direct.

For lower-offs that involve ‘dropping the rope in’ (instead of a requirement to untie and thread), there is the additional consideration of ensuring the rope remains captive, whereas, for rings, this is obviously not an issue. ‘Drop in’ anchors can be fitted with pairs of clips / mussy hooks / rams horns to reduce the potential for the rope to become detached.

Key point: However, a lower-off is configured, it should be straightforward to use and not endanger the user as a consequence of the design.

Whilst responsibility ultimately rests with the individual, decisions made by the developer obviously impact everyone choosing to climb the route after the first ascent. The rope attachment or thread point should be clear to identify, be achieved one-handed if possible and capable of being clipped into with a quick-draw.

Key point: Lower-off construction should also ensure rope retention regardless of action by the climber.

The Visual Impact of Lower-Offs

In some climbing areas, the visibility of fixed protection is enough of an issue to prompt the use of camouflaged bolts by developers. Generally, visual impact is less of a pressing issue in the context of other considerations (e.g. corrosion) however, anchors built using rope or chains are distinctly obvious compared to a pair of glue-in bolts fitted with rappel rings. Glue-in bolts have a significantly reduced profile compared to an expansion bolt because there is no flat reflective surface perpendicular to typical lines of sight. The adhesive used can obviously change everything. However, the majority of adhesives used by climbers match the natural rock colour. Hilti RE500 is often criticised for its vibrant red colour. However, the very persons criticising are often unaware of the change in colour this particular adhesive develops over time. Plated steel hangers are considered the worst in visual appearance and likewise for corrosion resistance, their use is discouraged by all North American climbing organisations and the UIAA.

Painting hardware is predominantly a North American practice and it is very uncommon to find painted hardware anywhere in Europe or elsewhere. Painting would seem a straightforward solution to camouflaging bolts and anchor setups; however, it is not that simple. Firstly, finding a paint that will resist damage from the clipping in of karabiners but of more significance is the reduction in corrosion resistance.

The Cost of Lower-Offs

This is perhaps single-handedly the most influential consideration faced by developers when selecting rigging configurations. Certain material types and grades will be more costly than others and linking hardware is required as well, so for developers in aggressive corrosion-affected regions, the requirement for high-grade stainless steel or titanium changes the economics of development considerably.

Following the initial installation, there will be ongoing maintenance costs, hopefully, limited to a pair of rappel rings for instance. The point is that the longevity of hardware is an important factor when determining what materials are used in the first place. If a route requires a complete re-bolt within the timeframe that an alternate material remains competent, the economics obviously indicate the alternate material is preferable and whilst perhaps more expensive initially, ends up proving more economical in the long term.

Why is there so much variety in the configurations encountered? Developers represent a significant range regarding anchor configuration and climbing gear companies supply what customers will buy. Some prefer pre-assembled configurations, others don’t. Discussions in online forums demonstrate there is little agreement from the user side on what is best so the manufacturers are effectively left in the position of having to offer all the options.

Climbing gyms that are free of the additional problems of climate, theft, rock quality and other local rock conditions have no particular agreement either. The solution is to recognise that lower-offs and bolt belays have different requirements and local conditions can reduce the possible configurations that will work. For example, arguing that a particular configuration is difficult to rappel from is perhaps less relevant if the scenario involves a single-pitch lower-off installed at the top of an overhanging sport route where ‘tramming’ is necessary in order to recover quick-draws.

Bolt Belays

Belays consist of two anchors and differ from lower offs in many ways:
  1. Both anchors provide a stance from which one climber belays another.
  2. Provide an anchor from which an abseil descent maybe required.
  3. Provides increased strength – two anchors are mandatory.
  4. Optimum view of the pitch below and consecutive pitch above.
  5. Positioned so that nothing can fall into the area of the station from above, if possible.
  6. May have additional anchor pairs fitted in the case of popular routes that experience heavy traffic.
  7. Could be used for rescue rigging.
  8. Will have more than one climber suspended from the belay.
  9. Positioned where pitches are broken into logical lengths and sections.
  10. Positioned to take advantage of obvious stances such as ledges to aid rope management.

Whilst climbing a multi-pitch route, there is the possibility of a leader generating a fall factor two onto the belay and loads could be imposed from different directions. There will always be two climbers suspended from the belay and often additional climbers, too, whether in the same party or from an additional team. For these reasons, belays must ALWAYS feature double anchors.

Should a belay be used for rescue rigging then a Static System Safety Factor (SSSF) commonly applied is 10:1 such that a very conservative rescue load of 2kN would require anchors rated to a minimum of 20kN. The premise is that a rescue system could experience 15kN when subjected to a worst-case event (there are specific circumstances describing this).

For these reasons, the UIAA recommends the fitting of two anchors for belays, but this should be considered a MINIMUM. Often it is advantageous to install three anchors per stance as this allows the second, upon arrival at the stance, to clip in directly off to one side whilst remaining on the belay, but not cramping the movement of the belayer.

Key Point: The third anchor can be placed slightly higher (than the other two belay anchors) to top rope the second on the pitch below but without any change; provide an immediate runner higher than the belay point to reduce any fall factor and ensure the leader falls away from the belayer if this should happen early on the next pitch.

Belay Configuration

Common rigging methods for sport belays involve equal-length slings or a pair of equal-length quickdraws attached to both belay anchors connected to the rope from the climber’s harness via a figure of eight knot (noting this is a constantly loaded anchor and not a top rope scenario).

Key point: belays are constantly loaded and we are not lowering from the belay where a single attachment point is usually required to prevent rope twisting.

Placing anchors horizontally is subject to rock conditions and there are drawbacks, as reviewed in the previous section on lower offs, however in the context of belays, there are advantages to this anchor configuration.

Key point: A convenient anchor orientation installs both belay anchors in a horizontal position to facilitate the use of equal-length quick draws and/or slings to rapidly rig a belay.

A second advantage for horizontal anchor placements is that the second, upon arrival at the belay, can attach directly to one anchor (whilst remaining on belay) and not be pulled in against the belayer, whereas an in-line arrangement would be pulling both climbers together due to the focal point.

Belays If Used For Abseil Decent

Any belay should be located to provide the cleanest hang, be offset from the pitch below and ensure the team can gain the next set of anchors. Placing the anchor higher can reduce rope drag when pulling the rope after rappelling. A single thread point for the rope creates less resistance (and twisting) when pulling the rope and an in-line arrangement may be better than placing both anchors horizontally in the context of reducing friction.

Belays should be positioned to protect the belayer from falling objects (climber / rock / dropped equipment) on consecutive pitches and this includes abseiling climbers descending over the top of a team while on ascent. Furthermore, the best anchor location during ascent may not be ideal for abseiling back down the line and on long routes, additional sets of anchors for descent may be required to prevent bottlenecks.

Establishing an independent abseil route is a prudent arrangement for routes on big walls and care should be taken to position each abseil station such that it is clearly seen from the previous station and not at the limit of standard rope lengths.