Failure mechanism
Anchor bolt pull-out
An anchor bolt in tension usually fails in the concrete before it fails in steel. The concrete breaks out in a cone that starts at the embedded end of the anchor and runs up to the free surface, so the capacity depends on how deep the anchor actually went in and how much concrete there was around it, not on the diameter of the bar. A bolt specified as a certain size is really a concrete cone of a certain size, and every millimetre of lost embedment or lost edge distance comes straight off the load it can carry.
How this failure works
An anchor in tension has several possible failures and the design strength is the smallest of them. The anchor itself can yield or rupture at its net section. The concrete can break out in a cone, and for a cast-in bolt of ordinary size this is usually the governing mode: the failure surface radiates outward and upward from the embedded end, so the resisting area grows roughly with the square of the effective embedment and the capacity does not follow the bar diameter at all. Pull-out and pry-out are separate modes in which the anchor pulls through a local cone or a short section of concrete, and splitting occurs where the anchor is close to a free edge or the concrete section is thin.
Edge distance and spacing are the two variables that remove capacity without anything visible having changed. A cone cannot form fully adjacent to a free edge, so the capacity is reduced by an edge factor; a corner is the worst case because two edges truncate the cone at once. In a group spaced closer than the individual cone fields can develop, the cones overlap, and the group does not carry the sum of the per-anchor capacities — the design has to work from the projected area of the whole group. These are geometric facts, and no drawing review will catch them if the anchors were moved on site to miss a reinforcement bar, a duct or a sleeve.
Cast-in and post-installed anchors do not develop tension in the same way. A cast-in bolt, headed stud or hooked bar carries tension by bearing its head or hook on the concrete, in a hole formed by the anchor itself with the concrete cast around it. A post-installed mechanical anchor expands a wedge or a sleeve against the wall of a drilled hole, and the friction and bearing that produce its capacity depend on that hole being the specified diameter, clean, and in concrete of the assumed strength. A hole drilled oversize, or drilled with a worn bit, gives partial expansion and a fraction of the published capacity, and the anchor still tightens — which is exactly what makes it deceptive.
The base plate is part of the mechanism, not a separate item. Under uplift or moment the tension reaches the anchor through a plate that may flex, and an unstiffened plate adds prying to the bolt tension in the same way an unstiffened end plate does in a steel connection. The plate also has to sit on grout that bears, because a plate carried on shims over a grout pad that is not in contact transfers its compression through the anchors instead of through bearing, and the anchor is then carrying a load it was never checked for. Shear has its own concrete capacity, and a shear load acting through an ungrouted gap bends the bolt and adds tension on top of the shear.
Illustration
Conceptual illustration: an anchor bolt in tension with the concrete breakout cone running from the embedded end to the surface, and the edge distance that truncates it.
Typical causes
- Effective embedment shorter than the design: the anchor set high, cut short, or stopped on reinforcement and left where it landed.
- Edge or corner distance reduced on site, so the concrete cone is truncated and the tension capacity falls.
- An anchor group whose spacing was never checked as a group, with overlapping cones carrying less than the sum of the individual anchors.
- Post-installed expansion anchors set in holes drilled oversize or with a worn bit, giving only partial expansion against the hole wall.
- An unstiffened base plate that flexes under uplift and adds prying force to the tension in the bolts.
- Base plates carried on shims with grout that does not bear, so compression is transferred through the anchors instead of the grout.
- Concrete below the design strength when the anchors were tensioned, or shear checked only as a bolt value and never against the concrete edge.
Warning signs
- A nut that turns freely or is loose after the plate was tightened, or a washer cupped under a nut.
- Rust bleeding from the grout line, or a hairline crack in the concrete running out from a base plate corner.
- Grout that sounds hollow when tapped, or a visible gap between the plate and the grout along one edge.
- A chip, spall or dished patch of concrete at the edge nearest an anchor.
- A base plate that is not level, or a bolt standing at a visible angle to the plate face.
- Threads that stop short of the nut on some bolts in a group, or one bolt sitting markedly lower than its neighbours.
Inspection methods
- 01Measure the embedment, edge distance and spacing of each anchor as installed against the drawing, and record the readings.
- 02Check the bolt group for plumb and the plate for fit before grouting, using the setting template as the reference.
- 03Confirm the concrete strength at the time the anchors were loaded from test records rather than from the mix design.
- 04Tap-test the grout for hollows and confirm that the plate bears on it around the full perimeter.
- 05Check for cracks in the concrete radiating from the plate, and for corrosion at the plate and grout line.
- 06For post-installed anchors, review the setting record against the anchor installation instruction, including bit diameter, hole depth and hole condition.
Preventive measures
- Design the anchor group for every mode — steel, cone breakout, edge and splitting, and shear — and specify the anchor type and the effective embedment.
- Set cast-in anchors from a template and check them before the concrete is placed; check post-installed anchors before the plate is fixed.
- Hold edge and corner distances as design requirements, and treat a moved anchor as a design change rather than a site adjustment.
- Follow the anchor installation instruction on bit diameter, hole condition and setting tool, and keep it at the work face.
- Detail a base plate stiff enough to limit prying, and grout it so the plate bears rather than standing on shims.
- Do not tension anchors until the concrete has the strength the design assumed, and record the tensioning that was done.
- Protect the thread and the surrounding concrete after installation, and recheck the group before the structure above is loaded.
Relevant standards
The codes and specifications a design is normally judged against. Naming them is not a claim that AKEN certifies, inspects or approves against them — the design authority for a project does that.
- IS 456 — concrete, including the base and foundation work the anchors sit in.
- EN 1992-4 — design of fastenings for use in concrete, including cone breakout and edge effects.
- ACI 318 — the concrete code whose fastening provisions cover tensile breakout and shear capacity.
- ETA — the European Technical Assessment for the anchor product, which states tested capacity and installation conditions.
- IS 800 — the steel side of the detail, including base plate and connection design.
Related AKEN services
Scope note
This article explains a failure mechanism for engineering education. It is general guidance, not a design check, an inspection report or a certification for any particular structure. Assessments of an actual structure must be carried out by the responsible design authority against the project’s own specifications.