Check concrete cone failure of a single anchor or anchor group in tension to AS 5216:2021 Section 6.2.3. Open the template and run your layout.
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About this AS 5216 Concrete Cone Failure Calculator
This calculator checks the design concrete cone resistance of a single anchor or a rectangular anchor group in tension to AS 5216:2021 Section 6.2.3. It starts from the cone of one isolated anchor, scales it by the projected area the group actually has once neighbouring cones overlap and member edges cut it off, then applies the factors for the nearest edge, closely spaced reinforcement, an eccentric tension and a compression force under the fixture.
- Structural engineer. Checking a base plate or bracket fixing on a slab or column, where the anchor group sits near one or two edges and the cone governs.
- Anchor design reviewer. Confirming the projected area and modification factors behind a supplier's or colleague's anchorage calculation, step by step.
- Precast or facade engineer. Sizing embedment and spacing for cast-in or post-installed fixings in narrow members, where three or more close edges reduce the effective embedment depth.
Every intermediate value is shown with units, and a plan and elevation redraw as you change the layout. It is an engineering-grade calculator on CalcTree that you can duplicate, adapt and keep with the rest of your project calcs.
More info on AS 5216 Concrete Cone Failure
Inputs
You enter the grid of anchors (number in each direction and the spacings), the four edge distances measured from the anchor centres, the effective embedment depth h_ef, the concrete strength f′c, the cone factor k1 and the capacity factor φMc from the anchor's approval, and the design tension on the group. Optional inputs cover the eccentricity of the tension resultant in each direction, whether the reinforcement at the anchorage is closely spaced, and whether a compression force acts under the fixture, with its lever arm.
Projected areas and narrow members
The ideal cone of one anchor projects as a square of side s_cr,N, giving A0_c,N. The group's actual projected area A_c,N is the product of its extent in each direction, with each edge distance capped at c_cr,N and each spacing at s_cr,N. Where three or more edges are close, Clause 6.2.3.8 replaces h_ef with a reduced depth h′_ef, and the page works this out from the close edges and spacings before any area is calculated.
Modification factors
Four factors adjust the area-scaled resistance: the edge factor ψs,N (Eq. 6.2.3.4) for the nearest edge, the shell spalling factor ψre,N (Eq. 6.2.3.5) where reinforcement is closely spaced, the eccentricity factors ψec1,N and ψec2,N (Eq. 6.2.3.6), and the compression factor ψM,N (Clause 6.2.3.7), which only applies when every edge is far enough away. Their product is shown as a single combined factor.
Resistance and design check
The single-anchor resistance N0_Rk,c follows Eq. 6.2.3.2 from k1, f′c and h_ef. Eq. 6.2.3.1 scales it by the area ratio and the combined factor to give N_Rk,c, and the capacity factor from Table 3.2.4 gives the design resistance, verified against the design tension under Clause 3.4.2 with a utilisation and a pass or fail result.
Common Calculation Errors to Avoid
- Using the wrong k1. The cone factor differs between cracked and uncracked concrete. Take it from the approval for the condition the design actually assumes, and use cracked unless you can show otherwise.
- Missing the narrow member rule. With three or more close edges the full embedment depth overstates the cone. Clause 6.2.3.8 requires the reduced depth h′_ef, which then also drives s_cr,N and c_cr,N.
- Overlapping cones counted twice. Adding single-anchor resistances for closely spaced anchors ignores the overlap. The projected area A_c,N of the group, not the sum of individual cones, sets the resistance.
- Projected area from anchors not in tension. Where only some anchors of a group carry tension, the area should be built from those anchors only, not the whole fixing pattern.
- Claiming the compression factor near an edge. ΨM,N only helps when every edge is at least c_cr,N away. Near an edge it must stay at one.
Engineering templates
Common calculators
Design guides
FAQs
What is concrete cone failure of an anchor?
It is the tension failure mode where a cone-shaped body of concrete breaks out around the anchor, with the anchor still intact. AS 5216:2021 Section 6.2.3 gives its resistance from the cone of one isolated anchor, scaled for the group's projected area and for edges, reinforcement, eccentricity and compression under the fixture.
Does this calculator handle anchor groups?
Yes. Enter a rectangular grid by the number of anchors and the spacing in each direction. The page builds the projected area of the whole group, so overlapping cones are treated correctly. For an irregular layout, build the projected area from the anchors actually in tension.
When does the reduced embedment depth h′_ef apply?
When three or more edges are closer than the characteristic edge distance. The page counts the close edges and, if there are three or more, replaces h_ef with h′_ef from the largest close edge distance and the largest spacing, per Clause 6.2.3.8.
Where do k1 and φMc come from?
Both come from the anchor's approval. Take k1 for cracked or uncracked concrete to match the design assumption, and φMc including any installation safety factor.
Does this check cover the other tension failure modes?
No. Steel failure, pull-out, combined pull-out and bond, and splitting are separate checks in AS 5216:2021, each with its own CalcTree template. Supplementary reinforcement designed to carry the tension is not covered.
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