AS 5216:2021: Combined Pull-out and Concrete Failure of Bonded Anchors in Tension

About this Bonded Anchor Combined Pull-out and Concrete Failure Calculator

This calculator checks the design resistance of a single bonded anchor or a rectangular group of bonded anchors in tension against combined pull-out and concrete failure, to AS 5216:2021 Clause 6.2.5. A bonded anchor, a threaded rod or bar set in mortar, fails by pulling out along the bond and taking a shallow cone of concrete with it. The page scales the bond resistance of one isolated anchor by the projected area the group actually has, then applies the group, edge, shell spalling and eccentricity factors.

• Structural engineer checks chemical anchor groups for brackets, base plates and post-installed connections, with every factor and the projected area shown.
• Anchor product engineer enters the bond strengths, k3 and sustained-load limit from a product approval and sees how each one drives the design resistance.
• Retrofit designer tests edge distances and spacings for bonded anchors placed near the edges of an existing slab or wall before committing to a layout.

Every step is a live equation on an engineering-grade CalcTree page, so the check can be audited, adapted to a new layout and saved with the rest of the project calculations.

More info on Bonded Anchor Combined Pull-out and Concrete Failure

Inputs from the layout and the approval

The anchors form a grid of n_x by n_y with spacings and edge distances measured from the anchor centres on all four sides. The effective embedment depth h_ef, the anchor diameter d and the concrete strength f′c describe the anchorage. From the anchor's approval come the bond strength for the design condition and the uncracked bond strength, both at the approval's reference concrete strength, the concrete strength exponent, the bond-cone factor k3, the sustained-load limit ψ0,sus and the capacity factor φMp. The design tension N*, its eccentricities and the reinforcement spacing at the anchorage complete the inputs.

Bond resistance and characteristic distances

The approval's bond strength is scaled to the actual concrete strength by the factor ψc, and reduced by the sustained-load factor ψsus where the sustained share of the tension exceeds the approval's limit. The bond resistance of one anchor, Eq. 6.2.5.2, is that bond stress over the embedded surface. The characteristic spacing s_cr,Np follows from the diameter and the uncracked bond strength, capped at 3 h_ef, and the characteristic edge distance c_cr,Np is half of it.

Projected area and modification factors

Each anchor projects a square of side s_cr,Np. The actual area A_p,N is the group's extent in each direction, with spacings and edge distances capped at their characteristic values, and a plan view draws it against the member edges. The group factor ψg,Np of Eq. 6.2.5.5 rewards close spacing where the bond strength is low against the concrete's own capacity τRk,c. The edge factor ψs,Np, the shell spalling factor ψre,N and an eccentricity factor in each direction complete Eq. 6.2.5.1.

Design check

The characteristic resistance N_Rk,p is multiplied by φMp to give the design resistance, and the page reports the utilisation against N* with a pass or fail for the verification of Clause 3.4.2.

Common Calculation Errors to Avoid

• Entering a bond strength already adjusted for f′c applies the concrete strength factor twice. Enter the approval's value at its reference strength and let the page apply ψc.
• Using the cracked bond strength for the characteristic spacing misstates s_cr,Np. The spacing expression takes the uncracked bond strength whatever the design condition.
• Ignoring the sustained part of the load overstates the bond resistance where long-term tension exceeds the approval's sustained-load limit.
• Treating a far edge as infinite can confuse the area calculation. Enter any distance of c_cr,Np or more for an edge that does not influence the anchors.
• Building the area from every anchor when only some are in tension overstates the resistance. For an irregular layout or partial tension, take the projected area, n and s from the anchors actually in tension.
• Mixing approval data from a different mortar, drilling method, temperature range or working life gives a resistance the installation will not achieve.

FAQs

What is combined pull-out and concrete failure?

It is the tension failure mode of a bonded anchor in which the bond between the mortar and the concrete or the rod gives way, and a shallow cone of concrete comes out with the anchor. AS 5216:2021 Clause 6.2.5 covers it for bonded anchors, separately from the concrete cone check.

Why are there two bond strengths?

The bond strength for the design condition, cracked or uncracked, sets the resistance of one anchor. The uncracked bond strength is used only in the characteristic spacing s_cr,Np, which the page always works out from the uncracked value.

How does the sustained-load factor work?

Where the sustained share of the design tension is no more than the approval's limit ψ0,sus, the factor is 1. Beyond that limit it reduces the bond resistance in proportion to the excess.

When does reinforcement spacing matter?

Close reinforcement spacing at the anchorage triggers the shell spalling factor ψre,N, which reduces the resistance for shallow embedments. Normal spacing, or no reinforcement, leaves the factor at 1.

Does this cover the other tension modes?

No. Steel failure, the concrete cone and splitting are separate checks to AS 5216:2021, and the governing ratio of all of them feeds the tension and shear interaction check. Seismic and fatigue actions are not covered either, as they need the reduced bond strengths the approval gives for them.