Prestressed anchor block

IRC SP 105 – I‑Girder End‑Block (Anchorage Zone) Calculator

Checks for Bearing, Bursting Steel and Spalling Steel for internal anchorages (per EC2‑2 Annex J approach adopted in IRC SP 105). All units are SI.

1) Global Inputs

Characteristic strand UTS, f_pk (MPa)

Typical 7‑wire strands

Stress at transfer (% of UTS)

e.g., 65–80%

Strand area, A_strand (mm²)

Per strand

Concrete at time of transfer

Cube strength at stressing, f_ck,cube(t) (MPa)

Cube→Cylinder factor

≈0.8 for normal concretes

Bearing limit factor

Limit = factor × f_ck,cyl(t)

Steel for bursting/spalling

Rebar f_yk (MPa)

Design strength f_yd = 0.87 f_yk (MPa)

Override if needed

γ_p,unfav (prestress factor)

Optional steel stress limit (MPa)

Use 0 to disable scaling

Serviceability note

Summary

Concrete cylinder at transfer

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Bearing limit (MPa)

—

Min. bursting steel per tendon (mm²)

—

Min. spalling steel per tendon (mm²)

—

Formulas: A_s,burst = 0.15·(P_max/f_yd)·γ_p (scale by f_yd/σ_s,lim if limit set); A_s,spall ≥ 0.03·(P_max/f_yd)·γ_p.

2) Tendons & Anchorage Prisms

Enter each tendon’s composition and its associated prism size C × C′ (associate rectangle) for bearing & transverse reinforcement checks.

#	Strands / tendon	Area / strand (mm²)	fpk (MPa)	% UTS at transfer	Pmax (kN)	C (mm)	C′ (mm)	Prism area (mm²)	Bearing stress (MPa)	Status	—

Status uses limit = bearingFactor × f_ck,cyl(t). Bearing stress = P_max / (C·C′).

3) Detailing Assistant (per Tendon)

Select tendon row

Provide bursting bars – dia (mm)

Provide bursting bars – count

Provide spalling bars – dia (mm)

Provide spalling bars – count (each dir.)

Provided A_s,burst (mm²)

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Provided A_s,spall (mm²) per dir.

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Bursting provision vs demand

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Spalling provision vs demand

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4) Notes & Assumptions

• Method per EC2‑2 Annex J as reproduced and used in IRC SP 105 for internal anchorages. Bearing check uses the associate rectangle immediately behind anchorage.
• For proprietary systems, comply with the anchorage supplier’s catalogue for minimum prism sizes, edge distances, and mandatory transverse (bursting) reinforcement. This tool assists verification and documentation.
• Use σ_s,lim ≈ 250 MPa when you prefer to avoid explicit crack‑width checks in the end‑block zone; the app scales A_s accordingly if enabled.
• All results are per tendon/prism. For multiple anchorages bearing on a common prism, ensure compatibility and check congestion and detailing clearances.

Prepared for educational/design‑aid use. Engineer to verify and take responsibility for final detailing and compliance with the latest IRC/IS provisions.