Ground anchor
One prestressed ground anchor (tieback) of an anchored wall or a tiedown by FHWA GEC 4, Ground Anchors and Anchored Systems (FHWA-IF-99-015, 1999): the design load, the bond zone capacity and the bond length it needs, the tendon against 60, 70 and 80 % of its specified minimum tensile strength, the unbonded length, the overburden over the bond zone and the movement expected at the test load. Loads are service loads: GEC 4 designs anchors with factors of safety.
Inputs of the ground anchor #
- Design load: entered, or from the anchor row's horizontal load per length of wall TH (from the apparent earth pressure envelope, GEC 4 5.2, not part of this calculation), the spacing and the inclination: DL = TH s / cos i.
- Bond zone: soil or rock and the ultimate load transfer rate: the presumptive values of Table 6 (small-diameter gravity-grouted soil anchors) or Table 8 (rock), a rate you enter (geotechnical report, preproduction tests), or an average ultimate bond stress of Table 7 times the drilled perimeter. The factor of safety is yours: GEC 4 uses 2.0 in soil and 3.0 in rock, and the calculation says so when you enter less.
- Tendon: 0.6 in. Grade 270 strands (Table 10) or ASTM A722 bars (Table 9); the maximum test load (1.33 DL for performance and proof tests) and the lock-off load as fractions of DL; the corrosion protection class (a note).
- Geometry: the unbonded length, the distance from the head to the critical potential failure surface along the anchor (from the stability analysis, or the active wedge at 45° + φ'/2), the wall height, the depth of the head and the jack length.
Auto-design varies the bond length (15 to 60 ft) and the tendon (1 to 19 strands, or the Table 9 bars); the load, the ground, the transfer rate, the factor of safety, the unbonded length and the geometry are kept.
Checks performed #
| Check | Basis |
|---|---|
| Tendon at the design load, the test load and lock-off | DL ≤ 0.6 SMTS, TL ≤ 0.8 SMTS, lock-off ≤ 0.7 SMTS (GEC 4 5.3.8) |
| Bond zone capacity | DL ≤ Lb × rate / FS (5.3.6); the bond length DL needs |
| Factor of safety and bond length range (advisories) | 2.0 soil, 3.0 rock; 15 to 40 ft in soil, 10 to 33 ft in rock (5.3.6) |
| Unbonded length | ≥ 15 ft (strand) or 10 ft (bar) (5.3.4); ≥ distance to the failure surface + max(H/5, 5 ft) (5.3.2) |
| Overburden (soil anchors) | ≥ 15 ft over the center of the bond zone (5.3.7) |
| Test acceptance | Elastic movement at the test load for the apparent free length range, jack + 0.8 Lfree to jack + Lfree + 0.5 Lb (7.4.5.3, Eq. 49) |
The drawing is a section through the wall with the anchor at its inclination, the unbonded length, the bond zone and the failure surface where it crosses the anchor.
Limitations #
- The earth pressure and TH, the stability of the anchored system (internal and external, 5.4 and 5.5), the wall, the waler or connection, group effects and creep testing are yours.
- PTI DC35.1 is not on disk: the limits are those GEC 4 quotes from PTI (1996) (to confirm against the code text). The tendon modulus (28,500 ksi strand, 29,000 ksi bar) and the alignment load of 0.05 DL used for the expected movement are our values (to confirm against the code text).
- Presumptive transfer rates are for preliminary design; every production anchor is proven by its test.
Verification examples #
Verification status: 17 values compared in 8 examples (6 published, 2 hand calculations from the cited equations): 17 within tolerance, 0 recorded deviations. Code basis: the provisions it uses were read in the target edition.
| Example | What it checks | Published | ILX Calcs |
|---|---|---|---|
| GEC 4 design example 1, upper anchor | DL = 168 kN/m × 2.5 m / cos 15° | 435 kN | 434.8 kN |
| same | Allowable bond, 12 m at 100 kN/m, FS 2.0 | 600 kN | 600 kN |
| same | 32 mm Grade 150 bar at 0.6 SMTS | 501 kN | 500.4 kN |
| Lower anchor | DL; bond length 445 × 2.0 / 100; 3 strands at 0.6 SMTS | 445 kN; 8.9 m; 469 kN | same |
| Tables 9 and 10 | Bar and strand capacities | as printed | same |
Hand calculations: the bond stress route (τ π d) and the depth of the bond zone center.