Concrete Anchor Steel Strength (ACI 318-19 17.6.1 / 17.7.1)
The steel strength of an anchor bolt, the other half of every 'least of steel and concrete' anchor check.
Example
You enter
- Anchor diameter da (in) 0.625
- Threads per inch n (5/8-11 -> 11) 11
- Steel yield fya (psi) 36000
- Steel tensile futa (psi) 58000
You get
- Effective thread area Ase 0.2260 in^2
- futa used (after the 1.9 fya / 125 ksi cap) 58,000 psi
- Nsa (lb) 13108.1
- Phi nsa (lb) 9831.1
- Vsa (lb) 7864.9
- Phi vsa (lb) 5112.2
Checked against published worked examples from: ACI 318-19.
Details, formula, and sources
ScopeAse = (pi/4)(da - 0.9743/n)^2 from the thread callout, futa capped at min(1.9 fya, 125 ksi), Nsa = Ase futa (phi 0.75), Vsa = 0.6 Ase futa (phi 0.65, cast-in headed/hooked bolts). A 5/8-11 A307 checks 9.8 kip design tension and 5.1 kip design shear. ACI Chapter 17's own steel check - the AISC J3 bolt checks are the structural-connection path, with different numbers.
Formula or tableAse = (pi/4)(da - 0.9743/n)^2; futa <= min(1.9 fya, 125,000 psi); Nsa = Ase futa, phiNsa = 0.75 Nsa (ductile, tension); Vsa = 0.6 Ase futa, phiVsa = 0.65 Vsa (ductile, shear, cast-in headed/hooked bolts); a cast-in headed stud takes Ase = (pi/4) da^2 on its shank and Vsa = Ase futa (17.7.1.2(a)).
Source and editionThe ACI 318-19 Sections 17.6.1 (steel strength in tension) and 17.7.1 (steel strength in shear, Eq. 17.7.1.2b) with Table 17.5.3 phi factors, by name; the Ase expression is the R17.6.1.2 commentary formula (also AISC Manual Table 7-18).
Free accessACI 318 is readable free through the ACI online reading room at concrete.org; the Chapter 17 anchoring provisions are in the published code.
What governsEstimate. AHJ and licensed professional govern.
API field namesanchor_dia_in, threads_per_in, fya_psi, futa_psi, ase_in2, futa_used_psi, nsa_lb, phi_nsa_lb, vsa_lb, phi_vsa_lb
- Effective area Ase = (pi/4)(da - 0.9743/n)^2 from the thread callout (5/8-11 -> n = 11)ACI 318-19 R17.6.1.2
- futa cap no more than the lesser of 1.9 fya and 125,000 psiACI 318-19 17.6.1.2
- Shear factor 0.6 Ase futa for cast-in headed and hooked bolts; a welded headed stud develops the full Ase futa on its unthreaded shank (anchor type select, added 2026-10-08)ACI 318-19 17.7.1.2
- phi (ductile) 0.75 tension / 0.65 shear; brittle elements take lower values (not offered)ACI 318-19 Table 17.5.3