Rebar Compression Development Length (ACI 318-19 25.4.9)
How deep compression dowels must embed: ldc = max(fy x psi_r / (50 lambda sqrt(f'c)) x db, 0.0003 fy psi_r db, 8 in).
Example
You enter
- Bar diameter db (in, #8 = 1.00) 1
- Bar yield fy (psi) 60000
- Concrete strength f'c (psi) 4000
- Lightweight factor lambda (1.0 normalweight, 0.75 lightweight) 1
- Confinement factor psi_r (0.75 with ties/spiral, else 1.0) 1
You get
- Term 1: fy / (50 lambda sqrt(f'c)) x db 18.97 in
- Term 2: 0.0003 fy db (floor) 18.00 in
- Compression development length ldc (8 in min) 19.0
Details, formula, and sources
Compression development is SHORTER than tension because the bar end bears on concrete. A #8 Grade 60 dowel at 4,000 psi needs 19.0 in (the fy/sqrt(f'c) term governs); raise f'c to 8,000 psi and it stops falling at 18.0 in because the 0.0003 fy db floor takes over -- doubling the concrete strength did not shorten the dowel. Confining ties (psi_r = 0.75) drop it to 14.2 in. A design aid; the engineer of record's detailing governs.
ldc = max( (fy x psi_r) / (50 x lambda x sqrt(f'c)) x db , 0.0003 x fy x psi_r x db , 8 in ).
The ACI 318-19 25.4.9.2 compression development length, with the 25.4.9.3 confining-reinforcement factor psi_r = 0.75 and the 8 in minimum, by name.
ACI 318 is readable free through the ACI online reading room at concrete.org; the 25.4.9 compression-development provisions are in the published code.
Estimate. AHJ and licensed professional govern.
Field names used by the API: bar_diameter_in, fy_psi, fc_psi, lambda, psi_r, term1_in, term2_in, ldc_in
- Governing terms the larger of the fy/sqrt(f'c) term and the 0.0003 fy db floor, never below 8 inACI 318-19 25.4.9.2
- Confinement psi_r = 0.75 only with 25.4.9.3 ties or spiral, else 1.0ACI 318-19 25.4.9.3
- Lightweight lambda = 0.75 for lightweight concrete, 1.0 normalweightACI 318-19 19.2.4