Anchoring to Grade Beams

Last updated February 27, 2023
By Ian Story

This article discusses the use of anchor reinforcement in grade beams to reduce the required footing size at high-capacity anchors (usually due to narrow shear wall segments, such as a garage door side panel).

Code References

All of the following code references are from ACI 318-14:

• 25.7.1.3: [For transverse reinforcement – stirrups] Anchorage of deformed bar and wire shall be in accordance with (a), (b), or (c): a) For No. 5 bar…and smaller…a standard hook around longitudinal reinforcement.
• Definition of “stirrup”: reinforcement used to resist shear and torsion forces in a member; typically deformed bars, or welded wire reinforcement either single leg or bent into L, U, or rectangular shapes and located perpendicular to, or at an angle to, longitudinal reinforcement.
• Definition of “tie”: (a) loop of reinforcing bar or wire enclosing longitudinal reinforcement… (b) tension element in a strut-and-tie model.
• 17.4.2.9: Where anchor reinforcement is developed in accordance with Chapter 25 on both sides of the breakout surface, the design strength of the anchor reinforcement shall be permitted to be used instead of the concrete breakout strength in determining /(/phi/N_n). A strength reduction factor of 0.75 shall be used in the design of the anchor reinforcement.

Practically, the minimum grade beam depth that can support anchor reinforcement is 16.5 inches. This consists of 6 inches above and below the concrete breakout surface (the minimum code-supported development length for a standard hook) plus 3 inches cover below and 1.5 inches cover above.

Capacity

Nominal capacity of one leg of #4 bar in tension with 6 inch development = 7,362 lb LRFD (see this article)

Actual capacity of one leg of #4 bar in this minimal configuration = (0.75)(7,362 lb) = 5,521 lb LRFD

Convert to ASD: 5,521 lb * 0.7 = 3,865 lb ASD

For 2 stirrups: 15,460 lb ASD (2 stirrups, 2 legs each)