5.2 Placement Tolerances for Effective Depth, Cover & Longitudinal Spacing (ACI 117)

5.2 Placement Tolerances for Effective Depth, Cover & Longitudinal Spacing

Effective depth $d$ is measured from the extreme concrete compression fiber to the centroid of the longitudinal tension reinforcement. Concrete cover is measured to the applicable steel surface. In a simple positive-moment region, moving a bottom mat upward increases bottom cover and reduces effective depth. A change can therefore satisfy one limit while exceeding another.

1. Select the correct reference dimension

For ACI 318-19 Table 26.6.2.1(a), use the smaller of 3/8 inch or one-third of specified cover when $d\le8$ inches, and 1/2 inch or one-third when $d>8$ inches. The table footnote limits the reduction at a formed soffit to 1/4 inch. ACI 117-10 Section 2.2.2 likewise caps a cover reduction at one-third of specified cover and limits reduction at formed soffits to 1/4 inch. Apply all governing requirements. A project can specify tighter tolerances.

Worked example: Thin formed slab

A 6-inch suspended slab has $d=5$ inches and specifies 3/4 inch of bottom cover. For reduction, check the 3/8-inch table limit, one-third of 3/4 inch, which is 1/4 inch, and the formed-soffit limit of 1/4 inch. The controlling reduction is 1/4 inch. The resulting lower cover limit for this example is $3/4-1/4=1/2$ inch. A 3/8-inch measurement is 1/8 inch below that limit. Check effective depth and support stability separately.

Worked example: Girder side versus formed soffit

A 20-inch girder with $d=17.5$ inches specifies 1-1/2 inches of cover at both a formed side and the formed soffit. At the side, one-third of the specified cover is 1/2 inch, equal to the reduction from the applicable table. The side cover limit from those checks is 1 inch. At the soffit, the additional 1/4-inch reduction limit leaves 1-1/4 inches. Record which face the measurement represents.

For a thick footing with 3 inches specified bottom cover, placed against soil with $d>8$ inches and total depth over 12 inches, one-third of 3 inches is 1 inch. The tables limit the reduction to 1/2 inch in this case, leaving 2-1/2 inches under the stated tolerances. This does not change the 3-inch specified cover or authorize selecting shorter supports.

2. Check longitudinal positions and clear spacing

ACI 117-10 Section 2.2.7 and ACI 318-19 Table 26.6.2.1(b) give ±1/2 inch at discontinuous ends of brackets and corbels, ±1 inch at discontinuous ends of other elements and ±2 inches at other locations. Apply the cover tolerance at a discontinuous end as well. ACI 117-10 Section 2.2.8 has separate reductions for embedded lengths and bar laps.

ACI 117-10 Section 2.2.5 gives the general nonprestressed reinforcement spacing tolerance of ±3 inches along the specified spacing line. It gives separate limits for stirrups and ties and requires at least the specified number of bars. The clearance check in Section 2.2.4 uses one-quarter of the specified distance capped at ±1 inch, subject to its stated clear-distance limits. For parallel nonprestressed bars in a horizontal layer, ACI 318-19 Section 25.2.1 also requires clear spacing at least the greatest of 1 inch, the bar diameter, and four-thirds of the nominal maximum aggregate size. Different limits apply to vertical reinforcement in columns and similar members and to other reinforcement configurations.

For No. 8 bars of nominal 1-inch diameter with 1-inch nominal maximum aggregate, the horizontal clear-distance criterion is $\max(1,1,4/3)=1\frac{1}{3}$ inches. At 2-1/4-inch center spacing, the clear spacing is only 1-1/4 inches. Even if center spacing appears acceptable, this clearance is too small. Bring the discrepancy to the contractor, verify the approved disposition and preserve the specified count. Routing conduits or pipes still has to meet ACI 318-19 Section 26.8.2(f) and the approved details.