6.1 Tension and Compression Lap Splices (Class A and Class B Requirements)

6.1 Tension and Compression Lap Splices

A lap splice uses the surrounding concrete to transfer force between overlapping bars. Cover, spacing and transverse reinforcement affect bond and resistance to splitting. The splice is also subject to the member-specific location rules. Check the approved splice schedule before measuring the installed overlap, and use the specified bar size, grade and member.

Start with the development length used in design

Tension development length is the anchorage length for a deformed bar to develop its specified strength at the critical section. ACI 318-19 Section 25.4.2 has a tabular method and a general equation. For the general equation, the confinement term $(c_b+K_{tr})/d_b$ is capped at 2.5. Here $c_b$ is the lesser of the center-to-nearest-surface distance and half the center-to-center spacing of the bars being developed or lap spliced. $K_{tr}$ accounts for transverse reinforcement crossing the potential splitting plane. Check any separate requirement for minimum transverse reinforcement for high-grade bars. Clear cover and the center-to-surface input are different measurements.

The applicable factors account for concrete density, bar size, grade, coating and casting position. Under Table 25.4.2.5, more than 12 inches of fresh concrete placed below horizontal reinforcement gives $\psi_t=1.3$. Epoxy- or dual-coated reinforcement uses $\psi_e=1.5$ when clear cover is less than $3d_b$ or clear spacing is less than $6d_b$, and 1.2 otherwise. Uncoated or zinc-coated bars use 1.0. The product $\psi_t\psi_e$ need not exceed 1.7. Use the conditions of the actual detail. Under Section 25.5.1.4, do not reduce the development length for excess reinforcement when calculating a lap.

Select a tension lap class

Here $\ell_d$ is the calculated length under Section 25.4.2.1(a) before its standalone 12-inch minimum. Apply the 12-inch minimum to the splice using Table 25.5.2.1. Column longitudinal bars also have member-specific splice requirements in Section 10.7.5. In a special moment frame beam, Section 18.6.3.3 adds location and transverse reinforcement requirements.

Worked beam example. For a No. 8 uncoated Grade 60 bar in 4000 psi normalweight concrete with at least one bar diameter of clear cover, at least two diameters of clear spacing and no more than 12 inches of concrete below the bar, use the qualifying No. 7-and-larger case in Table 25.4.2.3. The calculation is $60{,}000(1.0)/(20\sqrt{4000})\times1.000=47.43$ inches. If provided area is 2.4 times required but 60% of the steel area is spliced in one required lap length, Class B applies and the calculated lap is $1.3(47.43)\approx61.66$ inches. Check the detailed dimension, which may be rounded up. If more than 12 inches of fresh concrete were placed below these bars, the casting factor alone would increase the calculated development length to about 61.66 inches and the Class B lap to about 80.16 inches.

Check contact and noncontact geometry

For contact laps, verify the minimum clear distance from the pair to adjacent bars or splice pairs under Sections 25.5.1.2 and 25.2.1. Tie or support the assembly to maintain the approved position. For noncontact splices in flexural members under Section 25.5.1.3, compare the center-to-center separation of the paired bars with the lesser of one-fifth the required lap and 6 inches. For a required 20-inch lap, the centerline limit is 4 inches. For two No. 8 bars that corresponds to 3 inches of clear gap. Clear cover remains measured to the outermost steel at the applicable face.

No. 14 and No. 18 bars cannot be lap spliced in tension. Section 25.5.2.2 uses the greater of the larger bar's tension development length and the smaller bar's tension lap length for unequal sizes where tension lapping is permitted. Section 25.5.7.4 separately requires mechanical or welded splices in tension tie members and a minimum 30-inch stagger between splices in adjacent bars.

Calculate compression laps and transitions

Increase the lap by one-third for specified concrete strength below 3000 psi. For a Grade 60 No. 3 bar, $30(0.375)=11.25$ inches, so the 12-inch minimum governs before any low-strength increase. For a Grade 80 bar, $(0.0009(80{,}000)-24)d_b=48d_b$. Section 10.7.5.2.1 permits factors of 0.83 for qualifying tied columns with effective tie area at least $0.0015hs$ in both directions and 0.75 for qualifying spiral columns, with a 12-inch floor. Apply these reductions only where the design conditions are satisfied.

Sections 25.5.5.3–.4 allow No. 14 or No. 18 bars in compression to lap No. 11 or smaller bars. Use the larger of the larger bar's compression development length and the smaller bar's compression lap length. Confirm that the design treats the bars as compressive under the applicable load combinations.

Verify stagger, bundles and cover

Section 25.6.1.7 applies the 20% and 33% increases for three- and four-bar bundles to the applicable individual lap lengths. Individual splices within the same bundle cannot overlap. The whole bundle cannot be lap spliced as a unit. Check the actual stagger required by the approved detail.

Record the member, bar marks, measured longitudinal overlap and transverse spacing, and confinement and cover around the splice. Check the cage at the splice, where the outermost steel can differ from adjacent sections. Report discrepancies for correction or approved disposition before concealment.