Class A is 1.0 ld, Class B is 1.3 ld — tabulated, then computed for the cases the table cannot cover
← Back to the calculatorOverlap, lap and splice all mean the same thing on this page: the length over which two bars run side by side. A splice length is not a lookup in the code. It is a development length — computed from bar size, concrete strength and the bar's factors — multiplied by 1.0 or 1.3 depending on how much steel the section has and how many bars are cut at one place.
The table below does that multiplication for the ordinary case: Grade 60, uncoated, normal weight, bottom bars. If any of those is not true, section 04 lists what changes and section 05 does the arithmetic.
Inches. Class A is the development length itself; Class B is 1.3 times it. Every value is rounded up to the next whole inch and floored at 12 in, the minimum development length in ACI 318-19 25.4.2.1.
| Bar | ld 3,000 | ld 4,000 | ld 5,000 | Class B 3,000 | Class B 4,000 | Class B 5,000 |
|---|---|---|---|---|---|---|
| #3 | 14 | 12 | 12 | 19 | 16 | 16 |
| #4 | 18 | 16 | 14 | 24 | 21 | 19 |
| #5 | 22 | 19 | 17 | 29 | 25 | 23 |
| #6 | 27 | 23 | 21 | 36 | 30 | 28 |
| #7 | 39 | 34 | 30 | 51 | 45 | 39 |
| #8 | 44 | 38 | 34 | 58 | 50 | 45 |
| #9 | 50 | 43 | 39 | 65 | 56 | 51 |
| #10 | 56 | 49 | 44 | 73 | 64 | 58 |
| #11 | 62 | 54 | 48 | 81 | 71 | 63 |
Two things the table makes obvious. Concrete strength matters less than people expect: going from 3,000 to 5,000 psi takes #5 Class B from 29 in to 23 in, a 21 per cent cut, while going from #5 to #8 at 4,000 psi takes it from 25 in to 50 in, a doubling. Bar size dominates. If a lap will not fit the detail, the fix is usually more, smaller bars rather than richer concrete.
ACI 318-19 25.5.2.1 gives Class A only when both of these hold over the length of the splice:
Miss either one and it is Class B. In practice that means Class B, because a section with twice the steel it needs is a section that was sized by something other than bending. Detailers default to Class B for exactly this reason, and a drawing that says "lap 40 db" without a class is a drawing that has not been checked against this clause.
One consequence worth stating: Class A is not "the good splice". It is the shorter one, allowed only where there is spare steel. Using Class A where the conditions do not hold is the same as using 77 per cent of the required lap.
Five steps, and the last one is the one that separates a real answer from a table.
| Symbol | Meaning | Where it comes from |
|---|---|---|
| db | Nominal bar diameter | ASTM A615 Table 1 |
| ψs | Size factor, 0.8 for #6 and smaller, 1.0 for #7 and larger | 25.4.2.3 |
| ψt | Top bar factor, 1.3 or 1.0 | 25.4.2.5 |
| ψe | Coating factor, 1.0 uncoated | 25.4.2.3 |
| ψg | Grade factor, 1.0 for Grade 60 | 25.4.2.3 |
| ld | Tension development length | step 3 |
| lap | Splice length | step 5 |
1. db = nominal diameter from ASTM A615 Table 1 2. ψs = 0.8 for #6 and smaller, 1.0 for #7 and larger 3. ld = [ fy ÷ ( 25 × λ × √f'c ) ] × ψt × ψe × ψs × ψg × db 4. ld = max( ld , 12 in ) 5. lap = 1.0 × ld (Class A) or 1.3 × ld (Class B), rounded up
The divisor 25 is fixed. It is part of the expression, not a number to tune per detail, and it is the reason nothing in the table above moves with cover or bar spacing: neither appears in steps 1 to 5. That cuts both ways. It makes the table stable and repeatable, and it makes it narrower than the clause it comes from, because 25.4.2.3 is printed here in its simplified form only. If a detail is unusual — bundled bars, heavy cover, tight spacing — treat the tabulated lap as a starting point rather than the answer, and check it against the clause.
That is also why the development length page prints the same expression symbol by symbol instead of a single lookup: the factors are where a lap gets its length, and a factor can be checked in a way that a remembered number cannot.
ld = 37.95 × 0.8 × 0.625 = 18.97 in, rounded to 19 in. Class B is 1.3 × 19 = 24.7, so 25 in. A 40 ft run on 20 ft stock has one splice per bar, so each bar is 40 ft plus 2.08 ft of lap — 42.08 ft ordered per 40 ft of wall, 5 per cent extra steel.
ld = 37.95 × 1.0 × 1.410 = 53.51 in, so 54 in, and a Class B lap of 1.3 × 54 = 70.2, so 71 in. Same concrete, same class, 2.8 times the lap: the diameter is 2.26 times larger and ψs adds another 1.25 because #11 sits above the #7 step. That is the number that sends a job to couplers rather than laps, because 71 in of overlap does not fit a 4 in joint.
Overlap is the word on the drawing, in the yard and in the search box. The code calls the same thing a lap splice, and it is the same length: the distance two bars run side by side so the pair carries what one continuous bar would.
Because Class A needs both conditions at once — steel provided at least twice that required, and half or fewer of the bars spliced within the lap length — staggering supplies the second and not the first. A perfectly staggered job with ordinary steel is still Class B at 1.3 ld.
What staggering buys is the section, not the length: it keeps every splice from landing in one plane, which is where a crack would rather start. That is a reason to stagger, and it is not a reason to shorten anything. A drawing can be staggered and still specify Class B, and it usually does.
Measure the overlap along the bar, from the end of one bar to the end of the other — not along the member, and not from the tie. The bars stay in contact, or within one fifth of the lap and never more than 6 in apart for a noncontact lap (25.5.1), and the lap is tied at both ends: a 21 in lap tied only in the middle is a lap that can open before the concrete is placed.
overlap = 21 in (#4, Class B, 4,000 psi) per bar = 21 ÷ 12 = 1.75 ft weight = 1.75 × 0.668 = 1.17 lb 100 bars = 175 ft = 116.9 lb offset = more than 21 in to count as a separate section
Same run, 40 ft of bar on 20 ft stock, one splice per bar: the bar ordered is 41.75 ft for a 40 ft run. Detailed on 40 ft stock the order is 40 ft and the overlap is zero. Neither number is wrong; they are two different purchases, and the takeoff has to say which one it is counting.
16 in for Class A and 21 in for Class B, Grade 60 in 4,000 psi concrete, uncoated bottom bars. Class A is the development length ld and Class B is 1.3 ld. In 3,000 psi it is 18 and 24 in; in 5,000 psi 14 and 19 in. A top bar multiplies ld by 1.3 first, which makes the Class B lap 26 in.
Class A is 1.0 ld and Class B is 1.3 ld. Class A is allowed only when the steel provided is at least twice the steel required over the whole splice length and half or fewer of the bars are spliced at that section. Both conditions, not either. Everything else is Class B, which is why Class B is the default a detailer assumes.
No. Development length is what one bar needs to embed into concrete to reach yield. A lap splice is two bars doing that job side by side, so it is a multiple of ld: 1.0 for Class A and 1.3 for Class B. They are different checks and they sit on different pages, which is why this one covers laps and the development page covers embedment.
No. ACI 318-19 25.5.1 prohibits lap splices for #14 and #18 bars in tension, with a narrow exception for compression. Those sizes need mechanical or welded splices, so a heavy column that switches to #14 bars also switches its connection detail.
ACI 318-19 25.5.1 limits the transverse spacing of a noncontact lap to the lesser of one fifth of the required lap and 6 in. On a #5 Class B lap of 25 in that is 5 in, so the two bars sit within 5 in of each other, not one on each face.
Yes, 1.3 times longer, through the top bar factor ψt in ACI 318-19 25.4.2.5. A top bar is one with more than 12 in of fresh concrete below it during the pour, which includes most slab top steel. A #5 Class B lap goes from 25 in to 33 in, and that extra 8 in is what catches people who copied the bottom bar number.
One lap length per splice. On 20 ft stock and a 40 ft run there is one splice per bar, so a #5 Class B lap of 25 in adds 2.08 ft to a 40 ft bar: about 5 per cent of the run length and 5 per cent of the tonnage. On a long wall that is the difference between one truck and two.
Because ld is proportional to the bar diameter. #11 is 1.410 in and #8 is 1.000 in, a ratio of 1.41, and both are #7 or larger so the size factor is 1.0 for both. That is why 50 in of Class B lap becomes 71 in, and why the ratio between the two is 1.42 and not something larger.
21 in for Class B in 4,000 psi concrete, 16 in for Class A, and 26 in if it is a top bar. In 3,000 psi the Class B overlap is 24 in and in 5,000 psi it is 19 in. Class B is the default unless the steel provided is at least twice what the section needs and half or fewer of the bars are spliced there.
The code does not require staggering, and staggering on its own does not shorten the lap: ACI 318-19 25.5.2.1 allows Class A only when the steel provided is at least twice that required and half or fewer of the bars are spliced within the lap length. To count as separate sections the offset has to exceed the lap itself, more than 21 in for #4, 50 in for #8 and 71 in for #11, so a 12 in offset clears no bar size.