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. These rows are the first row of Table 25.4.2.3, and they apply only where the clear spacing is at least db and the cover is at least db with at least the code minimum of stirrups or ties along ld, or where the clear spacing is at least 2db and the cover is at least db. Outside those limits use the Other cases table below, which is 1.5 times as long. The 1.3 multiplies the computed ld and the answer is rounded up once — so a Class B value is not 1.3 times the Class A number printed beside it.
| Bar | ld 3,000 | ld 4,000 | ld 5,000 | Class B 3,000 | Class B 4,000 | Class B 5,000 |
|---|---|---|---|---|---|---|
| #3 | 17 | 15 | 13 | 22 | 19 | 17 |
| #4 | 22 | 19 | 17 | 29 | 25 | 23 |
| #5 | 28 | 24 | 22 | 36 | 31 | 28 |
| #6 | 33 | 29 | 26 | 43 | 37 | 34 |
| #7 | 48 | 42 | 38 | 63 | 54 | 49 |
| #8 | 55 | 48 | 43 | 72 | 62 | 56 |
| #9 | 62 | 54 | 48 | 81 | 70 | 63 |
| #10 | 70 | 61 | 54 | 91 | 79 | 71 |
| #11 | 78 | 67 | 60 | 101 | 87 | 78 |
The second row of Table 25.4.2.3. Same bar, same concrete, 1.5 times the length, because the concrete can no longer be counted on to confine the bars. The divisor becomes 3/50 for #6 and smaller and 3/40 for #7 and larger. This row applies where the clear spacing is below db, or the cover is below db, with no minimum ties along ld — and where the clear spacing is below 2db with the cover at db. Outside the first row's limits, use this table, not 1.5 times the table above: each value is rounded up on its own, so the ratio reads 1.47 to 1.52 rather than exactly 1.50.
| Bar | ld 3,000 | ld 4,000 | ld 5,000 | Class B 3,000 | Class B 4,000 | Class B 5,000 |
|---|---|---|---|---|---|---|
| #3 | 25 | 22 | 20 | 33 | 28 | 25 |
| #4 | 33 | 29 | 26 | 43 | 37 | 34 |
| #5 | 42 | 36 | 32 | 54 | 47 | 42 |
| #6 | 50 | 43 | 39 | 65 | 56 | 50 |
| #7 | 72 | 63 | 56 | 94 | 81 | 73 |
| #8 | 83 | 72 | 64 | 107 | 93 | 83 |
| #9 | 93 | 81 | 72 | 121 | 105 | 94 |
| #10 | 105 | 91 | 81 | 136 | 118 | 106 |
| #11 | 116 | 101 | 90 | 151 | 131 | 117 |
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 36 in to 28 in, a 22 per cent cut, while going from #5 to #8 at 4,000 psi takes it from 31 in to 62 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 Table 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 |
| 25 / 20 | Divisor, 25 for #6 and smaller and 20 for #7 and larger; 3/50 and 3/40, which is 1.5 times, when the spacing and cover limits are not met | Table 25.4.2.3 |
| ψt | Top bar factor, 1.3 or 1.0 | ACI 318-19 |
| ψe | Coating factor, 1.0 uncoated | ACI 318-19 |
| ψg | Grade factor, 1.0 for Grade 60 | ACI 318-19 |
| ld | Tension development length | step 3 |
| lap | Splice length | step 5 |
1. db = nominal diameter from ASTM A615 2. C = 25 for #6 and smaller, 20 for #7 and larger (3/50 and 3/40 if the limits are not met, 1.5 times longer) 3. ld = [ fy ÷ ( C × λ × √f'c ) ] × ψt × ψe × ψg × db 4. lap = max( 1.0 × ld , 12 in ) Class A or max( 1.3 × ld , 12 in ) Class B 5. round the lap up to the next whole inch
Note what step 4 does not do. The 12 in there is the lap’s own floor, from Table 25.5.2.1. ld carries a separate 12 in floor under 25.4.2.1, and step 3 deliberately does not apply it: multiply the expression first, compare with 12 in second, round up last. Floor the expression before multiplying and a small bar in high-strength concrete can come out an inch short.
The divisor is not fixed. It moves twice. Once with the bar: 25 for #6 and smaller, 20 for #7 and larger. Once with the detail: if the clear spacing and the cover do not reach the limits printed above the table, the divisor becomes 3/50 or 3/40 and the lap is 1.5 times as long. Both moves are in the table, in the row the detail actually sits in — which is why a tabulated lap is only as good as the assumption behind the row it was read from.
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 × 0.625 = 23.72 in, rounded to 24 in. Class B is 1.3 × 23.72 = 30.8 — the exact ld, not the rounded 24 — so 31 in. A 40 ft run on 20 ft stock has one splice per bar, so each bar is 40 ft plus 2.58 ft of lap — 42.58 ft ordered per 40 ft of wall, 6.5 per cent extra steel.
ld = 47.43 × 1.410 = 66.88 in, so 67 in, and a Class B lap of 1.3 × 66.88 = 86.9, so 87 in. Same concrete, same class, 2.8 times the lap: the diameter is 2.26 times larger and the divisor drops from 25 to 20 above #6, which adds another 1.25. That is the number that sends a job to couplers rather than laps, because 87 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, and the lap is tied at both ends: a 25 in lap tied only in the middle is a lap that can open before the concrete is placed.
overlap = 25 in (#4, Class B, 4,000 psi) per bar = 25 ÷ 12 = 2.08 ft weight = 2.08 × 0.668 = 1.39 lb 100 bars = 208 ft = 139.2 lb offset = more than 25 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 42.08 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.
A run that steps down from #6 to #5 is spliced at 31 in, the #5 Class B lap — not at the #6 lap of 37 in. Every adjacent pair works that way except one: #6 to #7 is spliced at 42 in, the #7 development length, because the divisor drops from 25 to 20 at #7 and the development length steps with it. The lap is the larger of two numbers, and at that one pair the larger number changes sides.
A splice between two sizes has two jobs and the drawing only has room to write one number. It has to develop the larger bar, because the larger bar is the one carrying force into the lap, and it has to be a legal Class B lap of the smaller bar, because the smaller bar is the piece being added. Take the larger of the two and both jobs are done:
lap, different sizes = max( ld of the larger bar , 1.3 × ld of the smaller bar )
| Run steps from | ld, larger bar (in) | Class B lap, smaller bar (in) | Lap to use (in) | Governed by | Extra steel per lap (lb) |
|---|---|---|---|---|---|
| #4 (0.500 in) to #3 (0.375 in) | 19 | 19 | 19 | equal | 0.60 |
| #5 (0.625 in) to #4 (0.500 in) | 24 | 25 | 25 | smaller bar’s lap | 1.39 |
| #6 (0.750 in) to #5 (0.625 in) | 29 | 31 | 31 | smaller bar’s lap | 2.69 |
| #7 (0.875 in) to #6 (0.750 in) | 42 | 37 | 42 | larger bar’s ld | 5.26 |
| #8 (1.000 in) to #7 (0.875 in) | 48 | 54 | 54 | smaller bar’s lap | 9.20 |
| #9 (1.128 in) to #8 (1.000 in) | 54 | 62 | 62 | smaller bar’s lap | 13.79 |
| #10 (1.270 in) to #9 (1.128 in) | 61 | 70 | 70 | smaller bar’s lap | 19.83 |
| #11 (1.410 in) to #10 (1.270 in) | 67 | 79 | 79 | smaller bar’s lap | 28.33 |
Grade 60, 4,000 psi, uncoated bottom bars, ψt = ψe = 1.0. The development lengths come from the same expression as the development length page at the 25 divisor through #6 and 20 from #7; the Class B lap is 1.3 ld rounded up, which is the table at the top of this page. The extra steel is the lap length at the smaller bar’s unit weight, because the larger run continues through the lap and only the small piece is added.
The divisor is the reason. Development length is divided by 25 for #6 and smaller and by 20 for #7 and larger, so ld jumps 45% between #6 and #7 — 29 in to 42 in — while the Class B lap of #6 is only 37 in. At every other adjacent pair the 1.3 in the Class B lap outruns the size step and the smaller bar’s row wins. At #6 to #7 it does not: 42 > 37, so the larger bar’s development length governs.
The consequence is a detail that reads as a typo. A drawing that says “#6 to #7, lap 37 in” because 37 is the #6 row is 5 in short. On a #6 bar that is 0.42 ft and 0.63 lb per lap, which is nothing on the invoice and everything on the inspection: the lap either develops the bar or it does not, and the weight of the shortfall says nothing about which.
Take a bottom run 36 ft long, #6 for the first 18 ft and #5 for the last 18 ft, 20 ft stock, 4,000 psi. Both segments come out of one 20 ft stick each with 2 ft of offcut, and the step costs one lap of 25 in of #5:
#6 18 ft × 1.502 = 27.04 lb #5 18 ft × 1.043 = 18.77 lb lap 31 in ÷ 12 × 1.043 = 2.69 lb total = 48.50 lb per line
Running #6 the whole 36 ft instead needs 39.08 ft of bar — 36 ft plus the 37 in lap — which is 39.08 × 1.502 = 58.70 lb. The step-down is worth 10.2 lb per line, 17%, and on a 20-line slab that is 204 lb. That is the whole argument for changing size mid-run: not that the smaller bar is cheaper per pound, but that the last 18 ft does not need the larger one, and the lap that joins them costs 2.69 lb instead of 4.63.
What the step-down does cost is a second bar mark. Two cut lengths instead of one, two lines on the placing drawing, two stacks on the deck, and one more place for the ironworker to put the wrong bar. On a 20-line slab that is 20 extra laps to tie and 40 extra ends to keep straight, against 204 lb of steel and the time to place it. Neither number decides it; the point is that both are on the page now.
The transition lap is a lap like any other, so it obeys the rule in section 07: two laps count as one section unless they are more than a lap length apart. For #6 to #5 that means the transition lap has to be more than 31 in clear of the next lap on the same run, or the pair is one section and the stagger was imaginary. It also has to sit away from the peak moment, which is a design call and not a length — a size change is usually placed where the bar it replaces is no longer needed, and that point is the design’s, not the table’s.
19 in for Class A and 25 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 22 and 29 in; in 5,000 psi 17 and 23 in. A top bar multiplies ld by 1.3 first, which makes the Class B lap 33 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 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 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 31 in, one fifth is 6.2 in, so the 6 in limit governs: the two bars sit within 6 in of each other, not one on each face.
Yes, 1.3 times longer, through the top bar factor ψt in ACI 318-19. 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 31 in to 41 in, and that extra 10 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 31 in adds 2.58 ft to a 40 ft bar: about 6 per cent of the run length and 6 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 sit above #6 so both are divided by 20. That is why 62 in of Class B lap becomes 87 in, and why the ratio between the two is 1.40 and not something larger.
25 in for Class B in 4,000 psi concrete, 19 in for Class A, and 33 in if it is a top bar. In 3,000 psi the Class B overlap is 29 in and in 5,000 psi it is 23 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 25 in for #4, 62 in for #8 and 87 in for #11, so a 12 in offset clears no bar size.
Take the larger of two numbers: the development length of the bigger bar, or the Class B lap of the smaller one. For #6 to #5 that is 31 in — the #5 lap — not the 37 in in the #6 row. For #7 to #6 it is 42 in, the #7 development length, because the divisor drops from 25 to 20 at #7 and ld jumps from 29 in to 42 in while the #6 Class B lap is only 37 in. Grade 60, 4,000 psi, uncoated bottom bars.