Class A vs Class B Tension Lap Splices Explained

A tension lap splice in Canada is either Class A (1.0 times the development length) or Class B (1.3 times the development length), and never shorter than 300 mm. CSA A23.3 makes Class B the default. A shorter Class A lap is only allowed where the steel provided is at least twice what the design needs at that spot and fewer than half the bars are spliced within the lap. Neither condition can be checked on site, which is why most laps on Canadian drawings are Class B.

This post explains what the two classes mean and how they are applied. For the full table of lengths by bar size, see our lap splice length chart for metric bars. For how the development length itself is worked out, see rebar development length explained.

What the two splice classes are

A lap splice passes force from one bar to the next through the concrete between them. If the bar is working hard and many bars are lapped at the same place, the concrete around the laps is more likely to split along the bars. A longer lap spreads that force over more concrete.

CSA A23.3 handles this with two classes, both based on the tension development length (ld) of the bar:

Two details matter here:

• The development length is calculated in full. A23.3 allows ld to be reduced for excess reinforcement when a bar is being anchored, but that reduction is not used when the length is for a tension lap. The extra steel is already accounted for by the Class A test, so it is not counted twice.
• The class only applies to bars in tension. Bars that are only ever in compression use a separate compression lap. Those are covered in the lap splice chart.

These classes apply to bars up to 35M. Lap splices are not permitted for 45M and 55M bars except in limited compression cases, so those bars are joined with couplers or welding. See rebar couplers vs lap splices.

The two conditions for a Class A splice

A Class A lap is allowed only when both of these are true at the splice location:

1. The area of steel provided is at least twice the area required by analysis. This is about where the splice sits along the member. Near a point of maximum moment, the bars are usually close to fully used and fail this test. Near a point where the moment drops toward zero, there may be far more steel than the design needs.
2. Less than half of the total reinforcement is spliced within the required lap length. If every bar in a layer is lapped at the same place, this fails. If the laps are staggered so that fewer than half of the bars have a lap inside any one lap length, it can pass.

Note the wording: "less than one-half". Splicing exactly half the bars within the lap length does not meet the condition in the clause.

Only the engineer knows the required steel area at each point, because it comes from the analysis. A contractor or detailer cannot look at a beam and decide that it has "plenty of steel" there. If a Class A lap is acceptable, the engineer shows it on the drawings or in the lap schedule.

Staggering to qualify

Staggering is how designers usually get to Class A. Take a wall with bars at a fixed spacing. If all the horizontal bars are lapped at the same location, the splice is Class B. If alternate bars are lapped at different locations, offset by more than the lap length, then only half or fewer of the bars are spliced within any one lap length. Combined with enough surplus steel, that can allow Class A laps.

Staggering has a cost of its own. Bar lengths vary, the bar list has more marks, and the placing crew has to follow the pattern exactly. On many jobs the engineer simply calls Class B everywhere because it is simpler to build.

Class A vs Class B lengths compared

Here is what the two classes give for uncoated Grade 400 bottom bars in normal-density 30 MPa concrete, in members with minimum stirrups or ties (the simplified method). The values are from the RSIC Manual of Standard Practice tables, which are calculated to CSA A23.3-19.

A few notes on these numbers:

• The RSIC tables use the bar designation number (15 for 15M) as the bar diameter, as the 2019 edition of A23.3 directs. Calculations using the actual nominal diameter, as in our chart post, come out a few percent longer.
• Top bars (horizontal bars with more than 300 mm of fresh concrete below them) are 1.3 times longer again. A 15M Class B top-bar lap at 30 MPa is 670 mm in the same tables.
• Grade 500 bar needs longer laps in proportion to its higher yield strength.
• Epoxy coating, low-density concrete and tight spacing all lengthen ld, and therefore both classes.

The structural drawings govern. These numbers show the size of the difference, not the lap for your job.

Splice classes in columns

Column bars follow a separate set of rules in A23.3, because a column's bars may be in compression under some load cases and in tension under others:

• Bar stress always compressive: a compression lap is used, not Class A or B.
• Tension not more than half the yield strength: Class B if more than half the bars are spliced at any section; Class A is allowed if half or fewer are spliced at a section and alternate laps are staggered by the development length.
• Tension more than half the yield strength: Class B.

The engineer works out the bar stress from the load combinations and shows the lap type. Columns in frames that resist wind or earthquake loads often need tension laps, and seismic frames add further limits on where laps can go. Our post on lap length and splice zones for beams and columns covers the locations.

Contact and non-contact laps

Lapped bars do not have to touch. Both forms are permitted:

• Contact laps. The two bars sit side by side and are tied together. This is the usual method, and RSIC standard practice says contact laps are to be securely fastened.
• Non-contact (spaced) laps. The bars are separated, for example when bars in a mesh are lapped at their normal spacing. They must be no farther apart than one fifth of the required lap length or 150 mm, whichever is less.

The spacing limit keeps the two bars close enough for the concrete between them to transfer the force. A lapped pair also takes up more room than a single bar. CSA A23.1's minimum clear distance between parallel bars applies between a contact lap and the next bar or lap, which is often where congestion shows up in beams. See minimum clear spacing between rebar in beams.

Bundled bars have their own increase. A lap of a bar within a bundle is the individual bar lap increased by 10% for a two-bar bundle, 20% for three bars and 33% for four, and the individual laps within a bundle must not overlap.

What this means on drawings and bar lists

Most lap problems start before the steel reaches site:

• Check the general notes or lap schedule. Many drawings give one Class B table for the whole job and note any Class A exceptions.
• If the class is not shown, ask. RSIC standard practice is that, where the splice type is not shown, the fabricator supplies Class B tension laps for horizontal and vertical bars in walls, slabs and beams, and compression splices for other vertical bars. That is a default, not a substitute for the engineer's direction.
• Do not shorten laps on site. Cutting a Class B lap down to Class A because the bars "look strong enough" changes the design. If bars come up short, get the engineer's direction.
• Follow stagger patterns exactly. A Class A lap only works if the stagger shown on the drawings is built.
• Allow for laps in the take-off. Every lap adds steel, and the jump from Class A to Class B adds 30% to each lap length.

If you are working from US drawings, the classes look familiar but the lengths are not the same. See ACI 318 vs CSA A23.3 lap splice requirements.

Getting laps detailed and cut correctly

We prepare bar lists and placing drawings from your structural drawings, with lap lengths and splice classes taken from the engineer's notes, then cut, bend, tag and bundle 10M to 55M bar to match. Where the drawings leave a lap unclear, we flag it before fabrication instead of guessing.

Send us your drawings for a free take-off and quote, or see our rebar services.