Envelope #30: “I feel like I should know this…” Concrete Refresher

Happy Thursday! Hope you are enjoying the summer (August is just about to be over – can you believe it!?). This is Back of the Envelope – the newsletter where I share things I learned (or relearned) recently, in 5 minutes or less.

Today, I am going to talk about something related to concrete reinforcing that you either:

  1. Already know it pretty well

  2. Kind of know it but never really looked into it.

  3. Used to know it but forgot

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What am I talking about?

I am talking about “development length” vs. “Class A splice” vs. “Class B splice”.

It’s one of those things that you say to yourself, “I feel like I should know this, and I kind of do, but not really…”

So let me lay it all out for you.

(Estimated read time: 3 minutes and 4 seconds)

Development Length vs. Class A Splice vs. Class B Splice

You’ve probably seen this in various versions every time you see a lap splice length table or development length table of some sort:





How does it all work?

The short version is:

  • Development length = Class A splice length

  • Development length x 1.3 (or Class A splice length x 1.3) = Class B splice length.

Here is a quick breakdown.

Development Length

This is one most of us are familiar with.

The official definition from ACI 318:

“Length of embedded reinforcement required to develop the design strength of reinforcement at a critical section.”

In other words, it’s the required embedment length such that when the bar is under tension, it will yield before it slips or breaks away from the concrete.

ACI 318 Table and Equation shows how this is calculated.

I'm not going to go into details here, so some snippets below just FYI to refresh your memory.

Lap Splices

Lap splice, on the other hand, represents the connection required to consider two separate bars as a single continuous bar. In this case, the two bars are “lapped” in the concrete by lap splice length.

For bars in tension, the lap splices can be classified as either Class A or Class B, and that’s where it gets slightly confusing.

ACI 318 Table (screenshot below) defines the two based on two parameters:

  1. As,provided/As,required over length of splice

  2. Maximum percent of As spliced within required lap length

What does that mean in plain English?

For the first one, “As,provided/As,required over length of splice,” it’s fairly straightforward:

You run the analysis and see how much steel is required. And if you provided at least double the amount needed, you are at the “>= 2.0” row.

The second one is not as clear. What is the “maximum percent of As spliced within required lap length”?

This is actually just a fancy way of saying that if you stagger your splice, the percent would be 50, and you’d be using Class A.

If not staggered, then you are Class B.

Here is a fancy graphic that depicts the difference:

As you can see, even though Class A requires less length, it needs to meet the As provided/required ratio, and it needs to be staggered.

So why would anyone ever consider using that?

According to the ACI commentary:

“The two-level lap splice requirements encourage splicing bars at points of minimum stress and staggering splices to improve behavior of critical details.”

“The tension lap splice requirements encourage the location of splices away from regions of high tensile stress to locations where the area of steel provided is at least twice that required by analysis.”

Makes sense… but I believe the common practice is to use Class B and splice at a location of minimum stress anyway.

Class A is generally avoided (as far as I know) since staggering requires extra labor and may be prone to errors.

Perhaps Class A could be helpful in highly congested areas? I am not sure.

Let me know your thoughts!

And that is all for now – we’ll end with a dad joke that my kids loved:

“How many tickles does it take to make an octopus laugh?




“Ten tickles!” 🐙

Enjoy the rest of the week & happy Labor Day weekend!

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