Development lenght of rebar in tension

[MrFurleyEIT]

I need feeback on the spacing or cover dimension,"c", used in the determination of development length for rebars in tension. "c"

ACI 318-02 states, "Use the smaller of either the distance from the center of the bar or wire to the nearest concrete surface, or one-half the center-to-center spacing of the bars or wires being developed".

It is the latter alternative I have a question with, i.e. "one-half the center-to-center spacing of the bars or wires being developed".

I thought the definition was simple enough. Just take the center-to-center spacing and divide it by two. For instance, my bars are #8 @ 4-1/2" O.C., then it will be 4.5/2 = 2.25 inches.

However, in one of the lecture notes I googled online (by T. Igusa), he had this particular definition written as, "clear spacing - diameter of bar" divided by 2. Now, is this correct? With my ealier method, I get a development of about 53.8 inches.

If my bars are i inch in diameter, and my center-to-center spacing is 4-1/2 inches, then the clear spacing is 4-1/2 - 1 = 3 - 1/2 inches. If I take another inch out of that, then I only have 2-1/2 inches. Dividint that by 2 gives me 1.25 which makes a humongous difference to my development length! Much larger that what I had gotten earlier!

Can anyone figure this out and let me know, please?

Thank you so much and have a great weekend.

 

[Taro]

Your first interpretation is correct. Without reviewing the particular situation in the lecture notes, I'm not sure why they are doing it that way but it doesn't look right.

 

[Taro]

After thinking about it for a couple minutes, I think the lecture notes probably just have a typographical error. It should read "clear spacing + diameter of bar" divided by 2.

 

[Lion06]

Isn't the "clear spacing" exactly that - the clear distance between teh edges of the bars or the clear distace between the edge of teh bar and the edge of the concrete? It is called clear cover, not center cover.

 

[MrFurleyEIT]

Thanks, Taro I am tending to go with you and say that the lecture most likely contained a typo, and that the second definition of "c" should be "clear spacing + diameter of bar"

divided by 2. That makes sense, and gives the same development lenght as dividing th center-to-center distance of the bars by two.

Yeah, StructuralEIT, the "clear spacing" would mean the clear distance between the edges of the bars. That was why I got confused when I read the lecturer's definition of "c" as being "clear spacing - diameter of bar" divided by
two. Somehow, it doesn't sound right.

However, if he had said that "c" was the "clear spacing + diameter of bar" divided by 2, that makes a lot of sense.

 

[Taro]

Definitely a typo. The (clear spacing + diameter of bar) divided by 2 is how it is worked in the PCA notes example.

 

[sdz]

It might not be a typo. It depends on the code being used for design. ACI318 uses c/c spacing whereas AS3600 uses clear spacing. End result is probably similar.

 

[civilperson]

The lecturer was correct in predicting the ACI code for 2005. When ties/stirrups are NOT available throughout the length of the bar, then 2 diameters of CLEAR spacing and one diameter of clear cover are required for the smallest development formula. Thus your 4.5" c-c example has 3.5" clear spacing between the bars. If the clear spacing is not less than than 2 diameters and the clear cover is not less than one diameter then development length is 50% longer than the smallest development formula. This is a function of splitting resistance.