Hooked development length (ACI/AASHTO) - space constraints, critical sections and length of hooked part

[hali]

AOA everyone. Recently I've been very confused with development length and how its provided in different components. I have a few conceptual confusions about it and then a few practical and design aspects of it. I'd really appreciate help on these.

1. Do we provide/check development length for simply supported beams (i.e. check it past the max moment?)
2. Where do we need to check for development length? (in my understanding, we need to check it almost everywhere, where you need to ensure full moment capacity)
3. What is a "critical section" when we talk about development length? (refer to the attached picture) I think its the point beyond which we need to ensure development
4. At a wall-slab joint where both the slab and wall are 8" thick, if #6 bars at 8" c/c need to be developed from the slab into the wall, I found that even with the 90 degree hook, the development length is at least 14". How do you satisfy development length in such a case?
5. For hooked development length ldh, is it the length up to the hook or is the length that includes the length of the hook? For example for a 90 degree hook, would it be 12db+the length beyond the critical section or just the length beyond the critical section up to the hook? I've seen in a few places on the internet that people include the 12db into the development length. 

This stuff has been eating up gray matter from my brain for a few days now. Any help is appreciated.

Thanks!

 

[Badar (BAZ)]

3 hours ago, hali said:

• Do we provide/check development length for simply supported beams (i.e. check it past the max moment?)
• Where do we need to check for development length? (in my understanding, we need to check it almost everywhere, where you need to ensure full moment capacity)

There are typical details of reinforcement, which can be found in various detailing manuals, that the design engineers follow around the world. They do not normally check development length for typical/usual member sizes with usual/ conventional loadings, instead they follow those reinforcement details.

 

3 hours ago, hali said:

What is a "critical section" when we talk about development length? (refer to the attached picture) I think its the point beyond which we need to ensure development

The critical section is the section with maximum tensile stress. For most beams or flexural members, it is the face of column or support.

 

3 hours ago, hali said:

At a wall-slab joint where both the slab and wall are 8" thick, if #6 bars at 8" c/c need to be developed from the slab into the wall, I found that even with the 90 degree hook, the development length is at least 14". How do you satisfy development length in such a case?

For this particular case, you can treat the wall-end as simply supported. But, there are other ways to develop the reinforcement such as mechanical anchorages (headed bar) or welding with end-plates.

 

3 hours ago, hali said:

For hooked development length ldh, is it the length up to the hook or is the length that includes the length of the hook? For example for a 90 degree hook, would it be 12db+the length beyond the critical section or just the length beyond the critical section up to the hook? I've seen in a few places on the internet that people include the 12db into the development length. 

As you can see in your attached diagram, the length of hook is not considered by American codes. Some codes do consider them explicitly. For American codes, any length beyond 12db does not contribute.

[UmarMakhzumi]

Hali,

The above reply is excellent. To make things more lucid, I will add the following to above answer.

7 hours ago, hali said:

I think its the point beyond which we need to ensure development

Correct. 

7 hours ago, hali said:

At a wall-slab joint where both the slab and wall are 8" thick, if #6 bars at 8" c/c need to be developed from the slab into the wall, I found that even with the 90 degree hook, the development length is at least 14". How do you satisfy development length in such a case?

There are different ways. Certain Codes allow low/ non-seismic applications to reduce development length by As provided\ As required ratio (including ACI). Other options include mechanical anchors, assuming pin condition in analysis like Baz mentioned above and increasing the size of members. 

7 hours ago, hali said:

For hooked development length ldh, is it the length up to the hook or is the length that includes the length of the hook?

Ldh is length up to hook. Don't add 12*Db. 

7 hours ago, hali said:

This stuff has been eating up gray matter from my brain for a few days now.

Haha. You are doing great. These are good signs.

Here is tip. If you already don't have, get a copy of PCA Notes on ACI, and see examples in that document. It will clear up any ambiguity that you have and also it serves as good reference.

Thanks.

 

[hali]

Thank you @UmarMakhzumi and @Badar (BAZ) for your responses. I'll apply these to a problem I'm dealing with at the moment. I'll probably have some more questions as well so I'll be back again lol

[hali]

Alright guys I'm back and here's the specific problem where I'm having confusion. The following link is a State standard detailed drawing of a range of different RCB (ranging in span lengths and heights). 

https://www.odot.org/bridge/2009-sb/brd_std_2009-lrfd-sb-531.pdf

I'm dealing with a double 6'x3' RCB. The A1 bars in the top slab are #6 bars, so their basic development length is about 14.5". Now this was not developed into the walls because I'm assuming you cast the slab last so the concrete in the walls has already matured. But shouldn't it have 180 degree hooked bars at least? 

Also I don't think this wall-slab connection can be considered a simply supported one since rebar is being developed from both layers from the walls.

[UmarMakhzumi]

Is the Barrel Section a vertical section or a horizontal section? Looks more like a horizontal section to me, but I could be wrong as not familiar with these.

On 5/1/2021 at 1:47 PM, hali said:

he A1 bars in the top slab are #6 bars, so their basic development length is about 14.5". Now this was not developed into the walls because I'm assuming you cast the slab last so the concrete in the walls has already matured. But shouldn't it have 180 degree hooked bars at least? 

A1 bar is for +ve moment (Sag, where tension is at the bottom) and likely the location of moment would be in the center (assuming) so you don't need to develop it with hooks as not required and rebar is developed at critical section.

 

On 5/1/2021 at 1:47 PM, hali said:

Now this was not developed into the walls because I'm assuming you cast the slab last so the concrete in the walls has already matured.

The dowels from the wall are protruding even if the wall is casted first. The dowels go in slab and will be covered by slab concrete.

 

On 5/1/2021 at 1:47 PM, hali said:

Also I don't think this wall-slab connection can be considered a simply supported one since rebar is being developed from both layers from the walls.

If there is no force reversal, you only need to develop the negative rebar for gravity loads to consider the wall-slab join as fixed which they have.

 

Thanks.

 

[hali]

19 hours ago, UmarMakhzumi said:

If there is no force reversal, you only need to develop the negative rebar for gravity loads to consider the wall-slab join as fixed which they have.

So shouldn't the negative rebar be developed adequately into the wall? According to AASHTO's development length equation it would be about 36" straight and 9.5" hooked. It looks like it wasn't properly developed

[UmarMakhzumi]

It should be. The B2 Bar X Dimension should be long enough to develop so that fixed end connection can be satisfied.

I am not sure what assumptions they had for the design. Do you have any way to get intouch with them and clarify this?

Thanks.

 

[hali]

I doubt anyone from the State will be available to address such minor things lol. My only remaining assumption about this would be that they used the modifier As-required/As-provided to reduce the development length. 

But regardless, thanks for helping me out with this. I think I'll go back to some textbook basics and see if I missed anything in there.  

[UmarMakhzumi]

2 hours ago, hali said:

My only remaining assumption about this would be that they used the modifier As-required/As-provided to reduce the development length. 

 

You're welcome. There is also a minimum hook development length requirement - the As required/ As provided can't reduce it beyond that amount.  I don't remember what is in ACI, but you can look it up. Also this might be unlikely as moment needs to be developed at the face of the wall but who knows what they were thinking. There might be a reason.

Thanks.

[abbaskhan2294]

Is there any benchmark of development length for the connection to be assumed rigid or pinned? I heard that 3/4 of full development length is the threshold between pinned and rigid connection.

Edited May 5, 2021 by abbaskhan2294

[UmarMakhzumi]

5 hours ago, abbaskhan2294 said:

Is there any benchmark of development length for the connection to be assumed rigid or pinned? I heard that 3/4 of full development length is the threshold between pinned and rigid connection.

There is no basis to the statement. You should develop it fully or if it is not developed fully, you should check for both extreme possibilities (that is, considering it is pinned). Such assumptions still work because of re-distribution of forces and that there are a lot of approximations in our analysis.

Thanks.

[hali]

I have not heard of this criterion. Is this in a code?