Reinforcement detail for a local thickening of a raft 1

[geo321]

Hello
I am dealing wirh a raft foundation with local thickening below the columns.
The general detail i found of bottom reinforcement inside the raft is shown in figure 1. However i dont understand the necessity of the extended part which i have clouded (useless in my opinion).
In such section the splicing is ensured by non contact splices. However the mat has a thickness of 600mm while the thickening is 900mm. I am using aci 318 which for non contact splices spacing  between bars being developed should be less than 150mm.
I was thinking of using the detail illustrated in figure 2 in order to ensure that bars are overlapping. What do u think ?
Thank u for the help

 

[jike]

Only two purposes that I can think of.....
1) to fully develop bars (by vertical + hook) if vertical distance is inadequate.
2) Horizontal extension can be used to support top reinforcing layer rather than requiring extra support bars.

 

[KootK]

I can think of three excellent reasons for the original detail:

1) the high shear in the localized thickening may need to be dragged up to the top of the raft at the transition to prevent a shear failure there.

2) running a tension bar right around a re-entrant corner us usually a bad idea. You'll pop the conceete at the corner when the bar tries to straighten out.

3) It's good detailing practice to anchor you bars in the compression zone whenever possible.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Also, in dirt, I'd go 1:1 or 1:2 on the sides for both the rebar and the concrete. Smoother transition and easier to build.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[geo321]

Thanks a lot kootk and jike. Clear enough.
If ur mat (having the smaller thickness in my sketch) has a thickness capable of developing your ld, is there still a need to hook the bottom bars or it can be omitted?
Thanks a lot once again.

 

[KootK]

That's pretty common. Personally, I consider this a detail where it's prudent to be generous with development/anchorage. These bars are analogous to your top mats in an elevated slab and their continuity is important.

I believe that the best way to detail this is to create a non-contact lap slice in the bars within the thicker part. I would extend the horizontal Ld to (1.3 Ld + 1.5 x Gap). And I'd develop the upturned bar into the thinner part as you've suggested. I'd really like to have vertical ties along the lap too but that's a tough sell.

What size are the bottom bars and how large is the thickening in plan?h

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[geo321]

Hi kootk the biggest bars are 25mm diameter Thickening in plan is 4x4 meters.
Is it possible to provide a small sketch regarding ur last post ?
I didn't get ur answer regarding if the raft is thick enough, does the horizontal bending still necessary ? I am talking in this case abt a raft having 2 meters thick - pit detail

 

[KootK]

The first sketch below is how I'd prefer to handle it. It's pretty tough to get contractors to go along with it however so compromises get made.

The second sketch is how I see the flow of forces with your detailing. I drew it upside down because, for some reason, that makes the issues jump out at me more.

The question of whether or not the hooks are required is a function of the design of the joint. If the joint is designed to transfer all of the required forces through the step successfully without the hooks, then they are not required. If I don't bother to explicitly design the joint for force transfer and just "detail it" instead, then I'll play it safe and provide the hooks. I'll also slope the transition.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[geo321]

kootk thanks for your clear explanation.

how do u check such joint is capable of transferring all the forces without or with hooks ?

 

[KootK]

The joint could be checked using the strut and tie method. That's a heap of work however. There are plenty of engineers out there who do feel that it is sufficient to just develop the bars either side of the joint. I'm not one of them, however, so I can't really speak to that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.