Reinforced Concrete: Wall to base connection detailing

[CJLCivilStruct]

Firstly, I appreciate that there are a few threads similar to this - but i'm hoping this is a little more specific.

Exam question: At what point (Design moment / utilisation ratio or other) are haunches / haunch bars required on a 90 degree reinforced concrete joint? Either Wall to Wall or Wall to Base connections with high moments (storm tanks / retaining walls etc).

Typically we detail joints using a 3 bar moment connection (example attached). However, I've been going down the rabit hole recently as there doesn't seem to be any rationale behind it (that i've found) other than it being standard practice in the UK. I've attached an example for reference.

For example, if you were designing and detailing a deep tank where corner or base to wall moments are in the order of 200 kNm, with approximately a 600mm thick section would you be comfortable without a haunch? If not, why not and how would you justify your choice?

Finally, if there are any examples shedding light on the subject I would be very greatful if you could highlight them to me?

Thanks in advance - just a young engineer trying to gain some knowledge.

 

[HTURKAK]

,

I have seen this post a few days ago and waited to see the responses of others. Just a simple question ; do you calculate this moment ( 200 kNm ) at the base of the wall ? If so , why not at the centerline of the wall and base ?

IMO , there is a merit for the use of haunches for the subject cases . Typical haunch depth is the same with wall thickness with 45 degr. The main reinforcement should not be more than about % 1.0 in order to avoid brittle failure of the corner. The use of haunch gives a possibility to design the stem wall reinf. at the beginning of the haunch.

In past i preferred haunches only at heavy culverts.
The following picture depicts the use of haunches at wall corner detail ( By Jose CALAVERA )

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[CJLCivilStruct]

I would typically check moments at the base of the wall, as this is the section which is being designed. I would argue not to design for forces at the centre line as these become reliant on member thickness - say a 1m thick base slab would result in much higher design forces?

Would you agree with this? I must say the question has got me thinking a bit!

Thanks for the response, I appreciate it.

 

[GeorgeTheCivilEngineer]

It's going back a few years but when I designed precast concrete culverts then standard moulds always have chamfers - because for us, concrete was cheap and steel was expensive.

The design checks were at the face of the wall using full depth of the haunch (large d) and at the start of the haunch. The corner reinforcement was two U-bars (like image C above) but with the haunch reinforcement going from outside face to outside face like in image A.

The balance as whether a haunch is needed would come back to crack widths. If you need 16mm+ to make the corner strong enough, you are probably better putting in a haunch, getting the steel back down to 12mm and having better control over crack widths (more, smaller bars giving better water retention than larger bars). As with anything, there's no definitive point where you should do one thing or another - it's engineering decision time.

 

[CJLCivilStruct]

Thanks George.

I think both replies have made the penny start to click where it's more of a decision based on effective depth and moving the critical section further from the joint, rather than the capacity of the joint itself.

 

[GeorgeTheCivilEngineer]

Yes - I think the different factors are:

- design resistance (lever arm, steel%, ULS & SLS, crack widths etc)
- effect on joint stiffness
- design complexity for additional locations to check the design
- formwork complexity especially insitu
- reinforcement complexity - extra haunch bars
- concrete / reinforcement quantities - but there may be savings in the slab away from the haunch

There's probably other things which I can't think of at the moment. Some of these are not usually significant (at all) but should still pass through your mind even if you don't explicitly do anything about them.