Cut horizontal bars at wall construction joints? 1

[mrengineer]

This is a survey - Do you normally cut a percentage of the horizontal bars at vertical construction joints in c.i.p. walls, when the walls span between deep foundations? (Situation I am looking at has long exposed walls in underground parking area).

 

[theonlynamenottaken]

"Normally" is the word I'm having trouble with in your question. Its based on what forces need to be transferred at that plane...

- Does the wall retain soils and if so does it just span vertically or does it behave in a two-way fasion?

- Do you expect any differential movement (settlement) between the two adjacent deep foundations?

- Does the wall serve as a flexural member supporting axial loads from above (Does the wall height vs. deep foundation spacing provide for compression struts or is the deep foundation spacing large enough that the wall acts flexurally?)

 

[mrengineer]

Wall spans one-way (vertically). No significant differential settlement expected. Wall height/span ratio most typically around 0.75.

 

[archeng59]

The purpose of the joint is to allow for shrinkage of the concrete between the joints. If you have any of the deformed bars pass thru the joint, you provide restraint that will partially inhibit the shrinking and potentially cause cracks between the control joints... defeating the intent of the control joint. If you want bars thru the joint, use smooth bars with the bar on one side of the joint greased or placed in a sleeve that allows the concrete to shrink. The smooth bar will slide in the hole as the concrete shrinks. PCA has a handbook about control joints for concrete that you might want to obtain a copy for your reference library.

 

[Taro]

Often, control joints in walls do little or no good. The wall is usually restrained from shrinking freely because it is tied into rigid continuous foundations and floor slabs at bottom and top of wall. Sometimes the best course of action is to eliminate the control joints and let the wall crack but put enough rebar in to hold the cracks tight. ACI 350 has some guidance on this.

 

[hokie66]

Taro is correct about control joints in walls on continuous footings, although there is no harm in providing vertical grooves to try to keep the cracks straight, as long as the grooves do not decrease cover to the reinforcement. If vertical bars are in outer layer, grooves between bars work.

But you asked about construction joints in walls spanning between deep foundations. I would not cut any of the bars. Joint best made over a pile.

 

[archeng59]

I can tell you from personal experience, you can be sued by the building owner if the wall cracks and you did not use control joints. I worked on a project where the windows leaked during rain storms because the contractor did not install the windows properly. During the investigation to determine where the leaks were originating, the investigator noted cracks in the walls. Even though it was eventually proven the cracks were not the source of the water entering the building, the owner sued my company for negligence (based on recommendations of another structural engineer) in not using control joints and allowing the wall to crack. During the trial, we showed that we met the standard of care regarding crack control based on ACI and PCA guidelines. We won, but it cost us alot of $$ to defend ourselves, not to mention the $$ spent by the architect who was dragged into the case.

 

[mrengineer]

hokie66:
Your post made me realize my original question was mis-worded-I am showing vertical control joints at 30' o.c., and meant to ask if you typically cut bars at all of the CONTROL joints. I have seen details where every other horizontal bar was cut at the joint, but that always bugs me when the wall is spanning between pile caps-not an insurmountable problem, just a feel-good thing.

 

[csd72]

Depend on whether you need the wall to act as a continuous entity or will separate sections be enough to resist overturning?

If they were precast walls, then 4 walls 1/4 of the length have only 1/4 of the overturning resistance due to the reduced lever arms.

If you need it continuous then you need to leave the rebar uncut, or preferably follow Taro's advice.