What is everyone's thoughts on placing sawcut contraction joints in suspended slabs? I routinely place them in slabs on grade but am unsure about the wisdom of placing planes of weakness in a structural slab. Most of my suspended slabs are designed with integral beams and removable forms and are tied to the walls. I have not been able to find any guidance on this situation. Thanks.
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Contraction joints in suspended slabs 3
- Thread starter Hemifun
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rowingengineer
Structural
I'm more of a mango man, we get our cyclone every 4-5 years, keeps you from thinking the wind code is to heavy.
I have used a similar concept for larger building like a shopping centre with a retaining wall external(precast), 200x150m slab, 3-4 contraction joints each way. What we did here was de-bonded the corners (and any returns) from the slab for about 5m each way, then we tied the walls into the slab to give us bracing. This was a post tension slab so the connection from the wall to the slab was only a dowel in a 50dia tube that could be grouted/sealed after 30days, to allow as much contraction as possible.
When in doubt, just take the next small step.
I have used a similar concept for larger building like a shopping centre with a retaining wall external(precast), 200x150m slab, 3-4 contraction joints each way. What we did here was de-bonded the corners (and any returns) from the slab for about 5m each way, then we tied the walls into the slab to give us bracing. This was a post tension slab so the connection from the wall to the slab was only a dowel in a 50dia tube that could be grouted/sealed after 30days, to allow as much contraction as possible.
When in doubt, just take the next small step.
rowingengineer
Structural
expect for the 5m at each corner debonded with no dowels here, yes, typical PT details.
When in doubt, just take the next small step.
When in doubt, just take the next small step.
rowingengineer
Structural
I like the way you design then.
When in doubt, just take the next small step.
When in doubt, just take the next small step.
Madengineer,
In order to accomodate the different volume changes between a long structural wall and a continuous floor slab, we detailed a slip joint at the ledge and cased the first 6" of reinforcement in pipe insulation to allow it to deflect. Additional reinforcement in the slab will not prevent cracking, it will just redistribute the cracking.
To all,
Unless you are doing a PT slab, there will be cracking over the supports for a continuous slab. For exposed slabs, these cracks become highways for salts to attack the top layer of reinforcement. Repair to this layer of reinforcement is expensive and makes your client very upset with you. Sawcut joints are not preferred over tooled joints, but sawcuts(on green concrete) are better than cracks.
In order to accomodate the different volume changes between a long structural wall and a continuous floor slab, we detailed a slip joint at the ledge and cased the first 6" of reinforcement in pipe insulation to allow it to deflect. Additional reinforcement in the slab will not prevent cracking, it will just redistribute the cracking.
To all,
Unless you are doing a PT slab, there will be cracking over the supports for a continuous slab. For exposed slabs, these cracks become highways for salts to attack the top layer of reinforcement. Repair to this layer of reinforcement is expensive and makes your client very upset with you. Sawcut joints are not preferred over tooled joints, but sawcuts(on green concrete) are better than cracks.
MaddEngineer
Structural
Thanks guys,
This isn't actually a project of mine but a colleague was asked to provide some preliminary engineering in hopes of getting the job (if it is within cost limits the project will get the green light). From what I understand it will be (3) stories and in a hurricane prone region.
This isn't actually a project of mine but a colleague was asked to provide some preliminary engineering in hopes of getting the job (if it is within cost limits the project will get the green light). From what I understand it will be (3) stories and in a hurricane prone region.
Over the years, I have been called in to investigate concrete slabs on composite metal deck. In all cases, a couple of years after construction, the concrete has cracked with a series of very load noises, startling the occupants. Some of these slabs have also had top reinforcing added over supports. No delamination of concrete to deck was found.
Has anyone elese experienced this? Has anyone used sawcuts or other joints to force the cracks to predetermined locations?
Has anyone elese experienced this? Has anyone used sawcuts or other joints to force the cracks to predetermined locations?
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