Contraction joints in suspended slabs 3

[Hemifun]

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.

 

[hokie66]

I can't think of a situation in which a saw cut in a suspended slab would be advantageous.

 

[Teguci]

Exposed parking decks, stadiums, unfinished office floors, roofs and other exposed concrete elements. If you are carpeting or tiling, don't bother. Contraction joints take the place of naturally formed cracks (ie you're going to have a crack near there anyhow).

The advantage of a contraction joint is that you can put sealant in it during construction, you can designate a profile for the joint (rounded edges), and it looks better.

A saw cut joint only works as a contraction joint when you saw cut the concrete before it cracks. This means you have to cut the concrete within hours of casting it (good luck with that).

 

[haynewp]

I don't put them in suspended slabs or composite steel beam/joist systems, I put them in topping slabs for precast.

 

[csd72]

Definately not in suspended slabs.

If you need movement joints in slabs then provide a full movement joint in the form of an edge supported on a corbel or double frames with a full joint between them.

Google it and you will find many references on this.

http://www.highbeam.com/doc/1P3-262575111.html

http://books.google.co.uk/books?id=...8JW6Cw&sa=X&oi=book_result&ct=result&resnum=6

http://www.concrete.net.au/publications/pdf/TN63.pdf

 

[ishvaaag]

Expansion joints at floors when wanted or required for proper account of rheological movements (temperature and shrinkage mainly) need be purposedly designed. The olden practice was to duplicate columns at each side of one of these joints, but today it becomes everyday more common to use shear contraptions sold by specialty providers that allow movement perpendicular to the face of the joint and horizontally as well if so wanted or required, whilst coercing vertical relative displacement and even support if one of the sides is rigid enough.

http://www.ancon.co.uk/main.asp?pageID=79

for example.

 

[hokie66]

ishvaag,
Provision of double columns at a movement joint is not an "olden practice". It is the right way to do it. Halving joints and corbels are a major cause of modern building structural distress and serviceability issues.

Who made this post so wide?

 

[apsix]

Ditto; use cast contraction/movement joints where required for large buildings.

Stay well away from sawcut joints for suspended slabs.

 

[dik]

For parkades, I've often used a "Goldblatt Groover" to provide a joint to be filled with a flowable joint filler. I've not used a saw.

 

[hokie66]

dik,

But the question was about suspended slabs. Surely you don't need a joint filler in suspended slabs where the top of the slab is in compression.

 

[dik]

Joints are made at supports and rely on the reinforcing steel. Yes, it's designed with this in mind.

 

[demayeng]

In suspended slabs we often control shrinkage (and flexural) crack widths by putting in enough reo to satisfy the crack control provisions in the codes rather than using contraction joints

 

[MaddEngineer]

I think we have a terminology issue.

What the OP described appears to be a CONTROL JOINT which essentially tries to control where the shrinkage cracks will occur making it more aesthetically appealing. I agree if the floors will be carpeted, etc. there is no reason to specify them.

However, the other type of joint is an expansion/contraction joint which actually allows differential movement to occur at a predefined locations in order to alleviate any themally imposed stresses. These are a totally differect animal than what was posted by the OP.

Since I am on the subject (and I think it is related to the original post), I am curious if anyone has utiized a two-way (flat plate) slab that was supported along the perimeter by concrete masonry bearing walls and interior by concrete columns. A collegue of mine has proposed this system and I thought there might be issues with expansion/contraction (as well as construction phasing) since the slab and exterior wall will be tied together. Any thoughts or experiences from anyone.

 

[csd72]

This is an old style of building a structure, it has fallen out of favor due to the delays in constructing the masonry walls.

 

[MaddEngineer]

thanks csd72,

other than phasing issues, do you know if there are also issues with thermal or shrinkage restraint. I think they are thinking they can shave costs, but as noted the sequncing issues are problematic. But any special considerations for movement detailing, etc. can add add'l costs... and if not detailed properly will probably be less forgiving I would think.

 

[hokie66]

MaddEngineer,

The term control joint is usually reserved for slabs on grade, not suspended slabs. Suspended slabs have movement joints, referred to by most as expansion joints, but their purpose is mostly for contraction.

Flat slabs and flat plates supported on masonry walls at the perimeter are still common where I am. You do have to design for the restraint, and this is normally done by increasing the shrinkage reinforcement in the slab and also reinforcing the walls accordingly.

 

[rowingengineer]

While it is an “old style” it is used in the north Qld a lot, home of 300 days of sunshine a year, it is currently frezzing in my office and i would love to go back right now.

but onto subject;
Your question is a touch open, i have done a few of these building and wall length and slab span are your deciding factors.

Depending on the layout of your walls (I will assume a square for now and not post-ten) you will get high restraint forces in your slab and walls (near the corners or any returns). Depending on the length of the building (if longer than 30-40m I would put a joint in the slab and walls and use some ancon dowels) but for discussion say 30m. I would full core fill the corners and put 1-3m of horizontal reo in to ensure that the wall doesn’t crack, (and all return walls).

Vertical joints in the walls can also help, will need a bit of extra reo in the slab at this points to stop reflection cracking, but I like to put some joints in the walls at corners if my wall length is above 20m.

Best to reinforce the slab as restraint to medium of high levels. Normally only a problem on the first two levels of your building then you’re sweet. To work out your restraint forces, there is a good bit of info in the PCI manual.




When in doubt, just take the next small step.

 

[MaddEngineer]

hokie66,

Thanks for the input... I understand they are contraction joints, however, what the original post described appeared to be a control joint (i.e., saw cut).

rowingengineer,

Correct the building will be about 30m or 100ft along one wall and 20m or 60ft along another. Thanks for the info. The proposed cmu will also be 8in and will have a stucco finish applied to the exterior. Concerning detailing at the masonry shelf, do you typically run the slab to the full thickness of the masonry or half and provide a 4in infill. Also (typically) would you run vertical rebar straight thru the slab and then hook additional bars into the slab or do you hook vertical bars into slab and then provide upturned bars from the slab into the next wall lift.

 

[rowingengineer]

normally I like to design/ detail it as a clean out block, that is if your slab thickness works equal to or less than a course, if your slab has a greater thickness than a course I would form the edge. but in your case it is parbably easier to just form the edge anyway, as you are facing the blocks.

Wall bars, I Like to continue the vertical bars and use a "u" bar at the edge of slab. But if i had a choice of your two options i would use the second.


When in doubt, just take the next small step.

 

[csd72]

Rowingengineer,

I never would have guessed you were a banana bender!

Last building I designed in north Queensland we had to tie down 12m high concrete panels against wind uplift. Some proper winds there.

MaddEngineer,

When I have previously designed large slabs sitting on brick walls I have used a sliding joint between the top of the walls and the underside of the slab. This in theory is flexible enough to give under thermal shrinkage but stiff enough to resist wind loads.

Not sure how this detail would work with multiple floors though.