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Warehouse Slab Design - Dowel-less joints

Warehouse Slab Design - Dowel-less joints

Warehouse Slab Design - Dowel-less joints

(OP)
We have a warehouse with light forklift traffic and a Contractor that has a lot of experience and success with placing 6 inch slabs, unreinforced and control joints at 12.5' oc without dowels with other engineers.

I'm fine with the plain concrete and joint spacing, but in my experience, I've always provided dowels for load transfer with wheeled traffic.

For the life of me, i cant find a reference for eliminating dowels for this type of loading. Does anyone have a reference or any helpful hints?

RE: Warehouse Slab Design - Dowel-less joints

You'd have to check your load near an edge (with construction joints). That winds up controlling for a lot of slab on grade designs. (That and corners.)

RE: Warehouse Slab Design - Dowel-less joints

I've ran into issue where the edges on slabs where no dowels were used started to curl i.e. shrinkage and the gap between the slabs would start to increase past the designed control joint width. The control joint would crack as designed, but due to various factors the shrinkage of the concrete pulled the slabs apart. It wasn't a strength issue, but definitely didn't look good

RE: Warehouse Slab Design - Dowel-less joints

Forklifts are characterized by small, very hard wheels and a high center-of-gravity driven all too often by nitwits (er, low-skilled, high testosterone young employees) carrying valuable loads in a warehouse surrounded by many near-by targets (er, other people and racks).

You do really need to keep the slab offsets to an absolute minimum. That other warehouses didn't report problems when the owner took over from the construction contractor doesn't mean there are no problems now, in the future, or problems that aren't reported from the forklift operators to the forklift foremen to any forklift supervisors to any previous operating managers to any previous executive officers to any previous owners back to this particular contractor, who wants to report to you that "there have been no problems in previous jobs".

That's a lot of filters between the guys who have to slow down at each joint and the contractor who doesn't want to put in dowels.

RE: Warehouse Slab Design - Dowel-less joints

Quote (racookpe1978)

surrounded by many near-by targets (er, other people and racks)

Don't forget about the building columns. Forklift drivers LOVE to hit those!

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RE: Warehouse Slab Design - Dowel-less joints

Dowels are typically not used at control joints, as there is no purpose for load transfer. The intent is for the concrete to crack at the control joint, and any reinforcing in the slab may or may not continue through the control joint. Dowels are required at construction joints where you have a cold joint through the entire slab thickness, thus you need some method to transfer load across the joint (shear key, dowel, or both).

RE: Warehouse Slab Design - Dowel-less joints

(OP)
@Motorcity, incorrect. Dowels are not usually needed in slabs using stationary loads at control joints. They are often used on slabs using wheel loads.


RE: Warehouse Slab Design - Dowel-less joints

Boiler106, I think we are saying the same thing. Reread my first statement "Dowels are not typically used at control joints". The use of dowels should not depend on the type of load (stationary vs. static). I provide dowels based on the magnitude of the load and the need to transfer load across a construction joint.

RE: Warehouse Slab Design - Dowel-less joints

I think you should just admit that you are wrong, MotorCity. No shame in that.

RE: Warehouse Slab Design - Dowel-less joints

No Dowels ? So a keyed joint ? Something they used years ago, probably as effective as dowelled joints IMHO.

RE: Warehouse Slab Design - Dowel-less joints

keyed joints are prone to breaking off the outside part when forklift wheels pass over. Thus the reason that centrally located dowels are better. Diamond plate dowels and square dowels with compressible material on the sides are better, as they allow for shrinkage parallel to the joint. Whatever kind of dowel is used, it is essential that they are normal to the joint.

RE: Warehouse Slab Design - Dowel-less joints

hokie66, what part(s) of my statement do you believe to be incorrect?

RE: Warehouse Slab Design - Dowel-less joints

(OP)
@civeng80 - no, we're talking sawcut control joints and diamond plate dowels at the construction joints. No keys.

@MotorCity - dowels are provided at control joints for load transfer in slabs subjected to wheel loads. Refer to the Army Manual CONCRETE FLOOR SLABS ON GRADE SUBJECTED TO HEAVY LOADS pg 5-12 fig 5-6 and ACI 360R-10 figure 6.5.

http://armypubs.army.mil/epubs/DR_pubs/DR_a/pdf/we...

I've since come across "aggregate interlock" in ACI360. Does anyone have experience with this?

RE: Warehouse Slab Design - Dowel-less joints

MotorCity,

The statement which is incorrect is your opinion that dowels are only used at construction joints, not control joints. This is clearly inaccurate in industrial slabs, which need a better mechanism than aggregate interlock.

All that said, I think joints in slabs at 12.5' centres will soon be a thing of the past. "Jointless" slabs are being used more and more. The main advantage is in reducing maintenance costs.

http://northrop.com.au/201704/big-pour-australias-...

RE: Warehouse Slab Design - Dowel-less joints

hokie

TNT would disagree with fibre reinforced pavements.

RE: Warehouse Slab Design - Dowel-less joints

You will have to explain that. A project that went wrong, perhaps?

RE: Warehouse Slab Design - Dowel-less joints

One of my firm's clients mentioned a facility of theirs that was constructed with a jointless slab using "Teqton". Link

RE: Warehouse Slab Design - Dowel-less joints

hokie66, contrary to your experience I have done countless industrial slabs w/ heavy uniform loads and forktruck traffic and cannot recall placing dowels at control joints. Have not received any complaints from owners or contractors. I must be in the minority of engineers described by the OP. We've done unreinforced, reinforced, and fiber reinforced slabs this way. For unreinforced slabs, load transfer is not an issue since the loads are usually small and/or the base is very good. For reinforced slabs, we place a layer of bars at mid depth and continue those bars across the control joint. If/when the slab cracks, the reinforcing across the joint transfer the load and prevent the crack from widening.

RE: Warehouse Slab Design - Dowel-less joints

Quote:

(MotorCity)

I have done countless industrial slabs w/ heavy uniform loads and forktruck traffic and cannot recall placing dowels at control joints.

Me either. The only time I use dowels is at construction joints.

RE: Warehouse Slab Design - Dowel-less joints

There has been no mention of the subgrade or it's preparation. To me, that is the "wild card".

If subgrade is "good" and correctly prepared... plain concrete, with or without dowels, will likely work just fine.

Or, as with most of my projects, if subgrade material is a "compromise" (but well prepared), even reinforced slabs, with dowels everywhere can have a hard life.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Warehouse Slab Design - Dowel-less joints

(OP)
@Motorcity, @Warose I'd be interested in knowing what design guidelines you follow for forklift and rack loading on your slabs with your approach.

It seems you're relying on aggregate interlock.

RE: Warehouse Slab Design - Dowel-less joints

Quote:

@Motorcity, @Warose I'd be interested in knowing what design guidelines you follow for forklift and rack loading on your slabs with your approach.


I figure the steel (in the slab) by the subgrade drag equation, then (as far as thickness goes) I will figure the moments & shears developed based on (assuming we are talking about forklift traffic) the load being near a edge or corner (which almost always controls) and compare that to the modulus of rupture and the shear capacities of a preliminary slab thickness (selected based on wheel load charts in ACI 360).

The corner and edge calcs I do with a program like STAAD (I have a generic file set up to do it every time; I just alter the subgrade spring constants and the loads from job to job).

As an alternative, I use to use a old concrete text by Winter & Nilson that has equations for point loads on slabs at those critical locations.

As far as the "aggregate interlock" question goes.....yes, I do use it. I've heard a lot of arguments against it....one being that the slab will curl and wheeled traffic hits the edge of the curl and cracks it.

Well my answer to that is: if it's going to curl....it's going to crack in a lot of places (from traffic and so forth) regardless of what you've got at the joints. A lot of times, that's as much as a QC issue as anything.

RE: Warehouse Slab Design - Dowel-less joints

It is usually an iterative process. I always start by assuming an unreinforced slab just to get an idea of a maximum thickness. If its reasonable I'll run with it. I use the PCA equations to determine a preliminary thickness (based on modulus of rupture and factor of safety). If I decide it to be reinforced, I will start with the minimum steel for temperature and shrinkage. To fine tune the design, I use a beam on elastic foundation spreadsheet and a radius of relative stiffness for interior, edge, and corner condition. I rely on aggregate interlock for unreinforced slabs, there is nothing else TO rely on other than a well compacted base. Curling is usually most critical when the slab is curing and until the building is tempered (i.e. when you have the potential for temperature variation across the slab thickness). For fiber reinforced slabs, we throw it to the fiber supplier to verify the slab thickness, fiber dosage, etc.

RE: Warehouse Slab Design - Dowel-less joints

Hokie,

Very bad cracking on the floor, to the point of it being unserviceable.
I believe there is a report on this somewhere (Len Stevens ex professor Melbourne University was the author).
I would never use fibre reinforced concrete and I believe many other designers would also not touch it.
I recall there was a thread on this years ago.
I also never have specified dowels in saw cut joints, just let the fabric go through. Only use dowels (round dowels) at construction joints.

RE: Warehouse Slab Design - Dowel-less joints

civeng80,

It is certainly a specialized area at the moment, but steel fibres in concrete have lots of advantages if done right. Now, if the case you are thinking about used useless plastic fibres, I won't try to find the report.

RE: Warehouse Slab Design - Dowel-less joints

My, my, there are some nasty little barbs in this thread! Along with MotorCity & WARose, I've done a lot of pretty heavily loaded industrial slabs, & I've only ever used dowels at construction joints, not control joints. I have worked for years for some companies that anyone in North America would know, along with a lot of smaller ones, so things must be working out. They've got lots of consultants to choose from if they don't like my work.
SRE nailed it though: the base is everything!! I drive on unreinforced concrete highways every day.

RE: Warehouse Slab Design - Dowel-less joints

If there is one thing that structural engineers disagree on most, it is slabs on the ground, which are really not structural elements at all.

RE: Warehouse Slab Design - Dowel-less joints

This stuff is such a black art. I don't know anyone who really knows how to design foolproof slab joints - be they dowelled, or otherwise.

Half our work seems to be dealing with busted joints.

RE: Warehouse Slab Design - Dowel-less joints

(OP)
@OldBldgGuy - can you fill us in on your design method?

RE: Warehouse Slab Design - Dowel-less joints

Can anyone explain to a young engineer WHY dowels are used at control/contraction joints? In my experience, we, like MotorCity, have only used them at construction or cold joints.

All of our control joints are sawn to 1/4 of the slab thickness, with the reinforcing continuous through it (most of the time, unless its very heavily loaded and has a top mat of rebar).

RE: Warehouse Slab Design - Dowel-less joints

Quote (bearjew)

WHY dowels are used at control/contraction joints?
All of our control joints are sawn to 1/4 of the slab thickness, with the reinforcing continuous through it...

For reinforced slabs, you are right, but the OP's question concerns "...slabs, unreinforced..."

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Warehouse Slab Design - Dowel-less joints

(OP)
@warose, @MotorCity - can you fill me in on how you treat wheel loads, rack loads at interior control joints? It seems like there is no guidance in the design guides.

Is it as simple as using the PCA nomographs for interior condition?

Im not concerned about my free edges and corners as i will have thickened slab edges, reinforcing and/or the loads will be sufficiently far from the perimeter.

RE: Warehouse Slab Design - Dowel-less joints

Quote:

@warose, @MotorCity - can you fill me in on how you treat wheel loads, rack loads at interior control joints? It seems like there is no guidance in the design guides.

Is it as simple as using the PCA nomographs for interior condition?

I might backcheck it with PCA charts.....but with the "rack loads" you are likely not going to have a pattern that matches the charts......ergo in such a case, I'd use a FEA program.

RE: Warehouse Slab Design - Dowel-less joints

I try to use the PCA charts if possible, otherwise I might use a program like PCA Mats or our beam on elastic foundation internal spreadsheet.

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