Warehouse Slab Design - Dowel-less joints

[Boiler106]

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?

 

[Boiler106]

@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.

 

[WARose]

@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.

 

[MotorCity]

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.

 

[civeng80]

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.

 

[hokie66]

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.

 

[OldBldgGuy]

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.

 

[hokie66]

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.

 

[hokie66]

This is a few years old and was a presentation by a dowel supplier, but has some useful information, I think.

http://www.concretepavements.com.au/Documents/16 - Bruce Ireland - Joint details.pdf

 

[Tomfh]

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.

 

[Boiler106]

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

 

[bearjew]

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).

 

[SlideRuleEra]

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..."

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Boiler106]

@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.

 

[WARose]

@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.

 

[MotorCity]

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.