Plain Concrete Design 1

[PT999]

I haven't any codes for plain concrete for slabs on grade.
Can anybody tell me shear and bearing stress requirements

 

[samdamon]

PT999-

What are your loads? What will the slab be used for? Some refs to get you started:

1. ACI committee 360- Design of slabs on grade. This publication discusses five different design methods for slabs and gives a few worked examples. Methods include those espoused by CRSI and PCA in other publications available to the public. (Engineering data report #33 for CRSI and PCA Concrete Floors on ground, 2nd ed.)

2. Army TM 5-809-12 "Concrete floor slabs on grade subjected to heavy loads". Some good typical details.

3. "Designing floor slabs on grade" by Ringo and Anderson (Hanley Wood is the publisher). Cited in othe recent posts here on the same topic. Lots of useful data.

4. Concrete International Magazine- About once a year they publish one of their monthly issues just on s.o.g. design and construction. Articels are on both design and construction of slabs.

Regards

 

[eric1037]

In the May 2001 issue of Concrete International, there is an article called "Design of Unreinforced Slabs-on-Ground Made Easy" by Wayne W. Walker and Jerry A. Holland. It's a very good reference.

Basically, the joint spacing is the key.

 

[antimatter]

Try researching the web for information on "Westergaard" equations. I seem to recall that he came up with equations for the use of plain concrete for slab on grade.

 

[3dboy]

I completely and totally disagree with 24676. Concrete, while it is relatively weak in tension, does have some capacity to resist tension. The argument can be made as well that the commonly included slab on grade reinforcement used in todays construction climate (wire mesh) is totally ineffective as its vertical location in the slab is dubious at best. I mean, at best you got a guy walking around with a hooked rebar pulling the wire up, if you are lucky. At worst they just dump the concrete on the mesh and you are left with the mesh in the worst place possible, underneath the concrete.

 

[MotorCity]

I would argue that plain concrete slabs on grade are the most common type of slab design: resident basement slabs.

 

[jheidt2543]

I'm aware of a 12" thick, plain concrete (no reinforcing whatsoever) slab on grade used in an industrial forgings plant that has stood up to steel ingot storage and heavy duty forktruck traffic for over 15 years and is still in good shape. The sub-grade is 24" of well compacted crushed limestone base material. So, plain concrete slabs are not just for residential construction, but like any other material or method, can be used under the right circumstances.

 

[SlideRuleEra]

Another example - 6" thick plain concrete road, on thick, well compacted subgrade, with sawed contraction joints every 15 feet (1" dia. steel dowels at the contraction joints) - no other reinforcement. Carries all the traffic into an electric utilitiy's equipment maintenance facility (examples - bulldozers on lowboys, truckcranes, loaded 18 wheelers). Constructed in 1985, I drove on it last month - no cracks, settlement or other obvious deterioration.

 

[UcfSE]

Some stores like Lowe's and Home Depot are designed with plain concrete slabs even with the fork lift load with those hard tires. The slabs are 6"-7" thick with saw cut control joints at about 10 feet on center max. Good control joint detailing is critical, and the slabs look great so far. I'm in Orlando by the way, for area reference.

 

[jheidt2543]

SlideRuleEra's comment on pavements reminded me that when major sections of I-90 were replaced in Wisconsin about 15 years ago, the Wisconsin DOT used many experimental sections. The designs ranged from continuously reinforced pavements to no reinforcing except shear transfer dowels at the construction joints.

 

[LittleWheels]

Five years ago, I was involved with the construction of the Pacific Motorway between Brisbane and the Gold Coast in Australia. The concrete pavement involved 250mm of plain concrete base, 125mm of lean mix sub-base, 150mm working platform and 300mm controlled subgrade. There wasn't any steel there, apart from dowels at joints.

 

[Ron]

MOST slabs on grade are plain concrete. While welded wire fabric is placed in them for "crack control", this does nothing for design tensile reinforcement.

Slabs on grade may be designed with a variety of approaches, the most common being Westergaard's procedures or some variant thereof. Elastic layer analysis is also used and finite element/difference procedures may be used as well.

The PCA and AASHTO both use the Westergaard approach for pavement design, as this is the most common application of design whether it is a roadway pavement or a warehouse slab serving as a pavement with forklift or other traffic.

I use the Westergaard approach and elastic layer analysis (ELA). ELA is quick and easy and gives me a cross-check on the PCA or AASHTO methods.

When faced with high static contact stresses, such as point load of shelving legs or similar, then a shear or bending design approach is more applicable, usually with shear being the primary controlling factor. In this analysis, code requirements for max. shear and bending should be used.

The performance of slabs on grade depends on several primary factors, including: proper thickness design for the anticipated loads, proper durability design in the concrete mix design (usually a minimum of 4000 psi concrete is necessary for durability requirements in pavement slabs or warehouse slabs subjected to traffic), proper placement techniques and thickness control, and proper joint design and placement.