Minimum reinf. for slab-on-grade

[Mossmobley]

Hi all. I'm in a pickle with a contractor. I have a pre-engineered building with a typical turndown slab over isolated footings foundation. Building shears are transfered to the slab with hairpin bars. Its my opinion the use of hairpin bars in this manner requires the slab be considered a structural element and subject to the minimum area of steel required by the code for structural concrete. The governing code is IBC 2000 which differs to ACI-318. ACI-318 does not address slabs on grade used in this manner. Does anyone have any information on this subject?

 

[JAE]

Go to this thread: thread592-152445 and read through it - also two other thread links within it. Hope these help.

 

[onteng]

For smaller frames, we have specified hairpins with wire mesh reinforcing in the slab. For larger frames with larger thrust reactions, we prefer continuous ties between the piers below the slab in a concrete duct.

If you are concerned whether the slab should be reinforced as a structural element with the hairpin approach, I suggest using the continuous tie.

 

[Mossmobley]

Thanks for the helpful comments Onteng. Sadly, design is not an issue here. The building collapsed durring construction and I have been hired by the owner to find fault. This project will likely end up a court room. Thats why I was wondering if anyone knows of a code requirement. Since I posted this thread, I found that 1901.2 of IBC 2000 states that slabs-on-grade which transmit verticle OR lateral loads to the soil must be designed in accordance with chapter 19. The commentary on this section specifically mentions preengineered buildings with hiarpin configurations. But...,I'm not sure what "in accordance to chapter 19" means. Does this requirement mean that slabs should be designed as oneway slabs, twoway slabs, or something else?

 

[kslee2006]

The hair-pin design is valid only when the column is supported directly on top of the pedestal of the footing, which in turn is seperated from the slab-on-grade through expansion joint. When the lateral load from the column causes the pedestal deflect outward, thus introduces tension in the hair-pin, the mass of slab-on-grade then provides resistance force through gravity and friction. This mechanism is a localized phenomenon, the slab-on-grade needs not to be reinforced entirely as a structural element for this purpose, provides that the hair pin legs are adequate to resist the tensile force, it has adequate development length (per code), and the concrete covers (per code)are adequate to avoid pull-out failure. However, for practical reasons, additional reinforcing may be provided accross the joint to reduce stress in the bars to minimize cracks on the slab near the joint.