International Concrete Abstracts Portal

At the department of Structural Engineering, formerly the Department ofBuilding Statics and Structural Engineering and the Department of Bridge Structures,Royal Institute of Technology in Stockholm (KTH), now merged into the Department ofArchitectural and Civil Engineering, research on punching of concrete slabs and plateshas been conducted for many years. The first report published in English was thefamous “Punching of Concrete Slabs without Shear Reinforcement” Nylander &Kinnunen (1961). [22] Since then, a large number of punching tests have beenconducted, many of these are unpublished or only published in Swedish. This paper willsummarize some of the tests performed and also present new findings in this importantfield.

The bearing capacity under punching shear has been addressed in a seriesof experiments. Three types of shear reinforcement of the stirrups type have beenopted as well as the most suitable position of the shear reinforcement. Reinforcementquantities have been modified versus code requirements. Tests results indicate that thesuggested shear reinforcement performed very well. Some requirements in the twoNorth American Codes appear to be conservative compared to the European codes.Bearing capacity assessment in some codes appears to be conservative (bearing inmind, however, that a variety of additional considerations are taken while codedrafting) . Behavior in serviceability was very satisfactory.

To improve punching shear capacity of slab-column connections, and toreplace other kinds of strengthening, the composite joints were introduced. The idea isto use combined head-and-column precast members and cast on site remaining partsof slabs. Following this general idea the precast parts from high performance concrete(HPC) and fiber reinforced high-performance concrete (FRHPC) were introduced. Thetests of such axi-symmetrical joints were done by the authors. The first two series ofmodels with the composite joints were tested to compare results and behavior ofconnections with HPC precast heads and formerly tested models with prefabricatesfrom ordinary concrete. The size of models was 2.7 ´ 2.7 m, with a total slab thicknessof 0.26 m. Three different sizes and two shapes of heads – circular and square – wereused in the tests. The final series of models was prepared with FRHPC heads. Eachseries contained four models, so the experimental data from 16 tests are presented.The heads from FRHPC appeared satisfactory to carry punching forces over two timesgreater than heads from the same HPC without fibers. Advantages from use of FRHPChead-and-column precast members have been confirmed in multi-phase failure as wellas in the significant increment of punching shear capacity of connections. On the basisof experiments, the analytical model of punching failure was calibrated. This model istaking into account the recent simplified physical models for homogenous slab-columnconnections and the failure criterion based on facture mechanics.

Flat slabs are an economical structural system for medium height residentialand office buildings. Prestressed concrete flat slabs have the additional advantagesthat longer spans can be used and deflection problems are less significant. Punchingshear of reinforced or prestressed concrete flat slabs is an undesirable mode of failurein that it occurs without warning and can lead to progressive collapse of large areas ofslab or even complete structures. Modelling the punching shear strength ofprestressed concrete slabs is conceptually difficult as the tendons can be bonded orunbonded, the tendons can pass through the columns or not, the stress in the tendonsat punching failure is indeterminate, the tendons can be banded and the tendons canbe draped resulting in beneficial vertical components of the prestressing force.The paper describes the code provisions, ACI 318-05, BS 8110-97, CEB-FIP 1990 ModelCode (Comité Euro-International du Béton and Federation International de laPrecontrainte), CSA A23.3-04, DIN 1045-1:2001-07 and Eurocode 2-2003. In general, thecode provisions for punching shear of prestressed concrete flat slabs are extensions ofthe punching shear provisions for reinforced concrete flat slabs. Most codes requiresupplementary bonded reinforcement in negative moment regions which complicatesanalysis.The literature has been reviewed to locate experimental results of punching shear testsfor isolated prestressed concrete flat plates, continuous prestressed flat plate systemsand tests of flat plate column connections under shear and moment transfer. Recentresearch has indicated that the limitations on concrete strength in code equations andignoring the precompression close to the slab edge are too conservative.The simplifications of an effective slab depth not less than 80% of the slab depthpermitted by ACI 318 and CSA A23.3 in calculations of the punching shear capacity ofprestressed concrete flat slabs should be eliminated.

The new DIN 1045-1 and the German Construction Approval Z-15.1-213 fordouble-headed studs are discussed and essential background information ispresented. In addition, comparison is made with the ACI 318-02 Code and the ACI 421report. Differences between the codes are reviewed and highlighted. Emphasis is seton the parameters affecting the punching resistance of slab-column connections, themoment transfer between slab and column, shear reinforcement (stirrups and headedbars), and punching of reinforced concrete footings. The code provisions are evaluatedby comparing with test results.