Researchers: Dr Robert Vollum, Prof. Ahmed ElghazouIi, Emeritus Prof. Paul Regan, Dr Guilherme Melo (University of Brasilia, Brazil ), Dr Maurício P. Ferreira, Universidade Federal do Pará Brazil, Dr Martin Eder, Luis Soares, Nivea Benevides de Albuquerque, Mohemmed Abdelsalam

Funding: Cidect, EPSRC studentship, Science without Borders

Background

Punching failure arises when a slab fails locally in shear around a column. Optimally designed flat slabs generally include shear reinforcement to increase punching resistance, which can also be increased by locally thickening the slab with a drop panel or locally enlarging the column with a column head. The latter measures are undesirable due to severe disruption to the construction process with consequential increased construction costs and loss of flexibility in the positioning of building services. There is no generally accepted theoretical treatment of punching shear and design is based on empirical methods given in design standards.

There are two strands to Dr Vollum’s research into punching shear. The first is his research with Prof. Elghazouli and Dr Martin Eder into the development of a ductile shearhead for connecting flat slabs to steel columns (see Figure 1). The second strand of Dr Vollum’s research into punching shear is focussed on developing improved design methods for punching through a combination of experimental (see Figure 2), numerical (see Figure 3) and analytical work.

This research compliments Dr Vollum’s activities on Task Group 4 of Working Group 1 of CEN/TC 250/SC 2 which is the European wide task group responsible for proposing revisions to the EC2 clauses on shear, torsion and punching. Dr Vollum’s current research with Soares is focussed on developing an understanding of the surrounding slab on punching resistance. His research with Benevides de Albuquerque is concerned with punching resistance at edge columns with outwards eccentricity.

Figure 1: Details of shearhead
Figure 1: Details of shearhead

Figure 2: Crack pattern in slab after failure of shearhead
Figure 2: Crack pattern in slab after failure of shearhead

Context and methodology

The research involves a synthesis of experimental, numerical and analytical work. The novel aspect of the system developed with Eder and Elghazouli is that a gap is left around the column as shown in Figure 2 to enable the slab to be connected to the column through isolated structural steel members which are designed to yield in shear before punching failure occurs.

The suggested connection could serve as a primary lateral resisting system in regions of moderate seismicity or as a secondary system in areas of significant seismicity. The detail offers enhanced ductility and energy dissipation compared with traditional forms. Eder1 developed a numerical procedure for modelling punching failure with DIANA8 that he calibrated using slabs with ACI shear reinforcement tested previously by Vollum3. He then used the procedure to successfully model the response of his slabs with shearheads3-6 including the shearhead as shown in Figure 3.

Research with Ferreira et al.7 compared the accuracy of the critical shear crack method of Muttoni9 and EC2 for slabs tested by Ferreira2 and others. The research showed the accuracy of the two methods to be comparable but the EC2 method is recommended for everyday use as it is considerably simpler to use. Current research with Soares is also considering the use of ATENA10 for modelling punching.

The emphasis is on assessing the effect of the overall slab behaviour on punching resistance as it is felt that conventional punching specimens like that that shown in Figure 4 underestimate the punching resistance of flat slabs where membrane and continuity effects are important. Evaluations of punching resistance are also being carried out using the critical shear crack theory of Muttoni10.

Figure 3: Plastic axial strain Epxx contours in longitudinal fuse at ultimate load of 615 kN
Figure 3: Plastic axial strain Epxx contours in longitudinal fuse at ultimate load of 615 kN

Outputs

PhD theses

  1. Eder M.A. (2011) Inelastic behaviour of hybrid steel/concrete column-to-flat slab assemblages, Imperial College London.
  2. Ferreira, M. P., “Punção em Lajes Lisas de Concreto Armado com Armaduras de Cisalhamento e Momentos Desbalanceados,” PhD thesis, Universidade de Brasília, Brasília, Brazil, 2010, 275 pp. (in Portuguese).

Journal articles

  1. Vollum R.L., Abdel Fattah T., Eder M. and Elghazouli A.Y., “Design of ACI type punching shear reinforcement to Eurocode 2”, Magazine of Concrete Research, 62, (2010), 3-16.
  2. Eder M.A, Vollum R.L., Elghazouli A.Y., Abdel-Fattah T., “Modelling and experimental assessment of punching shear in flat slabs with shearheads”, Engineering Structures, 32 (2010), 3911-3924.
  3. Eder M.A, Vollum R.L., Elghazouli A.Y., Performance of ductile RC flat slab to steel column connections under cyclic loading, Engineering Structures, 36, (2012), 239-257
  4. Eder M.A., Vollum R.L. and Elghazouli A.Y., Inelastic behaviour of tubular column-to-flat slab connections, Journal of Constructional Steel Research, 67 (2011), 1164-1173
  5. Ferreira M.P., Melo G.S., Regan P.E., Vollum R.L., Punching of RC flat slabs with double headed shear reinforcement, ACI Structural Journal, In press

Other references

  1. DIANA. Finite element analysis user’s manual. Release 9.2. TNO DIANA BV, Delft, The Netherlands, 2007.
  2. Muttoni, A., “Punching Shear Strength of Reinforced Concrete Slabs without Transverse Reinforcement”, ACI Structural Journal, V. 105, No. 4, 2008, pp. 440-450.
  3. Červenka V and Červenka J., ATENA user manual, Prague, 2013