2. Column slabs – A common “punch-out”, studs above column in suspended slab
2nd part of punching series is about punching prevention in column-slab interface. This is the most common joint in cast-in situ flat slabs, and it also is the first type, that rose large awareness on punching through catastrophic failure cases. The starting event can be accident, overload, earthquake, or corrosion, but the result is same – if not accounted, the shear resistance of slab quickly weakens with increased rotation and critical shear cracks develop and can lead to abrupt failure. (Olmati et al, 2017: punching failures from 1973 to present day in America, Asia, and Europe)
Figure 1.Non-punch resistant structure can quickly experience rotation based drop in shear capacity and bring down a building
Without shear reinforcement slab can fail abruptly Progressive failure. An initial failure can cause the complete slab to collapse (Wolverhampton, 1997, Middleton 2011), and finally continue vertically slabs falling on slabs collapsing even rest of the stories below (Skyline plaza, 1973, Sampoong, 1995)
Figure 2. Seismic failure of flat slab, Middleton (NZ, 2011), corrosion initiated progressive slab collapse, Pipers Row (UK, 1997)
In multi-story buildings, slim floors will optimize the buildings size and make it attractive in many aspects – technically, economically, and aesthetically. As seen from the picture (and subsequent video), large multistorey buildings can have large quantities of column-slab interfaces. Using conventional stirrup or bent reinforcement methods, installation can consume significantly material and work time.
Figure 3. Effective shear-enhancing of numerous intersection of multistorey slim column-slab (Solutions: https://www.youtube.com/embed/3kDI6lRDcuo?rel=0&wmode=transparent)
Stud-rails can be installed fast by 1-man team on site, or in precast half-slabs (eg. filigran), either before or after slab reinforcement. It is also common to reinforce post-tensioned continuous 1- or 2-way slabs. Compared to otherwise economic solution of bending bars up for slab hogging and shear resistance (“crank bars”), it is much faster and precise. In conventional way, longitudinal bars have to either be bended in multiple rows, or additional stirrups are added, which all can be alternatively substituted by studs.
Figure 4. Quick placing of studs in rail-elements is an easy one-man job compared to toilsome and un-accurate bent up bars or additional stirrup bending and placing (up-right corner: studs in post-tensioned column-slab)
Figure 5., comparison of punching reinforcement solutions on the bottom
Reference projects - example cases of column-supported flat slabs solved with studs.
Italy, Torino, Sede-unica, 2011 – the highest stud-reinforced bubbledeck reference
The “bubbledeck’ system made the flat slabs lighter, and together with stud-rail system, this type of deck was the ultimate efficient way to construct all 42 punch-resistant levels of the building.
Figure 6. Italy, Torino, Sede-unica, 2011, stud-reinforced column-bubbledeck slab More: https://www.archilovers.com/projects/84863/torre-regione-piemonte.html
Otherwise, typical column slab targets are up to medium-height offices and residential buildings.
Figure 7. Slovakia, Bratislava, New generation hospital, 2022 - typical punch-resisting flat slab
Figure 8. Magnum-business-center, Baltic, 12 floors, 2019 - PSB® - system reduced significantly the slab thickness in the complex
Figure 9. Kaunas - Algirdas Residential, Lithuania, 2017 – Large apartment complex, 4-11 floors, punch resistant flat slabs.
Figure 10. Residence 4 Torri Locarno, Locarno, Switzerland, 2016 – a precast and composite method constructed frame used stud rails to reinforced thick but flat cast-in-situ slab, that enabled faster construction and efficient spaces
References
- FEM analysis - reference to famous punching failures: https://www.researchgate.net/publication/331307321_Model_Uncertainties_in_FEM_Analyses_of_Punching_Failures_of_Concrete_Slabs
- Model with accidental action: https://core.ac.uk/download/pdf/76988557.pdf
- UK, Wolverhampton corrosion: https://www.hse.gov.uk/research/misc/pipersrowpt1.pdf
- New Zealand, Christchurch earthquake: https://www.scielo.br/scielo.php?script=sci_arttext&pid=S1983-41952015000300260&lng=en&tlng=en
- Skyline plaza, Virginia, Korea, Sampoong department total progressive collapse: https://www.nist.gov/el/skyline-plaza-building-failure-virginia-1973, https://safety.productions/2019/02/17/sampoong-department-store-collapse-south-korea/
Where else do concentrated loads occur - columns must end somewhere below too, right? In next post we will hopefully see more.
