Concrete has good compressive strength, but brittle (tension/shear). Punching in concrete occurs as a cone shape around point load, and if not reinforced against properly, can be catastrophic. Neglecting, simultaneous and progressive catastrophes have both happened in past. Nowadays, punching reinforcing has become routine, but if columns are numerous, amount of work on bending rebars (stirrups) or casted concrete can be high.
Slim cast-in situ floor slabs are attractive when space, building height and floor-count are limited or easy coordination of building technology and smooth installations below ceiling are needed.
Slabs can be made thinner (on any level, even on ground) than with bended rebar or more concrete as traditionally. In the process material is saved – for instance, a cubic meter (m3) of saved concrete reduces CO2 emission by 300 kg, comparable to driving 1500 km in a family car.
Concrete capitals are becoming past and any conventional transverse reinforcing, such as stirrups or bended flexural steel, to increase shear resistance of slab, can be troublesome to achieve.
Figure 1. Concrete drop-downs and capitals on columns
Figure 2. pictures on right: thickening and pedestal in foundations
Figure 3. Reinforcement type affecting slab thickness
Figure 4. Design can require heavy and thick hidden capitals. The size can be further controlled using steel profile capital element. Without studs the amount and required work for bending can however still be demanding.
What are the typical slim concrete structures prone to punching? Most typically a point-load or similar local reaction, most often through slab under or resting on a column. Other concrete members can be slim due space or economical limitations. In this series of blogs you will learn about punching tendency in buildings, using examples from construction sites from all around the world.
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