Solid R.C. slab (flat plate)
A floor made of a continuous reinforced-concrete plate, with no ribs or infill. When it bears directly on the columns, with no beams, it is a flat slab: a perfectly level soffit, free for the services, that carries loads in two directions. Simple and monolithic, its sizing is governed by deflection and, above all, by punching shear where the columns meet the slab.
Technical section of the system, from inside (left) to outside (right).
A floor made of a continuous reinforced-concrete plate, with no ribs or infill. When it bears directly on the columns, with no beams, it is a flat slab: a perfectly level soffit, free for the services, that carries loads in two directions. Simple and monolithic, its sizing is governed by deflection and, above all, by punching shear where the columns meet the slab.
A solid slab is a floor made of a continuous reinforced-concrete plate, with no ribs or infill: just concrete and steel. When it bears directly on the columns, with no beams, it is called a flat slab (flat plate) — a perfectly level soffit, free for the services.
The flat slab works in two directions: bending, it carries the loads to the columns along both axes, like a table resting on its legs. Removing the beams gains headroom and freedom of layout and simplifies the formwork; in exchange the plate is thicker and heavier than a lightened floor.
The critical point is where the column meets the slab: there the load concentrates and the column tends to punch through it in a brittle shear failure. It is resisted with special shear reinforcement, drop panels or column heads, and correct cover. It is the check that governs the slab thickness.
It is cast in situ on continuous formwork, with a double mesh of reinforcement, top and bottom; curing and striking must be respected. Being a thin plate over wide spans, deflection (and vibration) must be checked, because more than strength it is often the deformation that governs the sizing.
Why it works
Two-way · punching shearA solid slab on columns works in two directions at once: bending along both axes, it carries every load by the shortest path to the nearest columns, like a stiff plate resting on point supports. That is its elegance — no beams, a flat soffit, free services and regular spans. Its weakness is local and sits exactly at the columns: there the whole tributary load funnels into a small area and the column tends to «punch» up through the slab in a sudden, brittle cone of shear. So the slab is not governed by bending in the span but by this punching at the supports — answered with shear links, drop panels or column heads and careful cover — and, being a thin plate over wide spans, by its deflection. Get those two right and the rest is simple.
Free soffit and headroom
Comparison · insulantsNodal details
Critical junctions · sectionsAt the column the load concentrates and tries to push it up through the slab along a cone of shear. Vertical shear reinforcement — studs or links around the column — stitches the cone together; a drop panel or column head, and a generous depth and cover, complete the defence. It is the check that sets the slab thickness.
- Slab
- Column
- Punching cone
- Shear studs
- Top reinforcement
- Bottom reinforcement
The slab carries two ways with two mats of bars: the top mat is concentrated over the columns, where the slab hogs and the top fibre is in tension; the bottom mat runs in the spans, where it sags. Spacers hold the cover. Putting the steel where the tension is — top at the supports, bottom at mid-span — is the whole logic of the plate.
- Slab
- Top mat (over columns)
- Bottom mat (in span)
- Column strip
- Spacers (cover)
- Column
Installation controls
Specification · checklist01 · Formwork & props
02 · Reinforcement
03 · Cover & pour
04 · Curing
05 · Columns & fire
Recurring defects
Diagnostics · siteComponent materials
The network · materialsReference regulations
2 norms- D.P.R. 380/2001Consolidated Building Act (Testo Unico Edilizia)In force
- D.M. 16/02/2007Fire-resistance classification of construction products and elementsIn force
Informational links to the regulatory framework. Always verify the current text on the official source.