Solid or ribbed?Same slab — two solutions

The differences — step by step

Solid vs. ribbed

Step	Solid	Ribbed
Initial thickness	L₀/25, min. 15, rounded to the cm	The larger of L₀/18 of the slab and L₀/14 of the hidden beam, min. 20, rounded to the cm
K₁₁	1	From the table: h/tf against bf/bw
Self-weight	2.5·h/100	Via equivalent t, concrete 2.5 and block 0.5
Load for the moments	Fd,max per 1 m (t/m²)	Fd,max·bf per rib (t/m)
Reinforcement	Φ@spacing per 1 m, up to 25 cm	n·Φ per rib, 2 or 4 bars
The beam	Calculate the depth	Hidden — calculate the width

Same slab: 3 supports, 6.3 + 6.3 m

q = 0.3, Δg = 0.2, B30 limestone. L₀(max) = 0.8 · 630 = 504 cm in both. In the ribbed slab the section is given in the exercise (h = 30), so h₀ is not calculated.

The results (from the BST engine)

	Solid	Ribbed (5/15/65)
Thickness	21 cm	30 cm (given)
Required thickness	20.83 → 21 ✓	26.55 → 27 ✓
Self-weight	0.525 t/m²	0.385 t/m²
Fd,max	1.495 t/m²	1.299 t/m² → 0.844 t/m per rib
Moment over B	7.417 t·m per 1 m	4.187 t·m per rib
Top reinforcement	Φ16@19 (10.58 cm²/m)	4Φ12 in the rib (4.52 cm²)

The ribbed slab is 9 cm thicker — and still 27% lighter.

A fair comparison of reinforcement: a rib every 0.65 m → 4.52 / 0.65 = 6.95 cm² per metre, against 10.58 cm²/m in the solid slab.

3 traps

• Comparing A_s per rib with A_s per metre. First divide by b_f.
• 'Thick = heavy'. In a ribbed slab most of the volume is blocks.
• Calculating a depth for a hidden beam. It has the depth of the slab — calculate the width.