Slab Classification and Design Method
📌 Slab type classification is determined by the ratio of the longest span ($L_{max}$) to the shortest span ($L_{min}$): if $L_{max}/L_{min} > 2$, it is a one-way slab; if $L_{max}/L_{min} < 2$, it is a two-way slab.
📐 The design approach used for this slab calculation is the strip method (per meter strip).
🏗️ It is assumed that the slab and beams are cast monolithically, resulting in fixed (jepit) support conditions on all edges for elastic analysis.

Moment Coefficient Calculation (Two-Way Slab)
🔍 For a two-way slab with fixed supports on all sides, the moment coefficients ($C$) from PBI 1971 tables are used to find internal forces ($M_{ux}, M_{uy}$) based on span ratios.
💡 The required moments are calculated using the general formula (where $q_u$ is the factored load and $L$ is the span length):
$$M = C \times q_u \times L^2 / 1000$$
(The division by 1000 converts the result to $\text{kNm/m}$ for a design strip of $1000 \text{ mm}$).
🏠 Dead loads (SDL) include self-weight (concrete density $24 \text{ kN/m}^3 \times \text{thickness}$) plus finishes (e.g., tiles, ceiling), and live loads are based on the building function (e.g., $1.92 \text{ kN/m}^2$ for residential).
📉 The factored load combination used is $q_u = 1.2 D + 1.6 L$, which resulted in $8.30 \text{ kN/m}^2$ in the example.

Flexural Reinforcement Design (Field Area - Y-Direction)
📏 Design is performed for a $1000 \text{ mm}$ wide strip ($b=1000 \text{ mm}$) with slab thickness $h=130 \text{ mm}$ and concrete cover of $20 \text{ mm}$.
📏 The minimum spacing for longitudinal reinforcement must not exceed $2h$ (i.e., $260 \text{ mm}$). The assumed spacing was $200 \text{ mm}$ using $\phi 10 \text{ mm}$ bars.
⚙️ The required tensile reinforcement area ($A_s$) is determined by calculating the number of bars needed ($N = 1000 / \text{spacing}$) multiplied by the area of one bar. The required $A_s$ was $392.699 \text{ mm}^2$.
⚖️ The minimum reinforcement area ($A_{s,min}$) is governed by the yield strength ($f_y$); for $f_y < 420 \text{ MPa}$, $A_{s,min}$ is calculated using a specific PBI formula (resulting in $260 \text{ mm}^2$ in the example). The design must satisfy $A_{s,min} < A_s < A_{s,max}$.

Shear Design
⚙️ Shear forces ($V$) are exported from ETABS, requiring the stiffness modifiers for the slab elements to be set to 0.25 for all $M_{11}, M_{22}, V_{12}$, etc.
💥 The load combination for shear analysis uses serviceability factors: $1.0 D + 1.0 L$ (not factored loads).
✔️ The calculated shear capacity provided by the concrete ($V_c$) must be greater than the required factored shear force ($V_u$). In the example, the capacity was significantly higher than the demand, requiring zero shear reinforcement ($V_s = 0$) as is typical for slabs.

Deflection Check (Serviceability Limit State)
📊 Deflection ($\delta$) must be checked using service loads (1.0 D + 1.0 L, unfactored) applied to both spans ($L_x$ and $L_y$).
⚠️ For conservatism, the compression reinforcement area should be assumed as zero ($A'_s = 0$) when calculating the effective moment of inertia ($I_{eff}$).
📏 The instantaneous deflection check involves comparing the calculated deflection to the allowable limit (e.g., $L/360$). The example showed very small deflections ($0.17 \text{ mm}$ instantaneous deflection under dead load) indicating good stiffness.

Key Points & Insights
➡️ Assume Monolithic Construction: Assume the slab and beams are cast together to justify using fixed (jepit) support conditions for the design calculations, which influences moment coefficients.
➡️ Determine Slab Type Early: Quickly calculate $L_{max}/L_{min}$ to classify the slab as one-way or two-way, dictating the subsequent design tables and approaches used.
➡️ Reinforcement Placement Visualization: In two-way slabs, understand that reinforcement laid along the $L_{min}$ spans (e.g., $L_y$) runs perpendicular to the longer direction, and the calculated number of bars ($N$) is along the $L_{max}$ direction (e.g., $L_x$).
➡️ Shear Stiffness Modification: Before exporting shear forces from structural analysis software (ETABS), modify the slab stiffness coefficients to 0.25 as per code requirements for accurate shear calculation transfer.

📸 Video summarized with SummaryTube.com on Nov 20, 2025, 11:57 UTC