Understanding Load Transfer from Slab to Beams

Load transfer from a slab to beams is influenced by the geometric dimensions of the slab and the orientation of reinforcements. Various loads, such as self-weight, live load, and imposed dead load, are distributed across beams along their sides.

Load Expressions

Slab loads are measured in weight per unit area, while beam loads are expressed in weight per length of the beam. Regularly shaped slabs facilitate straightforward load transfer, but irregular shapes may require specialized software like SAP2000, SAFE, and ETABS.

One-way Slab

A one-way slab, typically rectangular, distributes its load equally between adjacent beams. The interior beam bears half of the total slab load on each side.

Load Distribution in One-way Slab

If a slab is supported on two sides or all four sides with a longer side-to-shorter side ratio greater than 2, it is categorized as a one-way slab.

Two-way Slab

In a two-way slab, loads are transferred to all beams on all sides. The slab is often divided into trapezoidal and triangular areas.

Load Distribution in Two-way Slab

Beam load is calculated by multiplying the segment area by the slab’s unit load divided by the beam length. Interior beams take loads from both sides.

Load Transfer Example

Consider a slab with a thickness of 150 mm supporting its weight, a 0.85 KN/m² partition, and a live load of 2.4 KN/m². The goal is to transfer the load to the beams on all four sides.

Solution Steps

  1. Calculate self-weight and total dead load on the slab.
  2. Distribute service load or ultimate distributed load to the beams based on load factors and ACI 318-19 specifications.
  3. Calculate the ultimate distributed load on the slab and transfer it to the beams.

Numerical Solution

  • Ultimate distributed load (Wu) = 1.2 * dead load + 1.6 * live load
  • Wu = 1.2 * 4.45 + 1.6 * 2.4 = 8.7 KN/m²
  • Load on beam (4 m) = area of triangle * Wu = 4 * 8.7 = 34.8 KN
  • Uniform distributed load on beam (4 m) = 34.8 / 4 = 8.7 KN/m
  • Load on beam (6 m) = area of trapezoid * Wu = 8 * 8.7 = 69.6 KN
  • Uniform distributed load on beam (6 m) = 69.6 / 6 = 11.6 KN/m

Complex Geometry Slab

For slabs with complex geometry, finite element modeling using software like SAP2000, SAFE, and ETABS is recommended for effective load distribution. This approach can also be applied to slabs with regular geometry.

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