The Effective Span of Slab Calculator helps structural engineers, architects, and construction professionals calculate the usable or effective length of a slab for accurate structural analysis and design.
When a slab spans between supports, not all of its length is used equally for resisting bending moments. The effective span is slightly longer than the clear span but shorter than the full support-to-support distance. This calculator makes it easier to find the correct value used in bending moment and shear force calculations.
This tool is part of the civil engineering and structural design calculator category, and is essential for those designing one-way or two-way simply supported slabs.
formula of Effective Span of Slab Calculator
Formula for Simply Supported Slabs (One-Way or Two-Way):
Where:
- L_eff = Effective Span of the slab (units: m, mm, or ft)
- L_n = Clear span, the distance between the inner faces of supports (units: m, mm, or ft)
- d = Effective depth of the slab, calculated as:
d = Total depth of slab (h) - Clear cover - (Diameter of main reinforcement / 2) - h = Total thickness of the slab (units: m, mm, or ft)
- Clear cover = Distance from the surface of concrete to the nearest rebar (typically 20-25 mm or 0.75-1 inch)
- Diameter of main reinforcement = Size of primary steel bars used in the slab (units: mm or inches)
- L_c = Center-to-center distance between supports (units: m, mm, or ft)
Key Design Rule:
Use the minimum value between L_n + d and L_c as the effective span (L_eff) for design purposes.
General Terms Table for Quick Reference
| Term | Description | Use in Design |
|---|---|---|
| L_eff | Effective span of the slab | Used for bending moment calculations |
| L_n | Clear span between support faces | Base dimension before adjustment |
| L_c | Center-to-center support spacing | Upper limit for effective span |
| d | Effective depth (h - cover - bar radius) | Important for moment resistance |
| Clear cover | Distance from surface to steel reinforcement | Protects steel from corrosion |
| Main reinforcement dia | Diameter of main bars | Affects slab strength and d calculation |
| h | Overall thickness of the slab | Needed for determining depth |
Example of Effective Span of Slab Calculator
Let’s calculate the effective span for a concrete slab with the following values:
- L_n = 3.5 meters
- h = 150 mm
- Clear cover = 25 mm
- Main reinforcement bar diameter = 12 mm
- L_c = 3.6 meters
Step 1: Calculate effective depth (d)
d = h - clear cover - (bar diameter / 2)
d = 150 - 25 - (12 / 2) = 150 - 25 - 6 = 119 mm = 0.119 m
Step 2: Calculate L_n + d
L_n + d = 3.5 + 0.119 = 3.619 m
Step 3: Compare with L_c
Minimum of 3.619 m and 3.6 m = 3.6 m
Answer: The effective span (L_eff) is 3.6 meters
Most Common FAQs
Effective span is used for structural design to determine accurate bending moments and shear forces. It improves safety and cost-efficiency in slab construction.
Effective depth is the distance from the top of the slab to the center of the main reinforcement bars. It is less than the total thickness of the slab.
Use it when the center-to-center distance between supports is less than the clear span plus effective depth. Always pick the smaller value between the two.