Enter Slab and Load Details
Use the stated units. The chosen moment case provides a transparent preliminary bending estimate.
Example Data Table
| Input | Example value | Purpose |
|---|---|---|
| Service point load | 50 kN | Applied concentrated load before factoring. |
| Load factor | 1.50 | Converts service load to the screen’s design load. |
| Loaded footprint | 200 × 200 mm | Defines contact area and local shear geometry. |
| Clear span | 5.00 m | Sets the simplified flexural demand. |
| Effective depth | 160 mm | Supports strip spreading and punching calculations. |
| Concrete strength | 30 MPa | Used in the simplified punching resistance screen. |
Formula Used
These equations provide a preliminary screen. They do not replace a local design standard or detailed finite-element analysis.
Ac = a × b / 1,000,000
q = Pu / Ac
beff = min(B, a + 2d tan θ)
Mu = C × Pu × L
Mu,m = Mu × 1000 / beff
b0 = 2[(a + d) + (b + d)]
vu = 1000Pu / (b0d)
φVc = 0.75 × 0.33√f′c × b0d
As = Mu,m × 106 / (0.90fy × 0.90d)
Symbols: P is service load, LF is load factor, a and b are footprint dimensions, B is panel width, d is effective depth, C is the moment coefficient, and f′c is concrete strength.
How to Use This Calculator
- Enter the service point load and the chosen load factor.
- Enter slab span and full panel width in metres.
- Enter the actual contact footprint in millimetres.
- Add effective depth, total depth, concrete strength, and steel strength.
- Select the simplified moment case nearest to the load location.
- Enter steel provided per metre in the governing direction.
- Review contact pressure, bending demand, punching stress, and screen results.
- Export the results only after checking the entered data.
Point Loads on Concrete Slabs
Point loads act over small slab areas. They can create high local stresses. A wheel, machine foot, rack leg, column base, or stored item may behave this way. The slab must transfer the force safely into supports and reinforcement.
The first concern is contact pressure. A smaller footprint increases pressure. The calculator divides the factored point load by the loaded area. This gives a useful pressure estimate. It helps compare pads, plates, wheels, and bearing strips.
Flexure is the next concern. A point load causes bending across the slab span. The result depends on load location and support behavior. A midspan load creates simple-span bending demand. Interior continuity can reduce positive bending. It may increase negative support moments. Review the support system before accepting this estimate.
Slabs distribute force in two directions. This tool uses a simplified effective strip width. It starts from the contact width. The load then spreads through the effective depth at the selected angle. The width cannot exceed the panel width. Real distribution depends on reinforcement, cracking, stiffness, joints, and edge distance.
Punching shear is critical near concentrated loads. It matters around bearing plates and column-like loads. The calculator checks stress around a perimeter outside the loaded face. It compares demand with a simplified concrete resistance. It only flags whether deeper technical review is needed.
Required steel is estimated per metre of slab width. The calculation uses the factored moment, steel yield strength, effective depth, and assumed lever arm. It also compares this with basic minimum steel. Enter proposed reinforcement to see approximate flexural capacity. The result guides detailing discussions, not final design.
Use consistent units. Enter load in kilonewtons. Enter dimensions in millimetres or metres as labelled. Confirm the footprint represents actual contact. Include any base plate or spreading pad. Select the moment case nearest the support arrangement. Use suitable load factors.
Construction conditions matter. Openings, saw cuts, sleeves, weak subgrade, and nearby edges reduce performance. Heavy equipment may add impact and repeated loading. Check settlement when slabs rest on ground. Check vibration for sensitive machinery.
This calculator is suitable for early planning, comparison, and education. It is not a final structural design. Use governing local codes, material properties, load combinations, and engineer review before construction. Always use qualified engineers for final slab design decisions.
Frequently Asked Questions
1. What is a point load on a slab?
A point load is a concentrated force applied over a small footprint. Examples include machinery feet, rack legs, wheels, posts, and base plates. The smaller the bearing area, the greater the local pressure and punching concern.
2. Does this calculator design a complete slab?
No. It provides a preliminary screen for contact pressure, simple bending demand, punching shear, and reinforcement demand. Complete design also requires code load combinations, support conditions, crack control, deflection, detailing, joints, and local engineer review.
3. Why does footprint size matter?
Footprint size controls contact area. A smaller footprint creates higher bearing pressure and a tighter punching perimeter. A larger base plate can reduce local stress by distributing the same load over more slab area.
4. What load factor should I enter?
Use the factor from the governing design standard and load combination. The default is only an example. Different dead, live, equipment, impact, seismic, and construction loads may require different factors.
5. Why is the effective strip width limited?
The calculator limits spread width to the full panel width. A slab cannot distribute load beyond its available width. Actual two-way distribution can be affected by stiffness, cracking, reinforcement direction, boundaries, and nearby openings.
6. What is punching shear?
Punching shear is a local failure mode around a concentrated load. A plug-shaped region can push through the slab. It is common near columns, small bearing plates, and heavily loaded support points.
7. Is a green screen result a final approval?
No. A green result only means the entered values satisfy this simplified screen. It does not verify every structural rule. Final approval requires a qualified engineer using the governing code and project information.
8. Can I use this for a slab on ground?
It can support early checks, but slab-on-ground design needs additional work. Subgrade modulus, joint layout, curling, edge distance, dowels, repeated traffic, and soil settlement can strongly affect performance.
9. What reinforcement value should I enter?
Enter the area of reinforcement provided per metre in the direction being checked. Calculate it from bar area, bar spacing, and effective width. Confirm bar placement, cover, development, and spacing separately.
10. Can a base plate improve the result?
Often, yes. A suitably designed base plate or spreading pad increases contact area. This can reduce contact pressure and improve local behavior. Its thickness, stiffness, anchorage, and bearing must also be checked.
11. Which checks need special attention?
Pay close attention to loads near edges, openings, joints, and supports. Also review repeated loading, dynamic effects, thin slabs, low concrete strength, short anchorage, and large cracks. These conditions can invalidate simple assumptions.