Calculator Inputs
Fields change units after recalculation.Formula Used
Dead-load strip moment: MD = wDL² / 8
Wheel-load strip moment: ML = PstripL / 4
Factored moment: Mu = (γDMD + γLML + γCMC + γSMS) × modifiers
Flexural resistance: φMn = φAsfy(d − a/2)
a = Asfy / (0.85f′cb)
Concrete shear resistance: φVc = φ × 0.17√f′cbd
Uniform-load deflection: δw = 5wL⁴ / 384EI
Center point-load deflection: δP = PL³ / 48EI
Bearing pressure: q = R / support width
Support, skew, void, settlement, and analysis modifiers are screening approximations. Replace them with governing agency procedures, refined analysis, and verified project criteria before final design.
How to Use This Calculator
- Select the unit system before entering project values.
- Choose calculate, check, or optimized thickness mode.
- Enter the clear span, slab dimensions, support width, and skew.
- Select the structural support and simplified analysis method.
- Enter traffic, axle, lane, barrier, overlay, and construction loads.
- Define concrete strength, reinforcement grade, cover, and spacing limits.
- Add settlement, possible void length, and subgrade stiffness information.
- Review resistance factors, load factors, deflection, crack, and fatigue limits.
- Calculate, then inspect every demand-to-capacity ratio.
- Export the summary and send it for professional project review.
Approach Slab Components
The approach slab bridges the transition between the bridge abutment and roadway embankment. Its performance depends on structural capacity, support continuity, backfill behavior, drainage, joints, and settlement control.
Worked Example
| Example input | Value |
|---|---|
| Clear span | 6.0 m |
| Slab width | 11.0 m |
| Concrete strength | 35 MPa |
| Steel yield strength | 420 MPa |
| Wheel load | 90 kN |
| Dynamic allowance | 33 percent |
| Differential settlement | 15 mm |
| Possible void length | 0.75 m |
The calculator converts surface loads into a one-meter design strip. It distributes the wheel load across an estimated effective width. It then checks flexure, shear, punching, deflection, cracking, fatigue, bearing, cover, and reinforcement.
The final thickness is the smallest searched value passing every enabled check. Optimization mode rounds that value to a practical increment. Final drawings still require code-specific detailing, joints, drainage, and geotechnical coordination.
Common Design Mistakes
- Using total slab length instead of the effective structural span.
- Ignoring dynamic impact, future overlays, barriers, or construction equipment.
- Assuming continuous soil support after backfill settlement occurs.
- Checking flexure while overlooking one-way or punching shear.
- Using gross-section deflection without cracking or support loss allowances.
- Providing adequate steel area but excessive bar spacing.
- Applying unsuitable load factors from another design standard.
- Ignoring skew, joints, drainage, corrosion exposure, and development length.
- Treating preliminary software output as sealed engineering design.
Frequently Asked Questions
What thickness does the calculator recommend?
It searches for the smallest thickness passing the enabled preliminary checks. The result also respects the user-entered minimum thickness.
Does this calculator follow one specific bridge code?
No. It uses editable factors and simplified mechanics. Final calculations must follow the governing transportation agency and adopted design standard.
Why can settlement increase required thickness?
Settlement can reduce support and create a partial void. The slab may then span farther and experience larger bending, shear, and deflection.
What does the load distribution width represent?
It estimates how traffic load spreads across the slab width. A narrower strip generally produces a more conservative wheel effect.
What is the governing demand-to-capacity ratio?
It is the largest ratio among enabled checks. Values above one indicate that the trial design does not meet the selected limit.
Can the calculator design reinforcement automatically?
Yes. It selects from common metric diameters and spacings. Engineers should replace those suggestions with locally available bars and approved details.
Why is punching shear checked?
Concentrated wheel loads can create localized two-way shear around the tire patch. This simplified check helps flag potentially thin slabs.
Does the material estimate include all reinforcing steel?
No. It approximates primary mats and distribution steel. It excludes laps, dowels, joint steel, barriers, haunches, and detailed waste.
Can this output be used for construction?
No. Construction documents require verified loads, code checks, geotechnical recommendations, reinforcement detailing, durability provisions, joint design, and professional approval.
Engineering Notice
This tool provides preliminary estimates only. A qualified bridge, structural, and geotechnical engineering team must verify the structural model, loading, combinations, support behavior, reinforcement, durability, drainage, joints, settlement provisions, constructability, and governing code requirements.