Calculator
Example Data Table
| Input | Example Value | Unit |
|---|---|---|
| Tributary Width | 8 | ft |
| Tributary Length | 10 | ft |
| Dead Load | 15 | psf |
| Live Load | 40 | psf |
| Snow Load | 20 | psf |
| Post Size | 5.5 x 5.5 | in |
| Post Height | 8 | ft |
| Footing Size | 24 x 24 | in |
Formula Used
Tributary Area: A = Tributary Width × Tributary Length
Uniform Service Load: Wu = A × (Dead Load + Live Load + Snow Load + Roof Live Load)
Total Service Load: Ps = (Wu + Point Load + Beam Reaction + Post Weight) × Contingency Allowance
Factored Load: Pu = [A × (D × Dead Factor + Variable Loads × Variable Factor) + Point Loads] × Allowance
Post Area: Ag = Width × Depth
Axial Stress: fa = Ps ÷ Ag
Euler Buckling: Pcr = π²EI ÷ (KL)²
Bending Stress: fb = (Ps × Eccentricity) ÷ Section Modulus
Interaction Ratio: fa ÷ Fc + fb ÷ Fb
Soil Pressure: q = Ps ÷ Footing Area
How to Use This Calculator
- Enter the tributary width and length supported by one post.
- Add dead, live, snow, roof live, point, and beam loads.
- Enter post size, height, material values, and end condition factor.
- Add footing dimensions and allowable soil bearing pressure.
- Press the calculate button to see service and factored results.
- Download the result as a CSV or PDF file when needed.
Understanding Post Loads
A post carries weight from beams, floors, roofs, decks, and equipment. The load first reaches joists or rafters. It then moves into beams. Finally, it travels down the post into the footing. This calculator follows that path. It uses tributary area, surface loads, point reactions, and post properties.
Why Tributary Area Matters
Tributary area is the floor or roof area supported by one post. A larger area creates a larger axial load. The tool multiplies tributary width by tributary length. It then applies dead, live, snow, and roof live loads. Extra point loads and beam reactions can be added separately.
Service and Factored Loads
Service load is the expected working load. It helps with stress and bearing checks. Factored load is higher. It supports strength design and code style review. The calculator lets you set dead load and variable load factors. You can also add a contingency percentage. This helps early estimates include uncertainty.
Capacity Checks
A post can fail by crushing, buckling, bending from eccentric load, or poor bearing. The calculator checks compression stress against the allowable compression value. It also estimates Euler buckling strength using post height, end condition, elastic modulus, and weak axis inertia. If load is off center, bending stress is added through a simple interaction check.
Footing Bearing
The footing spreads the post load into soil. The calculator divides service load by footing area. It compares that pressure with allowable soil bearing. A safe post still needs a suitable footing. Weak soil can control the design.
Reading the Result
Review the governing ratio first. A ratio below one suggests reserve capacity. A ratio above one means the post, footing, or assumptions need revision. Try changing tributary area, post size, bracing length, or load values. Each change shows how sensitive the design can be.
Practical Use
Use this calculator during planning, estimating, and concept review. It is not a replacement for local codes or engineered design. Real projects may need load duration factors, species factors, connection checks, lateral bracing, seismic loads, wind loads, and inspection. Still, the tool gives a first look. It shows where demand comes from. It also shows which limit controls. For final construction, ask a professional to verify assumptions.
FAQs
What is tributary area?
Tributary area is the portion of floor, roof, or deck area supported by one post. It is usually found from beam spacing, joist span, and post spacing.
What is service load?
Service load is the expected working load before strength factors. It is useful for checking stress, soil pressure, deflection, and general loading behavior.
What is factored load?
Factored load increases service loads with design factors. It helps compare demand against a strength design capacity or conservative review value.
Why does eccentricity matter?
Eccentricity means the load is not centered on the post. This creates bending stress. A small offset can increase the interaction ratio.
What does the interaction ratio show?
The interaction ratio combines compression and bending demand. A value below one suggests the entered post values pass that simplified check.
Does this calculator check connections?
No. It checks post load, stress, buckling, and footing bearing. Bolts, brackets, anchors, and uplift connections need separate review.
Can I use this for deck posts?
Yes, for early estimates. Enter the deck tributary area, surface loads, post size, height, and footing information carefully.
Is this a final engineering design?
No. It is a planning tool. Local codes, soil conditions, wind, seismic loads, and professional review may still be required.