Advanced Bolt Group Inputs
Choose a generated layout or paste coordinates. Positive x is right. Positive y is upward.
Formula Used
The calculator first locates the bolt group centroid. Each bolt coordinate is shifted to that centroid. Direct shear is shared equally by all bolts.
Vx,direct = Fx / n and Vy,direct = Fy / n
The eccentric torsional moment is:
Mz,total = Mz + ex × Fy - ey × Fx
The polar bolt group property is:
J = Σ(xi² + yi²)
Moment shear at a bolt is:
Vx,m = -Mz,total × yi / J and Vy,m = Mz,total × xi / J
Optional axial and bending tension is estimated with:
Ti = Fz / n + Mx × yi / Σyi² + My × xi / Σxi²
Final shear is the vector sum. The combined demand is √(V² + T²). The interaction ratio uses the entered allowable shear and tension values.
How to Use This Calculator
- Select a rectangular, circular, or custom bolt pattern.
- Choose the length and force units for the entire form.
- Enter the bolt spacing, bolt circle, or coordinate list.
- Add applied shear components and load eccentricity.
- Add torsion, axial load, or bending moments when required.
- Enter allowable bolt values for a quick utilization check.
- Press the calculate button. Review the critical bolt first.
- Use the table to compare every bolt in the group.
Example Data Table
| Input | Example Value | Meaning |
|---|---|---|
| Pattern | 2 rows × 3 bolts | Six bolt rectangular group |
| Spacing x | 100 mm | Horizontal bolt spacing |
| Spacing y | 80 mm | Vertical bolt spacing |
| Fx, Fy | 12 kN, 8 kN | Direct shear components |
| Offset x, y | 75 mm, 40 mm | Load point from origin |
| Allowables | 35 kN shear, 45 kN tension | Per bolt comparison values |
Bolt Group Force Distribution Guide
Why Bolt Patterns Need Force Checks
A bolt group rarely carries load evenly when force acts away from the centroid. Direct shear may look simple. Eccentricity adds rotation. That rotation makes far bolts work harder. A safe connection check must find the critical bolt, not just the average load. This is why coordinate based force distribution is useful for brackets, gusset plates, machine bases, lifting lugs, and small structural details.
Centroid Based Calculation
This tool treats the bolt pattern as a group of discrete fasteners. It builds the coordinates, finds the centroid, and measures each bolt from that center. This is important because torsional distribution depends on distance from the centroid. The larger the radius, the larger the moment shear contribution. A symmetric group usually has a centroid at the visual center. A custom pattern may not.
Direct Shear and Eccentric Moment
Direct shear is divided by the number of bolts. That part is equal for every bolt. Moment shear is different. It is proportional to each bolt coordinate and inversely proportional to the polar group property. A bolt far from the centroid may receive much more load than one near the middle. The final shear vector combines both effects.
Advanced Loading Options
The calculator also accepts extra torsional moment. This helps when a bracket, plate, or frame already has a known moment. Axial force can be added too. Optional moments about the x and y axes estimate tension from bending. Compression is not counted as positive tension demand. This keeps the demand table focused on bolt tension that may open the joint.
Reading the Result Table
Each row shows the shifted coordinate, shear components, total shear, positive tension, combined demand, and interaction ratio. The largest combined demand identifies the critical bolt. The highest interaction ratio checks the entered allowable limits. These two values may occur at the same bolt, but not always when tension and shear limits differ.
Use the CSV export to save a record of the load case. It helps compare alternate bolt spacings, larger fasteners, or changed eccentricity values during early design reviews. Keep notes with each saved result for clearer decisions later too.
Using Allowable Values
Allowable shear and allowable tension are entered per bolt. The tool reports a simple interaction ratio. A value below one suggests the entered allowable limits are not exceeded. A value above one warns that the selected bolt, spacing, material, or load case may need review. You can change the design load factor to study factored or service loads.
Design Judgment
This calculator is useful for preliminary physics and engineering checks. Real connections may include plate bending, slip, prying action, weld effects, hole clearance, fatigue, and code rules. Use the result as a focused calculation aid. Confirm final designs with the required standard and professional review. Check assumptions before fabrication, testing, procurement, or installation work.
FAQs
What is bolt force distribution?
It is the way applied load spreads through bolts in a group. Direct shear is often shared evenly. Eccentric moment creates extra shear based on bolt location.
Why does the critical bolt matter?
The critical bolt has the highest calculated demand. It controls the quick safety check because one overloaded bolt can govern the whole connection.
Can I use custom bolt coordinates?
Yes. Select custom coordinates and enter one x,y pair per line. The calculator shifts those points to the centroid before distributing the forces.
What units should I use?
Select one force unit and one length unit. Use those same units for every load, spacing, coordinate, and moment entry in the form.
What does eccentricity mean?
Eccentricity is the offset between the load line and the bolt group centroid. It creates torsion, which increases force in some bolts.
Does the tool include axial tension?
Yes. Enter Fz for direct axial load. You may also enter Mx and My to estimate tension caused by bending about the group axes.
What is the polar group property?
It is the sum of each bolt distance squared from the centroid. It controls how torsional shear is distributed across the group.
What does interaction ratio show?
It compares calculated shear and tension against allowable values. A lower value is better. A value above one indicates a failed check.
Can this replace a design code?
No. It supports calculation checks only. Final structural design should follow the governing code, material standard, and project requirements.
Why are some bolts in compression?
Bending can place one side in tension and the opposite side in compression. The table reports positive tension for demand checks.
How should I check multiple load cases?
Run each load case separately. Save the CSV for every case. Compare the highest interaction ratios and critical bolt locations.