Timber Connector Design Calculator
Bolt count, spacing, and capacity for timber connections using NDS lateral design values.
About this calculator
This calculator sizes a bolted timber connection from a base lateral design value (Z) for the chosen bolt diameter, drawn from the American Wood Council's NDS Table 12.3.1 for single-shear, wood-to-wood bolted connections in Douglas Fir-Larch, then adjusts it down through three real-world factors before dividing your Design Load by the result. Single Bolt Capacity starts from a reference value per bolt diameter (Douglas Fir-Larch baseline), scales by the square root of the species' specific gravity ratio, then by member thickness, then by the load duration factor (1.0 normal up to 1.6 wind/seismic). The member-thickness adjustment uses whichever of Main Member Thickness and Side Member Thickness is THINNER, not the side member alone -- a single-shear bolted connection's capacity is governed by whichever engaged wood member yields first in dowel bearing, so making only the thicker member thicker buys nothing once it already exceeds the thinner one.
Bolts Required then divides Design Load by that adjusted capacity, further reduced by a Group Action Factor (0.92 for up to 4 bolts, down to 0.78 for 8+) that accounts for uneven load-sharing across a bolt row -- which is why the bolt count uses an iterative check rather than one division. Minimum spacing, end-distance, and edge-distance values are all simple multiples of the selected Bolt Diameter (4D, 7D, and 1.5D respectively) and don't depend on load or species at all.
Inputs
Results
Bolts required
7
Single bolt capacity
980 lbs
≈ 6 adults
Figures current as of 2018. Source: American Wood Council, National Design Specification (NDS) for Wood Construction, 2018 Edition, Table 12.3.1 (Bolts) and Section 12.5 (Placement of Fasteners)
How to Use This Calculator
- Enter Design load, Bolt diameter, and Wood species.
- Set Load duration, Main member thickness, and Side member thickness.
- Review Bolts required and Single bolt capacity (lbs).
- Use Total connection capacity (lbs) and Demand/capacity ratio to inform your decision.
How the result changes with Design load
| Design load | Bolts required | Single bolt capacity |
|---|---|---|
| 2,500 | 3 | 980 lbs |
| 3,750 | 5 | 980 lbs |
| 7,500 | 10 | 980 lbs |
| 12,500 | 17 | 980 lbs |
What each input means
- Design load
- Total lateral load the connection must resist.
- Bolt diameter
- 0 = 1/2", 1 = 5/8", 2 = 3/4", 3 = 7/8", 4 = 1".
- Wood species
- 0 = Douglas Fir-Larch, 1 = Southern Pine, 2 = Eastern White Pine, 3 = Red Oak.
- Load duration
- 0 = Normal (Cd=1.0), 1 = Snow (1.15), 2 = 7-day construction (1.25), 3 = Wind/seismic (1.6).
- Main member thickness
- Thickness of the main (thicker) timber member.
- Side member thickness
- Thickness of the side (thinner) member or steel plate.
What each result means
- Bolts required
- Number of bolts needed including group action factor.
- Single bolt capacity
- Adjusted lateral design value per bolt (Z').
- Total connection capacity
- Sum of all bolt capacities with group action factor.
- Demand/capacity ratio
- Design load / connection capacity. Must be ≤ 1.0.
- Group action factor (Cg)
- Reduction factor for multiple bolts in a row.
- Bolt diameter
- Selected bolt diameter.
- Min bolt spacing (in row)
- Minimum spacing between bolts along the grain (4D).
- Min row spacing
- Minimum spacing between rows of bolts (4D).
- Min end distance
- Minimum distance from bolt to loaded end (7D for tension).
- Min edge distance
- Minimum distance from bolt to edge (1.5D loaded edge).
- Min member width
- Minimum timber width for single-row bolt pattern.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersDesign load = 5000, Bolt diameter = 2, Wood species = 0, Load duration = 0 = 6 input(s) provided
- Calculate Bolts requiredBolts required = ceil(designLoadLbs / (allowableZ * Cg))7 = 7
- Calculate Single bolt capacitySingle bolt capacity = adjustedZ * Cd980 = 980
- Calculate Total connection capacityTotal connection capacity = round(boltsNeeded * allowableZ * Cg)5831 = 5831
- Calculate Demand/capacity ratioDemand/capacity ratio = designLoadLbs / connectionCapacity0.857 = 0.857
Figures and sources
- NDS Table 12.3.1 lateral design values (Z) for single-shear bolted wood-to-wood connections, and Section 12.5 minimum spacing/edge/end-distance requirements (2018) — American Wood Council, National Design Specification (NDS) for Wood Construction, 2018 Edition, Table 12.3.1 (Bolts) and Section 12.5 (Placement of Fasteners)
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does a heavier design load change the Single Bolt Capacity?
No. Single Bolt Capacity is a property of the bolt and connection alone -- bolt diameter, wood species, member thickness, and load duration -- and never depends on Design Load. A heavier load only changes Bolts Required (more bolts needed to reach the same total capacity) and Demand/Capacity Ratio; the per-bolt capacity number stays fixed for a given diameter, species, thickness, and duration combination.
Why does increasing Main Member Thickness only sometimes raise the bolt capacity?
Capacity is governed by whichever of Main Member Thickness and Side Member Thickness is thinner, since that is the member that yields first in dowel bearing. Raising Main Member Thickness only helps while it is still thinner than the side member (or steel plate); once it matches or exceeds Side Member Thickness, the side member alone governs and further increases to the main member thickness have no additional effect on Single Bolt Capacity.
What is the Group Action Factor and why does more bolts sometimes need proportionally more than expected?
Group Action Factor (Cg) reduces the effective capacity of a multi-bolt row below the simple sum of individual bolt capacities, because load doesn't split perfectly evenly across every bolt in a row -- it steps down from 0.92 (up to 4 bolts) to 0.85 (5-7 bolts) to 0.78 (8 or more), which is why very large connections need slightly more bolts per unit of capacity than a simple division would suggest.
Do the minimum spacing requirements depend on the design load?
No. Min Bolt Spacing, Min Row Spacing, Min End Distance, and Min Edge Distance are all fixed multiples of the selected Bolt Diameter (4D for row and bolt spacing, 7D for tension-loaded end distance, 1.5D for edge distance), taken directly from Section 12.5 of the American Wood Council's NDS -- they describe wood-splitting geometry limits that apply regardless of how much load the connection actually carries or what species is used.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
Timber Frame Design Calculator
Beam sizing from span, load, and wood species using NDS bending and deflection checks.
Timber FramingTimber Grade Selection Calculator
Compare 9 species/grade combinations for a beam application with bending and deflection checks.
Timber FramingTimber Volume Calculator
Board feet, cubic volume, and weight from timber dimensions and species.
Structural EngineeringTimber Connection Calculator
Design bolted timber connections per NDS: calculate reference design values, group action factor, and connection capacity.
Structural EngineeringConnection Design Calculator
Calculate bolted steel connection capacity: shear, bearing, and overall connection strength for A325 and A490 bolts.
More in Construction & Building Trades.