Inputs
Example Data Table
| alpha | L | Tmin | Tmax | Tinstall | Shrink | Settlement | Seismic | Live | Capacity | SF |
|---|---|---|---|---|---|---|---|---|---|---|
| 12 µm/m/°C | 30 m | 5 °C | 45 °C | 25 °C | 4 mm | 2 mm | 6 mm (p-p) | 2 mm (p-p) | ±25% | 1.10 |
Formula Used
- Thermal movement (mm): ΔL = α × L × ΔT, with L in mm and α in 1/°C.
- Thermal opening: (Tinstall → Tmin) = α × L × (Tinstall − Tmin).
- Thermal closing: (Tinstall → Tmax) = α × L × (Tmax − Tinstall).
- Bidirectional allowances: Peak-to-peak / 2 applied to both opening and closing.
- Design movements: (opening and closing) × safety factor.
- Required nominal width (mm): W = max(open, close) ÷ (capacity% ÷ 100).
- Install gap (mm): G = W + (shrink + settlement) − α × L × (Tinstall − Tmid).
How to Use This Calculator
- Enter the movement length that contributes to the joint.
- Set Tmin, Tmax, and an estimated installation temperature.
- Add allowances for shrinkage, settlement, seismic drift, and live-load movement.
- Choose a movement capacity based on your selected joint system.
- Apply a safety factor to reflect uncertainty and tolerances.
- Click Calculate Movement to get the nominal width and install gap.
- Download CSV or PDF for submittals and field coordination.
Why expansion joint movement matters
Expansion joints protect slabs, façades, and waterproofing by providing a planned location for movement. Unchecked movement can transfer stress into finishes, causing sealant tearing, spalling at arrises, and noisy slip-stick behavior. Typical temperature-driven movement increases with both span length and seasonal range, so documenting the movement length and realistic Tmin/Tmax is as important as selecting the joint product.
Thermal movement inputs you should document
The calculator uses ΔL = α × L × ΔT, where α is the coefficient of thermal expansion and L is the contributing length. For reinforced concrete, α commonly falls around 10–12 µm/m/°C; for steel, about 12 µm/m/°C. Use site-specific temperatures when possible, especially for exposed roofs and podiums that can run hotter than ambient. Installation temperature affects the split between opening and closing movements. For façade joints, consider differential temperatures across materials and sun exposure; shaded elevations often move less, while dark cladding can amplify surface temperature by 10–20 °C.
Accounting for seismic and live-load drift
Seismic and live-load movements are often provided as peak-to-peak drift. This tool conservatively splits those values into ± half per direction, then combines them with thermal opening and closing. If structural drift is directional or tied to a specific limit state, enter the governing peak-to-peak value and apply an appropriate safety factor. Keep shrinkage and settlement as closing-only allowances.
Interpreting nominal width and installation gap
Joint systems are typically rated as a percentage of nominal width (for example, ±25%). The required nominal width is sized so the larger of design opening or design closing stays within that rating. The recommended installation gap then adjusts for installation temperature and adds one-way closing allowances. Always compare the output to manufacturer limits for compression, extension, and debris tolerance.
Quality checks and specification notes
Validate inputs by checking the thermal movement alone: a 30 m segment with α = 12 µm/m/°C and a 40 °C swing yields about 14.4 mm of total expansion or contraction. If your result is an order of magnitude higher, re-check length units and temperature range. For specifications, record assumed Tmin/Tmax, capacity rating, safety factor, and the installation gap target used for field setting.
FAQs
1) What movement length should I use?
Use the effective length of the structural bay contributing to the joint, typically between restraint points. For roofs and slabs, that may be the full panel length; for façades, consider the supporting frame span.
2) Why are seismic and live-load inputs peak-to-peak?
Many structural drift outputs are reported peak-to-peak. The calculator converts peak-to-peak to one-direction movement by dividing by two, then applies that value to both opening and closing so the joint is sized for either direction.
3) Should shrinkage and settlement always be closing-only?
In most building joints they reduce the gap over time, so treating them as closing-only is conservative. If your detail experiences net opening from other effects, document that separately and adjust allowances accordingly.
4) What capacity percentage should I choose?
Use the movement capability published for the joint system you intend to specify, such as ±25%, ±50%, or another value. When uncertain, choose the smaller capacity to size conservatively, then confirm availability with the supplier.
5) How do I pick a safety factor?
Safety factors reflect uncertainties in temperatures, tolerances, and drift estimates. Values around 1.05–1.15 are common for well-defined conditions; increase toward 1.25 when inputs are early-stage or variable.
6) Does the installation gap replace manufacturer setting tables?
No. Treat it as a planning target. Always cross-check with the manufacturer’s setting guidance for your joint profile, anticipated surface temperatures, and installation constraints, then coordinate with waterproofing and finish tolerances.