Centering is the temporary timber frame that carries every arch stone until the keystone is set, so it must be built to the full weight of the ring, on supports that cannot settle: a 6 m span arch 3 m wide has a ring of about 31 tonnes, and the soil under each post decides how many posts and how big a pad you need. Wait at least 1-2 weeks after the keystone when mortar is used, then lower the frame evenly rather than knocking it out. Most arch collapses during construction come from a centering that settled, twisted or was struck too early.
Part of Bridge Types & Construction
The temporary timber frame, curved to the intrados, that holds arch stones in position until the ring is closed and cured.
The Centering Must Be Strong And Stay Exactly Where You Put It
An arch cannot stand until the keystone is in. Until then each stone is held only by the frame under it. The parent states the requirements: build a semicircular wooden frame to the planned arch shape, support it on posts from the streambed in the dry season or from temporary piers, make it strong enough for the weight of all the arch stones before the keystone is placed, and after the keystone wait 1-2 weeks if using mortar before removing it.
Two failures matter. If the frame settles even a little while stones are being laid, the arch is built to a wrong shape and the error is locked in. If it fails outright, the whole half-built arch comes down on the people working on and under it. The sibling article Centering describes the rib construction, the lagging and the ±5 mm tolerance on the curve. This page covers the loads and the supports.
What You Need
- Ribs — curved timber members cut to the arch curve.
- Lagging — planks or poles laid across the ribs to carry the stones.
- Posts and sill beams — to carry the ribs down to the ground.
- Pads — wide timber or flat stones under every post.
- Wedges or sand boxes — to lower the frame evenly at the end.
- Diagonal bracing — so the frame cannot twist.
Method: Build And Strike A Centering
Step 1: Weigh the ring Compute the weight of all the arch stones, using the volume and a stone density from Common Material Weights. This is the load the centering must carry.
Step 2: Size the supports to the ground Divide the load by the number of posts, then divide each post load by the soil’s allowable bearing pressure to get the pad area. See Soil Bearing Capacity.
Step 3: Build the frame to the curve Cut the ribs to the curve, brace them in the horizontal plane, and check the curve with a string compass.
Step 4: Lay stones evenly from both sides Unbalanced loading, with stones placed on one side only, can twist or lift the frame. Raise both sides at the same rate.
Step 5: Strike slowly After the keystone and the cure period, lower the frame evenly with wedges or sand boxes, working from the crown toward the springings, and watch the underside for cracks.
Worked Example: Loads And Pads For A 6 m Arch
Inputs, mine: the 6 m span, 0.4 m deep ring, 3 m wide deck from Stone Arch Bridges; stone at 2,600 kg/m³, inside the granite and limestone ranges in Material Properties. Four ribs, three posts under each rib, so 12 posts. Allowable bearing pressures from Foundation Design: soft clay 500-1,000 lb per sq ft and sand 2,000-4,000 lb per sq ft, which at 0.0479 kPa per lb/ft² is 24-48 kPa and 96-192 kPa.
- Ring volume: π / 2 × (3.4² − 3²) × 3 = 12.1 m³.
- Mass: 12.1 × 2,600 = 31,400 kg, about 31 tonnes. Force: 31,400 × 9.81 = 308 kN. This is an upper bound, because stones near the springings partly stand on their own beds.
- Load per post if shared equally: 308 / 12 = 25.7 kN. The real shares are not equal, because a half-built arch loads the haunches before the crown. Allow a factor of 2 for that: about 51 kN on the worst post.
- Pad area on soft clay at the lower bound 24 kPa: 51 / 24 = 2.1 m², a pad 1.45 m on a side. At the upper bound 48 kPa: 51 / 48 = 1.1 m², a pad 1.0 m on a side.
- Pad area on sand at 96 kPa: 51 / 96 = 0.53 m², a pad 0.73 m on a side.
- A 15 cm square post 3 m tall, pinned at both ends, buckles at P(cr) = π² E I / L² = 9.87 × 9 × 10⁹ × 4.22 × 10⁻⁵ / 3² = 417 kN (E = 9 GPa recalled from the US Forest Products Laboratory Wood Handbook and not checked here). The post is not the weak point. The ground under it is.
Result: a post that would be safe on timber alone sinks if its pad is sized like a fence post. The pad area, not the post, is where the centering fails.
Common Mistakes
- Small pads on soft bed — a 0.25 m² pad on soft clay at 51 kN is 204 kPa, four or more times the clay’s allowable pressure. It sinks.
- Striking by the calendar — the parent says 1-2 weeks with mortar, and the sibling Centering gives 14-21 days for lime mortar. The two disagree; use the longer figure for lime mortar until the conflict is settled.
- Knocking the frame out — it drops a stress shock on the ring. Lower it with wedges or sand.
- No horizontal bracing — the frame twists under one-sided loading.
- Building in a flood season — a rising stream undermines posts in the bed. Work in the dry season.
Safety
Falsework collapse
If a partly built arch comes down, it falls on whoever is under or beside it, and each stone weighs hundreds of kilograms. Keep the area clear while stones are set and during striking. Never stand under the arch or the frame when wedges are being struck. Stop at once if a post settles or a rib bows.
What’s Next
- Learn next: Arch Principles — why a closed ring stands without support
- Combine with: Centering — rib construction and lagging