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Clapper Bridge

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Stone Arch Bridges
Permanent bridges that last centuries
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Clapper Bridge
Flat stone slabs on stone piers — simplest stone bridge

A clapper bridge is the simplest stone bridge: flat slabs laid across stone piers set 1.5-2 m apart, with no arch and no mortar, and it suits small shallow streams. Each slab must overhang its pier by at least 20 cm at each end, and the piers in the stream restrict the flow, so they invite scour and flooding. A slab fails by cracking across its middle from a hidden flaw, so choose slabs without cracks and test each before use.

Part of Bridge Types & Construction

A beam bridge of flat stone slabs resting on stone piers, used for short spans over small streams.

A Slab Is A Beam That Cannot Bend

A stone slab on two piers works like a log on two banks: the load bends it, the underside is stretched and the top is squeezed. Stone resists squeezing well and stretching poorly, so the underside decides. The parent article gives the construction and limits: stack flat rocks to make piers 1.5-2 m apart, which it gives as the span limit of most natural slabs, and lay large flat stones across them as the deck.

The weight of the slabs is a bigger practical problem than the loads they carry. A slab long enough to span 2 m and thick enough to be safe weighs about a tonne, and moving it needs levers, rollers and enough people to control it.

What You Need

  • Flat stone slabs — as long as the span plus 40 cm, sound, with no visible cracks.
  • Stone for piers — flat rocks that stack without rocking.
  • Levers, rollers and rope — to move slabs.
  • Extra stone — to weight the slab ends.

Method: Build The Piers And Lay The Deck

Step 1: Build the piers Stack flat rocks in the streambed to make piers. They must not rock or settle. Space them 1.5-2 m apart, measured between the faces.

Step 2: Select the slabs Tap each slab. A clear ringing sound means it is sound; a dull note means a crack. Reject cracked slabs.

Step 3: Lay the slab with its bedding horizontal Lay each slab so that its layers lie flat, as they formed. Stratified stone is weaker across the layers.

Step 4: Check the bearing Each slab should overhang its pier by at least 20 cm at each end.

Step 5: Weight the ends Stack stones on the slab ends over the piers, as the parent says, to stop the slab shifting.

Worked Example: A 2 m Slab

Inputs, mine: clear span 2.0 m; slab 2.4 m long (2.0 m plus 20 cm at each end) × 0.8 m wide × 0.2 m thick. Two stones from the table in Material Properties: granite at 165-175 lb/ft³ (about 2,640-2,800 kg/m³; use 2,650) with tensile strength 1,000-3,000 PSI (about 6.9-20.7 MPa), and sandstone at 120-145 lb/ft³ (about 1,920-2,320 kg/m³; use 2,100) with tensile strength 200-1,000 PSI (about 1.4-6.9 MPa). The conversion is 1 PSI = 6.895 kPa. A walker of 100 kg with a dynamic factor of 2 gives 100 × 9.81 × 2 = 1,962 N at midspan.

Granite.

  • Mass: 2.4 × 0.8 × 0.2 × 2,650 = 1,018 kg, about 1 tonne.
  • Self-weight on the span: 0.8 × 0.2 × 2,650 × 9.81 = 4.16 kN/m. M(self) = w L² / 8 = 4.16 × 2² / 8 = 2.08 kN·m.
  • M(person) = P L / 4 = 1.962 × 2 / 4 = 0.98 kN·m. Total 3.06 kN·m.
  • Section modulus S = b h² / 6 = 0.8 × 0.2² / 6 = 5.33 × 10⁻³ m³.
  • Stress = 3,060 / 5.33 × 10⁻³ = 0.57 MPa. Ratio to tensile strength: 6.9 / 0.57 = 12 at the low end, 20.7 / 0.57 = 36 at the high end.

Sandstone.

  • Self-weight: 0.8 × 0.2 × 2,100 × 9.81 = 3.30 kN/m. M(self) = 3.30 × 4 / 8 = 1.65 kN·m. Total with the person 2.63 kN·m; stress 0.49 MPa.
  • Ratio: 1.4 / 0.49 = 2.9 at the low end; 6.9 / 0.49 = 14 at the high end.

Result: a granite slab of this size has plenty of margin even at its weakest value. A weak sandstone slab is 2.9 times below its low-end tensile strength, which is under the parent’s safety factor of 3 for stone, and a hidden crack makes it worse. Thickness helps: stress on a slab falls roughly as the thickness squared, but weight rises with thickness.

Common Mistakes

  • Using slabs as found — a crack across the underside is invisible from above. Tap-test them.
  • Laying bedding vertically — the layers split under bending.
  • Short bearing — less than 20 cm lets the slab slide off.
  • Too many piers in the stream — each pier restricts flow and raises scour, as the parent warns. See Scour Protection.

Safety

Heavy slabs and sudden cracking

A slab of about 1 tonne can crush a foot or a hand in an instant, and a cracked slab breaks without warning under a walker. Move slabs with levers and rollers, never by hand lifting, and keep everyone clear of the span while it is laid. Do not allow a crowd on the bridge until the slabs have carried a test load.

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