The abutments or end seats keep the beam ends dry, and provide secure anchorage to prevent their movement in use and during flood. The diagram here shows how to calculate the correct distance of the end seat from the bank.
The end dam keeps surfacing and downslope material off the beam ends.
Various types of abutments and end seats are shown below.
Railway sleepers
In free draining ground, these can simply be embedded in a trench and securely backfilled. In heavy clay soils, excavate a drain from the trench to the ditch, so that the trench does not act as sump. Put the half sleeper in place, and backfill with free draining material.
In places where flooding may occur, the sleepers must be secured to the ground by stakes. Always bed the sleepers on a foundation of stony material to prevent uneven settlement.
Stone and mortar
Stone abutments should be mortared to give resistance against water erosion. Use the largest stones for foundations and corners, and fill the centre with rubble. Finish with a capping of mortar and set rag bolts in position for main beam attachment.
Cast concrete
A simple end seat can be made by excavating a trench and filling it with 1:4 concrete.
Normally a raised end seat will be required to give sufficient clearance for the beams. This should sit on a firm foundation of bedrock or subsoil. On deep soils where this is not possible, construct foundations as shown below.
The large end seat with concrete foundations is cast in one piece. Excavate the foundation, assemble and oil the shuttering, and then suspend it as shown on two beams. Fill with 1:4 concrete. The shuttering can be removed and re-used.
The shuttering for the end seat with ramp shown below can also be re-used. Use 15mm shuttering ply with 75mm x 50mm timber nailed along each edge to give extra strength. Drill as shown, for assembly with bolts 150mm long.
Carry shuttering to site, and set in levelled foundations about 100mm deep. Oil with shuttering oil. To save concrete, the central part can be filled with gravel, rubble or rubbish such as cans and bottles. Leave at least 100mm margin at the sides and 150mm at the head wall to give an adequate thickness of concrete. Layered bottles form a very strong structure.
Alternatively, another box of shuttering with a top can be made. This acts as a liner and makes a hollow abutment.
Fill with concrete in layers about 100mm thick and tamp to remove air bubbles. To give a neat finish, shuttering can be set into the head wall to form a recess for the main beams. Put rag bolts in place while concrete is still wet. Brush ramp to give rough surface before concrete has set.
Cover with damp hessian or sacking to slow drying, and leave for 24-48 hours in summer, and a minimum of 48 hours in winter. Remove bolts and knock shuttering away with a mallet.
Do not concrete if frost is likely within 12 hours.
Concrete block
Alternatively, concrete blocks can be used to build the end seat. These are easier than lintels to carry on site, but are more obtrusive because of the mortar joints.
Cast concrete steps require complex shuttering and concreting which is best left to an expert. If steps have to be made, build them out of concrete blocks, stone or wood.
H frame
Bridges crossing narrow watercourses with stable banks, such as drainage ditches on farmland, can rest on wooden H frames which are part of the bridge construction. These are simple and quick to construct.
Pre-cast concrete
Concrete lintels make good end seats, being of suitable dimensions and with a fairly smooth and unobtrusive finish. Holes for the main beam fixings should be drilled by an expert, using a suitable masonry drill and safety guards. If extra clearance is needed, mortar concrete blocks onto the lintel.
Abutments to stream level
These can be made of stone, concrete blocks, cast concrete or concrete pipes.
If the stream level is high, construct a sandbag dam to keep the worksite dry. Excavate foundations to at least 450mm.
The abutment must either be rounded or with wing walls to allow the smooth flow of water. Fill abutment with rubble and cap with concrete. Set rag bolts in position.
Cast concrete abutment with wing walls.
Piers
A pier may be necessary to span either a very wide shallow stream, or an asymmetrical stream valley. The two parts of the bridge superstructure should be independent, so that no stress is put on it if differential settling occurs. Piers can be built out of mortared stone, concrete blocks, cast concrete or concrete pipe. Designs are not given here because engineering advice will normally be necessary.
Fixing the main beams
Methods of fixing the main beams to the abutment or end seat are shown below. A damp-proof membrane of bituminous felt must always be placed immediately below the main beam.
Fixing to wood
Use any of the three following methods.
Fixing to cast concrete
Screw the rag bolts through a template to ensure that the spacing is accurate.
Logs can be secured as shown.
Stone and pre-cast concrete
Pre-cast concrete should be drilled in a workshop. Alternatively, galvanised straps can be used for concrete block end seats as shown here. A rock drill can be used to drill holes in rock outcrops.
