Friarton Bridge began to be constructed in 1975. The estimated cost was approximately £6 million. The final cost was approximately £8 million. Eight pairs of reinforced concrete pillars carry the deck. The nine spans of the bridge vary in length from 63m to 174m which is the river span. The navigation clearance is 25m as the bridge crosses the river. The bridge is a box-girder bridge. The girders were manufactured in Darlington and Chepstow.
When deciding where to place the bridge the Craigend Interchange to the south, Kinnoull Hill cliff to the north, the position of the railway line and the Salmon Fishing Bothy at Lairwell had to be taken into account. Another consideration was whether to cross the river with one span or in a number of spans with piers in the river. The cost of any bridge is directly related to the length of the main span. The longer the main span the more expensive the bridge. With piers in the river there is always the possibility of collisions. Dredgers use the river daily. There are fairly large cargo vessels which use Perth harbour. In September each year up to 50 in a month use the Harbour to transport grain to the Low Countries. The height of the biggest cargo ships determines the necessary clearance over the river. It was decided to erect a steel bridge as a concrete bridge would have required more piers.
On Friarton Bridge the width of the box girders is 4.3m. The girders form parallel lines. Each girder carries a reinforced concrete deck and the deck carries one carriageway of the new road. The carriageway is 7.3m wide. The box girders vary from 10.5m to 25m in length and were brought by road to the site. The ends of each box had a horizontal cut so that the girders fitted together smoothly for welding. This also allowed for the curve on the bridge and ensured that the girders would land exactly in the piers and also finally that each part of the bridge built from each river bank would meet and join in the centre at the mid-point. Between the piers the box girders were built on an upward curve so that they would deflect and sag between the piers when the concrete carriageway was laid. Very high precision accuracy was required. There is also weld shrinkage after the actual welding is done. This was calculated to be one quarter of an inch so that that length was added to each box girder. Constant checking and correction was necessary. Surveys for this were always done at dawn as during the day the sun would cause expansion of the box girders on one or other side of the bridge. It was essential to have parallel alignment. The climate (temperature) of the area also has to be considered. The expansion and contraction of the bridge must also be incorporated in the design. The steel girders are attached to the top of the concrete piers. As the girders expand or contract the columns (piers) will bend fifteen inches in either direction. The box girders were lifted by crane on the banks or hoisted by lifting beams from a raft on the river. All the welds were x-rayed to ensure there were no weaknesses. The concrete deck was formed of lightweight aggregate from the power station ash. Using this makes the bridge cost less. There was 200mm thickness of reinforced concrete. Then a layer (20mm) of maestro asphalt was laid (water proof) and finally a 45mm layer of rolled asphalt.
The consultant engineers were Freeman Fox & Partners.The main contractor was Cleveland Bridge & Engineering Company. Friarton Bridge cost approximately £8 million and was opened in summer 1978.