Is it important what kind of fixings are used on bridges?

There are a huge number of fixings and good design requires that the correct fixing is used in every place.

The fixings are made from different materials and have a range of different finishes.

Cost should never be allowed to be in conflict with safety and long term service. A cheaper fixing with poor resistance to weather will be a poor choice in 30 years time when its failure leads to the scrapping of an otherwise sound bridge.

The strength of some fixings needs to be greater than others. Some fixings are used in relatively more hostile environments.

The bolts used to assemble the sub structure of bridges for example have to have two features. Firstly they will be wet for much of their lives and a galvanised finish is the preferred one. BZP (bright zinc plate) will offer protection for almost 3 weeks before rusting begins! Secondly the bolt needs to be made from a High Tensile Steel (HTS). One indication of this is if 8:8 is cast into the bolt head.

 

Other considerations come into play when deck fixings are considered. Being able to remove deck boards to inspect the structure at regular intervals and if necessary to carry out maintenance is a huge help. This means using screws. Nails are dreadful crude things which damage the timber and admit insects and fungus into the components via splits. The removal should be possible over decades. In order to do this we prefer hex head coach screws as they can be more easily removed than posi/phillips or slotted head screws. Picking the grot from a screw on your knees on a cold wet February morning has little to recommend it. We are happy to pay extra for stainless steel deck screws to ensure they survive the hostile environment.

 

Coated fixings are usually poor value. They are trying to hide a cheaper steel. They are fine for light loads in doors but almost never for robust external use.

Nails are best avoided and particularly Screw Nails. These are driven in and rotate to create a fixing which can only be removed by destroying the deck board and stringer. Definitely a false economy.

 

To digress it is important to consider what fixings to use when oak is the timber. The sap is very aggressive and can quickly strip coatings, even galvanising, and eat away at the steel of the screw. Choose carefully who you choose to make your bridge. Don’t let them learn at your expense.

Making fixings user friendly is an important aspect of good design. Recessing the heads of deck boards into the a counter-bored recess prevents trips and makes unauthorised removal more difficult. Using a dome headed coach bolt prevent uncomfortable encounters on the hand rail of bridges.