Static Safety during Fire Exposure: Fire Protection for Stay Cable System

Fires caused by accidents or lightning quickly turn into a problem if the structural components of a bridge are not sufficiently protected from the impact of heat.

Fires that occurred on bridges in the past have increased the requirements for fire protection in some new bridge projects as well as for the reinforcement of existing structures. DYWIDAG has developed and tested a fire protection system that protects stay cable and steel parts such as anchorages reliably from this threat.

The aim was to protect the complete stay cable system against a 30 minute long hydrocarbon fire with a temperature of 1,100°C. During this time, all of the protected parts had to remain below a temperature of 300°C. The fire protection system is divided into two different areas. The protection in the free stay cable length and in the transition areas up to the anchorage is a highly fire resistant hydrophobic mat that is wrapped around the stay cable bundle and covered by an outer tube such as a weather resistant PE tube. An intumescent epoxy coating is used to protect the anchorage and the antivandalism tube. This coating can either be  brushed or sprayed on. At high temperatures, this coating increases its volume by foaming, thus isolating the anchorage material from high temperatures.

Two tests were conducted to qualify the system. The first test was to prove the efficiency of the fire resistance design, and the second test was to show that the cable force can be reliably maintained by the DYNA Grip® Anchorage at 300°C for a minimum of 30 minutes.

In the first test, a Type DG-P12 DYNA Grip® Stay Cable bundle was mounted on a steel frame for this purpose. The stay cable bundle was covered with the fire resistant mat, and the steel components were covered with the intumescent epoxy coating.

In order to measure the temperature, several sensors were mounted on the strands underneath the mat and on the bare steel components underneath the coating. This test frame was placed in a furnace chamber and exposed to a 1,100°C hydrocarbon fire for 30 minutes. After half an hour, the measured temperature of all sensors stayed below 300°C; in fact, in some areas, the temperature only reached 200°C.

In the second test, a Type DG-P12 DYNA Grip® Stay Cable bundle without any fire protection was mounted on a steel frame that was also placed inside the furnace chamber. Outside of the furnace, a hydraulic jack was placed in front of the steel frame to stress the complete specimen to a force of 45% of the cable’s guaranteed ultimate tensile strength (GUTS). At a temperature of 300°C, the system did not show any failure or strand slippage even after one hour.

The fire protected DYNA Grip® Stay Cable bundle is the first – and so far the only – system that has been successfully tested according to the new PTI guide line for stay cables.

The fire protection concept for the free stay cable length is currently in service on the DYNA Grip® Stay Cables of the Trois Bassins Extradosed Bridge on La Reunion and has served as a reinforcement measure for protecting the stay cables of the Galecopper Bridge in the Netherlands since 2014.