Carbon fibre for seismic strengthening of floors

Floor diaphragms transfer seismic forces between lateral load resisting elements like columns and walls. However, floor diaphragms in existing concrete buildings are often inadequate to transfer the tension forces, making them vulnerable to earthquakes.

The use of carbon fibre reinforced polymers (CFRP) is often used to improve the tension capacity of floor diaphragms due to their lightweight and unobtrusive qualities.

Del Rey Castillo, lecturer at the University of Auckland, says that while this material has been used for over 20 years, its application in floor diaphragms has not yet been studied – resulting in unsafe and inefficient designs.

“All the information we have on how to use this material is based on a very small scale,” says del Rey Castillo.

“For example, with columns you have half a metre by half a metre, and we are using that information to strengthen our very large-scale structural elements and the work I have done with some of my colleagues indicates that this is wrong.

“Additionally, we are loading whatever it is – a beam, a column – very slowly.”

On the other hand, says del Rey Castillo, an earthquake is not slow – meaning the effect of the loading rate on this is unknown.

“What the floor is doing during an earthquake is connecting everything together – it connects the columns and the beams from one side of the building all the way to the other side of the building. So, we have to look at the overall behaviour of the building.”

It is hoped that the main outcome of the team’s research will be a design methodology that engineers can use for safe, reliable, and efficient design of CFRP ties for existing diaphragms.

“I can test something in the lab, and it works really well, but how does it work when it is implemented in a real building?” says del Rey Castillo.

“The main hope is to give engineers the confidence that it works… that I can say with my hand on my heart, that yes this works.”