Precast Concrete Floors: Seismic behaviour of Connections and Wall-to-Floor Interactions

A photograph of damage to precast concrete floors as a result of the Canterbury earthquakes.

Damage to precast floors during the Canterbury earthquakes. Photos: Rick Henry, University of Auckland.

Floor diaphragms play an important role in the seismic response of buildings by transferring forces to lateral and vertical load-resisting systems.  However, the demands on floors during earthquakes can result in damage that compromises their connection to other elements in the building, and in extreme cases may result in the collapse of the floor.  In New Zealand, floor diaphragms in multi-storey building are typically constructed using precast concrete floor units.  Research in the past 20 years has highlighted a number of vulnerabilities in the connections of precast concrete floor systems that have resulted in significant changes to design standards and construction practice.

The Canterbury earthquakes reconfirmed several of the previously identified vulnerabilities in precast concrete floors and also raised new concerns that hadn’t previously been considered (See photo above).  The Canterbury Earthquakes Royal Commission made a number of recommendations related to loads and connection design in precast concrete floors, as well as a need to develop greater understanding of the interactions between structural elements.

This research project is focused on improving the seismic performance of connections in precast concrete floors and investigating the interaction between floor and wall systems as buildings deform during earthquakes.

Support connections

Experimental testing has been conducted to investigate the seismic behaviour of deep hollowcore precast units that use recommended support connection details. Additional testing is in progress to understand the seismic behaviour of precast concrete rib with timber infill floor systems that use typical pre-Canterbury earthquake detailing, as well as detailing recommended by the Structural Engineering Society (SESOC) after the Canterbury earthquakes.

The outcomes of these precast unit tests have resulted in recommendations that were used to support amendments currently proposed to the New Zealand Concrete Structures Standard (NZS 3101).

A diagram of structural interactions between concrete walls and floor systems.

Structural interactions between wall and floor systems. Left, a single cantilever wall. Right, a coupled wall system.

Wall-to-floor interaction 

The Canterbury earthquakes further highlighted the need to consider earthquake induced deformations on building and the interaction between different structural and non-structural systems.  For example, the deformation of reinforced concrete walls can be restrained by the floor diaphragms (See above).  Numerical models are being used to understand how the deformations induced in the floor and the restraint provided to the wall may alter the seismic loads on parts of the building and its overall behaviour during earthquakes.

Low-damage systems
A photograph a low-damage rocking wall system developed with USA colleagues.

Low-damage concrete systems can provide a more resilient building by minimising damage during large earthquakes.  Rocking concrete walls can provide excellent low-damage systems, but new wall-to-floor connections need to be developed to ensure that the floors are not damaged and to guarantee that the building can be reoccupied post-earthquake.  A novel isolating connector has been developed and a large-scale test of a rocking wall with a section of floor was recently completed by collaborators in the United States (Fig. 3).

Right: Low damage rocking wall system test at the University of Minnesota.

Project team

The project team includes Dr Rick Henry and Prof. Jason Ingham (University of Auckland), PhD students Sam Corney, Ericson Encina, Jonathan Watkins, and ME students Andy Ahn and Richard Malcolm.  The coupled wall analysis included collaboration with Prof. Des Bull (Holmes Consulting Group and University of Canterbury), and the low-damage wall tests were conducted in collaboration with Prof. Sri Sritharan (Iowa State University) and Prof. Cathy French (University of Minnesota).

A photograph of Dr Rick Henry's research team at the University of Auckland.

The research team takes a break from seismic testing of a hollowcore floor unit at Stresscrete. From left, Sandra Yassi, James Daniels, Sam Corney and Rick Henry.

Contact: Rick Henry, University of Auckland

All photos and data courtesy of the University of Auckland.

References: 

Corney SR, Henry RS, Ingham JM (2014) Performance of precast concrete floor systems during the 2010/2011 Canterbury earthquake series. Magazine of Concrete Research 66(11): 563-575. (♦ Link)

Sritharan S, Beyer K, Henry RS, Chai YH, Kowalsky M, Bull D (2014) Understanding poor seismic performance of concrete walls and design implications, Earthquake Spectra, 30(1), 307-334. (♦ Link)

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Last updated 20 Jan 2016