Retrofitting for reinforcement - reinforced concrete structures

Materials World magazine
,
3 Apr 2011
Damage caused by the Oklahoma City bomb, 1994

Making buildings and structures safer in the event of a terrorist attack is key to national security. Philip Isaac, Antony Darby, Tim Ibell and Mark Evernden of the University of Bath, UK, explore how to predict the behaviour of reinforced concrete columns that are strengthened with fibre-reinforced polymers.

The increasing threat posed to buildings and infrastructure in recent years from terrorist activity has heightened the need for engineers to develop safe and reliable methods for strengthening vulnerable structures.

Notable examples exist where high death tolls have resulted from the collapse of buildings due to explosions. In the case of reinforced concrete (RC) structures, these collapses are typically as a result of the severe damage sustained to the load-bearing columns on the lower floors, such as the structural collapse in the 1994 bombing of the Alfred P Murrah building in Oklahoma City, USA. It was estimated that 87% of the 187 people who lost their lives were in the collapsed portion of the building.

The pressure generated by an explosion can be many thousand times greater than the strongest hurricane, which a structure would typically be designed to withstand. In general, columns are made to carry loads axially rather than withstand the significant transverse loading that results from explosions. In these situations, columns must also resist both out-of-plane bending and shear forces. Given these significant changes in loading conditions, columns can be susceptible to catastrophic failure.

Brittle behaviour

Simulated blast tests carried out at the University of California in San Diego (UCSD), USA, have demonstrated how typical RC columns could fail in brittle shear. Research has also shown that the tendency for structures to fail in a brittle manner increases as the rate of loading increases. In cases where shear failure occurs, very little of the energy from an explosion is dissipated by the structure. In addition, the remaining column often has negligible residual load-bearing capacity, leading to the increased likelihood of structural collapse.

One method for preventing brittle failures and increasing ductility, common under static loading, is the use of fibre-reinforced polymers (FRPs). Aramid FRP is familiar to many people in the form of Kevlar, a material widely used in bullet-proof vests. However, it is carbon FRP that has gained favour in structural strengthening applications due to its high strength, which can be up to 10 times that of steel.

Since the introduction of these materials in Europe and Japan in the late 1980s, the material has been demonstrated to be effective in strengthening a multitude of structural members in a number of ways. One such application is the strengthening of columns with insufficient load bearing capacity. This is usually achieved by wrapping and curing a thin continuous sheet of FRP around the column to provide confinement for the concrete. In the case of strengthening columns against blast loads, the same concept has been successfully demonstrated in tests carried out at UCSD. In this case, the FRP significantly increases the column’s shear capacity.

Fibre reinforcement

Traditional methods of improving a building’s robustness against explosions have usually involved significantly increasing the members’ masses, or erecting large barriers to deflect the blast wave. However, these methods can be either impractical or detract from the aesthetics of the building. The use of FRP, on the other hand, involves minimal intervention. By wrapping and confining the concrete with FRP, the shear strength is increased, ensuring a more desirable ductile flexural response, dissipating significantly more of the blast’s energy. The concrete’s confinement also prevents rapid ejection of debris, which has the potential to cause severe injuries and reduce the columns’ residual load-carrying capacity.

However, retrofitting is a recent development, and suitable and reliable methods of analysing and designing these structural systems have yet to be developed. To engineer blast resistance, it is important to understand the strengthened column’s behaviour and predict lateral displacement accurately. In response to these concerns, work has recently commenced at the University of Bath, UK, on developing methods to analyse blast response.

One such method, based on the theory of plasticity, has demonstrated encouraging results for predicting the behaviour of FRP-retrofitted columns. Plasticity theory was first developed for the design of Morrison bomb shelters during World War II, allowing ductile structures to be analysed in a more efficient manner. From this, much greater efficiency can be achieved in material usage by predicting and designing for the collapse load, rather than limiting behaviour to the material’s elastic range. It is, therefore, usual to employ plastic analysis when deflections become large, as may be expected in blast situations.

Strength under strain

In flexurally deforming RC columns, distinct plastic hinge regions have been shown to form at both the mid-span and the supports. It is in these regions that the majority of the energy from the applied load is dissipated due to the high strains that develop.

A further important issue to consider in dynamic situations is the reinforced concrete’s properties at high rates of strain. Research has shown that the strength of both concrete and steel can increase considerably at high strain rates, as would be the case in blast situations.

The method developed at the University of Bath to predict the peak displacement of FRP-strengthened columns employs a time increment approach to analyse the column’s response over a series of small time steps. This technique assumes that the blast energy is dissipated by deformations in the plastic hinges, as the FRP prevents brittle failures for which energy dissipation modes are more difficult to quantify. As the displacement increases, the rotation in the hinges increases and more energy is dissipated.

A benefit of this method is that the strain rates can be accurately calculated, and the variation in strain rates during deformation can be easily accounted for – essential when including accurate material properties.

Results from this work on predicting the peak displacement of FRP-retrofitted columns tested under blast loading are shown in these pictures:

It can be seen that the theoretical work has been able to recreate the trends of the experimental data shown in this graph:

However, there are some discrepancies that are thought to be based on the assumption that all of the energy from the blast is dissipated in the plastic hinges. In reality, energy is likely to be dissipated in other modes, and ongoing research will address this.

It can be seen that the use of FRP materials provides engineers with an effective method for strengthening RC columns to withstand blast loads. The plasticity-based method of analysis has displayed encouraging results compared with test data, giving engineers renewed hope in protecting our buildings and infrastructure from terrorism.

Further information

BRE Centre for Innovative Construction Materials, Department of Architecture and Civil Engineering, University of Bath, BA2 7AY, UK. Researcher Phil Isaac (PhD student), Supervisors Dr Antony Darby (lead), Professor Tim Ibell and Dr Mark Evernden. Tel: +44 (0)1225 384495. Email: ace@bath.ac.uk Website: www.bath.ac.uk/ace