John Gales

A contemporary review of large-scale non-standard structural fire testing

In recent years, large-scale structural fire testing has experienced something of a renaissance. After about a century with the standard fire resistance test being the predominant means to characterize the response of structural elements in fires, both research and regulatory communities are confronting the many inherent problems associated with using simplified single element tests, on isolated structural members subjected to unrealistic temperature-time curves, to demonstrate adequate structural performance in fires. As a consequence, a shift in testing philosophy to large-scale non-standard fire testing, using real rather than standard fires, is growing in momentum. A number of custom made, non-standard testing facilities have recently been constructed or are nearing completion. Non-standard fire tests performed around the world during the past three decades have identified numerous shortcomings in our understanding of real building behavior during real fires; in most cases these shortcomings could not have been observed through standard furnace tests. Supported by a grant from the Fire Protection Research Foundation, this paper presents a review of relevant non-standard structural fire engineering research done at the large-scale around the world during the past few decades. It identifies gaps and research needs based both on the conclusions of previous researchers and also on the authors’ own assessment of the information presented. A review of similar research needs assessments carried out or presented during the past ten years is included. The overarching objective is to highlight gaps in knowledge and to help steer future research in structural fire engineering, particularly experimental research at the large-scale.

New Parameters to Describe High Temperature Deformation of Prestressing Steel determined using Digital Image Correlation

Abstract This paper describes the results from a series of high-temperature tension tests on prestressing steel under sustained load (creep tests). Both steady-state and transient heating regimes are used. A novel digital image correlation (DIC) technique is evaluated, validated and used to measure tendon deformation during the high-temperature testing. The tests demonstrate that DIC is a reliable method for measuring strain at high temperatures and is not hampered by some of the limitations that prevent the usage of traditional strain measurement techniques at high temperatures and for high strains. It has also been shown that DIC can capture the reduction in cross-sectional area that occurs during necking, which appears to govern the tertiary creep phase. Testing and analysis demonstrate the importance of accurate creep parameters for modelling stress relaxation in heated prestressing steel tendons made from modern prestressing steel; creep parameters available before this work were developed almost 50 years ago and so modern prestressing steel can have very different creep properties. New creep parameters are developed in this paper that considerably improve the accuracy of stress relaxation modelling.

Unbonded Post Tensioned Concrete Slabs in Fire - Part I - Experimental Response of Unbonded Tendons under Transient Localized Heating

The fire-safe structural design and construction of unbonded post-tensioned (UPT) flat plate concrete structures has recently come under debate in the UK, and questions are being raised regarding the response to fire of post-tensioned concrete slabs. Related to these concerns is the real world response of continuous UPT tendons inside such structures both during and after a fire, which is largely unknown and depends on many potentially important factors which are not currently accounted for in standard fire tests. Several credible concerns exist for UPT concrete structures in fire, most notably the potential for premature tendon rupture due to localized heating which may result from a number of possible causes (discussed herein). The research presented in this paper deals specifically with the time-temperature-stress-strength interdependencies of stressed UPT tendons under localized transient heating, as may be experienced by tendons in a real UPT building in a real fire. Nineteen high temperature stress relaxation tests on UPT tendons of realistic length and parabolic longitudinal profile are reported. It is shown that localized heating of UPT tendons is likely to induce premature tendon rupture during fire, even in structures which meet the prescriptive concrete cover requirements imposed by available design codes.

Unbonded post tensioned concrete slabs in fire - Part II - Modelling tendon response and the consequences of localized heating

This is Part II of a two part paper dealing with the current state of knowledge of the fire-safe structural design and construction of unbonded post-tensioned (UPT) flat plate concrete structures. Part I provided detailed results of nineteen transient high temperature stress relaxation tests on restrained UPT tendons of realistic length and parabolic longitudinal profiles. Experimentation identified several credible concerns for UPT concrete structures in fire, most notably the potential for premature tendon rupture due to localized heating, which may result from a number of possible causes in a real structure. The real world response of continuous UPT tendons both during and after heating is largely unknown, and is dependent on factors which are not currently accounted for either in standard fire tests or by available prescriptive design guidance. This second part of the paper presents and applies a numerical model to predict the time-temperaturestress-strength interdependencies of stressed UPT tendons u...

Structural fire performance of contemporary post-tensioned concrete construction

Post-tensioned (PT) concrete is excellent in providing optimized material use for stringent sustainability and aesthetic objectives in modern structures. The continual optimization of PT concrete structures combined with the ongoing development of stronger concretes has resulted in structures which require more engineering attention. This Springer brief intends to provide a more complete understanding of the structural and thermal response of contemporary PT concrete structures to fi re. Such an understanding is needed to help practitioners and researchers develop defensible fi re-safe designs for these structures and identify current knowledge gaps. Chapters on the following subjects pertaining to PT concrete are included: state-of-the-art summary of contemporary construction, review of previous structural fi re test research programmes and real fi re case studies, overview of the recent research programme conducted by the author(s), and a summary of current research needs.

Contemporary Post-tensioned Concrete Construction

Post-tensioned (PT) concrete is an increasingly popular technology since it allows for rapid construction using less material than conventional concrete. While its use has been widespread in the United States since the late 1950s, it has recently seen wider popularity in Europe, China, and the Middle East. PT concrete uses high strength cold-drawn prestressing steel tendons. The tendon configurations for PT concrete are bonded and unbonded. In both configurations, prestressing steel is used to pre-compresses the concrete slab before loading and results in longer spans without deformation. In comparison to conventional reinforced concrete slabs, PT concrete provides excellent control of in-service deflections (Khan and Williams 1995).

Localized Heating of Post-tensioned Concrete Slabs Research Program

Performance-based design is the growing paradigm in contemporary structural engineering, and structural fire safety engineering is no exception. Advocates of performance-based methodologies seek to adopt sophisticated fire strategies tailored to individual building needs. In particular, these strategies are being applied to optimized reinforced concrete buildings, including post-tensioned (PT) structures. The current understanding of prestressing steel behavior based largely on outdated research that fails to properly account for material property changes at elevated temperatures. Furthermore, real fires in real PT concrete buildings have the potential to induce unique failure mechanisms that cannot be observed or accounted for using standard fire tests, as seen through case studies of real fires (see Chap. 3). Current modelling tools used to establish structural fire safety engineering strategies lack realistic experimental validation and verification, leading to the development of potentially unconservative performance-based strategies for PT concrete buildings. In order to enable credible performance based design of PT concrete buildings current modelling capabilities need to be improved, through the analysis of densely instrumented experiments, which incorporate as many relevant structural properties (post-tensioning, continuity, restraint, realistic scale, unbonded reinforcement, etc.) of as-built PT construction as possible. This need is partly addressed in the current chapter by presenting experiments on three 3-span continuous, restrained PT concrete slabs (slab strips). The slabs were put under sustained service loading and exposed to severe localized heating using radiant heaters. This chapter considers an extensive recent experimental program conducted for this book over the years 2011–2013. This experimental program compliments those discussed in previous chapters. Three novel large-scale tests on locally heated, continuous PT concrete slab strips are detailed. In real and contemporary multiple bay and continuous unbonded post tensioned (UPT) concrete slabs exposed to fire, structural actions may play significant (often inter-related) roles influencing the response of this construction. This response is studied and explained herein.

Introduction to Contemporary Post-tensioned Concrete and Fire

In contemporary buildings, less is more. With new construction technologies and building materials, structures are being built with open floor plans and larger than ever before spans. Post-tensioned (PT) concrete is excellent at optimizing material usage, which helps designers meet stringent sustainability objectives while creating desirable large open spaces. PT concrete uses high strength cold-drawn prestressing steel tendons that are tensioned inside ducts in the concrete after casting. This compresses the concrete prior to loading and results in excellent control of in-service deflections which is far superior to that of conventional (non-prestressed) reinforced concrete. This tensioning process results in secondary support reactions in structural systems that balance deflection from loading. The post-tensioning process reduces the use of building materials enabling large spans. Typical PT concrete structures include bridges and buildings.

Structural Fire Test Research Programmes for Post-tensioned Concrete

In 1898, in the offices of the Roebling Company at 121 Liberty street in New York City (present day location of the ‘Ground Zero’ World Trade Center memorial), Abraham Himmelwright wrote the original necessary requirements for ideal structural fire testing to be of value. Structural fire tests performed at Himmelwright’s time were based upon an experimental methodology developed by mechanical engineer of the New York Building Department, Gus Henning in 1896 (Himmelwright 1898; Woolson 1902). Inspired to respond to the various ad-hoc structure fire tests of materials, the test series were conducted to develop “actual and relative efficiencies of different floor constructions” under fire (Himmelwright 1898). Henning would later denounce the early fire testing as fraud due to its non-representation of reality (Fig. 3.1).

Recommendations for Advancing the Fire Safe Design of Post-tensioned Concrete

As most jurisdictions move towards the adoption of performance-based structural fire design codes it is crucial that the behavior of all structures in fire is scientifically and rationally understood. Post-tensioned (PT) concrete structures are no exception. PT concrete is widely believed to benefit from its ‘inherent fire endurance.’ This belief, particularly for the unbonded configuration, is potentially problematic. It is based on results from standard fire tests performed on simply supported specimens more than five decades ago. The credibility of such tests is debatable. Not only are they unable to reflect the behavior of modern PT concrete construction materials, they cannot capture the true structural behavior of real buildings in real fires. The objective of this book has been to fill in some of the significant knowledge gaps regarding the structural and thermal response of PT concrete structures in fire, particularly the performance of the unbonded prestressing tendons in floor slabs. This knowledge is vital for the creation of defensible computational modelling of unbonded PT structures in fire.