Post-Tensioned Timber (PTT) frames have significant advantages over traditional timber frame systems especially where a low damage design and fast construction are desired. New Zealand practitioners designing timber structures for fire are accustomed to applying ambient design methods to an element cross-section reduced by a char depth based on a duration of Standard Fire exposure following NZS 3603 or AS/NZS 1720.4. The behaviour of PTT frames in fire remains a concern because this approach does not account for the actual mechanics of PTT connection and frame response under natural fires that will occur in the structure. This paper examines the individual and interdependent response of PTT connection components (tendon, dissipater, fasteners, etc) to fire. It is shown that the ambient analysis tool for PTT connections, the Modified Monolithic Beam Analogy, cannot be applied to the fire case by only using char reduced cross-sections of timber elements. This approach of combining ambient methodologies with reduced cross-sections does not account for the specific connection detailing, which result in unique damage in fire that may govern the structural response. The responses of two seismically detailed PTT connections are predicted using this approach and compared to a first principles assessment of connection behaviour to demonstrate that failure will occur earlier than otherwise predicted. Numerical thermal analyses of these two connections also qualitatively corroborate the damage that occurs. This investigation establishes that additional studies are required to understand the complex behaviour of these connections when exposed to fire before a design methodology can be developed.
<p>The Modified Monolithic Beam Analogy (MMBA) is a method for the analytical modelling of controlled rocking connections by establishing a displacement equivalence between the rocking member and an equivalent monolithic member. As member displacement is a function of the applied loads, the MBA must formulated for each loading scenario. The MMBA is extended to loadings scenarios with simultaneous seismic and gravity actions. This formulation can be used to analyse and design controlled rocking connections under combined seismic-gravity actions. The difference in connection response between seismic-only and combined seismic-gravity loadings is exemplified and the design implications for frames under this combined loading case is discussed.</p>
Structures are conventionally designed to maintain load‐bearing capacity during the heating phase of a fire. However, in structures with moderate or high thermal inertia, the thermal field which results in the lowest structural resistance is likely to occur after the heating phase. This is of particular interest for timber connections because the strength and elastic modulus of timber reduces until the formation of char while steel plates and fasteners, which transfer forces between elements, conduct heat through the connection. It is unclear how thermal fields develop in timber connections during the cooling phase of fires and what influence different cooling rates have. Experiments on identical timber beam‐column subassemblies exposed to the same heating duration but two different cooling phases are presented. The results show that exposed steel components conduct heat into the connection, which propagates a thermal wave through the elements. Although the thermal waves had similar speeds, the specimen absorbed more thermal energy during the longer cooling phase, resulting in higher temperatures. Since the strength and elastic modulus of timber decrease at temperatures below 100°C, these results provide evidence that the structural resistance of a timber connection decreases in the cooling or post‐cooling phases and that a longer cooling phase is more severe than a shorter one. Further investigation into thermal exposure during the cooling phase of realistic compartment fires and the response of a wide variety of timber connections is required to quantify the reduced performance and support the development of appropriate design methods.
Post-Tensioned Timber (PTT) is a structural system that has many advantages over conventional timber frames, particularly in high-seismic areas such as New Zealand. These include a low-damage, re-centring system and fast construction. Although PTT systems have been shown to perform well in ambient conditions, concerns remain about the performance of beam-column connections in fire. As part of a larger project to address these concerns, a full-scale loaded PTT beam-column subassembly was exposed to a Standard Fire. Connection moment resistance was lost when thread stripping at the anchorage nut. An analytical model was compared to the experimental results. The decompression moment decreased due to the reduced tendon force from heating, the tendon acting eccentrically and the reduced effective crosssection. The analytical connection rotation after decompression was a third of experimental rotation until thread stripping began. This analytical model can be improved by including more complex aspects such as softening of a heated rocking interface and determining the beam stiffness based on the reduced material properties beneath the char layer. A follow up study is being undertaken to address these improvements and to provide a method for designers to evaluate the performance of these connections in fire.
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