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The Influence of Tolerance Gaps on the Fire Performance of Aluminium-Wood Joints
Abstract
Tolerance gaps in wood connections are unavoidable, for reasons of constructability and the effects of natural shrinkage in timber elements with changing moisture content. During a fire, these gaps may lead to a substantial heat transfer to the metal connectors that are considered heat protected being embedded by the wooden components of the connection. Aluminium connectors are popular due to their ease of production and assembly, but they are particularly vulnerable to elevated temperatures. This study investigates the effects of tolerance gaps on the fire performance of aluminium connectors in beam-to-column/wall shear connections. Reduced-scale experiments were designed to study the temperature evolution of aluminium connectors during standard fire exposure for 1 mm and 6 mm tolerance gaps, as well the mitigation effects of additional intumescent fire protection in a 6 mm tolerance gap connection. For the 6 mm gap, the temperature of the connector increased much faster, reaching 286 ± 36°C after 80 min, at which time the connector with a 1 mm gap had only reached 97 ± 1°C. The addition of intumescent protection in a 6 mm gap case led to lower temperatures in the connection, in comparison to an equivalent tolerance gap without protection. Subsequently, two additional loaded fire tests were performed, for 6 mm and 22 mm tolerance gaps without fire protection, to investigate the critical failure mode of the connectors. In these cases, the failure occurred in the connectors at 87 min and 32 min, respectively, when their average temperatures reached approximately 315°C. This study demonstrates the critical influence of gap size on the fire performance of aluminium-wood joints.
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Tolerance gaps or slips in wood connections are unavoidable, for reasons of constructability and the effects of natural shrinkage in timber elements with changing moisture content. During a fire, these gaps may lead to increased heat transfer through the connection. Aluminium connectors are becoming more popular due to their high malleability and availability, but they are particularly vulnerable to elevated temperatures. Thus, the objective of this study is to investigate the effect of tolerance gaps on the fire performance of aluminium connectors in beam-to-column/wall shear connections. An experimental campaign was designed to study the temperature evolution of the aluminium connectors during standard fire exposure for 1 mm and 6 mm tolerance gaps, as well the mitigation effects of additional intumescent fire protection in a 6 mm tolerance gap connection. The results showed a clear and consistent impact of the connection gap size on the temperature evolution of the aluminium connectors. For the larger 6 mm gap, the temperature of the connector increased much faster, reaching 286 ± 36 °C after 80 minutes, at which time the connector with a 1 mm gap had only reached 97 ± 1 °C. The addition of intumescent protection in a 6 mm gap case led to lower temperatures in the connection after 40 minutes of fire exposure, in comparison to an equivalent tolerance gap without fire protection. This study shows that tolerance gaps can lead to a significant reduction in the capacity of aluminium connectors, but this may be mitigated with additional fire protection.
This paper presents the results of an extensive experimental campaign on the fire behaviour of beam-to-column timber connections loaded perpendicular-to-the-grain. The experimental campaign addressed the fire behaviour of beam-to-column timber shear connections in a systematic way, testing a wide range of common connection typologies, significantly enlarging their experimental background.
Fire safety has always been a major concern in the design of timber construction. Even though wood is a highly combustible material, timber members can perform adequately under elevated temperatures. The thermal response of timber connections, however, is in most cases poor and determination of their fire resistance is usually the crucial factor in evaluating the overall load-bearing capacity of wood structures exposed to fire. The analysis of timber joints under fire conditions can be challenging due to their complexity and variety. After presenting the variation of the properties of timber with temperature, this paper reviews the fire performance of various connection types, such as bolted or nailed wood-to-wood and steel-to-timber joints. Results from relevant experimental programs and numerical studies are discussed in detail and future research needs are highlighted. The effect of several factors on the fire resistance of timber connections, such as the fastener diameter, timber thickness and joint geometry, is investigated and useful conclusions are drawn. Based on these, preliminary guidelines for the efficient design of timber connections under fire exposure are presented.
Buildings constructed from engineered timber are becoming more prevalent globally as building designers, owners and architects realize the sustainability opportunities with timber construction and the overall aesthetic of a completed timber building. As timber buildings are planned to be taller than many model codes permit, the National Fire Protection Association (NFPA) Fire Protection Foundation commissioned research entitled “Fire Safety Challenges of Tall Wood Buildings”, with the aim of understanding where the current gaps in knowledge are and how the research agenda should be prioritized.
This paper investigates the influence of gap size (openings/spaces between two structural members) and the presence of intumescent sealants on the development of temperature when timber connections are exposed to fire conditions. The experimental results of 21 samples representing a concealed steel‐to‐timber connection configuration that was exposed with a 0, 3, 6, and 10 mm gap to the ISO834 standard fire for 120 min are presented. Half of the samples in each sample group were protected with an intumescent fire protection sealant. The temperatures in the timber were measured at various locations around the gap and directly next to the steel component. The experimental results show that the presence of a gap increases the temperature of the timber more than predicted by the thermal penetration models available, with current code guidelines (e.g., Eurocode 5) possibly being non‐conservative. The use of an intumescent fire protection sealant in the gap is shown to be an effective protection method, but that the application configuration is important to ensure effective protection, especially as the gap becomes smaller. The results also show that the use of an intumescent fire protection sealant in a gap increases the predictability of the temperatures in the timber in longer duration fires. Unprotected samples and 0 mm gap samples exhibited large variability in the thermal development outcomes over time.
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.
This New Casebook contains ten essays written about Blake's poetry since 1970 selected to show the diversity of Blake criticism during the last twenty years and the ways in which contemporary critical theories open up new readings of his work. Essays representative of Marxist, psychoanalytic, deconstructionist, feminist and new historicist criticism are included. David Punter's Introduction places these in the context of recent developments in critical theory and shows how today's student can best engage with Blake's complex and rewarding work.
This paper describes fire tests on loaded glued laminated timber columns in which the structural response was measured during the heating and cooling phases. Identical columns with 280 × 280 mm2 cross‐section and 3.7 m length were tested under various heating durations in a standard furnace to investigate integrity to full burnout. Two of the columns were subjected to ISO 834 heating until failure and their measured fire resistance was 55 and 58 min, respectively. Two columns were subjected to 15 min of ISO 834 heating followed by controlled cooling; these columns failed during the cooling phase, respectively after 98 and 153 min. Flame self‐extinction occurred after approximately 40 min while smoldering continued locally. Two columns tested under 10 min of ISO 834 heating both survived the defined heating–cooling exposure. Thermocouples inside the columns show sustained temperature increases for hours after the end of the heating phase. These full‐scale furnace experiments show that timber columns may fail during the cooling phase after exposure to standard heating for about 25% of the standard fire resistance duration. These results, in line with previous numerical predictions, highlight the need for further investigation into fire safety until full burnout for timber structures.
The current understanding of the thermo-mechanical response of cross-laminated timber (CLT) walls to fire is insufficiently developed. This paper presents results obtained using a novel experimental methodology on fire-exposed CLT walls under sustained loads. The findings demonstrate that global instability is likely to be the dominant failure mode for CLT walls in fire. Use of a polyurethane adhesive resulted in earlier structural failure than use of a melamine urea formaldehyde adhesive. Three-ply walls failed significantly earlier than those with five plies. The combination of these two factors caused a halving of failure time between different walls under identical heating conditions. In addition, CLT walls were found to collapse during artificially induced cooling phases. It is concluded that these findings are centrally relevant considerations for fire design of CLT.
When a thermocouple is embedded in a material of lower thermal conductivity, under certain heating or cooling conditions, the presence of the thermocouple can distort the surrounding temperature field. As a result, the measured temperatures may be very different to the ‘undisturbed’ temperatures that would exist without the thermocouple. This study presents the results of a sensitivity analysis of key factors influencing this thermal disturbance. A series of heat transfer models and accompanying experiments are used to demonstrate the effects of thermocouple geometry, contact conditions, thermal properties, and heating regime on the temperature measurement error. These tailored finite element models were validated against experiments on vermiculite insulation board, which confirmed the accuracy of the models in simulating the thermal disturbance for inert heating conditions. Also, a simplified version of the finite element model was used to calculate the thermal disturbance error for a number of conditions, and subsequently to predict a range of corrected temperatures for the experimental measurements. This correction method was found to greatly improve the accuracy of the results for inert heating conditions. Since the method does not account for the effects of moisture in heat transfer, a creep of uncorrected errors could be observed.
The anticipated growth and urbanization of the global population over the next several decades will create a vast demand for the construction of new housing, commercial buildings and accompanying infrastructure. The production of cement, steel and other building materials associated with this wave of construction will become a major source of greenhouse gas emissions. Might it be possible to transform this potential threat to the global climate system into a powerful means to mitigate climate change? To answer this provocative question, we explore the potential of mid-rise urban buildings designed with engineered timber to provide long-term storage of carbon and to avoid the carbon-intensive production of mineral-based construction materials.
This paper reports on two experiments conducted in a fire resistance furnace to study the differences in the boundary conditions, the fire dynamics and the fuel required to run the furnace when a combustible timber specimen as opposed to a non-combustible concrete specimen is tested. In both experiments measurements were taken in the furnace to evaluate the difference in the environments of the furnace and the response of the elements being tested. These include non-control plate thermometers distributed throughout the furnace; O2, CO2 and CO gas measurements taken at different distances from the specimen surface and in the furnace exhaust; instrumentation of one of the bricks comprising the furnace lining with thermocouples at different depths from the exposed surface; and mass loss of the combustible timber specimen. Thermal exposure of elements in a furnace is discussed, as well as the impact of the different materials on the similarity of thermal exposure. This is done through analysis and discussion of the different measurements taken and the apparent influence of the specimen being tested on the boundary condition of the heat diffusion equation. We conclude that; (1) the fire dynamics in a furnace are dependent on the specimen being tested; (2) that the test with the combustible specimen requires less fuel flow to the burners such that the control plate thermometers follow the ISO 834 temperature–time curve compared to the non-combustible specimen, however that this is not only a result of the combustibility of the specimen but is also a consequence of the different thermal inertia of the two materials; (3) that the boundary condition for heat transfer to a test object in furnace tests is dependent on the properties of the specimen being tested; and (4) that the timber when placed on the furnace experiences smouldering combustion after the char layer has formed. A fire resistance test of combustible construction of a given period represents a significantly less onerous test in terms of energy absorbed or fuel made available than one of a non-combustible construction, implying that the existing fire resistance framework may not be appropriate for timber structures and that an alternative approach may be required.
Multi-storey mass timber buildings constructed with cross laminated timber and glulam are being developed globally. Where engineered timber such as glulam is utilized, the column to beam connections need to be constructed with a fire resistance rating equal to that of the connecting members. The preferred glulam connectors are either a concealed steel plate with bolts and dowels; or a concealed proprietary screw-in sleeve type connector. The fire resistance of connectors for glulam members is an unresolved design issue, as there is no clear methodology to assess their capacity under fire, when the timber is exposed and not clad behind fire protective plasterboard. There is limited fire test data on concealed connectors under shear forces, which is the normal loading condition within a constructed building. Fire test data is also limited on full-size specimens. Correlations developed to date to calculate concealed connector fire resistance have only limited application.
A methodology for the design of glulam beam to column connections has been developed based on an extensive literature review, examining the key issues for connection failure. It has been determined that char rate for the timber at the connection needs to be increased above the normally accepted design values, due to the influence of the steel connectors. Secondly, the reduction in timber strength behind the char layer needs to be accounted for, by including a greater depth of reduced strength and stiffness timber, such that the connection can effectively transfer the applied forces through the timber to the steel connector. The methodology detailed within this paper provides a simple approach to evaluate the timber cover to the concealed steel connector, where the timber strength and stiffness are effective.
This research investigates the added value or negative impact of mass timber construction (MTC) when compared with traditional site built construction. The project documented mass timber case studies and collected cost and schedule data for each built case from three stakeholders – architect, general contractor, and mass timber fabricator. This data were compared with site built cost and schedule performance data provided by cost estimation firm Cummings Corp. for building types of similar size. The study found that mass timber improves project cost when compared with traditional site built construction. In addition to the quantitative comparison, a survey protocol of qualitative opportunities and challenges associated with MTC was solicited from project stakeholders. This resulted in an analysis of frequent lessons learned and best practices for mass timber project delivery for to be taken into consideration by stakeholders.
Purpose
This paper aims to present the results of an extensive experimental programme on the fire behaviour of timber beam-to-column shear connections, loaded perpendicularly to the grain.
Design/methodology/approach
The experimental programme comprised tests at normal temperature and loaded fire resistance tests on beam-to-column connections in shear. Twenty-four full-scale tests at normal temperature were performed covering nine different connection typologies, and 19 loaded fire resistance tests were conducted including 11 connections typologies.
Findings
The results of the fire resistance tests show that the tested typologies of steel-to-timber dowelled connections reached more than 30 and even 60 minutes of fire resistance. However, aspects such as a wider gap between the beam and the column, reduced dowel spacing, and the presence of reinforcement with self-drilling screws all have a negative influence on the fire resistance.
Originality/value
The experimental programme addressed the fire behaviour of timber beam-to-column shear connections loaded perpendicularly to the grain in a systematic way testing a wide range of common connection typologies significantly enlarging their experimental background.
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