Behavior of beams in bolted column‐tree frames at elevated temperature

Column‐tree moment resisting frames, as the efficient shop‐welded and field‐bolted structural systems, are used in many countries. Very limited research has been carried out on such systems under fire conditions. This paper presents experimental and numerical investigations of the behavior of beam and bolted splice connections in steel column‐tree moment resisting frames exposed to fire. Two full‐scale steel sub‐frames with different splice connections were tested under International Organization for Standardization (ISO 834) fire. The observation of thermal and structural fire behaviors including temperature histories, temperature deflection of the beam, temperature rotation of splice connections, and failure modes was investigated. The beam splice connection failed because of shear fracture of top bolts at temperatures beyond 750 °C, while beam underwent large deflections of more than span/20. In addition, detailed 3‐D finite element models were developed to simulate the structural behavior of the specimens in fire. Obtained numerical results from the finite element analysis successfully simulated the experimental fire test results. Copyright © 2015 John Wiley & Sons, Ltd.     

Fire tests of steel-to-timber connections with high fire resistance

This paper presents the results of twenty-one fire tests on unprotected steel-to-timber connections with dowel-type fasteners and one or two slotted-in steel plates. The experimental results of this type of connections available in existing studies have been mostly limited to a fire resistance duration of 60 min. The tests performed in this study target a significant increase of fire resistance to reach 90 and 120 min. Two configurations of connections with one and two slotted-in steel plates, i.e., two or four shear planes, were tested. The temperatures were measured at different locations in the wood members and along the steel plates. Two load levels for fire tests were determined on the basis of tests performed at ambient temperature. The experimental results show that the specimens proposed for steel-to-timber connections with dowel-type fasteners are suitable for achieving fire resistance of 90 and 120 min.     

Temperature distribution in a nailed gypsum-stud joint exposed to fire

Performance of wood-stud walls depends on the integrity of nailed connections between the sheathing and the framing members. The performance of nailed connections has been studied at room temperature, but the effects of intense thermal loads, such as those from fire, are still poorly understood. This study examines the temperature distribution within nailed joints exposed to fire; this information is essential in modeling strength and stiffness of connections in wall systems. The finite-element method was used to determine the effects of wood density, nail size, and type of gypsum board on temperature distribution within a set of connections. Temperature distributions were verified in nailed joints exposed in fire tests conducted in accordance with ASTM E119-88. The principal path of heat flow through the connection was along the nail, rather than directly through the interface between gypsum board and wood. Wood species, type of gypsum board, and nail length did not change temperature distribution significantly.     

Experimental fire analysis of steel‐to‐timber connections using dowels and nails

The paper describes and discusses the results of an extensive testing programme on the structural behaviour of timber connections under ISO‐fire. The results of reference tests performed at normal temperature are also presented. From the variety of timber connections multiple shear steel‐to‐timber connections with dowels and slotted‐in steel plates and connections with steel side plates and annular ringed shank nails were experimentally studied. Particular attention was given to the analysis of the efficiency of different strategies in order to increase the fire resistance of the timber connections. The test results showed that unprotected multiple shear steel‐to‐timber connections with dowels designed for normal temperature reached a fire resistance of about 30 min. A reduction of the load level applied during the fire did not lead to a significant increase of the fire resistance. By increasing the side timber members as well as the end distance of the dowels by 40 mm the connections reached a fire resistance of more than 70 min. Connections protected by timber boards or gypsum plasterboards showed a fire resistance of around 60 min. Thus, from a fire design point of view these strategies were favourable in order to increase the fire resistance of the connections significantly. Unprotected connections with steel side plates and annular ringed shank nails failed already after about 12 min due to large deformations of the nails and the steel side plates directly exposed to fire. By protecting the steel side plates using an intumescent paint the fire resistance of the connections was increased to around 30 min. The test results enlarged the experimental background of timber connections in fire significantly. Copyright © 2009 John Wiley & Sons, Ltd.     

Fire performance of gusset connections in glue-laminated timber

This paper describes a comprehensive experimental investigation of the fire performance of nailed gusset connections between large glue-laminated timber members. Both plywood and steel gusset plates were investigated with a range of loaded and unloaded test methods. The principal conclusions are that unprotected gussets have poor fire performance, but that a layer of solid wood or gypsum plasterboard will provide at least one hour of fire protection to typical joints.     

Tensile and fatigue investigations of timber joints with glued-in FRP rods

Glued-in rods are an effective method to form timber connections that are increasingly in the focus of research. Compared to steel rods, fibre-reinforced polymer (FRP) bars provide higher resistance against corrosion, reduced weight, and lower heat conductivity. Despite excellent mechanical performance, high fire resistance, and improved aesthetics, they are, however, not yet widely used due to lack of design regulation. This is particularly true for cyclic loads, where determination of fatigue characteristics depends upon time-intensive experimental procedures. In this research, 50 glued-in FRP rod specimens with different embedment lengths were manufactured and tested in uniaxial tension: a first set under quasi-static load and a second set under cyclic load. For the fatigue tests, a new approach based on a stepwise load increase was used to estimate fatigue strength, a method that aims at reducing the experimental effort usually associated with Woehler curves. Experimental results indicated that quasi-static and fatigue strength increased with the overlap length up to an apparent maximum. The estimated fatigue strength from the load increase tests (LITs) was approx. 41% of the quasi-static strength. Additionally, constant-amplitude tests were performed and the obtained data points fit well into established Woehler curves for timber.     

Postfire mechanical properties of Galfan-coated steel cables

Galfan-coated steel cables are widely used in prestressed structures due to their excellent mechanical properties and corrosion resistance. Their postfire mechanical properties are important to evaluate the residual load-bearing capability of the structures after fire. However, the research on the postfire mechanical properties of Galfan-coated steel cables is stillscarce, especially the cable in stress state. Hence, a research based on experimental study was carried out to investigate the deterioration of mechanical properties of Galfan-coated steel cables in stress state after experiencing elevated temperatures in this paper. Eighteen tensile tests were conducted on specimens exposed to elevated temperatures varying from 100°C to 500°C and then cooled down to ambient temperature in air. Both twisting characteristic and constant stress level of Galfan-coated steel cables were considered in this study. The residual nominal yield strength, ultimate strength, elastic modulus, fracture strain, and stress-strain curves of Galfan-coated steel cables after experiencing elevated temperatures were obtained and compared with the existing researches. The results show that the postfire mechanical properties are obviously decreased when the fire temperature exceeds 300°C. Equations for the residual nominal yield strength, elastic modulus, ultimate strength, and fracture strain of the Galfan-coated steel cables were proposed. Furthermore, a modified two-stage Ramberg-Osgood model for Galfan-coated steel cables after experiencing elevated temperatures was established, which can provide reference for the safety assessment and repairment of prestressed structures after fire.     

高温下钢筋与混凝土粘结锚固性能试验研究

为研究温度对钢筋与混凝土粘结性能的影响,通过制作30个标准立方体试块、8个温度场试件及15个中心拉拔试件,分别完成了室温(20℃)、100℃、200℃、400℃、600℃下标准立方体试块抗压试验与抗拉劈裂试验、拉拔试件温度场试验及中心拉拔试验.分析了高温作用对混凝土抗压强度与抗拉强度的影响,根据温度场试验研究结果,提出一种简易的高温下中心拉拔试验方法,在此基础上研究了高温下钢筋与混凝土粘结性能退化规律.基于Harajli模型综合考虑温度对粘结强度、峰值滑移及试件破坏模式的影响,提出了高温下钢筋与混凝土粘结-滑移本构模型.试验结果表明:高温下混凝土强度、钢筋与混凝土的粘结强度随温度升高整体呈下降趋势,但在100℃时发生陡降现象,高温下钢筋与混凝土的粘结强度变化趋势与混凝土抗拉强度相近.最后提出了高温下钢筋与混凝土的粘结-滑移本构模型,并验证了模型的适用性.     

Strength of polybenzimidazole and phenolic laminate-to-metal joints

Stress concentration effects and strengths of bonded and bolted butt joints were investigated for a glass fabric polybenzimidazole lalminate at room temperatuer and 700°F for a gloass fabric phenolic laminate at room temperature and 500°F. Specimen configurations included: (1) standard tensile specimen, (2) stress concentration specimen, (3) bolted double shear butt joint, (4) bolted single shear butt joint, (5) bonded double shear butt joint and (6) bonded single shear butt joint. Both polybenzimidazole and phenolic laminates exhibited high room temperature tensile strengths and little degradation of that strength occured as a result of elevated temperature exposure. However, low joint effencies (22 to 32%) were obtained for bolted butt joint specimens. Although bonded joints exhibited higher efficiencies, they suffered from a thermal expansion mismatch between the plastic laminate and the Inconel butt plates.     

玄武岩纤维增强塑料筋混凝土黏结性能的梁式试验研究

设计制作了14个玄武岩纤维增强塑料筋（BFRP筋）混凝土梁式试件和2个钢筋混凝土梁式试件,通过梁式试验分析了影响BFRP筋混凝土黏结性能的主要因素。结果表明,（1）BFRP筋的受力过程可分为微滑移段、正常滑移段、加速滑移段和下降段;（2）当BFRP筋的锚固长度相同时,随着混凝土强度的提高,黏结强度随之增大;（3）当混凝土强度相同时,随着BFRP筋锚固长度的增加,黏结强度明显减小,并且试件的破坏模式也发生了改变;（4）BFRP筋直径的大小对黏结强度的影响不明显;（5）当筋直径、锚固长度和混凝土强度相同时,BFRP筋混凝土的黏结强度与钢筋混凝土基本相当;（6）BFRP筋的外形对BFRP筋混凝土的黏结性能有着较大的影响。     

Retained properties of concrete exposed to high temperatures: Size effect

Concrete as a construction material is likely exposed to high temperatures during fire. The retained properties of concrete after such exposures are still of great importance in terms of the serviceability of structures. This paper presents the effects of high temperatures on the physical, mechanical, and microstructural properties of concrete. Specimens with different sizes were exposed to high temperatures ranging from 200 to 1200^°C. The compressive strength, splitting tensile strength, ultrasonic pulse velocity, and rebound numbers of the specimens were determined. The microstructures of the specimens were examined by scanning electron microscope (SEM) analyses. The test results indicated that the retained compressive strength of concrete considerably decreased with increase in temperature. The effect of specimen size on the retained compressive strength was not pronounced. The retained splitting tensile strength of concrete remarkably reduced as the temperature was increased. The specimen size played an important role on the retained splitting tensile strength of concrete up to 400^°C. The test results revealed that ultrasonic pulse velocity (UPV) test can be successfully used in order to check the uniformity of fire‐damaged structures. The rebound numbers decreased with increase in exposure temperature. SEM studies on specimens exposed to 800^°C revealed significant changes in the microstructure of the concrete. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental study on insulative properties of intumescent coating exposed to standard and nonstandard furnace curves

This paper reports the results of an experimental study on two types of intumescent coating exposed to the ISO834 standard fire and three nonstandard fire curves. The nonstandard fires were all less severe than the standard fire. A total of 72 intumescent coating protected steel specimens were tested. The expanded thickness of intumescent char was measured, and the pore feature was observed. Constant thermal conductivity for each specimen was calculated based on the measured steel plate temperature. Thermogravimetric analysis (TGA) test was carried out, and the results show that more gas is trapped within the coating due to better matching of thermal behaviour between gas evolution and polymer viscosity as the rate of heating increases. The constant effective thermal conductivities for the intumescent coating under the nonstandard fires were 65% (type‐W) and 35% (type‐S) higher than that under the standard fire, which resulted in an overestimation of the coating failure time up to 15 and 11 minutes, respectively. Therefore, it is sometimes insecure to use results from standard fire tests guiding the design of coating thickness for steel elements under nonstandard fire conditions.     

Effect of temperature on mechanical properties of ultra-high performance concrete

High temperature mechanical property data are needed for evaluating fire resistance of structural members. Being a relatively new construction material, there is a lack of temperature-dependent mechanical property data on ultra-high performance concrete (UHPC). To address this knowledge gap, this paper presents results from an experimental study on the effect of temperature on mechanical properties of UHPC. Specimens made of two UHPC mixes: one with only steel fibers (UHPC-S) and the other with hybrid fibers, that is, both steel and polypropylene (UHPC-H), were tested under different heating conditions in 20 to 750°C temperature range. Compressive strength, tensile strength, stress-strain response, and elastic modulus of UHPC were evaluated at various temperatures. Results generated from these property tests on UHPC were compared with property relations specified in design codes for conventional normal strength concrete (NSC) and high strength concrete (HSC). The comparisons show that UHPC experiences faster degradation in compressive strength and elastic modulus as compared to conventional concrete. However, UHPC exhibits slower degradation in tensile strength and ductility at elevated temperatures due to the presence of steel fibers. Data generated from these property tests were utilized to propose relations for expressing the mechanical properties of UHPC as a function of temperature and these relations can be used as input to numerical models for evaluating fire resistance of structures made of UHPC.     

Load-bearing Capacity of Nailed Joints Exposed to Fire

Nailed joints with wood members were exposed simultaneously to standard fire and constant load. Different loads were applied in the range of 0.1 to 0.6 of the estimated failure load at normal temperature. Measurements of the rate of charring and the temperature were used to determine the temperature profiles and further to estimate the reduction in the strength properties. The test results are compared with calculations based on the theory of K. W. Johansen. Three different relations between the reduction of embedding strength and the temperature were investigated. For one of these relations, the theory agrees well with the results obtained in the fire tests. The results of the calculations also show that there is a considerable reduction of the embedding strength during the first period of fire.     

Effect of temperature and cooling regime on mechanical properties of high‐strength low‐alloy steel

This paper presents results from experimental studies on the effect of temperature on mechanical properties of high‐strength low‐alloy ASTM A572 steel commonly used in structural members in bridges. A set of high‐temperature tensile strength tests and post‐temperature exposed residual strength tests is carried out on ASTM A572 steel coupons in 20–1000 °C temperature range. The residual strength tests on high‐temperature exposed steel coupons are carried out after subjecting the coupons to two methods of cooling, namely, air cooling and water quenching. Results from these tests indicate that temperature‐dependent strength and stiffness degradation in A572 steel follow the same trend as that of carbon steel but with some variations. A572 steel recovers almost 100% of its room temperature yield strength when heated to temperature up to 600 °C, regardless of the method of cooling, while the extent of strength degradation in coupons subjected to heating beyond 600 °C is dependent on heated temperature and method of cooling. Data generated in these tests are utilized to generate high‐temperature stress–strain and residual stress–strain response of A572 steel. These results are also utilized to propose temperature‐dependent strength, elastic modules, and residual strength reduction factors of A572 steel, which can be used in evaluating residual response of fire‐exposed steel structures. Copyright © 2015 John Wiley & Sons, Ltd.     

Adhesive joints in glass structures: effects of various materials in the connection,thickness of the adhesive layer,and ageing

This paper reports on an investigation of glued joints in glass load-bearing structures, with reference to the effect of various substrates (glass, steel, stainless steel, aluminium) and their surface treatment (sandblasting for the glass surface) on the adhesion of selected adhesives. The thickness of the adhesive layer and the effect of artificial ageing – a simulation of 5 years of ageing in outdoor central-European conditions – are also discussed. Tensile and shear tests were carried out on three series of specimens with various adhesives and substrates – two series for tensile and shear tests, and one series for shear tests on specimens exposed to ageing. Our results show that sandblasting the glass surface can improve the adhesion, and thus the strength values, of an adhesive joint in cases where, with a smooth glass surface, cohesive failure is not reached. The thickness of the adhesive layer had a significant effect for a semi-rigid acrylate adhesive, where the joint achieved higher strength values with less thickness of the glue. The effect of ageing varied according to the adhesive. The most visible changes were observed for a two-component acrylate adhesive and for methacrylate UV-adhesives. One of the selected glues was marked as unsuitable for load-bearing connections due to significant worsening of its mechanical properties after ageing.     

复合材料单钉搭接单剪与双剪的挤压破坏试验研究

复合材料机械连接在航空结构中应用广泛,在受力过程中,挤压破坏为主要关注的破坏模式。对于复合材料单钉连接结构,单剪有面外弯曲存在,双剪可视为孔边纯挤压受力模式。采用试验的方法,研究了相同孔径、厚度及铺层的单钉单剪与单钉双剪的破坏模式、挤压变形及屈服和挤压强度,发现单剪形式的二次面外弯曲对强度及变形影响大,双剪结构大幅提高了复合材料机械连接结构的屈服强度和挤压强度。在不改变铺层比例的前提下,通过改变铺层数量,探讨了层合板厚度对单钉单剪受力及破坏的影响,发现层合板的厚度增厚对单钉单剪的屈服及挤压强度提高不显著。     

Bond durability of wood-glue joints on Kapur (Dryobalanops spp.) and Yellow Seraya (Shorea spp.) bonded with phenol-formaldehyde resin adhesive

Bond durability of wood-glue joints on Kapur (Dryobalanops spp.) and Yellow Seraya (Shorea spp.) bonded with phenol formaldehyde resin was investigated. The tensile shear strength of wood-glue joints bonded and tested thirteen years ago were compared with samples tested after thirteen years conditioning. The effect of treating wood (boiling for two to thirtytwo hours) on the tensile shear strength of the wood-glue joint was also taken into consideration. The tensile shear strength of Kapur and Yellow Seraya treated for 20–24 and 15–20 h, respectively, before gluing was higher than that of the untreated samples which were bonded and tested thirteen years ago. The retention ratio of tensile shear strength of Kapur after 13 years was lowest at 20–24 h boiling. Likewise, results for specimens similarly tested showed the same pattern for both dry and wet conditions. However, variation of boiling time on the solid wood of both raw materials did not affect their ratios of decrease in tensile shear strength. Bond durability of Yellow Seraya wood-glue joints was higher thanthat for Kapur wood-glue joints as indicated by the higher retention ratio of the tensile shear strength of the former compared with the latter.     

Using the microstructure and mechanical behavior of steel materials to develop a new fire investigation technology

In the study, an A36 steel board, a frequently used building construction material, was heated to a high temperature, and then a metallographic replication experiment and tensile experiment were performed to obtain the composition and proportion of the microstructure and the mechanical behavior of fire‐damaged steel boards. When the steel board was heated to 800°C or higher and then rapidly water cooled, significant changes were found in its composition and proportion. More specifically, pearlite was completely lost, ferrite was reduced from 80% to 30%, bainite was increased to 30%, and martensite was also increased to 40%. The significant increase in the martensite phase altered the structure of the fire‐damaged steel board by making its structure more delicate and loose. Even though the yielding strength and tensile strength showed a tendency to increase, element ductility dropped from 32.5% to 15%. Reducing the extensibility substantially can make the steel board more likely to crack suddenly. The aim of the study is using changes in the structure and mechanical behavior of these steel components because of high‐temperature burning to reconstruct fire spread in fire investigation technology.     

Effect of thermal cycling on tensile properties of degraded FML to metal hybrid joints exposed to sea water

In the present paper, the mechanical properties of hybrid bonded bolted joints between Fiber metal laminate (FML) and stainless steel adherends are investigated using experimental tensile tests. Three and five layered FMLs were fabricated using 430 stainless steel sheets and fiberglass prepreg layers. The adherends were bonded by AD-314 resin mixed with HA-34 hardener as adhesive and steel bolt was used for the mechanical fastening. The specimens were immersed into the sea water for 30 days and degradation of the mechanical strength of the joints was studied. Thermal cycles including heating (40 °C to100 °C) and cryogenic (−100 °C to −40 °C) cycles were applied in order to study their effects on the strength of the degraded joints. The failure mode for the adhesive bond was mixed failure and that of the bolted joint was the net-tension failure. The results showed 52% strength recovery in hybrid joints subjected to heating cycles. Cryogenic cycles also caused a 50% improvement in the tensile strength of the hybrid joints. In addition, the joint stiffness and absorbed energy of the specimens were improved significantly for both heating and cryogenic cycles. Moreover, the effect of FML stacking sequence on the results was also investigated. The results revealed that the mechanical fastening failure load for 5 layered FML joint is more affected by thermal cycles in comparison with 3 layered FML joint.