Post Elastic Behaviour and Moment Redistribution in a Double Span LTP200 Steel Trapezoidal Sheet

Cold Formed Steel Trapezoidal sheets are very popular in Sweden as a roofing system for stadiums and arenas. To effectively utilize the span and field moment capacities, Lindab International uses Gerber system of joints for one of their products LTP200 Trapezoidal Profile. In this joint, a hinge is placed in span at a predefined location near the internal support where the bending moment is theoretically zero. Within the working limit states, this joint works very well as it is not subjected to any moments. But as the section over the mid-support fails, the stiffness of this section decreases dramatically and so does its moment capacity, which is normally the case with cold formed section. As the moment capacity of this section drops, there is a redistribution of bending moments in the whole system and the joint will be subjected to moments in this post elastic scenario. Previously, the joint was not able to sustain these bending moments. Now, Lindab is modifying the design of this joint by using a combination of screws and sheet overlap. In this design, the joint will work as a hinge in working limit states, but as the section over midsupport fails, contribution of overlap is attained and the joint will now work as continuous sheet. The amount of moments this joint is subjected to; depends on the variation of stiffness of the section over mid-support and reserve capacity of this section. The aim of this thesis is to study the behaviour of the section over mid-support after its elastic limit and make conclusions about inelastic reserve capacity and stiffness variation. In the end, a moment redistribution diagram will be made for a double span system at ultimate state which will enable the design of joint against the varying internal forces.Cold Formed Steel Trapezoidal sheets are very popular in Sweden as a roofing system for stadiums and arenas. To effectively utilize the span and field moment capacities, Lindab International uses Gerber system of joints for one of their products LTP200 Trapezoidal Profile. In this joint, a hinge is placed in span at a predefined location near the internal support where the bending moment is theoretically zero. Within the working limit states, this joint works very well as it is not subjected to any decreases dramatically and so does its moment capacity, which is normally the case with cold formed section. As the moment capacity of this section drops, there is a redistribution of bending moments in the whole system and the joint will be subjected to moments in this post elastic scenario. Previously, the joint was not able to sustain these bending moments. Now, Lindab is modifying the design of this joint by using a combination of screws and sheet overlap. In this design, the joint will work as a hinge in working limit states, but as the section over midsupport fails, contribution of overlap is attained and the joint will now work as continuous sheet. The amount of moments this joint is subjected to; depends on the variation of stiffness of the section over mid-support and reserve capacity of this section. The aim of this thesis is to study the behaviour of the section over mid-support after its elastic limit and make conclusions about inelastic reserve capacity and stiffness variation. In the end, a moment redistribution diagram will be made for a double span system at ultimate state which will enable the design of joint against the varying internal forces

Shrinkage behavior enhancement of infra-lightweight concrete through FRP grid reinforcement and development of their shrinkage prediction models

Infra-lightweight concrete (ILC) is an efficient alternative of normal concrete (NC) for structural applications with low strength but high thermal performance requirements. Three types of ILCs with dry densities 600, 700 and 800 kg/m3 have been manufactured by using high water to cement ratio (w/c) and expanded clay lightweight aggregates (ECLAs). Experimental studies showed that shrinkage strains in ILCs can be up-to 1.51 mm/m as compared to 0.2–0.8 mm/m for NCs, which affects the structural durability of ILCs. To control shrinkage strains of ILCs, ILCs have been then reinforced with two types of fiber reinforced polymers (FRPs): carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP). For each type of FRP, two different grid arrangements of size 21 × 21 mm and 25 × 25 mm have been used. Although all FRP grid reinforcements are effective as they reduced the shrinkage strains significantly and close to the shrinkage strains of NCs, CFRP reinforcement with 25 × 25 mm grid is most effective as it reduced the shrinkage strains up-to the maximum level. Also, experimental shrinkage strains have been compared with five commonly used prediction models and it has been found that each model failed to accurately predict the shrinkage strains. Therefore, a new prediction model has been developed by modifying one of the existing model i.e. B3 model for shrinkage prediction of ILC which considers the effect of water content, compressive strength and dry density. Another prediction model for FRP reinforced ILCs has also been developed to incorporate the influence of any type of reinforcement in ILC. A comparison of modified prediction models with experimental results has shown that the models can predict shrinkage accurately and can be utilized for normal ILCs as well as reinforced ILCs

Bond behaviour improvement between infra-lightweight and high strength concretes using FRP grid reinforcements and development of bond strength prediction models

The structural performance of a newly developed lightweight and thermally efficient alternate of normal concrete (NC) i.e. infra-lightweight concrete (ILC) had been under question due to its low elastic modulus, surface roughness, and cracking. In the present study, the structural performance of ILC has been improved by using a layer of high strength concrete (HSC) on each side of the ILC. As the efficiency of the ILC-HSC composite structure depends on the bond between them, therefore, an extensive study has been performed to assess and improve the bond strength in two parts. In the first part, shear (push-out) and tensile (pull-off) bond strength tests have been conducted on ILC-HSC specimens which revealed that the interfacial bond strength is weaker than the weakest material i.e. ILC550. Hence, the bond strength has been improved by carbon and glass fiber-reinforced polymers (CFRPs and GFRPs) with two different grid dimensions i.e. 25 mm and 38 mm in the second part. Test results indicated that both the CFRPs and GFRPs significantly improved the bond strengths and this improvement depends on the reinforcement ratios. Maximum bond strength has been achieved for GFRP-25 reinforced ILCs where shear and tensile bond reinforcement ratios of 0.492% and 0.445% increased the shear and tensile bond strengths by 331% and 456% respectively as compared to un-strengthened specimens. In addition, the comparison of experimental shear bond strengths with five commonly used prediction models revealed the inaccuracy of all the presently available models. Moreover, there is no prediction model available for tensile bond strength prediction. Therefore, two new prediction models have been developed for shear and tensile bond strengths. The comparison of experimental results with developed models has revealed the accuracy and applicability of these models for both the un-strengthened and FRP strengthened ILC-HSC composite structures

Experimental investigations on inelastic behaviour and modified Gerber joint for double-span steel trapezoidal sheeting

Cold-formed steel trapezoidal profiles provide efficient solutions for roofing and often use the Gerber joint to effectively utilize capacities. The previous design of Gerber joint was sensitive to uneven distribution of loads and accidental loads, which imposed bending moments in the joint and lead to its failure. In this experimental program, the design of Gerber joint has been modified to work as a hinge under service loads and carry moments in accidental conditions. Also, the design of CFS is based on elastic methods that underestimate their capacity, especially for multi-span systems. Full-scale tests were conducted on highly stiffened double-span trapezoidal sheeting profiles with modified Gerber joint to investigate elastic capacity, inelastic behaviour, moment redistribution in the post-elastic phase, ultimate load capacity and feasibility of modified Gerber joint. Comparison of elastic load capacity with EWM and DSM predictions revealed that EWM design predictions were conservative by 30% while DSM predictions were accurate. For multi-span application, residual moment capacity ratios of 0.76 and 0.81 in the post-elastic phase allowed for moment redistribution and increased ultimate load capacity by 7.14% and 8.80% for 0.85 mm and 1 mm thick profiles respectively. Performance of modified Gerber joint to behave as a hinge under service loads and as continuous in the post-elastic phase was also found to be satisfactory. The study concluded that the economy in design and capacity utilization of multi-span CFS profiles can be improved by allowing for moment redistribution and using the modified Gerber joint

Effect of Elevated Temperature on the Behavior of Amorphous Metallic Fibre-Reinforced Cement and Geopolymer Composites

To improve the tensile, flexural, and ductility properties of geopolymer composites, amorphous metallic fibres (AMF) are used to reinforce these composites, and the behavior of these composites at elevated temperatures has been assessed in this study. Four types of composites, i.e., cement, reinforced cement, geopolymer, and reinforced geopolymer composites have been prepared. The composites have been reinforced using AMF with a fibre volume fraction of 0.75%. The composites have been assessed for change in mass loss, cracking, compressive strength, and flexural strength at four elevated temperatures of 200 °C, 400 °C, 600 °C, and 800 °C, and conclusions have been drawn concerning these composites. The results have shown that an increase in temperature has an adverse effect on these composites, and geopolymer composites exhibit higher performance than their counterpart cement composites at elevated temperatures. The mass loss and surface cracking were significantly lower in geopolymer composites, and the fibre reinforcement had a negligible effect on mass loss. Also, the residual compressive and flexural strength of reinforced geopolymer composites was significantly higher than that of the reinforced cement composites. In addition, scanning electron microscopic images also showed that even at higher temperatures, the geopolymer matrix is present on the AMF fibre, which results in higher residual strength than the cement composites in which a negligible amount of matrix is present on the fibres

Performance and design of steel structures reinforced with FRP composites: A state-of-the-art review

Fiber-reinforced polymer (FRP) composite materials have gained popularity in civil, mechanical, aircraft, and chemical engineering domains due to their superior mechanical properties and durability. They have been used for strength and durability enhancements of civil structures and a wide range of steel structures subject to static (flexure, compression) and dynamic (fatigue, impact, and seismic) loads have been strengthened and retrofitted. The strength enhancement provided by FRP composites to steel structures depends on several parameters including fiber types, fiber orientations, number of fiber layers, steel section types (geometry and grade), member slenderness etc. Although the superior properties of FRP are sometimes affected by severe environmental conditions that the structures are exposed to, these adverse effects can be minimized. This paper provides a comprehensive review of various techniques to improve the performance and design of steel structures using FRP composites. Strength prediction models under a range of loading and environmental conditions are presented in this single document for the evaluation and safe design of FRP strengthened steel structures and thereby minimise their vulnerability to failur

A novel winding–wedge anchorage for CFRP straps: Conceptual design and performance evaluation

Carbon fibre-reinforced polymer (CFRP) straps can be utilised as tension members in structures on account of their high-strength properties. The development of an effective anchorage device for CFRP straps that can fully utilise the strength of straps before anchorage failure is an ongoing technical challenge. To address such knowledge gap, a novel anchorage device termed winding-wedge anchorage that combines winding and wedge anchorage mechanisms is reported herein. For the winding anchorage component, different numbers of winding turns are applied. For the wedge anchorage component, three different types of wedge anchorage systems are investigated namely bonding, friction clamping, and friction non-clamping. Initially, the working mechanism, theoretical winding–wedge anchorage model (herein theoretical model), and design parameters are determined. Experimental and finite element analyses are then reported to assess the behaviour of the anchorage including failure modes and anchorage efficiencies. The accuracy of the theoretical model is also reported. It is concluded that the efficiency of the anchorage device is primarily dependent on the wedge anchorage component. Moreover, the bonding wedge anchorage component performed better than the other two wedge anchorage systems. The theoretical model and experimental results revealed that 1.5 winding turns results in an optimal interaction between the winding and wedge anchorages components