Mechanical behavior of a K-type truss joint of ordinary or high strength steel grades

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Insinööritieteiden korkeakoulu | Master's thesis
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Master's Programme in Building Technology (CIV)
This thesis studies the behaviour of a K-type joint made of high strength steel (HSS) as a part of the truss, which is often integrated into slim floor systems to allow longer spans and ease the installation of building service systems. Manufacturing the truss of high strength steel (HSS) can further improve strength-to-weight ratio of a building system. However, as current design standards may not be applied to design a truss joint of HSS, the study compares the behaviour of the HSS joint to the joint made of mild steel. The studied K-type joint consists of tubular sections as braces and a plate as a lower chord. Both experimental and numerical studies were conducted in order to analyse the joint behaviour. For the experimental studies, two scaled-down specimens made of HSS with different chord lengths were tested. Both specimens were loaded at a tension brace. The supports of compressed braces were pinned, and the chord was clamped. Load-deflection curves were used for analysing the results. The tested joints were studied by three-dimensional finite element models considering both material and geometrical non-linearities. Residual stresses and strength reduction in heat affected zone were included in material modelling. The scaled-down models were validated by the test results. By reducing the complexity of the model, the results between a scaled-down joint model and a full-scale joint model were compared, and the scaling factor was verified. Strain criteria were used to determine the limit loads for the full-scale joints of HSS and mild steel. The test results confirmed that the chord length of the joint affects its failure mode. The joint with a shorter chord failed in the tension brace, but the joint with a longer chord failed in the compression brace. The maximum load observed in the tension brace during a tension-controlled failure, which is also the joint ultimate strength, was 4% higher than the maximum load of the compression brace or the joint limit load in a compression-controlled failure. The compression failure let to more ductile joint behaviour than the tension failure. The FE models based on the measured material properties overpredicted the limit and ultimate loads of the joint by 18% and 23%, respectively. If the strength measured for materials were reduced by 10% because of residual stresses, the capacities calculated and measured for the joints matched well validating the FE models. The further simplification of the FE models by removing the endplates, stiffening plates and welds reasonably predicted the load-deflection curve up to the maximum load. The simplified FE models created for the joint in both downscaled and full-scale dimensions predicted the maximum load and the corresponding deflection similarly considering the corresponding scaling factors. Further studies indicated that the use of HSS increased the ultimate strength of the joint about 67% and the limit load with a plastic strain of 5% by about 63% compared to the joint of mild steel. The limit load estimation for the joint of HSS was proposed by adjusting the equation for effective width calculation. Further parametric studies are necessary for the further use of the proposed equation in practical design.
Puttonen, Jari
Thesis advisor
Lu, Wei
Saremi, Pooya
K-type joint, high strength steel, HSS, FEM
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