Reliability of cold-formed steel screwed connections in tilt-and bearing

[摘要] ENGLISH ABSTRACT: The South African National Standard for the structural use of cold-formed steel (SANS 10162-2)provides capacity prediction models for screwed connections. Screwed connections aredesigned against shear failure of the screw(s), section tear-out, net section failure and tilt-andbearingfailure. Previous studies (Rogers & Hancock, 1997) showed that the capacity is typicallydetermined by the tilt-and-bearing type failure mode. The aim of this document is to report onthe reliability of single screwed connections in cold-formed steel against this critical failuremode.Predicted nominal capacities depend on the ultimate tensile strength of the steel, the thicknessof the connected plates and the diameter of the screw. Design capacities are obtained bymultiplying the nominal capacities by a capacity reduction factor of 0.5, according toSANS 10162-2. Reliability is assessed by means of FORM analyses, taking uncertainty in theprediction model and variability of input parameters into account.Laboratory testing of 222 single screwed connections allowed to statistically describe the modelfactor, i.e. the ratio of actual tested- over unbiased predicted capacity. For each connection, thesteel strength, plate thickness and screw diameter were measured, with the measured valuesused to predict capacity. This implies that the model factor accounts for uncertainty in theprediction model and experimental setup, while the variability of input parameters is separatelyaccounted for through appropriate statistical modelling.Variability in the input parameters was described using appropriate statistical distributionsfrom expert literature (Holicky, 2009:199; JCSS, 2000). For steel strength, the mean value andstandard deviation were obtained from tensile tests, while mean values and standard deviationsof the plate thickness and screw diameter were obtained from the above mentionedmeasurements.The experimental work and numerical analysis resulted in a model factor with a mean justexceeding unity and a small standard deviation. This suggests that the design code underconsideration is able to accurately predict the nominal capacity of screwed connections. TheFORM analysis resulted in computed reliability indexes significantly higher than thecorresponding target values which suggest conservative and reliable design formulations.Die eksperimentele werk en numeriese analise het gelei tot 'n model faktor met 'n gemiddeldhoër as een en 'n klein standaardafwyking. Dit dui daarop aan dat die ontwerp-kode onderoorweging in staat is om die nominale kapasitiet van skroef verbindings akkuraat te voorspel.Die betroubaarheid analise het gelei tot betroubaarheidsindekse aansienlik hoër as dieooreenstemmende teiken waardes wat daarop dui dat die ontwerp formulerings betroubaar enhoogs konserwatief is.

[发布机构] Stellenbosch University