Pascual, Romero, and Tizani 2015

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This paper discusses the effect of fire on blind-bolts which are used to connect concrete filled tubes (CFT) in tension. There has been minimal testing done on blind-bolts because they typically have a lower temperature than the columns that are jointed due to their lower section factor A/V. The two types of bolts to be tested are hollo-bolts (HB) and extended hollo-bolts (EHB).

Analytical Study

Three-dimensional numerical models were created using FEA package ABAQUS in order to validate the calculations of the connections studied. The two connections studied were a single blind-bolt, and two T-stubs, and the procedure attempted to reproduce the procedure for the experimental study. Primarily, the torque was applied to the hollow steel section (HSS) and was followed by a tension load being applied after the concrete was added, which led to failure when the velocity of deformation increased very quickly. This model displayed differences in ratio ξ for the maximum load of below 11% for all bolts tested, proving that the simulation is an accurate representation of thermal behaviour. Concrete and steel plasticity models were performed in order to get various property values for each specimen. As the simulation provided accurate measurements for connection failure at elevated temperature, a thermal and structural analysis, were performed for both HB and HSS column, HB and CFT column, and EHB and CFT column. Three-dimensional eight-noded heat transfer brick elements were used to conduct heat through the specimens. For the structural analysis, a tensile load of 50% of the maximum load was applied to the blind-bolts. It was concluded that with an increase in temperature, steel, normal steel, and high strength steel bolts display varying failure modes and fire resistance ratings (FRR).

Results of Single Blind-Bolted Connections

The failure either occurred at the shank bolt or the sleeve. At the shank bolt, the temperature was highest and the steel strength was lowest, whereas the highest stress occurred at the sleeve. In the HB connection to HSS column, failure occurred at both the shank and the sleeve at the same time, whereas in both connections with CFT columns, the failure at the shank occurred first because the stress was distributed throughout the concrete as well as the sleeve. The ultimate strength capacity was reached at the shank bolt in all three connections. Fire resistance rating represents the time that the connection sustained the load before failure. Connections with CFT connections had a FRR increase of 16-20% (4-5 minutes) compared to HSS connections. Furthermore, the connections to CFT columns reached an FRR of 36 minutes with a 20% load.

Results of Double T-Stub Connections

For the double T-stub connections, plastic deformation must also be noted, and helps detect failure mode. With the HSS columns, concrete prevented deformation, whereas the connections for the CFT columns failed at the shank bolt, and high plastic strain occurred in the sleeve of concrete. The FRR increased by an average 25% (4 minutes) as compared to unfilled columns. Furthermore, no difference was observed between Hollo-bolt and the extended-hollo bolt.

Parametric Study

In order to determine the parameters on the connection performance, the properties of steel bolts were varied on the steel capacity and the connection response at different temperatures was determined. Fire resistant (FR) steel bolts were studied, and it was concluded that their chemical composition allows for better strength retention, and provides an increase in 4 minutes FRR compared to normal steel in the single blind-bolted connection, and an increase of 2 minutes FRR for the T-stub connections.

References

Pascual, A., M., Romero, M., L., and Tizani, W. (2015). “Fire performance of blind-bolted connections to concrete filled tubular columns in tension.” Engineering structures 96 (1), August, pp. 111-125. doi:10.1016/j.engstruct.2015.03.067