A New Improved Type of Friction Connection : An Experimental Study

This master thesis is a part of a European research project named “Prolonging life time of old steel and steel-concrete bridges” or simply “PROLIFE”. As the name indicates the purpose is to strengthen existing bridges. The part of the project that this thesis deals with is developing and testing a new innovative friction grip connection being used to connect a truss with the bottom flanges of beams in order to make two or more beams to act like a steel box girder. The thesis also evaluates the friction grip connection experimentally at the research facility Complab, the largest laboratory at Luleå University of Technology. By increasing the effective friction coefficient the entire friction connection can withstand larger shear forces.The hypothesis is to increase the friction coefficient between the plates in a lap connection by inserting small indenters between the plates. In this thesis three types of indenters have been used, aluminium oxide, stainless steel and tungsten carbide. All types of indenters have been tested in a compression test between two plates. The aluminium oxide and stainless steel indenter types have also been loaded in shear in a friction connection after compression of the two plates.The necessary pre-load to impress an indenter of 2.5 mm in diameter 2.3 mm into two plates is 11 kN. The remaining 0.2 mm gap between the plates is due to the crater developed at an impression point. The protruding edges of craters on opposite plates are brought into contact with each other at 2.3 mm indentation.The aluminium oxide increases friction but the effect is inconsistent and uncertain, since the indenters are crushed and indenting is not identical between different tests. With small indenters made of stainless steel the effective friction coefficient (µ) of the friction connection can also be improved. How much (µ) can be increased by small indenters in a friction connection depends on the existing surface condition or treatment of the plates in a joint. The tests indicate that the effect of the indenters almost equals that of a shot or grit blasted surface with loose rust removed and no pitting. A connection with indenters provides further a certain safety against overloading and the definition of the procedure is unambiguous. In a reference test with just as rolled surface conditions and no indenters, the slip resistance was 54.5 kN and the effective friction coefficient μ=0.09. With 58 indenters and an M30 bolt with a pre-loading force of 320 kN the slip resistance can be expected to slightly exceed 250 kN and the effective friction coefficient to be somewhat smaller than μ=0.40, at the acceptable slip of 0.15 mm, according to the current Eurocode standard. The slip resistance from the tests with different number of indenters and without indenters is shown in Figure 36 and Figure 37. Without the slip criterion the expected slip resistance for the same configuration can be 310 kN and the effective friction coefficient somewhat smaller than μ=0.50. See Figure 39.Validerat; 20160629 (global_studentproject_submitter