experiments of bolted cover plate connections with slotted holes

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  • CTU Reports, Vol. 6, 2/2002 Praha, ISBN 80-01-02526-8



    Key words: Steel structures, Bolted connections, Slotted holes, Cover plate connections, Experimental observations, Bolt bearing resistance, Bolt shear resistance, Eurocodes. The work describes the tests of the bolted connections with slotted holes perpendicular to the acting force. Two sets of tests simulating behaviour of components of cover plate connections were carried out in laboratory of Czech Technical University. This paper summarises these tests with components and results of the tests of TU Nottingham and of CRIF laboratory, Liege. The results of preliminary statistical calculations made with Annex Z procedure are presented. The obtained reduction factors applicable to the bearing resistance of bolted connections are ready to be incorporated into the European design rules.

    1. Introduction The slotted holes are used in bolted connections of steel structures to compensate the tolerances during the erection. Since the first application of high-strength friction bolts in 1947, the oversize bolt holes 2 mm larger than the bolt diameter have been used for assembly [Kulak et al, 1987]. Restricting the nominal hole diameter to 2 mm in excess of the nominal bolt diameter can impose rigid alignment conditions between structural members, particularly in large joints. The erection problems occur when the holes in the material do not line up properly. Occasionally, steel fabricators have to pre-assemble structures to ensure that the joint will align properly during the erection. With a larger hole size, it is possible to eliminate the pre-assembly process and save both time and money. An oversize hole provides the same clearance in all directions to meet tolerances during erection. However, if an adjustment is needed in a particular direction, slotted holes are used, as shown in Figure 1. Slotted holes are identified by their parallel or transverse alignment with respect to the direction of the applied load. The application of NC cutting machines and the digitalisation of the geometry of elements bring rather lower tolerances into the production of the structural elements [Kersten, 1997]. This allows use of long slotted holes and slots in inclination at the primary beams and columns for connection to different endplates of particular cross-sections of connected beams. Deformation of the connection in direction of the slot is prevented by preloading of the bolts. Smooth tightening of the non-preloaded bolts reach up to 20% of the tensile resistance of the bolt and together with the corrosion pretends structural use of the slip in the joint [Bickford, 1995].

    The design requirements limit the bolt end and pitch distances. For regular slotted holes for M16 and M24 bolts, short slotted holes may not be greater than (d + 2) mm by (d + 6) mm, where d is the nominal bolt diameter in mm. Long slotted holes may not be greater than: (d + 2) mm by 2,5 d. Extra large slots according to British rules may not be greater than (d + 2) mm by 3,5 d.

    Drilling, punching, or cutting form the holes. Punching of the holes in steelwork is faster than drilling but cracks may appear in the material. Therefore, the holes are not punched to full size but 2 mm less in diameter and then reamed. New punching machines, 79

  • CTU Reports, Vol. 6, 2/2002 Praha, ISBN 80-01-02526-8

    which operate at high speeds, induce less distortion in the material, and it is expected that punching will be used extensively in the future. The punching is approved for material up to 25 mm in thickness provided that the hole diameter is not less than thickness of the material, if there is no other specification. The burrs should be removed from the holes before the assembly. It can be omitted when the holes are drilled in one operation through parts clamped together which would not be separated after drilling. The gas and plasma cutting may also form the holes. In this case, similarly to fast drilling/punching, influence on material properties has to be studied experimentally.

    Limited knowledge is available about the design of connections with slotted and oversize holes. Resistance of bolted connections loaded by shear force in plane of the plates is limited by failure of bolts in shear and in bearing. The same design limits at service limit state may be applied when high strength friction bolts are preloaded and slip is allowed in ultimate limit state. When the high strength friction bolts are used and no slip is allowed, bearing resistance needs to be checked at ultimate limit state to eliminate the end shear failure. The bearing resistance represents the resistance of internal and external bolts in most design procedures. Based on experimental evidence, shear and bending type of failure may be recognised on the bearing failure. The bearing resistance is reduced in case of the slotted holes. The reduction factor is based on the best engineering practice and few unpublished tests only. In the European design rules [ENV 1993-1-8, 2001], the slotted holes are taken into consideration by calculation of the slip resistance of preloaded bolts only.

    e4 1,5 d

    e3 1,5 d


    0,5 d0




    Figure 1 : Geometry of the slotted hole and the requirements for the end distances

    a) b) Figure 2 : Test arrangements a) and points of measurement b) [Mazura, 2002]


  • CTU Reports, Vol. 6, 2/2002 Praha, ISBN 80-01-02526-8

    a) b) Figure 3 : Bolt bearing failure of component tests [Mazura, 2002], a) shear failure,

    b) bending failure 2. Tests with Components 2.1 Main Tasks

    Three kinds of tests were carried out in view to determine the bearing resistance, the deformation stiffness and the ductility. At CRIF, Liege the assemblage with more bolts has been tested [Piraprez, 2000]. The plates were connected with bolts in each hole of the plate made by different technology. At University of Nottingham the experiments were carried out in order to investigate the behaviour of the very long slots in the plate [Tizani, 1999]. At the Czech Technical University of Prague it was planned to study the plate bearing capacity by the component method [Wald F., Sokol 1999] and [Mazura et al, 2001]. Thus double cover plates were used with only one bolt in an internal/external position. The design prediction model of stiffness and resistance including gap and using component method is under preparation at Czech Technical University with cooperation of Universit B. PASCAL Clermont, laboratory LERMES-CUST.

    The available tests are shown in Tables A1.1, A1.3 and A1.5. The original name of test specimens is kept in order to refer to the original paper. The bolt diameter, the number of bolts and the slot length follow the original name to describe the major parameters of tests. Additional information is included: the bolt presence in the holes, the thickness of the cover plates t, the slot length and the hole in the internal plate, the joint geometry (e1; e2; p1), the method of production of the holes and the threaded or unthreaded part of the bolt in the shear plane. The measured values are summarized in Tables A1.2, A1.4 and A1.6: the reached deformation exp;ult. the maximal measured set-up resistance, the ratio to the predicted value with circular hole and the failure mode.


  • CTU Reports, Vol. 6, 2/2002 Praha, ISBN 80-01-02526-8


    2.2 Test Set-Up

    3A, 3B, 3C 1A, 1B, 1C

    Test 3A-16-1- 3d Test 5B-16-1-2,5d Test 1A-16-1-d+2

    5A, 5B, 5C

    Tests with the bolts in the external holes were the major tests [Mazura, 2002] and tests with the bolts in the internal holes complete the observations [Moal, 2001]. The test arrangements and the points of measurement can be seen at Figure 2. Test machine FPZ 300 with load capacity 300 kN was used for the tests. The geometry is summarized at Figure 4 and Table A1.3, which also include the measured values. The characteristic values are shown in Figure 4. The bolt bearing failure of the component tests [Mazura, 2002] is described according to the type of the bearing failure as bending or shear failure [Bijlaard D02, 1989]. It was observed, that the slotted holes tends to fail in bending failure of the plate, see Figure 3.

    Figure 4 : Geometry of the test set-up test 3A-16-1-3d, 5B-16-1-2,5d and 1A-16-1-d+2 [Mazura, 2002]

    3B) 3C) Figure 5 : Bending failure of component test 3B-16-1-3 and 3C-16-1-3 [Mazura, 2002]

  • CTU Reports, Vol. 6, 2/2002 Praha, ISBN 80-01-02526-8


    0 5 10 15 20 25 30 35 40Displacement, mm

    Force, kN

    slotted holes,

    circular holes,


    (test 1C - 16-1-d+2) (test 5C - 16-1-2,5d)

    Figure 6 : Examples of force - displacement diagrams of test with circular holes

    (test 1C - 16-1-d+2) and slotted holes (test 5C - 16-1-2,5d)



    0 5 10 15 20 25 30 35

    Displacement, mm

    Force, kN Test 2A - 16-1-2,5d



    0 5 10 15 20 25 30 35

    Displacement, mm

    Force, kN Test 2B - 16-1-2,5d



    0 5 10 15 20 25 30 35

    Displacement, mm

    Force, kN Test 4C - 16-1-2,5d

    2A, 2B, 4C

    dimensions in mm,

    slots 2,5 d

    Figure 7 : Force - displacement diagram of the tests with repeated loading, test 2A - 16-1-2,5d, 2B - 16-1-2,5d, and 4C - 16-1-2,5d, [Mazura, 2002]


  • CTU Reports, Vol. 6, 2/2002 Praha, ISBN 80-01-02526-8

    2.3 Repeated loading Three tests specimens were loaded by repe