Prying Action
Design Model for Bolted Moment End Plate: The behaviour of a connection where tensile loads are transferred to fasteners through an end plate is highly dependent on the rigidity of this plate. This is demonstrated in Figure 2 where two stub tee connections are shown. In the first connection, which comprises a rigid end plate, minimal deformation occurs when the tensile load (P) is applied, with the plate remaining virtually parallel to the connecting surface.
Design Model for Bolted Moment End Plate Connections using Rectangular Hollow Sections
The second connection, which contains a flexible end plate, deforms as shown when loaded, generating compressive (prying) forces between the contacting surfaces which raises the tensile bolt forces correspondingly.Design Model for Bolted Moment End PlateThe behaviour of a connection where tensile loads are transferred to fasteners through an end plate is highly dependent on the rigidity of this plate. This is demonstrated in Figure 2 where two stub tee connections are shown. In the first connection, which comprises a rigid end plate, minimal deformation occurs when the tensile load (P) is applied, with the plate remaining virtually parallel to the connecting surface.
The second connection, which contains a flexible end plate, deforms as shown when loaded, generating compressive (prying) forces between the contacting surfaces which raises the tensile bolt forces correspondingly.Design Model for Bolted Moment End Plate: The behaviour of a connection where tensile loads are transferred to fasteners through an end plate is highly dependent on the rigidity of this plate. This is demonstrated in Figure 2 where two stub tee connections are shown. In the first connection, which comprises a rigid end plate, minimal deformation occurs when the tensile load (P) is applied, with the plate remaining virtually parallel to the connecting surface.
The second connection, which contains a flexible end plate, deforms as shown when loaded, generating compressive (prying) forces between the contacting surfaces which raises the tensile bolt forces correspondingly. Design Model for Bolted Moment End PlateThe behaviour of a connection where tensile loads are transferred to fasteners through an end plate is highly dependent on the rigidity of this plate. This is demonstrated in Figure 2 where two stub tee connections are shown. In the first connection, which comprises a rigid end plate, minimal deformation occurs when the tensile load (P) is applied, with the plate remaining virtually parallel to the connecting surface. The second connection, which contains a flexible end plate, deforms as shown when loaded, generating compressive (prying) forces between the contacting surfaces which raises the tensile bolt forces correspondingly.
The study by Nair et al. (1974) into the effect of tension and prying forces found that the load capacity of bolted connections can be substantially reduced by prying action. The factors found to govern the magnitude of this prying force include the geometry and material properties of the end plate, and the size and strength of the bolts.
If it is assumed that the connection fails due to tensile fracture of the bolts, the failure load for the rigid end plate can be easily calculated by determining the tensile strength of the bolt group. For the flexible end plate, the reduced failure load (Pu) is defined as the ultimate tensile load in the bolts (Bu) minus the
prying force at ultimate load (Qu).
The magnitude of the prying force (Q) depends on the flexibility of the end plate. As seen previously, Q is zero for the rigid end plate, while for the flexible end plate Q ranges from zero to Qmax, where Qmax is the maximum attainable prying force which occurs at the formation of a plastic hinge through the line of
the bolts.
While the connections described in this report are not a stub tee, prying forces are considered important. By modifying the stub tee analogy, a model that effectively predicts the connection strength considering the prying forces can be developed for tubular end plate connections.
Experimental Study
While two types of end plate connections (termed Type A and Type B) were investigated experimentally at the University of Sydney (Wheeler, Clarke and Hancock, 1995a, 1995b, 1997), this report deals only with the Type B connection containing four bolts, as shown in Figure 3. The connections were tested in pure bending by subjecting a beam, containing a beam splice connection (Figure 1) at mid span, to a four point bending test.
The parameters varied in the experimental programme include the plate size (Wp, Dp), the plate thickness (tp), the section shape (square or rectangular), and the position of the bolts with respect to the section flange (so) and the section web (c). Each test contained two rows of bolts, one above and the other below
the section. The dimensions of the end plates and the type of sections used for all Type B specimens in the experimental programme are given in Table 1.
The distance from the edge of the plate to the centre of the bolts (ae) was constant for all tests and set at 30 mm according to the edge distance limits specified in the Australian Standard for Steel Structures, AS 4100 (SA, 1990). All holes were clearance holes (diameter 22mm) for M20 bolts. The end plate material was 350 grade steel, to AS 3678 (SA, 1981b) with a nominal yield stress of 350 MPa. The measured static yield stress (fy) and ultimate tensile strength (fu)
for the end plates obtained through coupon tests are listed in Table 2.
The bolt and nut assemblies were M20 structural grade 8.8 assemblies (grade 8.8/T), manufactured to AS 1252 (SA, 1981a). The measured yield and ultimate tensile loads of the bolts were 195 kN and 230 kN, respectively. Further details on these bolt assemblies can be found in the manufacturerís catalogue (Ajax
Fasteners, 1992).
The connections were prefabricated to AS 4100 (SA, 1990), with a combination fillet/butt weld joining the section to the end plate. This weld was SP category and qualified to AS 1554.1 (SA, 1991c), with a nominal leg length of 8 mm for the fillet.
Upon assembly of the connection, the bolts were tensioned to 145 kN (60% proof stress). An incremental load was then applied to the connection by means of a stroke-controlled servo until failure occurred. As the sections were not susceptible to local buckling, the ultimate load of the specimen was limited to
connection failure, which occurred either when the tensile bolts fractured, or when the longitudinal deformations of the end plate were deemed excessive. The ultimate moment (Mcu) and the failure mode for each test are listed in Table 1.
In most cases the ultimate failure mode for the tests was tensile bolt failure, with excessive deformations in the end plates only occurring in those specimens containing the thinner, more flexible, end plates. In all tests, the formation of yield lines was evident well before the ultimate load was reached. As each test continued, the end plate deformations increased until either excessive deformations occurred or fracture of the tensile bolts was imminent.
Changes in the end plate width (Wp) and thickness (tp) resulted in significant changes in the ultimate load. An increase in plate thickness (tp) increased the strength of the joint (compare SHS specimens #11, #12, and #13, and RHS specimens #14, #15 and #16). An increase in the plate width (Wp) (corresponding to moving the position of the bolts away from the line of the webs as denoted by the parameter c in Table 1) reduced the stiffness and strength of the joint (compare SHS specimens #12 and #18, and RHS
specimens #15 and #21). The effect of the position of the bolts was further demonstrated through their proximity to the section flange (parameter so in Table 1). As the bolts were moved closer to the flange of the section, the connection stiffness and strength also increased (compare SHS specimens #23, 12 and #24, and RHS specimens #25, #15 and #26).
For the test specimens with the 12 mm end plate (specimens #11, #14, #17 and 20), the loads in the tensile bolts were well below their ultimate values when a yield line mechanism formed in the plate. This mechanism was such that the load transfer to the bolts was minimal, and fracture of the bolts would only
have occurred if the test was continued until very high rotations were experienced. In practice, these tests were stopped prior to the deformations becoming excessively large.
Design Model | Design Model for Bolted Moment End Plate
The design model presented here relates to a bolted moment end plate connection connecting square or rectangular hollow sections. The connection is assumed to consist of two rows of high strength structural grade bolts (8.8/TB), which are tensioned to the minimum bolt tension as specified in AS 4100 (SA,
1990). The layout of the end plate is shown in Figure 14, with the depth of the beam section (d) assumed to be no greater than 400 mm.
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