Behaviour of Web Moment Connections

The four specimens were designated as 14–1 to14−4; 14–1 and 14–2 are flange-welded web-bolted connections, 14–3 is a fully-bolted connec-

FIG. 6.24. Web connection test setup.

tion and 14–4 is a fully-welded connection. The test setup for these specimens is shown in Fig. 6.24. The connection assemblages were designed in such a way that, at the predicted plastic limit load, the beam would resist a beam plastic moment Mp and a beam shear V approximately equal to 0·81 Vp, whilst the column would resist an axial load P approximately equal to 0·50Py. The material used for these tests was A572 Grade 50 steel (Fy=55 ksi (379 MPa)).

Figures 6.25 and 6.26 show the joint details of tests 14–1 and 14–2, respectively. In connection 14–1, the beam flanges are groove welded to the flange moment plates which in turn are fillet welded to the

FIG. 6.25. Joint details of connection 14–1 (all dimensions in mm).

column flanges and web. The beam web is connected to a shear plate by seven 7/8-in (22 mm) diameter A490 high strength bolts. This shear plate is attached to the column web and flange moment plates by fillet welds.

For connection 14–2, the beam flanges are groove welded directly to the column web and the beam web is connected to the column web by

FIG. 6.26. Joint details of connection 14–2 (all dimensions in mm).

two structural angles of size (89 mm×89 mm× 9·5 mm). These angles are fillet welded to the beam web and bolted to the column web by eight 3/4-in (19 mm) diameter A490 bolts.

The load-deformation behaviour of these two connections is shown in Figs. 6.27 and 6.28, respectively. From Fig. 6.27, it can be seen that although connection 14–1 possesses the required strength and stiffness,

FIG. 6.27. Load-deflection curve of flange-welded web-bolted connection 14–1.

FIG. 6.28. Load-deflection curve of

flange-welded web-bolted connection

14–2.

it lacks ductility due to fracture of the tension flange moment plate near the groove weld that joins with the tension beam flange. From Fig. 6.28, it is clear that connection 14−2 lacks both strength and ductility. This is attributed to the severe out-of-plane deformation of the column web and flanges. The stress concentration that builds up in the beam tension flange finally causes fracture of the column web. In order to reduce the out-of-plane deformation and to alleviate the stress concentration, back-up stiffeners and moment plates can be used. It has been shown (Rentschler, 1979) that the presence of back-up stiffeners on the other side of the column can reduce the column web deformation substantially and alleviate stress concentration as they attract forces from the beam. The use of moment plates alleviates the problem by delivering the beam forces to the column flanges as well as the web, so that both the column flanges and web can take part in resisting the beam forces.

In these figures, the symbol Vmp is the shear force required to produce the plastic moment Mp in the beam. Vmp at the critical section is not always equal to Vmp at the column web centreline, because the critical sections of these connections do not necessarily coincide with the column web centrelines. The critical section for connection 14−1 is at the juncture of the beam and the moment and shear connection plates. The critical section for connection 14−2 is at the column web. Therefore, Vmp (critical section) and Vmp (column web) have the same value for connection 14−2, but have different values for connection 14−1.

Connection 14−3, which is a fully-bolted connection, is shown in Fig. 6.29. In this connection, the beam is connected to the moment and shear connection plates by high strength bolts. The moment connection plate is connected to the column web and flanges by fillet welds, and the shear connection plate is connected to the column web and to the top and bottom moment plates also by fillet welds. The critical section of this connection is at the outer row of flange bolts.

The load-deformation behaviour of this connection is shown in Fig. 6.30. Note that two distinct elastic slopes are observed. The occurrence of the second shallower slope is due to bolt-slip into the bearing. After this second slope, the connection starts losing its stiffness due to local yielding of the assemblage elements. Failure of this connection is again due to fracture at the tip of the tension flange connection plate.

This fully-bolted connection is not proper for a plastically designed

FIG. 6.29. Joint details of connection

14−3 (all dimensions in mm).

FIG. 6.30. Load-deflection curve of fully-bolted connection 14−3.

structure, because of the significant reduction in stiffness in the working load range due to bolt slip, and the inadequate ductility due to fracture in the tension flange connection plate. However, these shortcomings can be remedied by designing the joint as a friction-type connection and by using an extended moment flange connection plate. This bolt-slip phenomenon is less significant in a friction-type joint and the use of an extended moment plate will reduce the high stress concentration at the beam tension flange adjacent to the tips of the inner faces of the column flanges.

Figure 6.31 shows the joint details of the fully-welded connection 14−4. The beam flanges and web are groove welded to the moment and shear connection plates, which in turn are connected to the column by fillet welds. The critical section for this connection is at the column flange tips.

Figure 6.32 shows the load-deflection behaviour of this connection. It can be seen that this connection possesses the required stiffness and strength. Although local buckling of the beam compression flange and cracks in the area of the groove weld joining the tension flange of the beam to the flange moment plate were observed, the connection exhibited sufficient ductility and no fracture occurred.

FIG. 6.31. Joint details of connection

14−4 (all dimensions in mm).

FIG. 6.32. Load-deflection curve of fully-welded connection 14−4.

6.4.3 Concluding Remarks

From the above discussions, it is clear that fracture at the junction of the tension beam flange and the moment plate near the tip of the column flange is the main problem which limits the ductility of a web moment connection. Fracture occurs as a result of a triaxial tensile stress state that develops there. Figure 6.33(a) shows that the longitudinal stress across the width of the beam flange at section A-A is highly uniform. This non-uniformity arises as a result of shear lag. The part of the moment plate that joins the inner face of the column flange is, relatively, stiffer than the part in the middle. Thus, stress tends to migrate to the edges of the moment plate. At section B-B (Fig. 6.33(b)), the stress is fairly uniform because the stiffness across the width of the beam flange is more or less constant. The shear stress along section C-C is also non-uniform (Fig. 6.33(c)).

When a tensile force is applied to the moment plate, transverse and through-thickness strains will be induced due to the Poisson effect. However, because of the constraint of the column flanges, these strains can not be released along the welds of the moment plate and the column flanges. Consequently, a triaxial tensile stress state will develop there.

This undesirable stress state, compounded by the high stress concentration at section

FIG. 6.33. Stress distributions (a) longitudinal stresses on section A-A;

(b) longitudinal stresses on section B-B; (c) shear stresses on section C-C.

A-A and aggravated by the shear stress at section C-C, will ultimately cause fracture at the intersection of A-A and C-C.

To remedy this problem and to improve ductility for these types of web moment connections, the following suggestions were made (Driscoll and Beedle, 1982).

(1) Use oversized moment plates (Fig. 6.34(a)) to reduce the non-uniformity of tensile stresses across the width of the plate.

(2) Use a back-up stiffener (Fig. 6.34(b)) to reduce stress concentration at the column flange tip.

(3) Use an extended connection plate (Fig. 6.34(c), (d) and (e)) to avoid intersecting beam flange butt welds with the column flange fillet welds.

FIG. 6.34. Possible approaches, for use individually or in combination, for improving performance of tension flange connections to column web (Driscoll and Beedle, 1982).

(4) Use a tapered moment plate (Fig. 6.34(d)) or a moment plate with reduced width (Fig.

6.34(e)) to move the beam flange-moment plate juncture away from the critical section.

A test programme using the above suggested web connection details is now underway at Lehigh University. The results of these tests will give the designer more insight into the behaviour of web moment connections.