Connections Workshop - Case Study 2

Building with Steel, in cooperation with TC10, organized a connections workshop for structural engineers at Kampstaal in Emmeloord.

The teams consisted of structural engineers from engineering firms and steel fabricators, and were each guided by an experienced connection designer. After the groups presented their design, we from IDEA StatiCa were given the opportunity to model the connections with the Connection application. That way we were able to analyze the results immediately and discuss them together.

We explain the designs and results in more detail below. In this second article, we will discuss Case Study 2, in which the engineers were given the following task.

Case 2

The second case study involves a column footplate connection in combination with a diagonal brace. The strut can be made in three different profiles and is loaded with a compressive force of 500 kN. The column itself experiences a significant compressive force of 2000 kN.

The focus is on the connection of the diagonal to the column, and the design of the base plate, including the anchors and foundation. Below we summarize the proposed solutions. Based on the sketches and presentations received, the connections were modeled and calculated in IDEA StatiCa. In the remainder of this article, we discuss the connections, expert comments and calculated results.

In this design issue, we again see that multiple connections are possible, with no single correct solution. Below we give an overview of the different connections, including the results from IDEA StatiCa. Then we discuss the main design considerations, treating the different groups together rather than separately.

Connection brace to column

For the connection of the brace, three groups (A, C, E) chose a head plate with butt connection, and the other three groups (B, D, F) chose a sketch plate with bolt connection.

The butt joint (Stub) design provides direct transfer of compressive force without complications in the connection. By opting for an HEA profile, bolt assembly is easily feasible and the body of the strut runs in line with the body of the column. As a result, the stresses are well transmitted into the column, as seen in the solutions of groups A, C and E (see figure).

In contrast, Groups B, D and F chose a sketch plate connection (Gusset plate). This considered turning the column a quarter turn so that the brace can be connected inside the column without taking up too much space. However, in that case, the gusset plate is connected directly, but transversely, to the body of the column, and due to the high compressive forces, peak stresses can then occur in the body of the column. The calculations in IDEA StatiCa show that this is just about sufficient, but the structural engineer must be alert to this. If the web begins to deform plastically, it is advisable to turn the column, thicken the web or add stiffeners.

In the designs with the gusset plate connection, it is advantageous to make the connection symmetrical and not let the plates protrude too far, for the same reasons we discussed in Case 2. Connection B has an asymmetrical connection, but thanks to the 20 mm thick plate and the use of six bolts, the additional moment is sufficiently absorbed, limiting stresses.

Column base plate

There are also important considerations in the design of the footplate and foundation. Due to the high compressive forces, it is crucial that the stresses are well distributed through the footplate into the concrete. This can be achieved by choosing a thicker footplate and making it wider than the column profile so that the stresses are better distributed.

The figure below compares the stresses in the base plate and the contact stresses in the concrete for a base plate 40 mm and 10 mm thick. With a footplate that is too thin, the stresses concentrate around the column profile and are not effectively distributed through the slab thickness. The same effect is seen in the stresses in the concrete. Poor stress distribution leads to exceeding the permissible compressive stress.

Column foundation

We see different foundation solutions, with or without mortar joint, and anchors with or without anchor plate. The anchors used range from M20 to M30.

The calculations in IDEA StatiCa show that none of the connections are satisfactory for testing the anchors. As a default, the shear forces are set to be transmitted through the anchors. M20 anchors are found to be insufficiently strong and cannot withstand the shear forces. In contrast, anchors M30 8.8, in combination with a follower plate, are sufficiently strong to transfer the shear forces. Nevertheless, the test is still not satisfactory, because the problem is now not in the steel, but in the failure of the concrete.

The shear forces on the anchors cause edge failure of the concrete, with the anchors breaking laterally out of the concrete. IDEA StatiCa Connection calculates, in accordance with EN 1992-4, with unreinforced concrete, so concrete failure at higher forces is unavoidable.

If the forces cannot be reduced, three possible solutions remain.

1. Modify the concrete block( increaseedge distance or concrete class).
2. Transfer shear forces through friction rather than through the anchors. The high compressive force in the column provides sufficient frictional resistance.
3. Apply additional reinforcement in the concrete block so that the reinforcement absorbs the tensile stresses and prevents the concrete from breaking out.

As shown in the sketch, only Group E had reinforcement included in the design. Want to know how to model and test reinforcement with IDEA StatiCa? Then read this article on analyzing foundation blocks including anchors and reinforcement.

Final word

The steel connection of case 2 was designed by 6 groups, modeled in IDEA StatiCa and discussed with experienced structural engineers. We were able to analyze the results through the use of IDEA StatiCa, in which a number of concerns emerged. This workshop shows that many connections can be designed in an infinite number of ways and that there is never one correct solution. We experienced the importance of drawing to scale and following the path of forces in the connection. Analyzing the stiffnesses and visualizing how the joint will deform is a good thought experiment to understand how a joint will behave.

"Imagination is more important than knowledge" a man named Albert Einstein once said. And that certainly applies to steel joint design as well. Anyone who can imagine what the joint looks like, how it will be made, whether the proportions are right, how the forces will flow and how the joint will deform is already one step closer to becoming the best joint designer.

In case you missed the previous article, be sure to check out Case Study 1, in which the engineers had to design a complex column-girder connection with edge bars.