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Fundamental Concepts of Joints in Design of Steel Structures

Regarding joints, the first fundamental concept the engineer must be clear about when he or she starts to design is which connections will develop moment resistance and which can be executed as simple pin joints. To do this, it is necessary to clarify the lateral load resisting system.

1.1 Pin Connections and Moment Resisting Connections

1.1.1 Safety, Performance, and Costs

Steel structures should be safe, able to perform, and be cost-effective.

They must be safe because they act as canopies, mezzanines, buildings, skyscrapers, bridges, and much more that give shelter, protect, and be welcoming to men and women. A structural collapse is extremely dangerous and likely to cause severe harm to anyone in the surrounding area.

Structures must also effectively serve their commercial purpose while efficiently and comfortably (for the users) maintaining their design features over time. These are the basic notions of serviceability limit state design specifying that, just as a nonlimiting example, deformations will not damage secondary structures or that excessive vibrations will not make users uncomfortable.

Poor performance might also decrease the structure's value and harm the property owner.

Simultaneously, the market logic requires that the structural system be economically sound and cost-effective when compared to alternatives using different materials and design. Being economically sound is a complex matter that must take into account many factors in the building design. However, the engineer must make the structure as cost-effective as possible without compromising safety and performance. The service and expertise that engineers are expected to deliver should include reducing costs while maintaining high standards of functionality and protection.

For the principles stated, the design of connections is a focal point and it must be well defined in the engineer's mind from the commencement of the project.

1.1.2 Lateral Load Resisting System

The choice of connections is related to the choice of the lateral load resisting system.

Taking a closer look at this key point, we consider these initial hypotheses: that the structure geometry is defined, that steel will be used as structural material, and that the design loads are provided. This means that the engineer can set up the analysis model with the finite element software available. However, before building the model wireframe, the engineer must have a clear vision of the lateral resisting system(s). This choice influences costs and architectural restraints.

Lateral load resisting systems can be diverse and variously combined among themselves. Each horizontal direction can have its own system, one that may be different from the other direction.

The basic lateral resisting systems (Figure 1.1) are as follows:

• Braces (bracings)
• Moment connections (portals)
• Base rigid restraints (cantilever columns or inverted pendulum)
• Connection to an existing structure or another ad hoc structure built with different materials (say a concrete staircase, masonry or concrete walls, etc.).

Figure 1.1 Lateral load resisting systems.

The structural engineer attentive to fabrication logics usually tries to adopt bracings as this will deliver maximum cost performance. The main advantages of using braces are as follows:

• The structure is easily sized against horizontal forces (mainly wind and earthquakes) allowing less weight for beams and, most of all, columns (braces take care of lateral forces and the column can work only in compression).
• Connections can roughly be just in shear or axial action and so are light and economic.
• Lateral deflection control is excellent.
• Seismic response is good (given that the necessary detailing is provided).

At the same time bracing has some disadvantages:

• It laterally obstructs the transit, limiting windows or gates.
• The architect or the owner might not like it for esthetic reasons.

This last problem might be solved by "highlighting" the braces and assigning architectural importance to them. Some famous examples can be found, such as landmark skyscrapers (Figure 1.2) and more "ordinary" buildings (Figure 1.3), where the architect was able to create an interesting contrast with materials that nicely emphasize the braces.

Figure 1.2 Braces emphasized esthetically in the John Hancock Tower of Chicago.

Source: From Wikipedia; photo courtesy of "Akadavid", 2008.

Figure 1.3 Valorization of internal braces (InterPuls, Reggio Emilia, Italy).

The problem of transit obstruction is usually bypassed by choosing one specific bay for braces, if possible. This is done either in the middle or at the end of the building system. Horizontal braces are implemented to bring forces to the localized braces. (This book does not discuss the layout of horizontal braces. Rather it discusses one of their main functions, beyond limiting flexural torsional buckling of beams, that is, to connect unbraced bays to braced ones.)

Another method to limit the obstruction in the space occupied by the braces is to adapt their geometry to the challenges of architectural restraints using different schemes and shapes (V, inverted-V, X, K, Y, and more).

Having given the many advantages of using braces and the importance of informing the owner and the other players about this solution in order to have it approved, in many situations it is not possible to use braces, especially in both directions. As a consequence, it is necessary to use portals or base rigid connections or a combination of them, if not different additional schemes such as shear steel walls or other concrete or composite systems that are outside the scope of this book.

The main advantage of using portals and rigid bases is what made braces undesirable; that is, there are no obstacles in fully exploiting all the space of the bays. In addition, moment resisting systems (by the way, it is not trivial to underline that a system made by trusses and columns is a specific case of a portal) have the following advantages:

• Possible savings (at the expense of the dimension and cost of the columns) in beam depth since the moment connection allows a better exploitation of the beam strength along the full length.
• A more "convincing" look of the columns that, being heavier, seem safer.
• Pin (hinge) connections at the base, then savings in foundation work (larger even compared to braces, which could give an uplift and require more expensive tension details and some "ballasting" of the plinths).
• Reasonable seismic resistance (if the necessary detailing is followed).

Disadvantages of portals might be the following:

• Moment connections are required and they are usually complex and more expensive.
• Additional encumbrance may be provided by the beam-to-column connection (net height at the eaves is impacted and this could make it mandatory to raise the whole structure); also, the obstruction given by trusses is similarly and evidently large.
• On average, the weight per unit of area will worsen.
• Lateral deflections should be checked carefully.
• Buckling length of columns worsens.

A lateral resisting system having the columns rigidly connected to the base may have the following benefits:

• No obstructed bays, as already mentioned.
• Larger columns inspiring more confidence in the safety of the building.

The following are some of the disadvantages:

• Expensive foundation work required: large plinths, piles likely mandatory
• Lateral deflections to check (but usually better than portals)
• More material (steel) necessary to build the structure
• Longer buckling length of columns
• Poor seismic performance (for example, the American Society of Civil Engineers () basically bans this system for buildings if the area is highly seismic (see Ref. [1] for more precise information)).

Sometimes, to solve the problems of lateral deflections and column buckling length (see Figures 1.4 and 1.5 for reference values), both systems are contemporarily adopted.

Figure 1.4 Buckling length coefficients (effective length factors) for braced systems.

As Figure 1.5 shows, the buckling length of columns is two times the physical length in each system when taken by itself, but it goes back to almost unity (braced systems have 1) when used in a combined system.

Figure 1.5 Buckling length coefficients (effective length factors) for unbraced systems.

Every situation is different and braces are not always the best option. For example, if the structure has large bays (beyond 20?m, or 60?ft) and that direction already uses trusses in its architectural layout, it is already a moment resisting system that can be exploited as a lateral resisting system. Braces can be used only in the orthogonal direction, effectively restraining the weak side of the columns.

An engineer with a clear understanding of a lateral load resisting system will correctly prioritize and evaluate, in any situation, the...