How to Choose Economical Steel Frames for Buildings and Structures?

There are various structural steel frame systems from which designers can choose the most economical type for the specific building. The complexity and economy of each frame system are different.

This article would provide guidance regarding economical choice of structural steel frame system for the specific building.

Fig.1: Economical Steel Frame Structure Construction, UK

How to Choose Economical Steel Frames for Buildings and Structures?

  • Different types of economical structural steel frames include:
  • Braced frames in simple construction
  • Braced frames with continuous construction
  • Unbraced sway frames
  • Discrete stability frames

Braced Frames in Simple Construction

This steel frame type is the most economical and least complex among other types. Braced frames use the simple beam for column connections.

It provides several advantages such as simplicity, economical beam to column connection, columns are small both in size and mass. Beams in this system are subjected to sagging moments only, that is why composite construction would considerably suitable to be utilized.

Simple analysis can be used for the steel frame because the structure is determinate that is why both beams and columns can be easily optimized.

Apart from advantages provided, there is a number of disadvantages. For example, the beams in braced frames will be costly if serviceability governs the design of steel structure frame. It is not the suitable option for small-scale structure when the design is governed by strength.

The size of the bracing element increases with the increase the building height and number of a story are increased. This may make the bracing element uneconomical. Therefore, it is necessary to proportionate bracing elements carefully.

Fig.2: Simple Braced Frame Steel Structure

Braced Frames with Continuous Construction

This type of steel frame is economically suitable for the case where the stiffness of floors controls the design. Considerations required to be accounted for are the same as those of braced frames with a simple construction.

Braced frame with the simple construction is more economical than the braced frame with continuous construction.

One of the most outstanding benefits of braced frames with continuous construction is that stiffness of floor system is considerably improved due to continuous beam to column connections.

However, continuous beam to column connection is uneconomical and mass of external columns should be considered large to be able to withstand imposed moments.

Finally, beams and columns cannot be optimized due to the complex analysis method that is suitable for such system.

Fig.3: Continuous Steel Frame

Unbraced Sway Frames

Un-braced sway frame system as shown in Figure-4 is considered when the bracing system cannot be used because it is not possible architecturally.

So, either partially rigid or continuous steel frame system should be used to resist horizontal forces. But it should be kept in mind that the construction of the semi-rigid frame is considerably costly.

Moreover, the complexity and cost of un-braced sway frame is greater than both braced with simple construction and braced with construction frame. To make the un-braced sway frames economical, it is required to size the structural elements properly.

Architectural configuration can be established without the use of bracing elements. Despite this advantage, there are certain disadvantages that should be considered by designers.

For instance, beam to column connection is neither simple nor economical, external columns with substantial masses should be utilized to withstand applied moments, and the size of structural steel elements cannot be optimized easily because the analysis is considerably complicated.

Fig.4: Unbraced Sway Steel Frame Structure

Discrete Stability Frames

It is considered to be the most uneconomical and complicated steel frame type compared with others explained above.

Architecturally, strong discrete stability frame is favorable to withstand horizontal forces and provide adequate stability.

It is necessary to use substantially strong beams and columns because discrete stability frame must offer complete and sufficient sway stiffness and sway resistance.

When discrete stability frames are considered, then strong columns need to support applied moments and beam to column connections may be reduced.

Consequently, the more economical frame would be constructed compared with the case where the entire frame structure is assigned to resist horizontal force because all beams and columns should be strong in this case.

Moreover, architectural configurations can be established without triangulated bracing. Beam to column connections in a partialized frame is costly and complex.

The size of steel elements cannot be optimized due to the complexity of the analysis used. Masses of external columns in the partialized frame should be substantially large to be able to withstand the moment.

Fig.5: Discrete Stability Frame: (1) Discrete Stability Frame with Rigid Connection, (2) Frame with Simple Connection