A steel structure is exposed to numerous different load conditions. In simple terms, they can be categorized as Vertical or Horizontal and may be either static or dynamic.
Vertical loads, also referred to a Gravity or static Loads include the self-weight of the structure including all parts that are permanently connected to, and forming part, of the structure. This would include: Internal walls or partitions, floor materials, roofs, finishes, permanent mechanical equipment such a plumbing and electrical services – in fact anything the will be in place for the lifetime of the building.
Horizontal or Lateral Loads are essentially dynamic in nature and include: Wind and Seismic Loads.
Gravity Loads are, when taken in isolation, fairly straightforward to resolve, but in the real world there will always be some form of dynamic load acting on the structure affecting its stability. The magnitude of these loads will increase proportionally with the height of the structure. Wind Loads however, increase exponentially with an increase in height until, with High-Rise structures, it will become the dominant consideration.
These lateral forces will induce lateral deformation of the structure, this can take the form of a lean or sway and if not dangerous to the integrity of the structure can be very uncomfortable for its inhabitants. Another consequence may be vibration - and steel, being flexible, is an excellent conductor. This can be caused by gusting wind, rotating or vibrating machinery, seismic activity, and even road or rail traffic in close proximity to the building. Generally, tall buildings, or those with long unsupported spans are more susceptible, because the taller the building columns – and/or the greater the floor span its natural frequency will decrease, making the structure less resilient to vibration. Other side effects to vibration include Joint fatigue which left unchecked will ultimately lead to failure.
So stabilization can be viewed as the key element in the design and construction of safe, stable, resilient and fit-for-purpose buildings.
To counter these affects, the structure will need to be stabilized in both the vertical and horizontal planes by distributing and transferring these loads through the stabilizing elements, into the external columns – and thereby into the ground via the foundations. To achieve this there are a number of options, which include:
- Vertical stabilizing using Braced-Frames
- Vertical stabilizing using Rigid or Moment-Frames
- Vertical stabilizing using Leaned-Frames (Internal cores or external frames)
- Vertical stabilizing using Shear or Infill Walls
- Horizontal stabilizing using Diaphragms (composite steel/concrete floors)
- Horizontal stabilizing using Horizontal Bracing