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Anatomy of steel construction


Multi-level steel construction

 
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Steel-framed construction is a very popular structural solution for multi-level buildings; the material offers considerable flexibility thanks to its inherent strength and light weight.

The term 'multi-level' applies to structures with more than one storey, and covers buildings used for a variety of purposes including:
  • office developments
  • retail centres
  • car parks
  • schools and universities
  • hospitals
  • hotels 
The two main components of multi-level steel buildings are the composite steel floor system and the lateral load-resisting system. As a general rule of thumb the most cost-effective solutions are produced when the steel framing supporting the flooring and the lateral load-resisting system are kept separate. This allows expensive complex connections to be limited to the lateral load-resisting system and simple cost-effective connections to be used for the remaining structure.


Flooring systems

Steel-concrete composite floor construction will generally produce the most efficient floor system as it combines the benefits of both materials. Designing the floor slab to act compositely with the steel beams can increase the bending strength of the beam by a factor of 1.5-2 and increase its stiffness by a factor of 3-4.5. This results in lightweight floor solutions with good spanning capability. In New Zealand there are three common forms of steel-concrete composite construction:
  1. Precast flooring on steel framing
  2. Rolled steel joist on steel framing
  3. Metal deck slab on steel framing.
The advantages of the latter two options are that they result in a lighter superstructure and, subject to appropriate span limits, the concrete slab can be poured without needing to prop the floor. This has speed advantages in buildings with significant follow-on trades such as heating and ventilation.

In recent years there has been greater demand for buildings with large uninterrupted floor areas, providing flexibility in their end use. An increase in the number of companies in New Zealand manufacturing customised cellular or welded beams has seen an increase in long-span composite beam solutions for multi-level commercial projects such as car parks and offices. These beam spans are typically in the range of 12-25m.
 

Lateral load systems

Steel-framed buildings are typically braced against lateral loading such as wind or earthquakes by braced (concentric or eccentric) or moment-resisting frames. Eccentrically braced frames are the most popular form of braced steel frame used in New Zealand, and this will likely continue due to their very good performance during the Canterbury earthquakes.
 
Braced frames will generally result in a cheaper overall structural cost but will not always be architecturally acceptable. Both types of steel lateral load systems feature low-damage technology, further enhancing their seismic performance. The design of such systems is well established in New Zealand, with good seismic design resources available for structural engineers.

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