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Reduced Beam Sections (RBS)

ArcelorMittal – All Regions

Structural members with long uninterrupted spanning and large loading capacity are needed in the design of tall buildings, convention centres, sport arenas and airport concourses. The latest product developments made by Arcelor Mittal permit the use of large jumbo steel profiles. When such profiles are used in areas of high seismicity, the welded connection design requires a careful development from welding performance specifications (WPS) point to ensure that the required rotational capacities to reach interstudy drifts can be developed.

One technique used to control the flexural demand from the beam is to utilise reduced beam sections (RBS), which effectively limit the demands at the beam-column interface (see figure below).

The RBS concept was proposed initially in the late 1990s to alleviate problems encountered in the 1994 Northridge Earthquake with conventional welded connections. In the RBS, the beam may be deliberately weakened at some distance from the column, by trimming the flanges. The plastic hinge is then displaced away from the column flange, and the stress concentrations in the connection are separated from the plastic strains that develop in the plastic hinge.


Whilst removing material may seem something of a paradox and indeed potentially uneconomical, in fact, beam sections are normally sized to meet deformation requirements under gravity and earthquake loadings, often providing more resistance than is needed (‘overstrength’). The only effect of adopting RBS is therefore to consume part of this excess. It also :

  • reduces very slightly the stiffness of the structure (between 4% and 9%),  ecause sections are only reduced over very short lengths of the beams
  • normally does not require any change in the section sizes of the structural elements in order to compensate this minor stiffness reduction
  • reduces the ultimate strength of the structure, but not significantly because, as noted above, there is normally a high excess of resistance anyway
  • allows column section sizes to be reduced, assuming they have been sized by the ‘strong columns-weak beams’ capacity design condition
  • allows the dimensions of any stiffeners needed in the columns for the transmission of bending moments and shear in the connection zone to be reduced, which can result in a significant reduction in fabrication costs.


Prevention Solution, Earthquake

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