Structural fundamentals | Expansion joints and stability

Words by Steven Bailey, Monday 17 October 2016

Steven Bailey, BSBG's Head of Structures, provides insight into some structural fundamentals, specifically the consideration process for expansion/movement joints and stability in structures.

Expansion joints

In structural terms, an expansion/movement joint is a vertical mid-structure separation which allows the separated parts to move independently and thus reduce stresses arising from:

Sway caused by wind
Seismic forces
Ground settlements
Thermal expansion and contraction due to temperature variations

When ascertaining if a structure needs movement joints some initial factors to consider are:

Is the building plan maximum dimension <50-60m?

If yes, joints are not required

If no, then do consider joints in the structure. It may be possible without, but additional thermal factors to be considered in the design can lead to increased reinforcement to control cracking, leading to additional cost

Is the building of uniform height and <50-60m in plan?

If yes, joints are not required

If no, then do consider joints. This is especially important for shallow (non-piled) foundations where differential settlement may be an issue

Stability

The ability of a structure to resist undesirable movements like sliding, overturning and collapse is dependent upon support conditions and the arrangement of members – in particular vertical elements. This structural stability is normally ensured by the incorporation of core/shear walls (RC) or cross-bracing (steelwork).

For a regular structure, rectangular in plan, some typical stability element arrangements can be assessed as below:

Key: Shear wall
Direction of lateral force
Shear reaction

Regularly spaced perimeter shear wall system
Centre of stiffness coincides with, or is very close to, centre of mass
Even distribution of stiffness
Good load transfer
Low out of balance forces
GOOD STRUCTURAL LAYOUT

Central core system
Centre of stiffness coincides with, or is very close to, centre of mass
Low out of balance forces
Slip form potential
GOOD STRUCTURAL LAYOUT

Offset core system
Variable lateral stiffness
High out of balance torsion
Core details extremely sensitive
POOR STRUCTURAL LAYOUT

To overcome the issues with the offset core system, introduce an additional shear wall to resolve the torsion issue
Centre of stiffness is moved closer to centre of mass
GOOD STRUCTURAL LAYOUT