Backstay Effect

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Buildings with more parking requirements tend to have multiple basements or podiums levels.

Usually, these parking levels are extended beyond the tower area and into the non-tower part increasing the floor diaphragm significantly.

[1] This essentially help in lateral load resisting system as it introduces an intermediate support at the podium/basement level and reducing the cantilevering height of the building. (see figure below)

[2] This reduction in span/height by converting the structural system to propped cantilever from pure cantilever is termed as backstay effect.

[3] Although this effect reduces forces in the lateral load resisting walls/system at bottom most story but there is sudden increase in story shear at podium where diaphragm extent is increased.

[4] Additionally the diaphragm at podium level needs to resist the forces being pushed into, which increases the forces in the periphery beams/frames of tower area.

[5] Backstay effect is thoroughly discussed in clause 8.1.3.3.1 of IS 16700 (IS code for structural safety of concrete tall buildings).

But it becomes tricky on how to simulate this in the analytical model, one of my friends while working with similar situation asked me this. How do you generally take in factor for backstay effect in your design?

This is a small post on backstay effect. Any comments or suggestions are welcomed.

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