Back Stay Analysis in Tall Buildings | Direct vs Indirect Lateral Load Path Explained
Back Stay Analysis in Tall Buildings | Direct vs Indirect Lateral Load Path Explained
Back Stay Analysis in Tall Buildings | Direct vs Indirect Lateral Load Path Explained What is Backstay?
For high-rise/tall buildings shear walls/core walls are provided to resist lateral loads like wind & earthquake for stability.
Columns & beams connected to these shear walls help to resist lateral loads indirectly. These are called Backstays.
They carry tension/compression forces when the wall tries to bend due to lateral forces.
Core shear wall
Lateral force →
Deflected behaviour
Main backstay diaphragm (left and right)
Soil (on both sides)
Foundation (bottom)
Symbols: M (moment) and V (shear)
Lateral Load Path
(A) Direct Load Path
(B) Back Stay Load Path
Load transfers through tower elements directly.
Indirect load path through back stay elements.
Main structural elements are frames, columns & shear walls.
Main structural elements are core walls, diaphragm, and retaining wall.
Applicable to low-rise or no-basement structure.
Applicable to high-rise buildings with basements connected.
Loads go straight to the ground.
Loads travel sideways through slab & walls before reaching the ground.
Advantages of Back Stay Analysis
It reduces the drift of the tower.
It reduces bending in core walls.
It mobilises the basement & podium structure in resisting lateral forces.
It ensures the floor slab behaves like a rigid/semi-rigid plate that pulls the basement walls during lateral sway.
It ensures a real boundary condition.
Why Upper & Lower Bound Stiffness Modifiers are Required in Back-Stay Analysis?
The stiffness of podium walls, diaphragms, soil restraint, and tower system is uncertain and changes due to cracking, construction sequence, and load level.
So, a single model with a single stiffness assumption does not represent the true behaviour.
Hence, IS 16700:2023 recommends using two extreme stiffness conditions (modifiers):
(A) Upper Bound → 0.5 (B) Lower Bound → 0.15
Upper Bound
Lower Bound
Diaphragms, podium & basement walls will be stiffer.
Diaphragms, podium & basement walls will be softer (less engaged with soil).
More backstay force transfer.
Less backstay force transfer.
Higher shear in diaphragms & collector beams.
Higher core-wall moments, less shear in diaphragms & collectors.
Lower tower drift.
More drift for tower.
Higher forces in foundation elements under basement walls.
Higher forces for the tower core foundation.
Conclusion
Back stay analysis is not optional; it is a critical load path in tall buildings.
Without back-stay modelling → underestimation of diaphragm forces & overestimation of core demand are observed.
Two separate ETABS models (Upper & Lower Bound), along with tower + non-tower + periphery retaining walls modellin,g are required to conclude member sizes.
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