In normal analysis, geometry of structure don't change when applying forces.
So Internal forces are computed using original geometry.
But in reality, when structure deflects, loads act on deformed shape, producing secondary moment and increased stresses.
This is called P-Delta effect \(M = P \times \Delta\) , here P = Axial load acting on structure and Delta = Lateral displacement
P-Delta Analysis is extremely important in case of tall buildings where there is large amount of lateral drift or structure with slender columns or frames or structure with high flexibility
seismic or wind-loaded buildings are also affected by this.
Types of P-Delta effects
GLobal P-Delta effect
Overall lateral sway of the entire structure
for example wind load on tall building
Local P-Delta effect
Local bending of individual members
Column bending between floors
How Modern software deals with it?
All the modern software like ETABS, SAP2000 and STAAD Pro supports P-Delta analysis.
This software first runs first order analysis and then calculates additional movements using deflections.