Design of Hollow Octagonal Steel Poles. Is it Required to use of Rigid Inplane Rotation Command

Question

Design of Hollow Octagonal Steel Poles. We have modeled 15 m high vertical steel pole using Finite Plate Elements. The Hollow Steel Pole is connected to Base Plate fixed on a Concrete Pedestal with Four No. of Bolts with Self Weight and Wind Loads applied as Reference Load. Our Query is as follows : 
 1. When we use the rigid inplane rotation command applied to all plates. The values of maximum Deflection results in 100 mm. 
 2. If we do not use rigid inplane rotation command applied to all plates. The value of maximum reflection results in 1 metre. 
 Is there any specific reason for variation in value of deflection in both the cases ?

Kris Sathia · Answer

If you consider the elements of the tower that are resisting the lateral load: 
 
a) there are some whose plane is at 90 degrees of the direction of loading and hence they resist the load entirely by flexure (out-of-plane bending stiffness). 
 
b) there are some whose plane is aligned with the direction of loading and hence they resist the load entirely by their inplane shear stiffness which is based on the element properties and the modulus of rigidity. This is usually a small value, but not zero. 
 
c) there are some whose plane is at an inclination to the direction of loading (between 0 and 90) and hence resist the load partly through type (a) and partly through type (b) action. 
 
These values are computed by STAAD when assembling the stiffness matrix for the element. 
 
When you use the specification ELEMENT RIGID INPLANE, its treats item (b) as having an infinite value. This increased inplane stiffness causes the deflections to come down.