Contents


Stiffness modifiers for bar and shell elements

The cracked-section analysis is a non-linear calculation, which is very time consuming. Therefore, for large models (such as high rise buildings), standards suggest stiffness reductions on parts of the stiffness matrix to model and replace the real cracking behavior of RC beams and shells. A so-called stiffness reduction factor has been implemented in FEM-Design 20, which therefore provides a more realistic rigidity distribution in linear calculations and faster results of displacement and internal force calculations.

Of course, stiffness reduction can be applied to a structural element of any material (not only for concrete material) in order to reduce the internal force effects in some directions.

Availability

Stiffness modifiers are available for the following structural objects (Structure tab):

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Stiffness matrix components can be managed with modifier factors on the new Stiffness settings page (Default settings).

  • Bar-type elements:
     

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  • Shell-type elements:
     

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Calculation method

Pictures help us understand the process of stiffness modification and the role of factors.

  • Bar-type elements:

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  • Shell-type elements:

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  • When applying stiffness reduction for selected elements, their stress results are invalid! (The method is recommended for displacement and internal forces and design calculations based on them.)
  • In case of stiffness reduction, cracked-section analysis is unnecessary and does not give a correct result.

Separate stiffness definition for each calculation type

In some parts of the national standards it is stated that different limit states require different stiffnesses in structural analysis. It means that the rigidity of supports, foundation elements, connections, or other structural objects can be different in different limit states / calculation types. Besides stiffnesses, the plastic limits can also be calculation-type-dependent.  

FEM-Design 20 allows setting different stiffnesses (stiffness modifications; see above), springs or plastic limits for selected elements by calculation types.

Availability

This functionality can be controlled by unchecking or checking the "Same for all calculations" option by analytical model elements or their types. 

Supports and Connection objects (Structure tab):
 

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  • Different spring stiffnesses can be assigned to following calculation types: 
    • 1st order analysis: U, Ua, Us 
    • 1st order analysis: Sq 
    • 1st order analysis: Sf 
    • 1st order analysis: Sc 
    • 2nd order analysis: U, Ua, Us 
    • 2nd order analysis: Sq 
    • 2nd order analysis: Sf 
    • 2nd order analysis: Sc 
    • Stability analysis 
    • Dynamic analysis 
       
  • Different plastic limits can be assigned to regular limits states as well:
    • 1st order analysis: U, Ua, Us 
    • 1st order analysis: Sq 
    • 1st order analysis: Sf 
    • 1st order analysis: Sc

Wall and Slab foundation, Bar and Shell objects (Structure tab):
 

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  • Different stiffness modification can be done by the following calculation types:
    • 1st order analysis: U, Ua, Us 
    • 1st order analysis: Sq 
    • 1st order analysis: Sf 
    • 1st order analysis: Sc 
    • 2nd order analysis: U, Ua, Us 
    • 2nd order analysis: Sq 
    • 2nd order analysis: Sf 
    • 2nd order analysis: Sc 
    • Stability analysis 
    • Dynamic analysis

The Copy function helps quickly set the same spring, plastic limit or stiffnesses for more than one analysis types.
 

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Results

Running analysis will give results based on the adjusted properties in the selected calculation types, one-by-one.

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Improvements in Eigenfrequency analysis

Mode shape normalization

In FEM-Design 20, vibration shapes can be normalized not only to the modal mass (knowledge of previous versions, where the modal mass is one for each vibration shape), but now also to the max amplitude of the vibration shape (Unit option). 

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Effective mass 

FEM-Design 20 lists effective masses in Eigenfrequencies results. 
 

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Average vibration shapes by Storeys

FEM-Design 20 calculates the average vibration shape/amplitude by building Storeys.

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Parent topic: FEM-Design 20 New Features

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