I have a Solver > Model > Analysis Properties > “Invalid Material Properties - Poisson’s ratio is X.XXX” error. Why do I get this and how do I resolve it?

Question

I have a Solver > Model > Analysis Properties > “Invalid Material Properties - Poisson’s ratio is X.XXX” error. Why do I get this and how do I resolve it?

 

Answer

Background

The Material database contains the concrete grade properties when the chosen material is used in the model.

In the screenshot below, the Modulus of elasticity, E is specified as 34000 N/mm2 for C32/40 concrete.

  

Tekla Structural Designer uses the theory of isotropic linear elasticity so the modulus of elasticity, E is related to the shear modulus, G by way of Poisson's ratio, v.

Poisson’s ratio is the ratio of lateral strain to longitudinal strain in a material subjected to loading and is typically in the range of 0.1 to 0.2 for concrete. This is input as 0.2 in the screenshot above.



The shear modulus, G is related to the Elastic modulus by way of the following relationship:

G = E / (2 x (1+ v)).          Equation (1)

or rearranged for validation checks later

v = E / (2G) - 1                 Equation (2)

Hence, in the screenshot above using Equation (1) the Shear modulus G is calculated from the specified Modulus of elasticity, E and Poisson’s ratio, v values as G = 34000 / (2 x (1 + 0.2)) = 34000 / 2.4 = 14167 N/mm²

Modification Factors

Analysis modification factors are available for each of the analysis types, to allow you to adjust the modulus of elasticity, E and shear modulus, G used in the analysis. These can be reviewed and edited via the Analysis ribbon tab > Options > Modification factors page.

 

Note. This article will not go into the reasons for the different element type values or what they should be. They do need to be highlighted, however, since the “Invalid Material Properties - Poisson’s ratio is X.XXX” validation error is a direct consequence of edits that have been made on this page for a specific analysis (Building analysis, FE chase-down etc.).

The Poisson's ratio for a wide range of materials fall within a lower limit of -1.0 and an upper limit of +0.5. Since most common materials become thinner in cross section when stretched, the Poisson's ratio is positive and hence in Tekla Structural Designer we only allow values within this positive range [> 0 < 0.5].

Example

To demonstrate this, we will consider different flat slab modification factors. The slabs are meshed in the 3D analysis and hence the modification factors on the Building analysis page are as follows:

E modification factor, E_mod = 0.2 (default)
G modification factor, G_mod = 0.15, 0.16, 0.2 (default),0.22, 0.24, 0.25
(Here we have only edited the modification factor for G, however we could have edited E or both E and G).

Thus, using Equation (2), v = E / 2G -1  where modification factors are now applied to the value of the elastic modulus, E and the shear modulus, G specified earlier for use in the analysis - we have:

Poisson’s ratio, v = (E_mod * E) / ( 2 * G_mod * G) - 1

where E = 34000 N/mm² and G = 14167 N/mm²

So for E_mod = 0.2 and G_mod = 0.15, Poisson's ratio, v = (34000 * 0.2) / ( 2* 0.15 * 14167) -1 = 0.6

In tabular form the results for all the different modification factors are:

GMod	0.15	0.16	0.2	0.22	0.24	0.25
Poisson's ratio, v	0.6	0.4999	0.2	0.09	0	-0.04

Since Tekla Structural Designer only allows values in the +ve range up to +0.5 we can clearly see that our adjusted modification factor when Gmod = 0.15, 0.24 and 0.25 are invalid. Hence the solver error along with the calculated poisson’s ratio is reported. (Screenshots below).

 


 

 

Resolution

To resolve issues of this type, ensure that the calculated poisson’s ratio, v = (E_mod * E) / ( 2 * G_mod * G) - 1 lies within the range [> 0 < 0.5]

We recommend that the same modification factor adjustment is made to both the Elastic modulus, E and Shear modulus, G,  as is the case with the default values in the table. This then ensures the poisson’s ratio, v is within the required limits.