I am trying to calculate the coupled mode shapes of my tower structure with BModes and right now i am facing two questions.
The first deals with the attribute sec_loc, which represents “the location of the section measured with respect to the beam root and normalized by the beam length”. For both the blade and the tower, the first section (root) is always located at 0.0 and the last section (tip) at 1.0. My question here is how come that there are values like the mass density already given at the height 0. Regarding a onshore monopile tower structure, are the values given by the part of the tower underground?
The second deals with the execution of BModes. I got the an error message: Interpolation failed for TWX in struct(). After doing some research in the code I found out that if the attribute id_tw=0 I will get this error message. I think this error message correlates with my selection of sec_loc. Is someone familiar with this problem? I divided my tower into 16 sections.
Regarding your first question, the distributed mass should be specified only for the portion of the support structure represented as the beam within BeamDyn (presumably this is the tower). Mass and stiffness properties for anything below this beam (e.g., the offshore substructure) should be specified in the “Properties of tower support subsystem” section of the BModes input file. The distributed mass density (
mass_den) is the mass per unit length along the member, and, so, should nonzero both at the root (
sec_len=0) and tip (
sec_len=1), as well as other stations in between these extremities.
I’m not familiar with the BModes source code and don’t know the answer to your second question.
thank you for your quick reply.
To my first question, i understand that at the root and at the tip the values should be nonzero and only the tower structure itself is considered. Lets say I divide the tower into two sections, then I got three points to define the two sections: top, middle and root. How do I allocate these two sections properties to three different points? Either the top or the root has to have the same values as the middle point, am I right?
The distributed mass specified at each station should be independent of how many stations you’ve discretized the structure into because the value is a property of the cross section itself. For example, for a tower composed of an isotropic material,
mass_den equals the cross-sectional area of a given cross section times the mass density of the material.