# 4.2.11.6. Twenty Eight Node Brick u-p Element¶

Twenty_Eight_Node_BrickUP is a 20-node hexahedral isoparametric element.

The eight corner nodes have 4 degrees-of-freedom (DOF) each: DOFs 1 to 3 for solid displacement (u) and DOF 4 for fluid pressure (p). The other nodes have 3 DOFs each for solid displacement. This element is implemented for simulating dynamic response of solid-fluid fully coupled material, based on Biot’s theory of porous medium.

element('20_8_BrickUP', eleTag, *eleNodes, matTag, bulk, fmass, permX, permY, permZ, <bX=0, bY=0, bZ=0>)
 eleTag (int) unique element object tag eleNodes (list (int)) a list of twenty element nodes matTag (int) Tag of an NDMaterial object (previously defined) of which the element is composed bulk (float) Combined undrained bulk modulus Bc relating changes in pore pressure and volumetric strain, may be approximated by: $$B_c \approx B_f/n$$ where $$B_f$$ is the bulk modulus of fluid phase ($$2.2\times 10^6$$ kPa (or $$3.191\times 10^5$$ psi) for water), and n the initial porosity. fmass (float) Fluid mass density permX, permY, permZ (float) Permeability coefficients in x, y, and z directions respectively. bX, bY, bZ (float) Optional gravity acceleration components in x, y, and z directions directions respectively (defaults are 0.0)