Model types available

Which model type should I use?

The table below summarises the three available model types and the situations each is best suited to. All three are created through create_grillage(); only the model_type keyword (and a few type-specific keyword arguments) differ.

Model type	model_type kwarg	Best suited for	Additional inputs required
Beam only	(default) "beam_only"	Routine bridge deck grillage analysis; fastest to set up and run; well understood by practitioners.	None beyond the standard arguments.
Beam with rigid links	"beam_link"	Composite sections where the neutral axes of longitudinal and transverse members are offset from the grillage plane (e.g. Super-T girders).	beam_width, web_thick, centroid_dist_y
Shell & Beam	"shell_beam"	Studies where two-dimensional slab behaviour is important (punching, local bending); highest fidelity but most computationally expensive.	max_mesh_size_z, offset_beam_y_dist, link_nodes_width; shell member must be assigned via set_shell_members().

Rule of thumb: start with Beam only to verify boundary conditions and loading, then switch to Beam with rigid links or Shell & Beam once the global model is validated.

For the example code on this page, ospgrillage is imported as og

import ospgrillage as og

Beam Elements Only — beam_only

This is the traditional modelling approach that is comprised of beam elements lay out in a grid pattern, with:

• longitudinal members representing composite sections along longitudinal direction (e.g. main beams);

• transverse members representing slabs or secondary beam sections.

This is the default model type if model_type keyword argument is not specified to create_grillage()

example_bridge = og.create_grillage(bridge_name="Super T grillage", long_dim=10, width=7, skew=-42,
                                num_long_grid=7, num_trans_grid=5, edge_beam_dist=1, mesh_type="Ortho")

More information of this model type can be found here.

Beam with Rigid Links — beam_link

This is a modified version of the traditional beam element model with the following features:

• Offsets (in x-z plane) for start and end nodes along direction of transverse members - using joint offset.

• Offsets (in vertical y direction) for start and end nodes of longitudinal members - again using joint offsets.

Figure 2 shows the details of the aforementioned model type. Figure 3 shows the model type created in a similar commercial software SPACEGASS.

To create this model, set model_type="beam_link" in create_grillage().

example_bridge = og.create_grillage(bridge_name="Modified bridge grillage", long_dim=10, width=7, skew=-12,
                                    num_long_grid=7, num_trans_grid=5, edge_beam_dist=1, mesh_type="Ortho",
                                    model_type="beam_link",
                                    beam_width=1, web_thick=0.02, centroid_dist_y=0.499)

The joint offsets are rigid links. Information can be found in OpenSeesPy’s geomtransf

Table 1 outlines the specific variables for the beam link model.

Keyword argument	Description
beam_width	Width of the beam/longitudinal members — used to define the offset distance in the z direction.
web_thick	Thickness of web — used to define the offset distance in the z direction.
centroid_dist_y	Distance in the y direction to offset longitudinal members (exterior and interior main beams).

Table 1: Input arguments for the beam link model.

Note

As of release 0.1.0, OpenSeesPy visualization module vfo and opsvis is unable to visualize the joint offsets.

Shell & Beam Elements — shell_beam

This is a more refined model using two element types — shell and beam elements — with the following features:

• Shell elements lay in grids to represent bridge decks.

• Beam elements modelled with an offset to the plane of shell elements to represent longitudinal beam sections.

• Beam elements linked to shell elements at two corresponding locations using constraint equations — OpenSeesPy’s rigidLink command.

This model has advantages in modelling slabs using shell elements which are well-suited to represent two-dimensional slab behaviour. Figure 4 shows the details and variables of the shell beam hybrid model.

When model_type="shell_beam" is selected, ospgrillage automatically determines the position of shell elements within the grillage plane. Users only have to define and assign the section of the shell element via create_section() and set_shell_member() respectively. The following example code shows the steps to create the shell model type:

# create section of shell element
slab_shell_section = og.create_section(h=0.2) # h = thickness
# shell elements for slab
slab_shell = og.create_member(section=slab_shell_section, material=concrete)
# create grillage with shell model type
example_bridge = og.create_grillage(bridge_name="Shell grillage", long_dim=10, width=7, skew=0,
                                    num_long_grid=6, num_trans_grid=11, edge_beam_dist=1, mesh_type="Orth",
                                    model_type="shell_beam", max_mesh_size_z=0.5, offset_beam_y_dist=0.499,
                                    link_nodes_width=0.89)
# set shell members to shell elements
example_bridge.set_shell_members(slab_shell)

Table 2 outlines the specific variables for the shell hybrid model.

Keyword argument	Description
max_mesh_size_z	Max mesh size in the z direction. ospgrillage automatically determines the mesh size based on this value and the spacing of link nodes.
offset_beam_y_dist	Distance between offset beams and the grillage shell plane.
beam_width	Width between link nodes (left and right links to offset beam elements) in the global z direction.

Table 2: Input arguments for the shell hybrid model.