Meshing and Segmentation in RAM Frame or Concrete

General

Lateral walls and semirigid diaphragms are composed of shell elements. The subdivision of these elements into smaller pieces is commonly referred to as meshing. In Ram Frame, the size of the mesh is controlled by the “Maximum distance between nodes” parameter in Ram Frame – Criteria – General. When a lateral beam has an internal node at a column or brace end, the beam is segmented in the finite element model and the beam is represented by two beam segments. These finite element segments are denoted by the use of I’ and j’ in the results. The first segment ends are I and j’ and the second segment ends are I’ and j. Therefore, I’ and j’ are always the ends at the internal node.

Common Meshing and Segmentation Issues

When semirigid diaphragms or lateral walls are present in the model, Ram Frame will go through a meshing process when you enter Ram Frame or change your meshing parameters. Occasionally, errors or warnings occur during this process. Typically, these errors are related to the deck or surface load polygons. Excessive overlapping polygons and subtle overlaps between polygons can cause problems for the program algorithms. Also, deck polygons that do not extend to the slab edge will cause problems if the semirigid diaphragm is not being clipped to the exterior beam loop. To resolve these types of issues, apply a deck and/or surface load to the entire floor. That will delete the existing polygons and should resolve the meshing issue. Then, carefully go back and remodel the partial polygons. Enter Ram Frame after each modification to make sure you have not recreated the problem.

Other meshing and segmentation issues stem from subtle discrepancies in member end coordinates. These problems present themselves when initializing Ram Frame or during the analysis and can produce bogus results. Common error messages during the analysis as “invalid distance between nodes” or "less than 2 nodes found for beam..." Bogus results (alphanumeric values) may show up when you review forces and can cause crashes in the steel post processor. When members are modeled accurately, there is a single node at the intersection of the member ends. However, if there is a slight misalignment, there will be two nodes that are close together. This could cause problems for the mesh if there is a semirigid diaphragm. These misalignments are most common in imported models or models with where the end of a brace misses the beam/column joint or if chevron braces don’t intersect the beam at the same location. A good way to identify if there are close nodes is to make the diaphragms rigid and view the node numbers graphically in plan and elevation. If you have two numbers that appear to be on top of each other as shown below, then you know there are two nodes very close to one another. Go to Ram Modeler and review the member end coordinates using the Layout – Show command.

A similar issue can occur with wall openings. If there is a thin sliver of wall between the edge of an opening and the edge of a wall, this will produce a poor mesh. Look for poor wall meshes by using Ram Frame – View – Meshed Walls. Modify the size of the opening in elevation mode in Ram Modeler so the opening aligns with the wall edge.

It is imperative that sloped semirigid diaphragms lie in the plane of the supporting framing. The plane of the diaphragm is defined by three points which are selected based on the way the deck polygon was modeled. If you have sloping framing, the column offsets must be very precise. If there are multiple planes, there must be a separate deck polygon that corresponds to each plane if the diaphragm is semirigid.

Recommendations

Generally, the finer the mesh is, the more accurate the deformations are. However, using a very fine mesh increases analysis time. To determine an adequate mesh size, systematically decrease the maximum distance between nodes until you see an acceptable level of convergence in the displacements.

When you have one way decking, the slab edge is offset from the perimeter beam/wall loop. If there are lateral beams/walls on the perimeter, there will be a poor mesh in the area between the edge of the slab and the lateral beam/wall if the slab edge offset is small. To avoid this, use the “Use Beams for Exterior Boundary” option in Ram Frame – Criteria – Diaphragm.

Avoid using sloped semirigid diaphragms. If the roof has a subtle slope, conservatively model the roof flat at the highest elevation.

Tips for Correcting Problems

In the Modeler under the integrity menu are two powerful commands, Align Columns and Align walls. These tools allow you to reposition walls or columns on some or all levels so that they align through the height of the structure. The objects can be aligned to one another, or to a grid intersection or any user provided coordinate.

This is similar to moving grids. When the supports are moved all of the supported framing is also adjusted or stretched. Since this can have unexpected effects, it's always best to back up the file before using these adjustments.

Another tip is to use the Layout - Slab - Deck Assign - Change Polygon option so that you can stretch the corners of the deck polygons. The edge of deck polygons should typically align with beams or walls. Deck changes should not occur mid-bay except when using 2-way decks. At the edges of the structure it fine for the decks to extend you to the slab edge vertices or even beyond, but in cases where the slope is complicated, it may be simpler to stop the deck at the perimeter framing. In these cases the program will assume a like type deck on the overhanging portion of the slab edge.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

RAMSS Semirigid Diaphragms

RAMSS Polygon Intersection Errors

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

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