Frame Walls FAQ

Lintel Beams

There are 2 approaches for modeling lintels beams in walls. The first approach is to model the whole wall and then place an opening in the wall. The second approach is to model walls on each side of the opening, and then span across the opening with a beam.

Wall with opening

When a wall opening is used the area above the opening is meshed along with the rest of the wall using shell elements and using the mesh parameters set under Criteria - General. The beam is assigned the same properties and cracked section factors as the rest of the wall.

Lintel beams of this type can be designed as Coupling Beams in the Ram Concrete Shear wall Module only, though you can cut sections through the beam to get forces at specific sections.

Generally speaking this approach gives you the stiffest overall wall compared to other methods. For that reason it's preferred for beams that are fairly stout, i.e. when the depth of the beam is > 1/4 the span.

Beam between walls

When modeling the lintel as a beam, the linear finite element of the beam connects to the corner nodes of the wall only. Consequently, the total stiffness of two walls coupled with a beam is less than the method above. (For RAM SS version 14.06, a modification to the analysis options is being implemented so that a small rigid end zone is created to provide greater resistance to rotation at this node.)

Beams modeled this way can be designed in the Ram Concrete Beam module only, though forces are reported the same as other frame beams.

Generally speaking, this approach is recommended for long and skinny beams between walls, e.g. when the beam depth is < 1/4 of the span.

Note, a two-way slab between walls can also couple wall together in a similar fashion. A rigid diaphragm will tie walls together and can provide for shear force transfer from one wall to another, but a rigid diaphragm alone does not actually couple the walls together.

Combining Columns and Walls

In the Modeler, you can freely model a column at the end of any wall without warning.

Where a column pilaster like this is modeled it's important to note that:

1. The full self weight of both will be counted.
2. If the columns and wall are lateral, then they will be meshed together and act compositely in Ram Frame (this is also true of gravity members analyzed in Ram Concrete.)
3. The design of the wall is based on the forces in the wall only, while the design of the column is based on the forces in the column only.
4. Where boundary element design is intended, it is better to model using walls only.

The presence of pilasters inside shear wall systems introduces a much different distribution of loads that has to be looked at closely.  In the Figure below, P1, P2, P3, P4, P5, P7, P9 and P11 represent the axial loads in the columns whereas P6, P8 and P10 represent axial loads in walls 1, 2 and 3 respectively. R1, R2, R3 and R4 are the external reactions. 

 Axial forces in Walls with Pilasters

A finite element analysis of the wall system shown in the figure above will give axial loads P5, P7, P9 and P11 which are much smaller than P1, P2, P3 and P4 respectively, because these loads are transferred not only to the columns also to the shear walls below.  In fact, the shear walls will often take most of the load leaving the column pilasters with very small internal forces. Many engineers want the pilasters to be designed for at least the loads that come directly from the columns above. There is no single tool to accomplish this in the program, so we recommend hand-checking that the pilaster below has at least the same size and area of reinforcement as the column above.

The frame reactions are the total reaction at a node. These reactions include the forces from the columns and the walls supported. For this system shown, the sum of R1-R4 is equal to the sum of column loads P1-P4, but forces will be distributed differently due to the presence of the walls.

Walls on Beams

When a wall is being supported by a beam, it is important to understand the way forces are transferred through the walls and into the supporting structure.

Gravity Walls

Starting in version 10 of RAM Structural System, gravity walls can be used to transfer loads from level to level.

When using Ram Steel analysis methods, any load is applied to the top of a gravity wall, that load is transferred straight down to the supporting member below (another wall or a beam for example). The applied loads are not fanned out or redistributed in any way.

This simplistic approach works nicely for simple bearing walls.

When using Ram Concrete analysis, the gravity members are part of the finite element analysis and the behavior is similar to that of lateral walls described below.

Lateral Walls

Loads are tracked down through lateral through finite element analysis in RAM Frame (or RAM Concrete).

A frame wall is a shell element capable of spanning from support-to-support. If a frame wall on an upper level is supported by frame columns on a lower level, then that wall is able to span from column to column like a very deep beam. Consequently, if a beam is modeled on the lower level from column to column as well, that beam will not be directly loaded by the wall. Think of it like a small flange welded to the bottom of a very deep plate.

Bending forces in the beam still occur because the wall is meshed (based on the settings under Criteria - General) and bending deformation of the whole system is still possible. Axial tension under gravity load is expected, though a rigid diaphragm or stiff two-way slab would inhibit those forces as well.

To reiterate, the forces within a frame wall from the RAM Frame analysis are not delivered to the supporting beam in the form of an external line load. The only external loads shown on the Report - Gravity Loads are:

1. member self weight
2. line loads applied directly to the beam
3. the loads applied to the deck supported by the beam
4. reactions of gravity members supported by the beam

If the supporting beam is longer than the wall above, then the beam still acts like a flange but we can expect sudden increases in the shear and moment beyond the end of the wall.

Here the program creates additional nodes on the beam where the two finite elements are connected together and alone the length of the wall based on the mesh criteria. As the wall is vertically loaded, assuming everything is symmetric, the basic deformed shape of the beam will now look like a trapezoid (although the true deformed shape is actually a continuous curve and not “kinked”). Nodes N2 and N3 remain level and the same distance apart, thus there will be large shear forces and moments in the end segments of the beam, at the face of the wall. If the wall is broken up into smaller elements, then there will be additional nodes between N2 and N3. In this case, there can be relative displacement between the ends of the wall, but the deformed shape will still be basically the same when the wall is stiff in comparison to the beam.

Behavior of a Wall Supported by a Beam – Centered

For situations where the wall is not centered upon the beam, or where the system is otherwise asymmetric, the situation is further complicated. Where the beam supports one end of the wall, significant vertical displacements can be expected, as opposed to the column support which is presumably much stiffer. A net rotation of the wall results throwing shear and overturning moments into that wall and the supported structure. When a rigid diaphragm is present, other frames may even experience lateral shear due to this rotation.

Behavior of a Wall Supported by a Beam - Off Center

If there is another level of framing or a rigid diaphragm at the top of the wall, that could limit the rotation and affect the forces throughout the whole system as well.

Special Considerations for Ram Concrete Analysis

As noted in the Analysis Types wiki, Ram Concrete Analysis is also a finite element analysis, but it works by analyzing one floor at a time. Consequently some of the complex multi-story, truss like effects from having multiple levels of walls transferred on one slab with not be captured by the Ram Concrete analysis.

See Also

RAM SS Analysis Types

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?

Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!