This article explains how to build a flat slab, floor-to-floor ramp in RAM Structural System. There is no specific tool in RAM Modeler that facilitates the creation of a ramp, and so the method discussed here is a work around, requiring some intermittent steps and procedures. This is an advanced technique and great care should be taken in assuring that the expected load path and member connectivity results from the model developed. While the method documented here may produce sufficiently precise results in a specific case, it should be emphasized that this an approximation of what a formally implemented feature would produce.
The ramp geometry this tutorial produces is shown in the image below.
The low edge of the ramp is at the lower level, grid line 2/A-B, and the high edge is at the upper level, grid line 3/A-B. It is intended for the ramp to run continuously from the low edge to the high edge. As we shall see however, the work around in RAM Modeler requires that a sliver of wall be used to connect the two discontinuous pieces of the ramp that meet at mid-height of the story.
In addition to the real concrete columns in the structure, four "dummy" columns are present in the model: at 2/A.5, 2.5/A, 2.5/A.5, and 3/A.5. These columns are present only to facilitate the modeling of the ramp, and have been given near-zero stiffnesses so that they have negligible impact on the analysis. The same is true for the six "dummy" beams within the ramp, which are at A/2-3, A.5/2-3, 2/A-A.5, and 3/A-A.5.
We start by building the lower level. At this level we model only the lower half of the ramp, leaving the plan extent where the upper half extends open (similar to how the floor would look on a structural floor plan). One of the keys to this work around is to leave a small gap between the north (in this case) edge of the ramp and the slab immediately to the north. This will allow us to change the slope of the slab at A-B/2-2.5 to accomplish the ramping.
The two images below illustrate this.
Strictly speaking, RAM Modeler does not allow the elevations of deck edges to be changed relative to the story height. Users can shorten or heighten only columns and walls. However, any deck polygon edges that are coincident with a beam or wall that changes elevation or slope (as a result of the support columns being shortened or heightened) move with the beam or wall. So we can create a pitched slab by forming a grid of "dummy" beams and columns around the slab and then raising or lowering the columns as needed to form a ramp.
The next key step in this procedure is to lay down a separate deck polygon for the ramp itself. Equally important is to modify the polygon for the floor slab so that there is no overlap of the floor slab and the ramp deck polygons (see image below). This is necessary to allow the ramp to "hinge" at the lower edge.
Now the columns at 2.5/A and 2.5/A.5 can be raised using Layout -> Columns -> Modify Elevation. The story height in this example is 12'-0". It would seem that 6'-0" would be the appropriate height to raise the columns, as this is half of the story height. However, the east edge of the ramp deck at the lower level must be connected with the west edge of the ramp at the upper level using a wall element (otherwise there would be no connectivity between the free edges at each level). As such, we must leave a small vertical space within which to place the wall element (zero height should be avoided). In this example the columns at the lower floor are raised 5.9'.
The image below shows in the resulting 3D view after this is done.
The next step is to model the upper floor, and the same general process is repeated. For the upper level, the deck representing the ramp is placed within 2-3/A-A.5, while the area to the immediate west is left open.
Dummy beams and columns are placed just as was done with the lower level. This time, rather than raising the columns at 2.5/A and 2.5/A.5, we lower them by little bit smaller height that the columns at the lower level were raised. (e.g., if you raised the columns at the lower level by 5 ft, lower the columns at the higher level by 4.9 ft)
Now we need to connect the free edges of each ramp extent with a very stiff concrete wall. A very high Ec can be used to accomplish this. Note that in order to transfer lateral loads down the ramp level to level, the wall must be assigned as "Frame" rather than "Gravity".
This is the resulting deflected shape of the structure in RAM Frame under dead load.
A check of the column axial forces in RAM Frame confirms there is no load taken by the "dummy" columns.