by Josh Taylor
Solutions Discussed: Design of concrete walls in buildings
Products Discussed: RAM Structural System
**NOTE: Coupling beam design was added as a new feature in r14.03.00.00 of RAM Structural System. This makes some elements of the workarounds discussed in this article unnecessary. Refer to the r14.03.00.00 release notes, available here: http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/ram-ss-v14-03-release-notes.aspx, for more information.
Doorways and windows in concrete walls present some specific challenges when selecting reinforcement to resist shear and flexural forces. The header region directly above the opening is likely to be stressed more highly than other regions of the wall panel, thereby requiring particular attention and, often times, heavier reinforcement. To bring down the forces and reinforcing demand in the header, some engineers assign a crack factor to the header that is smaller than the value chosen for the regions where the wall is vertically continuous. The rationale here is that the headers will crack substantially more than the surrounding regions of the wall when subjected to cyclic earthquake loading, causing a shedding of loads from the header.
In RAM Structural System, an opening can be assigned to a wall element in RAM Modeler during the creation of the building geometry. After the modeling is complete, the RAM Frame analysis will be run, where the wall will be automatically meshed, and the gravity and lateral forces acting on all lateral elements for all load cases under consideration will be formulated. After the RAM Frame analysis is complete the design of concrete walls is done in the RAM Concrete Shear Wall Design module.
The shear wall design module treats the wall that was created in RAM Modeler as a single wall panel for design purposes. Assigning a bar pattern (e.g., #7@12" o.c.) to this wall panel produces uniform reinforcing over the entire wall panel, even the header. This means that if you are letting the program size the reinforcement for you, you may see larger reinforcement requirements than you expect. This is because the program will look for a single bar pattern that satisfies the strength requirements at all locations in the wall panel, including the header. So the vertical and/or horizontal bars in the vertically continuous portions of the wall could end up being significantly larger than required in some cases.
RAM Structural System V8i, r13 provides capabilities to avoid this unnecessary over sizing of the reinforcement. There are three approaches outlined below that should result in more economical designs of walls with headers.
The first approach is to go ahead and let the program size the reinforcement for all wall design groups as discussed above. Once this has been done, the engineer can use the manual reinforcement command, new to V8i, to refine the reinforcement. The manual reinforcement command allows multiple vertical reinforcement zones to be laid out in a wall panel. Thus, the engineer can inspect the demand to capacity ratios within the header and compare them to the demand to capacity ratios in the adjacent piers. If this evaluation determines that the pier reinforcement can be brought down significantly, the user can create different reinforcing zones for the piers and the header, each with its own bar size and spacing.
Figure 1 - Assigning different reinforcing zones to piers and header using the Assign -> Manual Reinforcement command
The second approach involves laying out the wall differently in RAM Modeler. Rather than using a single wall element, the physical wall can be created by using three separate wall elements. One element is used for the header, and a wall element for each of the piers. Then an opening can be assigned to the middle wall element (the one that represents the header), that extends the full length of that element. Now a different stiffness multiplier can be assigned to the element representing the header. RAM Concrete will still treat all three wall elements as a single wall panel since they all have the same thickness. The design forces for the header will likely be less than with the first approach discussed due to the smaller stiffness multiplier for the wall header.
Figure 2 - Properties of a wall panel with opening, created using a single wall element in RAM Modeler
The third approach builds off the second approach. In addition to creating a separate wall element for the header, this wall element can be given a slightly smaller thickness (I recommend using 0.01" less) than the pier wall elements. Why do this? The shear wall design module forms wall panels automatically from the walls that were laid out in RAM Modeler. Walls with identical thicknesses that are contiguous and lie in the same plane will be formed into a single wall panel. By giving the header wall a slightly smaller thickness, the wall element will be assigned to a separate wall panel. Since each wall panel can have its own bar pattern, the header can now be assigned a bar pattern independently of the adjacent wall piers. This is true during the optimization process as wall, so that the heavier requirements in the wall header will not drive an unnecessarily heavy reinforcement requirement in the wall piers.
Figure 3 - Properties of header wall panel (orange), created by using a slightly different wall thickness than the adjacent wall piers (yellow).