by Josh Taylor
Solutions Discussed: Structural design of concrete-framed buildings
Products Discussed: RAM Structural System
This article is intended to give engineers who are not familiar with the concrete design capabilties of RAM Structural System an insight into how boundary elements and confined regions in concrete walls are dealt with in the program. The screen captures shown are taken from RAM Concrete, the concrete design module in the RAM Structural System. In order to design walls in RAM Concrete, an analysis of the model must be conducted in RAM Frame, the frame analysis module within RAM Structural System. This is where the design forces used by RAM Concrete are obtained.
Horizontal Extents of Boundaries
The wall boundary design method that is implemented in RAM Concrete involves limiting the neutral axis of a section under a given axial load to a prescribed value. The Extents page calculates the actual and limit neutral axis distance for each load combination considered during the design (see Figure 1). If the neutral axis exceeds the limit, a boundary is required to confine the compression region(s) of the section. The required length of the boundary, projecting at an angle β from the point of maximum compression, is listed in the last column of the spreadsheet. If a boundary is required for a load combination, the corresponding row will be colored red.
Figure 1 - Boundary Element design sheet showing required boundary extents
Clicking on a row in the spreadsheet will draw the following in the cross-section sketch at the lower part of the dialog:
• The corner of extreme compression will be denoted with a black dot.
• The required neutral axis will be displayed according to the calculated required length and angle β listed for the respective load combination in the table above.
The Design Warnings Page will list all load combinations where the required boundary region encompasses reinforcing zones not designated as boundaries. The user may then adjust the reinforcing zones accordingly to meet the code requirements. For example, for the case shown in Figure 2 below, the required boundary region (outlined in green) extends past the zone denoted as a boundary, and thus a design failure will be listed for this load combination on the Design Warnings page.
Figure 2 - Required boundary region (outlined in green) versus user-defined boundary zone
Tie (Link) Design
The Tie/Link Design page provides information related to the design of confinement ties for zones specified as boundaries (see Figure 3). Zones not specified as boundaries will have no tie design performed. Reinforcing zones are created using the Assign -> Manual Reinforcement command (discussed in the next section). Each reinforcing zone is referenced to a Wall Panel and a Section Cut Segment in the calculation page to identify where they occur. The zone start and end locations listed are with respect to the Wall Panel in which the zones occur. The tie size used may be changed in the design code criteria menu.
Figure 3 - Confinement tie calculation sheet
Specifying Boudary Zones in Wall Panels
The Assign Manual Reinforcement dialog allows the engineer to assign reinforcement to Wall Panels such that vertical bars are placed in one or more zones along the length of the Wall Panel, each of which can have different properties. Each vertical zone created may be specified as a boundary, in which case any code-prescribed confinement calculations are performed for the respective vertical zone (see Figure 4).
Figure 4 - Wall Panel (plan view) with three reinforcing zones. The outer two zones are specfied as boundaries.
If a zone is specified as a boundary, the program will design the vertical reinforcing zone as a boundary according to the provisions of the selected design code. Among the possible conditions that will be evaluated are reinforcing ratio (percentage), boundary extents, and confinement. In addition, the design of ties or links required to confine the vertical reinforcing in the zone per code will be performed.
I have 2 question about Boundary zone for Special Concrete Shearwalls. In the manual it says that if “c” exceeds “c limit” then boundary zones will be required. It also states that the output line will be in RED. When I run my model I notice that in all my cases the “c” value is less than “c limit” but it lists a “Req’d bnd length” in the output and it does not list it as RED. Could you help provide some clarity on this issue? See below for screen shot of my output
My second question is in the ACI for checking boundary zones we have 2 methods; 18.104.22.168 and 22.214.171.124. It is understood that RAM uses the neutral axis method (126.96.36.199). If we have cases were our wall piers are not continuous to the top of structural wall as noted in 188.8.131.52 or if our wall h/l does not exceed 2, how do use RAM to check the provisions per 184.108.40.206 to check if 0.2f'c is exceeded?
For users looking for a way to replicate the manual reinforcing for several identical walls, try using Process - Copy Reinforcing.