I have a 9" conventionally reinforced, 2-way slab that I am running in RAM Concept and I am running into a problem when I start to specify user reinforcement in some areas.
As you can see here, I have run the model and everything checks out fine. I have a full top and bottom mat and additional program reinforcing is being shown at the top mat:
Now on this support location of interest all I did was change the 19#6 bars from program to user reinforcement, and all of a sudden the strips are failing per 10.3.5 and a ridiculous amount of bottom bars are provided:
Any idea what could be causing this? I have this happening at several other locations with no particular rhyme or reason.
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This issue seems related to the forum post that can be found here:
Like the previous forum post, span detailing is the cause of the added bars. However, this case is slightly different. The bottom bars are added at the sections at the support face, where additional reinforcement is required for strength but there are no extended bars into the section due to detailing requirements.
To understand how span detailing is associated with the result, an understanding of the design process is required. An initial design pass, Pass 0, is completed for the purpose of span detailing. When determining the span detailing requirements, we did not want an excessive amount of user reinforcement to create excessive span detailing requirements over the entire span. As a result, the Pass 0 design excludes the user reinforcement. The resulting Pass 0 program reinforcement represents the minimum reinforcement at the cross section, which is then span detailed along the span based on the code requirement. After Pass 0, the program completes a Pass 1, Pass 2, and Final Design. For the Pass 1 design, the Pass 0 reinforcement is not considered. The user reinforcement is always considered in this pass and additional program reinforcement is designed as required. Pass 2 uses the cumulative program reinforcement designed in pass 0 and pass 1, in addition to any user reinforcement. These passes involve shear and ductility design. The Final Design includes any rebar designed in Pass 1 and 2, but not less than the rebar required in Pass 0. The Pass 0 and Pass 1 and 2 results are not summed, but rather enveloped.
In your initial run before converting the top reinforcement to user reinforcement, the top program reinforcement was added in Pass 0. When viewing the controlling criteria on Layers - Design Status - Top Reinforcement Plan using the Visible Objects dialog, you will see "Det" prepended to the governing ACI provision. This stands for detailing and means that the rebar at each station was added for span detailing requirements.
In your model, you can make one small change that will eliminate the bottom bars completely. When viewing the properties of the user reinforcement and the program reinforcement, you will see that the user top bars are at an absolute elevation of -1.875 inches and the program top reinforcement is at an absolute elevation of -2.249 inches. I believe the intent is for both user and program reinforcement to be placed in the same plane. If so, then the span segment should be edited and CS Top Cover should be set to 1.5 inches (1.5 + 6/16 = 1.875 in). The cover adjustment reduces the number of program top bars that are added and bypasses the ductility failure. When the program reinforcement and user reinforcement are at different elevations the enveloping of the Pass 0 and Pass 1 and 2 can be problematic in some cases.