January 28, 2015

Understanding the Rebar Design in RISAFloor ES

The Detail Report is the best place to start to investigate the reinforcement design.  It will present the forces for the Total Design Strip which is the summation of forces over the entire Design Strip.  The Total Design Strip is broken into regions of reinforcement design; Column Strip, Mid-Rt and Mid-Left.

Design Strip

Inside the Design Strip, it is automatically sliced into Auto-Cuts drawn perpendicular to the support line.  The summation of all the forces along the Cut is used to design the reinforcement.



Top Reinforcement Design

Step 1: Find the required length by analysis determined by the point of zero moment. This location indicates the no demand for tensile reinforcement to resist flexure. The zero moment location is indicated on the moment diagram as a vertical red line and the location of that line is listed above. This dimension is an overall dimension from the start of the support line to that point.

Step 2: The development length is calculated using ACI 318-08 12.2.2. Check the development length is longer than the minimum required by ACI 318-08 12.10.3, if not use this value as the development length.

Step 3: Add the required by analysis length (Step 1) and development length (Step 2) and check that it meets the minimum extensions listed in ACI 318-08 Fig 13.3.8.  If the total minimum length is not met then the additional length will get added to the length required by analysis.


The table displayed in the Detail Report is organized by support point.  The top reinforcement must be the same spacing on both sides of the support.  There is a red line indicated on the reinforcement drawing at the governing location for each support point.

Let’s take a closer look at the middle support point CS6_L1 which is designed for #6@15.   The governing location and cut is shown on the left side of the support but the Moment diagram shows that there is a higher on the right side of the support line (Cut S2-X1).  Why isn’t it governing?  The answer is explained by looking at the Design Cuts.  The Unity Check is shown to be higher in Cut S1-X50 versus Cut S2-X1.  The length of the cuts are different which means that the number of bars that fit into each cut is different.  The Top As Provided shows S1-X50 can fit 6#6 bars, while S2-X1 can fit 8#6 bars thus the unity check is lower.


Area of Steel Required

The program calculates the area steel required based on the steps listed below and the green text shows the governing area.

Step 1 (Analysis): Calculate the area steel required by analysis. Select a spacing and number of bars that fit the design region.

Step 2 (Min Flex): Check that the minimum reinforcement for flexure is being met per ACI-08 10.5.1.

Step 3 (4/3): Check the 4/3*As required by analysis per ACI-08 10.5.3. The ACI explains this exception is used when Min Flex would be excessive so this will only govern when it’s less than Min Flex.

Bottom Reinforcement Design

The bottom bars are considered continuous along the entire Design Strip throughout the entire support line.   The table in the Detail Report shows the different spans and the maximum cut that governed.  Since there are three different Area of Steel Required values, only the “green” or governing value is shown in the table.


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