| Product(s): | RAM Concept | ||
| Version(s): | Various | ||
| Environment: | N/A | ||
| Area: | Design | ||
| Original Author: | Bentley Technical Support Group |
Why do I get an error about "Too many slab shapes intersecting the column shape"?
The location in the error message is the location of a column and punching shear check in the model. The error usually occurs where multiple beams of varying thickness intersect the column creating an overly complex punching shear failure plane.
Often in those cases no punching shear check is even required (provided the beams have sufficient one way shear capacity in both directions). If that's the case simply delete the offending punching shear check to proceed. In cases where a punching shear check really is required, try simplifying the geometry to limit the number of changes in thickness at the column. For edge columns, the concrete beams or slabs should typically cover the full column area.
Modifying the punching check by reducing the Search Radius or changing the Edge Treatment could potentially help as well.
The warning message "No column critical sections were found at a punch check" or "No cutoff critical sections found" can also occur under the same conditions.
This warning is more likely when the search radius is so small that the critical section a distance "d" from the face of the support is bound to be beyond the search radius.
Why do I get an error "Punching Critical Section has Ix (or Iy) = 0 at..."?
This error occurs when there is a potential failure plane that is a single line (in plan). As such it's not a valid punching check, though one-way shear should certainly be checked using a design strip or design section.
In some cases, reducing the punching check radius so that the circle does not cross 2 opposite slab edges is a solution.
The Punching Shear Check has the box for "Design SSR if Necessary" checked. However, there is a punching shear failure and stud rails are not designed.
Stud rails are extended to the extents of the search radius of the punching shear check. If the rails are extended to this point and the design is not satisfied, then no stud rails will be designed by the program.
The ACI code limits the shear strength, Vn, of the slab. See ACI 318-08 11.11.3.2, for example. If the shear demand exceeds this maximum strength, then no stud rails will be designed.
Can Ram Concept check punching shear for a pair or group of columns?
No, in Ram Concept each column reaction is used in an independent punching check, No group punching for close columns is considered.
Punching around walls is also not considered. For this reason we do not recommend checking punching for column pilasters at wall locations.
How does Ram Concept calculate the demand for punching shear calculations?
Refer to the program manual, section 66.2 "How does RAM Concept handle punching shear?" and 66.2.1"Step 1: Determine the force envelopes to be checked" for details. In brief, the program uses the column reactions, less any point loads or column above reactions inside the column shape. Surface loads around the column within the punching failure plane are not discounted from the demand.
How does Ram Concept calculate the critical section properties for circular columns?
The critical section for circular columns is approximated as an equivalent polygon with 16 sides and the properties are calculated using the resulting linear segments. This approximation is used so that the program can handle critical sections with irregular shapes.
When Concept reports the punching shear allowable stress, is it already reduced by phi?
Yes, for example, the ACI equation 11-34 indicates:
Vc = (as * d / bo + 2 ) (root f'c) bo d
but phi = 0.75
So Concept reports ( phi Vc / bo d ) as the allowable stress.
Can Ram Concept design a standard reinforcement cage to prevent a punching failure?
No, Ram Concept designs stud rails (including the Ancon Shear fix) but not a traditional shear cage.
Is the beam shear reinforcement considered in the punching check design?
No, the punching check, and potential stud rail design, is performed independently from the one way shear reinforcement in the design strip design. The one-way shear reinforcing is not considered in the punching check, and the stud rails are not considered in the one way shear check.
See Also
RAM Concept Design Strips [TN]