| Product(s): | RAM Concept | ||
| Version(s): | Any | ||
| Environment: | N/A | ||
| Area: | Design | ||
| Original Author: | Bentley Technical Support Group | ||
Why does my flat slab (or flat plate) model have one-way shear reinforcement or fail the shear check?
When some engineers design flat slabs by hand, they ignore the one-way shear checks. They decide that punching is all that is appropriate. (This is often decided without much consideration – it just “seems right”).
Concept does not make this decision, as nowhere does the code advise to ignore one-way shear checks in a flat slab or flat plate. In fact, ACI 318-02 rule 11.12.1.1 specifically requires a one-way shear check in flat plates. Nonetheless - you should decide what the possible failure mechanism is and so what is appropriate. It may, or may not, be appropriate to ignore the one-way shear results.
In Ram Concept, one-way shear checks are performed for design strips and sections independent of the punching shear check. Transverse shear reinforcement is detailed by the program if required for one-way shear capacity. Stud rail reinforcement is detailed if required for punching shear capacity. Transverse steel reinforcement is not considered in the punching shear check, and stud rails are not considered in the one-way shear check.
One of the biggest factors in thick slabs (especially in mats) is the fact that the program designs for shear at each cross section, while the design codes generally allow the designer to ignore shear some distance from the face of the support (typically "d"). The program cannot know what d is in advance of creating the cross sections, so it cannot precisely place a section for this purpose. In cases where program designed shear reinforcement is only required in the last section at the face of the support it can be ignored.
The results have a lot more shear reinforcement than expected.
This is likely to be a shear core issue. Refer to “About shear core” on page 99 and “Shear core in slabs” on page 100 of Chapter 22, “Defining Design Strips” or the design strips technote. In summary, when the shear core is small relative to the total section area and demand, a lot of shear reinforcement may be required.
For a post-tensioned beam, the reason could be that Concept is deducting a fraction of the (bonded) duct from the web width per the appropriate code rules.
Concept calculates the number of duct by dividing the Strands per tendon by the Max strands per duct (as specified in the Materials) and rounding up to the next integer.
Refer to the following sections for an explanation of Concept’s shear web calculation:
- For AS 3600, “Section 8.2 Shear Design” on page 415
- For BS 8110, “Section 3.4.5 Design shear resistance of beams” on page 432.
- For IS 456, “Section 22.4 Design shear resistance of beams” on page 460.
Note: There is no ACI 318 rule concerning deduction of ducts.
What does this audit text mean: “Depth “d” is zero - replacing with “column” effective depth. Depth is still zero - giving up.”?
The is likely a combination of two things:
- there is net compression force and a small moment, and as such the bending designer does not provide any reinforcement, or
- the minimum designer has been turned off. If this is the case, you should consider turning the minimum designer back on.
A similar condition tends to occur near the inflection point of a span. In these locations the moment can be very small and a flexural capacity check can pass with very little top reinforcement, but if there is no tension reinforcement at the tension face, or of there is a lot more reinforcement on the compression face, this skews the calculated depth to the tension reinforcement (d) and drastically reduces the shear capacity.
Can I model user shear reinforcement?
Starting with Ram Concept V8i SELECTseries 9, release 5.2 user defined transverse reinforcing can be specified.
A wiki on user defined shear reinforcement in RAM Concept is available at the following link:
Prior to release 5.2, transverse reinforcement is always designed by the program and cannot be modeled as user reinforcement. Program designed transverse reinforcement also cannot be changed by the user.
Does the reinforcing weight in the Estimate Report include transverse reinforcement?
Yes, transverse reinforcement is included in the reinforcement weight in the Estimate Report.