CROSS Safety Report
Punching shear design spreadsheets
This is a technical note from a reporter about the methods their firm uses for punching shear design because there was concern about punching shear design spreadsheets possibly leading to potentially unsafe situations.
Key Learning Outcomes
For civil and structural design engineers:
- The appropriateness of any software should be checked particularly where the design case is different to typical construction
- This should include reviewing the output to check that it is consistent with what would be expected from engineering judgement, as well as independent calculations
- Be aware that proprietary design packages for shear rails may not calculate Beta factors and care needs to be taken to get this right
- Guidance is provided on the detailing of shear links in flat slabs in The Institution of Structural Engineers Standard method of detailing structural concrete
For the construction and site inspection teams:
- The importance of shear links in flat slabs cannot be overstated and they must be fixed exactly as detailed
- Where there are difficulties in positioning reinforcement the designer should be consulted
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This is a technical note from a reporter about the methods their firm uses for punching shear design because there was concern about punching shear design spreadsheets possibly leading to potentially unsafe situations. A key issue is that input values which are outside the assumptions of the spreadsheets may not be flagged, particularly for large openings located near columns resulting in the spreadsheets incorrectly showing valid designs.
In addition, it is difficult to check the spreadsheets in cases where the formulae have been edited. The recommendation from the reporter is to use reputable proprietary design packages for shear rails.
These systems use studs on rails arranged in either radial or cruciform layouts, but can be converted to shear links where conventional reinforcement is required. Such software may not calculate Beta factors though and care needs to be taken to get this right. The Beta factor in the EC2 punching shear rules accounts for the uneven distribution of punching shear stress around the control perimeter, and plays an important part in determining the required punching shear reinforcement.
Such software may not calculate Beta factors though and care needs to be taken to get this right.
When loose links are specified, there is potential for these to be installed incorrectly on site and there is much more scope for them to be installed with the wrong spacings compared to longitudinal reinforcement.
Shear rails are much more robust, says the reporter, as the layout and spacing is set on the prefabricated units which greatly reduces the risk of incorrect installation on site. The potential issues identified by the reporter also highlight the importance of carrying out regular technical reviews and checking designs well in advance of construction.
Shear rails are much more robust, says the reporter, as the layout and spacing is set on the prefabricated units which greatly reduces the risk of incorrect installation on site.
Where designs are based on a radial arrangement, but loose links are being detailed, the Concrete Centre guidance must be followed to convert the radial design into an orthogonal grid of shear links. Ideally, a cruciform layout would be designed as this translates directly into an arrangement of loose links. If loose links are specified, then a radial arrangement must not be detailed.
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The appropriateness of any software should be checked particularly where the design case is different to typical construction. This should include reviewing the output to check that it is consistent with what would be expected from engineering judgement, as well as independent calculations.
Shear rails are more robust than loose links, but because of this they need to be fully coordinated with the longitudinal reinforcement and other inserts around the column. Loose links do allow minor local adjustment. In either case checking of the placed reinforcement is required.
Ideally the layout of links/studs should match the design intention. The reporter notes methods of going from radial layouts to orthogonal layouts, but this also depends on the design approach with some radial packages for studs taking advantage of their superior performance is certain circumstances. Whilst the reporter mentions the use of the cruciform layout, there is no reason not to use a rectangular arrangement as was normal with previous UK codes of practice; again providing the design intent is followed.
There have been previous reports about punching shear concerns in flat slabs, including:
1050 Concerns about punching shear in a flat slab
950 Inadequate punching shear reinforcement in flat slabs
906 Missing punching shear reinforcement in concrete slabs