As of version 2015, the verification of the ultimate limit state of punching shear according to code criteria has been included. Additionally, the existing method of point “tangential stresses” is maintained as a more accurate method, carrying out the sizing of the vertical reinforcement of vertical strengthening bars, if necessary.
- Point tangential stress method (punching shear prior to version 2015): Tangential stress is checked on surface areas which are concentric to the perimeter of the support at a distance from half the effective depth (d/2) and at successive surface areas every 0.75 times the effective depth (0.75d). The value of the tangential stresses at the points of the punching shear perimeter is obtained from the shear stresses at the nearby nodes of the discretised mesh. (25×25 for slabs and 1/3 of the rib spacing in waffle slabs) by means of linear interpolation. This stress is compared to the maximum punching shear tangential stress resistance, which is analysed using the corresponding concrete code.
This approach, based on point tangential stress verifications at the previously mentioned perimeters, may differ from the punching shear stress verification provided by the different concrete codes, which tend to be more simple when considering the stresses that the slab transmits to the support.
- Punching according to code criteria: The codes generally use nominal tangential stress on a critical surface concentric to the loaded zone, which is analysed considering the reaction of the support and the relative position of the support, and, occasionally, the moments transferred by the support to the slab.
The final result obtained by the two procedures may not coincide.
The additional reinforcement analysed by the program according to the point tangential stresses can be used as a reference to enter punching shear reinforcement (reinforcement inclined at 45º or beam type reinforcement) should the user decide to adopt the punching shear check according to code criteria. The reinforcement by tangential stresses must be removed manually or by using the “Delete reinforcement placed by tangential stress” option to avoid duplication both in the drawings and in the quantities.
Note: The two methods coexist at the same time, and are independent of one another. The results obtained by the two procedures need not necessarily coincide, which is justified in this comparative document, to be obtained from each method, and depending on the case to be checked.
From this point onwards the following reference will be made:
Method A = Point tangential stress verification
Method B = Punching shear in accordance with code criteria
|Item to be compared||Method A||Method B|
|Basis of the method||Elastic.||Plastic.|
|Implementation date||All versions.||Version 2015 or higher.|
|Method description||Starting from the first section of the punching shear verification (0.5 d) and on parallel surfaces at a distance of 0.75 d, the shear verification is carried out on the entire surface of the slab, until all the radiating surfaces are found starting from the support edges. If reinforcement is necessary, the number and diameter of the reinforcements to be placed (vertical branches) are indicated with the same type as indicated for the punching shear.|
Similarly, if this is not complied with, a red line appears indicating that the maximum punching shear stress limit has been exceeded, displaying the following text: “Insuf.”. In this case, the depth, the support size or the concrete strength must be increased.
The treatment of foundation slabs is identical to normal solid slabs in terms of reinforcement design.
|Generally, codes use nominal shear stress, acting on a critical surface concentric to the loaded zone. It is analysed by taking into account the reaction transmitted by the slab to the column, usually, the increased axial force depending on its position (centred, intermediate, corner), and in some codes, with the transmitted moments.|
|Where to get additional information on the method||“CYPECAD Calculations” PDF file available via the “Help” menu|
See page 67, Transverse reinforcement.
|The chosen code.|
|Sizing||Sizes the reinforcements automatically if necessary, during the analysis of the job.||It does not carry out the sizing of the reinforcement automatically during the analysis, it is a user option after the analysis, it only performs the check assuming that there is no reinforcement.|
|Check||The reinforcement generated by the program is always compliant. The program does not check whether the reinforcements modified by the user are compliant. If the thickness is not sufficient with the placed reinforcement due to oblique compression problems in the concrete, the program will mark the area with “Insuf. Sect.” and a red line.||The user must reinforce the indicated zones and use the program to verify them. When reinforcements are entered, they are checked for compliance.|
|Check command||“Waffle floor slabs” menu > “Views” > “Tangential stress reinforcement”||“Waffle floor slabs” menu > “Punching shear”|
|Identification of error messages after the analysis. Final design report||– a) Sections with shear or punching shear reinforcement.|
– b) Insufficient sections due to shear or punching shear.
|– a) Supports “P1, P2, P3 y P4” fail a punching shear check.|
|Message interpretation. Final design report||– a) The program has generated the necessary reinforcements.|
– b) Very serious. The thickness of the slab is not sufficient and it will break under oblique compression.
|– a) With or without the punching shear reinforcement entered by the user, with or without the reinforcement area of the beams reaching the reversed column with the edit command of the punching shear menu, the program checks the punching shear and provides a justification report.|
|Command to view results||“Results” > “Flat/Waffle slabs” > “Views” > Activate/Deactivate > “Tangential stress reinforcement”||“Results” > “Flat/Waffle slabs” > “Punching shear” > “Checks”|
|How to modify the reinforcement||“Results” > “Flat/Waffle slabs” > “Tangential stress reinforcement”, we have several options.|
Also using the “Delete reinforcement placed by tangential stress” command from the “Punching shear” menu from Method B.
|“Results” > “Flat/Waffle slabs” > “Punching shear” > “Generate reinforcement” > “Add reinforcement”|
|Plans for the reinforcement||Floor plans, type of layout plan, configurable.||Punching shear reinforcement.|
|Reports||Non-existent due to its complexity. Must be justified with the presented contour plots of the total shear design forces.||The “Punching shear checks” report does this.|
|Advantages||– For almost any type of geometry on plan.|
– Checks shear in waffle slab concrete ribs.
|– Generates reports.|
– Includes 2 types of reinforcement, with 45º inclination and beam-type reinforcement.
|Disadvantages||– Does not check the modified transverse reinforcement.|
– Does not generate reports.
– Only has one type of reinforcement.
|– Does not work for some complex geometries, with closely spaced columns and an irregular opening distribution.|
– For concave polygonal contours the adjacent surface is not analysed correctly.
-No sizing, only checking.
|Applied supporting elements||Around columns, shear walls, walls, starts, beams.||Only around columns.|
|Known limitations||The insertion point of the grid may have a slight influence on the results for some complex outlines.||– The check does not automatically take into account the ratios of the transverse reinforcement of the beams in contact with the column (Asw/s).|
– Existing loads in the area of the punching shear perimeter are not discounted for the check, the method remains on the safe side by taking into account the reactions in the support.