RC design

Contents

• Eccentricity increment
• Automatic buckling length calculation
• Punching for wall ends
• Change direction of concealed bar
• Ignore torsion in RC bar design
• Symmetric bar reinforcement
• Change of bar cross section during design
• Bar reinforcement summary
• Improved auto-design of RC shells
• Auto-update of RC shell results
• Reinforcement colour settings
• Improved Quantity estimation of RC elements

Eccentricity increment

The design of reinforced concrete bar, shell and composite column is supplemented with several eccentricity increase options that address design issues such as the reduced imperfection parameters of prefabricated elements or the imperfection of the RC walls when calculating the required reinforcement.

RC bar and concealed bar

The Calculation parameters of bar and concealed bar have been supplemented with an option to manually enter a value of the eccentricity increment data of imperfection and minimum design. The manually entered values override the code-defined values.

The application of the specified values is also reflected in the detailed design results:

RC shell

The Calculation Parameters of RC shell have been supplemented with the option to manually enter eccentricity increments which will be taken into account in the required reinforcement calculation and optionally in the shell buckling calculations. Reinforcement design moment will be modified by compression forces and eccentricity increments, which will be applied in the most unfavorable side in the direction of the reinforcement.

Example - Design moment for x’ reinforcement direction:

m_x’,bottom = max(n_x’ * e_{min,x,mx’,bottom} + n_x’ * e_add,x)

m_x’,top = max(n_x’ * e_{min,x,mx’,top} - n_x’ * e_add,x)

where n_x’ is the maximal compression force in the x’ reinforcement direction. If n_x’ is tension, no eccentricity increment will be applied.

The Design forces table is supplemented by the applied eccentricity increments in the detailed results of the calculation of the required reinforcement.

Note: This feature is not suitable as a replacement of a corbel element or any other structural connection that causes eccentricity in a general model geometry.

Composite column

The Calculation parameters have been supplemented with an option to manually enter a value for the eccentricity increment data of imperfection and minimum design. Manually entered values override code-defined values. Minimal eccentricity is not declared in EN 1994-1-1, thus, only the manually entered value can be taken into account.

The application of the specified values is also reflected in the detailed design results:

Automatic buckling length calculation

So far, the wall buckling length value had to be entered manually in the program. Now however, it is possible to automatically calculate buckling lengths of selected RC walls, depending on their lateral restrains and the ration of their edge sizes according to Table 12.1 of EC 1992-1-1 12.6.5.1.

To do so, start the Buckling length function, click the Calculate buckling length function and select the wall(s). The calculated β values can be overwritten manually at any time using the Properties function.

Algorithm rules:

• The automatic calculation can only be applied to reinforced concrete walls, so other types of shells (e.g., timber, steel shells, RC plates) are excluded from the selection.
• Any shell that makes an angle of 30° to 150° with the wall is considered as a lateral constraint of the wall.
• If there is only one lateral constraint in the direction of buckling (cantilever wall), the calculated buckling length multiplier is 2.0.
• According to the note in Table 12.1 of EC 1992-1-1, openings with an area of less than 10% of the total wall area or with a height of less than 1/3 of the total wall height do not affect the calculation, so the wall is considered to be without an opening during the calculation.
• Walls with openings, where the dimensions are above the upper mentioned limits, can be divided into several sub-regions. For non-rectangular sub-regions, the calculated value is 0.0. Opening edges are considered as free edges.

Punching for wall ends

When designing reinforced concrete structures with wall-supported slabs, the wall-slab connections can be critical against punching. From now on, FEM-Design offers a possibility to design and check the wall-end and wall-corner type connections of the slabs for punching. Along with the new wall punching function, the column punching (already available in previous versions) is expanded with two methods to calculate the design forces:

• Nodal force-based method
• Shell internal force-based method

Wall end / corner definition

The definition of the wall end and wall corner can be done with the Punching tool of the Punching reinforcement design mode of the RC design tab.

Pick wall endpoints on shell element. If the point definition is successful, the punching region and the coordinate system are created automatically based on the geometrical parameters set in Punching dialog:

• Distance of perimeter (of the punching region
• Perimeter length limit
• End limit: maximum perimeter length along the wall, which affects the considered shell region for the checking perimeters

All geometric parameters can be declared in effective depth (d_eff) for ease of use.

Stirrup segment edit ray is an aid for easy manual and automatic design of all types of stirrup types with the appropriate geometry. We can move it along to the wall baseline to fine-tune the stirrup setting. This tool does not affect the checking perimeters.

Limitations:

• Wall end punching region cannot be modified.
• Wall corner case works for maximum of two walls, if the smaller enclosed angle is no more than 135°.
• In case of multi-storey buildings, depending on the state of the Downward option (Punching tool palette), the Define procedure controls the walls above (checked state) or under (unchecked state) the storey level.
• Wall thickness must be constant.
• The punching region is created by the analytical region of the wall, not its physical extent.
• The newly created wall does not affect the already placed punching areas.

Design force calculation for column and wall punching

Two methods of design force calculations are available for punching:
 

Nodal force-based method (only for column punching)

This method calculates the shear forces and moments at the finite element node closest to the punching reference point from the difference between the normal forces of the connected columns and the total external loads acting on this node. Since the shear force is uneven along the wall connection line and the shell, the node force-based calculation can underestimate the design force when calculated for the node closest to the origin. For this reason, the nodal force-based method cannot be chosen for walls, only for columns.

Shell internal force-based method

This method calculates shear forces and moments from the internal forces of the shell along an adjustable perimeter segment. This design force calculation circumference may differ from the checking perimeters.
 

Following calculation methods for the shear force along the perimeter can be chosen:

Integration methods take over the integration of internal forces across the design perimeter.

Max method selects the maximum value on the perimeter and multiplies it by the length of the perimeter. When choosing this method, consider the followings:

• The method natively takes into account the effect of uneven load distribution (the maximum value is taken along the design line); therefore, it is recommended that the Beta increase factor be set to 1.0 at Calculation parameters. (Beta is the factor in Eurocode that represents the increase in load due to uneven distribution).
• However, the method has a major drawback: if the perimeter of the design force intersects a singularity point or a highly disturbed region, the result can be overestimated.

The main differences between the two calculation methods are the accuracy and the handling of external distributed loads.

• Nodal force-based method is high-precision, almost mesh-independent, but it is increased by the external distributed loads in the punching region.
• The accuracy of the Shell internal force-based method is highly dependent on the accuracy of the shell internal force (basically the mesh size) and is not increased by the external distributed loads. Therefore, smaller design forces can be obtained. Consequently, refining the mesh around the punching region is strongly recommended to use this method.

Change direction of concealed bar

From now on, it is possible to change the original direction of the concealed bar. Use Change direction option at Concealed bar function.

Ignore torsion in RC bar design

According to EC 1992-1-1 6.3.1 (2), for reinforced concrete bars, if the torsion is only due to compatibility and the structure is statically indeterminate, it is not necessary to take torsion into account in the ULS load combinations. FEM-Design 21 is supplemented with such option that can be found in the Calculations parameters of RC bar.

If the option is checked, a relevant comment will be displayed in the detailed calculation results:

Symmetric bar reinforcement

Symmetrical reinforcement allocation within the bar cross-sections is many times a preferred solution. Therefore, from now on, in case of RC bar Auto design, the symmetrical rebar allocation can be chosen at the RC bar, design parameters.

Change of bar cross section during design

It is now possible to change the bar's cross-section during the reinforcement (manual and auto-design) process, in order to find the most optimal one.

Auto design

In the Cross-section tab select the possible cross-section(s) for Auto design. In the Reinforcement tab, set the allowed reinforcement ratio. FEM-Design will start the design process by considering the smallest allowed cross-section, then will design reinforcement and check if reinforcement ratio is below the set limit. If not, the next cross-section will be checked. In case of the biggest cross-section, user-defined reinforcement ratio limit is not considered.

Note: By default, only the original cross-section is selected as possible one.

Manual design

If a new cross section is selected during design, it will be displayed in both the Utilization table and in the model view.

Apply design changes in the analysis model

If a different cross-sections (from the original one) is selected during the design, following warning message will be displayed upon leaving the RC design tab.Upon pressing Yes, the original cross-section of the bar will be modified to the applied one.

Bar reinforcement summary

Detailed summary of reinforcement applied to RC bars, concealed bars and composite columns can be now listed in table format.

Bar reinforcement data is available under Input data for RC design and Composite design.

Improved auto-design of RC shells

A significant work has been done towards optimizing the auto-design of RC shells in FEM-Design 21, which results in about 15-20% less reinforcement in general, and up to 50% less in special cases.

Improvements

• The algorithm starts with the regions containing smaller diameter reinforcements or larger spacing, depending on the selected strategy (...by fixed diameter or ....by fixed space).
• Optimization is done in the automatically generated reinforcement regions, focusing on reducing the amount of reinforcement.

New algorithm

A new shape called Minimal area of rectangles has been added to the Design parameters of RC shell.

The new algorithm tries to cover the region, where any reinforcement is required, by summing all the regions into a polygon and then dividing it into rectangles that target the smallest total area. The algorithm ignores the number of generated  rectangles (unlike the Minimal number of rectangle algorithm, where the aim is to cover the region with the least number of rectangles).

The division where the width of the new rectangle would be less than the b_min parameter is not completed. In this case, the geometry of the remaining non-rectangular region can be modified. For b_min parameters that are large enough, the result could look like the figure above.

For previously defined regions, the algorithm automatically checks if their rebar collides with the rebar in the automatically generated reinforcement regions. If so, the concrete cover of the previously defined regions will be slightly modified to avoid collision. This operation is also logged in an information message (e.g., "In this previously defined applied reinforcement region the x' or r bottom reinforcement's cover was increased from 30 mm to 35 mm to avoid the rebar collision.")

Auto-update of RC shell results

A new option Auto-update RC shell results is added to the Reinforcement layer / option. When it is activated, the design results such as design forces, required reinforcement and missing reinforcement will be automatically updated upon any changes in the applied shell reinforcement.

The animation below shows an example of how modifying the characteristics of an existing reinforcement automatically changes the amount of the missing reinforcement.

Reinforcement colour settings

In order to visually separate the individual surface reinforcement areas (for example the base net from the additional reinforcement), reinforcement Colour settings has been introduced in FEM-Design 21. From now on each area reinforcement type (base / additional), in each plane (bottom / top / center) and each direction (x' / y') can be given a unique color.

Colour settings in Auto-design

Colour settings in Manual design

Change appearance

Surface reinforcement colour can be also modified with the Change appearance command (Modify menu). The selection of the surface reinforcement has been improved, and from now on, it is enough to select only one object of the surface reinfocement (e.g. edge of the area) in order to change the colour of all area.

Improved Quantity estimation of RC elements

From now on, more RC data is available in the Concrete tab of Quantity estimation.