Improvements in concrete design EN 1992
Version 18.1 of SCIA Engineer brings good news to those who would like to calculate the long-term deflection (with creep). In this version, a new improvement has been implemented to take the effects of ribs in the calculation of the deflection for the beam-slab connection.
Based on the new approach, the user can implement the beam as a rib in his floor/slab system and the software will calculate the accurate stiffness for the ribs and the slab independently. Moreover, this new system will calculate the total deflection as well as the deflection due to creep at the same time.
So with this improvement, the user will not waste the time to have a sub-model to calculate the deflection for any multi-story buildings including the ribs effects.
Calculation of long-term deflection for a rib-slab system according to EN 1992-1-1
Improve the calculation of the compression zone based on NEN notional annex.
A new methodology for the calculation of long-term deflection of the slab containing ribs.
It is clear to us that today we have many clients using the system of ribs when the construct a floor or slab system. The system is quietly used by the multi-story buildings where the beam and the slab are cast at the same time. With this system, specific requirements should be taken into account for the calculation of the stiffness of that system. In the previous versions of SCIA Engineer, the user always get a message to inform him that the deflection of this system is based on the linear stiffness of that system. This means that the deflection will be much less due to the fact that the stiffness for the flange of the beam is calculated twice within the connection with the slab.
Therefore in this version, we have decided to improve the calculation of the long-term deflection by taking into account the stiffness on final section (part of the slab + 1D member) with internal forces recalculated to the center of gravity of final section. The stiffness of a 1D member has calculated only for the 1D member without the part of the slab. The stiffness of 2D meshes inside effective width and in direction of the rib has calculated with using the height of compression zone, which is calculated for the final cross-section.
Procedure of calculation
The new procedure for the calculation is quite simple and the user has to do nothing except model his/her ribs within the floor/slab system. The rest of the procedure is implemented automatically by the software in two scenarios:
First scenario: Ribs is not taken into account
If the checkbox RIB is off, then deflection will be calculated separately for 1D member (member on eccentricity which is linked to the slab) and for the 2D member. For calculation deflection of the 1D member following preconditions will be used:
• Only cross-section (rib cross-section), which is defined in properties of the RIB will be taken into account.
• The forces in the center of gravity of rib cross-section will be used for calculation deflection and stiffness.
• Only longitudinal reinforcement inside of rib cross-section will be used for the calculation
For calculation deflection of the 2D member following preconditions will be used:
• The forces in the center of each FEM element will be used for calculation.
• The sum for reinforcement defined for 2D member (2D reinforcement meshed, free bars) and transformed reinforcement from the flange of the 1D final cross-section.
Second scenario: Ribs is taken into account
If the checkbox RIB is ON, then deflection will be calculated for the equivalent cross-section
The final cross-section of the rib will be divided for two or three entities (for rib aligned to the center of the slab). Equilibrium for SLS will be calculated for the whole final cross-section, but cross-section characteristics and stiffness will be calculated only for selected entity.
Advantages of this solution:
There are several advantages in this version regarding the new solution for the calculation of long-term deflection for the rib-slab system:
1. The user can model the beam as rib and the software will calculation the deflection correctly for that system.
2. The reinforcement which defined in a flange (part of the slab) of final cross-section will be taken into account for calculation stiffness of 2D elements inside of effective width.
3. The reinforcement designed (required or provided reinforcement) in a flange (part of the slab) of final cross-section will be taken into account for calculation stiffness of 2D elements inside of effective width. This reinforcement will be transformed on the background on standard 2D reinforcement and will be added to reinforcement which is designed on 2D member.
4. The new solution takes into account the following effects for the calculation of stiffness for the equivalent cross-section:
- The height of compression zone calculated for quasi-permanent combination (value Xs).
- Maximum tensile stress inside slab entity of final cross-section calculated for characteristic combination (crack forces, with tension).
- Maximum tensile stress in reinforcement in the whole cross-section calculated for quasi-permanent combination (stiffness forces, without tension).
- Distribution coefficient for the whole cross-section calculated for quasi-permanent combination .
5. Take the influence of the ribs for the calculation of the stiffness for the 2D members.
6. Take into account the calculation of stiffness of 2D meshes inside effective width for more ribs
More improvement in this version:
Not only the calculation of the ribs has been improved but also, SCIA Engineer brings some good news to our clients who use the NEN-EN 1992-1-1 on a daily base.
Improvement of the calculation of the compression zone based on NEN national annex:
Small but very important improvement has been done in this version of SCIA Engineer, this regards the calculation of the compression zone (betondrukzone xu).
This improvement includes the followings changes:
1. Add an option to deactivate the calculation of the compression zone:
Based on the user requests, SCIA Engineer version 18.1 has the option to deactivate the calculation of the compression zone. If the user deactivated the compression zone check then SCIA Engineer will ignore if the hight of the compression zone is ok or no. this is the quite useful case when you have beam supported directly on the soil.
2. Calculate the compression zone based on the action forces. With this option, the user can calculate the hight of the compression zone and the internal forces NEd, MRd will be used for that. Useful case to have more conservative results.
3. Calculate the compression zone based on the resistance of the cross-section and, MRd. With this method, the software will calculate the capacity of the cross section and use the resistance to calculation the hight of the cross-section. This method gives results closer to the hand calculation.