Comparison between the IBC and EC Codes in Reinforced Concrete Structural Design

Introduction Buildings are designed and constructed all over the world. In the UAE, every day we see buildings that are under construction given the significant property growth. Buildings can be constructed for various objectives and uses; i.e. a building can be constructed to serve as residential apartments, hotel and office building including retail and educational facilities. It is also known that any project needs to be designed properly by using professional software like ETABS which makes the process of design easier and efficient before proceeding with the construction of the project. In this project we have chosen to design the structural frame of a reinforced concrete building which is approximately 30.65m tall and consists of a ground floor, mezzanine floor and 6 typical floors. The main objective of the project is to see the differences between IBC and EC codes for the case of a real reinforced concrete structural design that was performed here in Abu Dhabi. The model of the building was constructed with the use of ETABS and AutoCAD. We are going to use the IBC and EC codes to apply different load types in order to study the mechanical behavior of the building and see the differences that derive in the design when accounting for seismic loads. In reinforced concrete buildings design realistic constraints apply like owner’s demand budget, time frame and material availability in the UAE related to the concrete and steel rebars. For the architectural constraints, we did not alter the geometry of the architectural drawings which is something that creates many conflicts between the civil engineer and the architect. The main objectives of this work were: • Practice with ETABS commercial software. • Perform a literature review on IBC and EC. • Construct the numerical models that will be used in order to compare the two

design codes through parametric analysis.

IBC A building code is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings. The International Building Code (IBC) is a model building code developed by the International Code Council ICC). A model building code has no legal status until it is adopted or adapted by government regulation. The IBC provides minimum standards to insure the public safety, health and welfare insofar as they are affected by building construction and to secure safety to life and property from all hazards incident to the occupancy of buildings, structures or premises. Eurocode The “Structural Eurocodes” comprise a group of standards for the structural and geotechnical design of buildings and civil engineering works. They cover execution and control only to the extent that is necessary to indicate the quality of the construction products, and the standard of the workmanship needed to comply with the assumptions of the design rules.

Geometry of the RC Building

For nearly 30 years ETABS has been recognized as the industry standard for Building Analysis and Design Software. Today, continuing in the same tradition, ETABS has evolved into a completely Integrated Building Analysis and Design Environment. The System built around a physical object based graphical user interface, powered by targeted new special purpose algorithms for analysis and design, with interfaces for drafting and manufacturing, is redefining standards of integration, productivity and technical innovation.

Fig. 2 Side View Fig. 1 Front View

Fig. 3 Plan view of the ground floor (Architectural drawing)

Fig. 4 Plan view of the typical floor (Architectural drawing)

Fig. 5 Plan view of the ground floor (Structural drawing)

Fig. 6 Plan view of the typical floor (Structural drawing)

The finite element method (FEM) is a numerical analysis technique used by engineers, scientists, and mathematicians to obtain solutions of differential equations that describe, or approximately describe a wide variety of physical (and non-physical) problems. Physical problems range in diversity from solid, fluid and soil mechanics, dynamics and electromagnetism or chemical reactions. As shown in the following figures, some of the practical implementation found in the international literature and the internet that use the FEA so as to study different type of structures in different scientific fields are presented.

Fig. 7 FEA Applications: Aerospace Engineering (AE)

Fig. 8 FEA Applications: Civil Engineering (CE)

Fig. 9 FEA Applications: Biomedical Engineering (BE)

Fig. 10 FEA Applications: Electrical Engineering (EE)

Fig. 11 FEA Applications: Mechanical Engineering (ME)

Presented by: Mohamad Walid El Said ID: 201010014 Supervised by: Dr. George Markou

Modeling of Structures

Fig. 12 Simulation of a Structural System

Fig. 13 A simplified view of the physical simulation process

3- Finite Element Discretization 2- Mathematical modeling 1- Structure

As it is shown in Figs. 12 & 13, the modeling and simulation process of a structural system are presented.

IBC and Eurocode

Commercial Software (ETABS)

Numerical Implementation

Fig. 15 Deformed Shape (Dead Loads)

Fig. 20 First fundamental mode. Rotational oscillation of the top floor

In Fig. 15 , we can see the deformed shape of the structure for the case of the model that was selected from the mesh sensitivity analysis (1m meshing size for the case of the shell finite elements). As it derives, the first fundamental mode is rotational, which indicates a significant eccentricity along the y-axis.

As it was shown in this work, developing the FE model of a RC structure requires experience and knowledge in modeling and the finite element method. The procedure that was implemented so as to achieve an error free numerical model is tendious and difficult especially for users that do not comprehend the basic know-how that relates to the problem at hand. A convergence analysis was performed by using the preliminary model of the under study structure. It was found that several columns and slabs were not connected with each other (mesh discontinuities), thus remedial measures were implemented so as to address these problems. Furthermore, a mesh sensitivity analysis was carried out so as to select the optimum mesh size for the case of the shell elements. It was found that when using a meshing size of 1m for both slabs and shear walls, the numerical accuracy and the computational efficiency were efficient. Finally, by observing the modes that derived from the model analysis, the numerical results indicate that the frame geometry derives large eccentricities and this is the reason why fundamental modes 1 and 2 include rotational deformations.

Conclusion

Fig. 16 Displacement = 0.329mm (IBC and Eurocode, Dead load)

Fig. 17 Displacement = 0.395mm (Eurocode Live Load)

Fig. 18 Displacement = 1.29mm

(IBC and Eurocode, Self Load) Fig. 19 Displacement = 0.263mm

(IBC Live load)

Numerical Implementation

As we can see from the above figures, the dead and self loads resulted displacements that were the same for both Eurocode and IBC, due to the fact that they foresee the same load values. In the other hand, the live load displacements in Eurocode are larger than in the case of the IBC code which points that Eurocode is stricter than the IBC code regarding the loading magnitude. The numerical behavior of the two models underline that both models have no numerical errors or mesh inconsistencies, thus they are set to be used for the numerical investigation that is going to be performed in capstone II

We performed a convergence analysis of our model. We used three sizes of auto meshing (0.5m, 1m, 1.5m), for the case of the shell finite elements, while comparing the derived numerical results after the analysis.

Fig. 14 Complete model of the building

Fig. 21 Mode 1 - Period = 0.5096 sec (meshing size 0.5m)

As we can see from the figures, we used a mesh size of 1m as to apply the loads in our model given that it provides computational efficiency and an acceptable numerical accuracy in comparison to the model that used a shell mesh size of 0.5m. In addition to that when using a shell mesh size of 0.5m, the computational cost increases significantly. The total required computational time for the case of 0.5m was 1 hour; while for the case of 1m was only 5 minutes. Finally, the model that used 1.5m as a shell meshing size derived a computationally efficient numerical model, but the accuracy was not adequate given the large size of the elements.

Fig. 22 Mode 2 - Period = 0.4358 sec (meshing size 0.5m)

Fig. 23 Mode 3 - Period = 0.3675 sec (meshing size 0.5m)

Fig. 24 Mode 1 – Period = 0.4758 sec (meshing size 1m)

Fig. 25 Mode 2 – Period = 0.4104 sec (meshing size 1m)

Fig. 26 Mode 3 – Period = 0.3468 sec (meshing size 1m)

Fig. 27 Mode 1 - Period = 0.4708 sec (meshing size 1.5m)

Fig. 28 Mode 2 - Period = 0.4078 sec (meshing size 1.5m)

Fig. 29 Mode 3 - Period = 0.3451 sec (meshing size 1.5m)

Senior Design I Fall 2013

Finite Element Method