Utilisation Factor of a Reinforced Concrete Slab

This workflow is about calculating the degree of utilisation of a reinforced concrete slab. You’ll find a guideline and how to represent the utilisation factor within WINGRAF as a filled area.

Let’s jump in and get started.

Basically, the calculation of the degree of utilisation is a comparison of two different design results. For this workflow, the focus is on reinforcement results.

The entire workflow can be broke down into three steps:

  • Definition of existing reinforcement
  • Design of required reinforcement
  • Determination of utilisation factor

Step 1. Definition of Existing Reinforcement

First of all the existing reinforcement of the structure needs to be stored in the database as a design result. so after setting up the model, it becomes necessary to assign the reinforcement parameters to the quad elements. It sounds much more complicated as it actually is, and can be done quite easily based on the assigned group numbers of the areas.

Design Parameters – Area Elements

To assign the reinforcement information to the different groups and as well layers,  add the task “SOFiSTiK: Design Parameters – Area Elements” to the project tree in Sofistik Structural Desktop (SSD). Depending on the selected template while creating a new project, the task might be already available. Keep in mind the task can be inserted into the Project only once.

Within the task, the tab “Common” allows selecting the different groups. Adding a new line assigning different details to use the right-handed “New” button. In the column “Reinforcement” modifying the details for each reinforcement layer within the selected group is possible. In the dialog box “SOFiSTiK: Reinforcement” enter the existing reinforcement amount for each layer at “Min. Reinforcement”. The input field “max. Reinforcement” is referred to a non-linear analysis only – it doesn’t have any effect in this particular workflow.

The system is complete and its reinforcement information assigned. Let’s focus on how to save the reinforcement information into the database as a design result case 101.

Therefore, performing the design based on a dummy load case is necessary. The only purpose of this dummy load case is avoiding any increase of the defined reinforcement. So its essential the forces are negligible.

Load Combinations

You can use the task “SOFiSTiK: Combine Loads” to create a single design load combination which is used afterwards. Select the Type of the Result “(D) ULS fundamental Combination” and create a new combination (1001) – the dummy load case –  on the right-hand side. Only add the dead-load case and assign a factor of 0.01 to get those negligible results.

Click on “OK” to create the load combination.

Analysis of Load Combination

To perform the analysis of the load combination (1001) the task “SOFiSTiK Analysis of Combined Loadcases” is required. Select the result type, tick the checkbox left to the load case number 1001 and activate “Linear Elastic” analysis.

Click on “OK” to run the linear elastic analysis.

Perform the design to save the existing reinforcement

To save the existing reinforcement into the database add the task “SOFiSTiK: Design ULS – area elements” to the project tree. Enter the design result case number 101, and select load combination 1001. Selecting specific load cases is possible when activating the option “Manually” at the top of the load case list.

Click on “OK” to start the design.

The existing reinforcement is stored in the database as design result case 101, and representing it in WINGRAF is possible.

Step 2. Required Reinforcement

In this section, the actual design is performed based on the loads. Depending on your experience in SOFiSTiK your preferred method might be a bit different. In this workflow, the graphical task is used.

Load Combinations and Superposition

SOFiSTiK provides tasks to perform an automatic superposition to generate the design load cases based on the design code. Two tasks are required to add which are the same task, but with different available Tabs. “Define Combinations” with the activated tab “Combinations Rules” and “Superpositioning” with the activated tab “Superposition Commands”.

If you want to learn more about the task to combine loads the following posts.

Superposition Manager – Combination Rules

Superposition Manager – Combination Rules – Action

Superposition Manager – Combination Rules – Load Cases

Design

Add the task “SOFiSTiK: Design ULS – area elements” to the project tree to design the area elements using the generated combinations. It is the same process as at the end of step 1. Except for the fact that instead of a single load case a bunch of design results are available to select. Important, enter a different design result case (111) to save the reinforcement results in a new load case.

Click on “OK” to design the area elements.

The results of design case 111 and the lower principal reinforcement look a bit different as in step 1. What makes sense as it represents the required principal reinforcement based on the load combinations.

Step 3. Utilisation Factor

All the analysis and design work is done, and the comparison of the two results can be performed. It’ll be done in WINGRAF using the comparison feature in “Properties”.

Right-click on “Lower Principal reinforcements (1st layer)” and select Properties. Double click will do it as well. Select the “General” tab if not active on default.

On the right of the dialog box Select “Single: Quotient” in the drop-down menu below “Comparison”. An additional drop-down menu will pop up on the left to select a second result case. Select the two different result case (101 and 111).

WINGRAF will present by default a textual output of the quotient. To change the representation, select the “Filled Area” in the “Representation” drop-down menu and switch to the tab “Filling”. In the section “Changes single values” the colour and range can be changed.

Wrap-up

The above workflow shows the required steps to do a comparison of two different results in WINGRAF. In this specific case, the comparison of two reinforcement results to end up with the utilisation factor. You can utilise it with other results such as forces too.

The entire workflow is represented in the below graphic as a recap.

Software version: SOFiSTiK FEA v2018-06.