How to design a reinforced concrete building: model, structural analysis and theoretical reinforcement

A typical reinforced concrete building can be taken through a single program from an empty screen to the calculation note. This tutorial shows the whole path in the “Building” mode of IgentForm on a simple example: the storey model, loads and combinations, structural analysis, theoretical reinforcement areas, the calculation note and the step up to a multi-storey building.

The steps follow the order of our Polish video with voice-over, which you will find at the end of this tutorial; the English video above shows the same workflow. The screenshots come from the Polish version of the program — IgentForm is also available in English and French.

Step 1. The storey: slab, walls, columns and beams

You start with the slab of the first storey. Its contour can have any shape. Snap options and a grid, which you can modify freely, help you pick the points.

IgentForm is a solid modeler, so the slab you draw immediately becomes a solid. In practice one solid corresponds to one storey. This has a concrete advantage: you select all the external contour lines in one move and define the external walls on them in one go.

Then you:

• pick points and add the internal walls,
• add columns and beams — after this step the static scheme of a simple building is ready,
• insert door and window openings from the opening database, which you can extend with your own dimensions.

If you have a floor plan in a DXF file, you do not need to draw from scratch: the program loads DXF backgrounds and lets you model the structure on top of them.

The finished storey: a slab with an arbitrary contour, walls and openings. On the left, the section and opening database.

Step 2. Loads, load patterns and combinations

Dividing the slab into panels

Before you apply loads, it is worth dividing the slab into panels. The division can be done manually or automatically — along the internal walls. This is controlled by the option that includes an internal line in the division of the slab and loads. When it is on, the program updates the division after every change to the model. In the example the main slab was divided into three slabs this way.

Loads

You assign the live load to the whole slab or to selected panels into which it has been divided. The load value can be changed at any time.

Live load assigned to the slab of the storey.

Load patterns

You can build load patterns manually by choosing the loads that belong to them. It is faster to use automatic patterns: with one click the program generates all possible load arrangements, so that the most unfavorable internal forces are obtained later.

Code combinations

Finally you select the code for the combinations and click “Generate”. All the combinations generated according to the code are listed in a table.

Step 3. Structural analysis — three methods

Before the calculations you choose the design code. The French code BAEL 91 mod. 99 and the Eurocode (EC) are available.

Then you choose the calculation method. There are three to choose from:

Method	How slabs are calculated	Good to know
Load TakeDown — envelope method (influence surface method)	The load from the slab is distributed to the supports according to envelopes	Results: envelopes, load distribution on the storey, vertical forces in the elements, moments in beams, reactions on the foundations
Load TakeDown with slabs calculated by FEM	Slabs are calculated by the built-in finite element solver	Additionally: displacements and bending moments of the slabs
Autodesk Robot	FEM calculations with the Autodesk Robot calculation engine	The method is available to users of Autodesk Robot

The envelope method and the influence surface method are two names for the same method. In the “Calculation method” list of the program, the three methods are the items for Load TakeDown, Load TakeDown with FEM slabs and Autodesk Robot.

Results of the envelope method

After the calculations you can display the envelopes and the load distribution on the storey.

The envelopes show which part of the slab sends its load to a given wall, beam or column. On the left, the “Calculations” panel with the choice of method.

In the structure view the program shows by default the vertical forces transferred to the elements and the moment diagrams in the beams. At the end you display the reactions on the foundations — on strip and spread footings.

Vertical forces in the elements and bending moment diagrams in the beams.

Results for slabs calculated by FEM

When you choose the method with slabs calculated by FEM, the program shows the vertical displacements of the slab by default. You can also display the maps of bending moments needed for design.

Result map for a slab calculated with the built-in FEM solver.

For every slab and every load case you can display the extreme values: maximum deflection, maximum bending moments, and the areas of bottom and top reinforcement in all directions.

Options that affect the results

Structural and reinforcement calculations can be run automatically after any parameter is changed. This way you see the effect of every decision immediately:

• FEM mesh size — after a change the calculations are run again.
• Slab support type — a wall or a beam can be defined as a pinned or a fixed support. The change immediately affects the deflections and the deformed shape, so it is easy to check whether the model behaves correctly.
• Beams as rigid supports — by default beams are treated as rigid supports of the slab. The option can be switched off, which is immediately visible in the deflections.
• Beam stiffness coefficient — reducing it lets you approximately take the cracking of the beam into account, and the results become more reliable.

Step 4. Theoretical reinforcement areas

Slabs: reinforcement crosses

For slabs the program shows “reinforcement crosses”, i.e. the required reinforcement areas at the characteristic points of the slab. You can see at once where more steel is needed than the minimum reinforcement.

Reinforcement crosses: theoretical reinforcement areas at the characteristic points of the slab.

These values are used in the next stage: you open the saved model in IgentDraw and select the actual reinforcement of the slab on their basis.

Beams: two options worth knowing

The theoretical reinforcement areas of the beams are displayed on the model and in a table. Two options affect the result:

• Reduction of moments to the face of the support. By default the internal forces and the reinforcement are calculated at the axes of the supports. With this option on, the program detects the widths of the supports itself and reduces the moments to their faces. The design moments are smaller, which lets you reduce the reinforcement area of the beams.
• Slab interaction. The option is on by default. The beam is then designed not as a rectangular section but as a T-section, and the program recognizes such a section automatically.

Moment diagrams in the beams. The “Calculations” panel shows, among others, the options for reducing moments to the face of the support and the stiffness coefficients.

Step 5. The calculation note

The calculation note is generated in XLS format. For beams, characteristic points are selected: the sections over the supports and the extreme values in the spans.

The note contains:

• a summary of the materials and sections used,
• the parameters of all elements,
• the Load TakeDown results for columns, beams, walls and foundations,
• the theoretical reinforcement areas.

The calculation options used are given for every element. For columns the note contains values that help verify the correctness of the calculations. For beams all the sections used are listed — rectangular and T-sections — so you can check which flange widths were adopted.

The reinforcement sheet of the note: calculation options, rectangular and T-sections, and the design forces.

Step 6. From one storey to the whole building

A single storey very quickly becomes a multi-storey building. It is a good idea to switch off the automatic calculations while you change the model.

1. Increase the number of storeys; you can change their heights freely.
2. Copy the selected storey to the others, giving the number of copies. The polygon copy option then lets you give each storey a different static scheme.
3. Select a level and change its layout — for example, remove the beams and arrange the columns differently.
4. When defining elements, display the storey below or above as a reference.

After the calculations are run, the result maps for the changed storeys differ from the others. The same can be seen in the envelope method: the envelopes on the changed storey have a different shape.

A multi-storey building with result maps on the floors of successive storeys.

Summary

The whole design path of a typical reinforced concrete building fits in one model:

1. a storey with a slab, walls, columns, beams and openings,
2. loads, automatic load patterns and code combinations,
3. structural analysis with one of three methods,
4. theoretical reinforcement areas of slabs and beams,
5. a calculation note in XLS format,
6. duplication of the storey into a multi-storey building.

Because the calculations can be run after every parameter change, the model is well suited to quickly checking variants — a different support type, a different beam stiffness or a different column layout.

What next

You open the saved model in IgentDraw, where the actual reinforcement and the drawings are created from the calculated areas. This is described in the tutorial Reinforcement drawings of a reinforced concrete building: slabs, beams, columns and walls.

Would you like to go through these steps on your own example? Download the IgentForm demo version — it works with full functionality for 30 days.