Structural design of a concrete frame joint (ACI)

The analyzed frame joint will be part of a real reinforced concrete frame structure. The frame is 14 ft high, the span is 26 ft and both columns will have fixed supports.

1 New project

Start a New project in IDEA StatiCa Detail.

In the Discontinuity Region Wizard, set the concrete strength and reinforcement grade and define the Concrete cover thickness. Select the Frame joints template.

From Frame joint templates select Knee joint.

By choosing a predefined template the software will automatically generate the geometry, load, and reinforcement.

2 Geometry

Start with the modification of the geometry. The predefined knee joint consists of a Column and a Beam. Change the cross-section of the column by clicking on Edit parameters.

Let’s change the dimensions of the cross-section.

Then add a new cross-section for the beam:

Again, change the dimensions:

3 Loads

In the navigator, proceed to the definition of loads. Two load cases and five nonlinear combinations were automatically added by the software. You need two load cases to distinguish between dead (permanent) and live (variable) loads and three combinations to cover ULS and SLS checks.

Let´s modify load case LC1 for dead load effects. At first, remove the line load from the column.

Then you can modify the line load on the beam:

Switch to LC2 load case by combo box and repeat the steps with different values.

Now go to the Internal forces tab and apply them in the cross-section, which is placed in the intersection of the column and the beam. You can get these internal forces by modeling and calculating the whole frame with the corresponding line load (2.04 kip/ft for LC1 and 1.56 kip/ft for LC2) in any CAE/FEM software in minutes.

At the end come back to combinations and check the combination rules. There you can modify partial factors or keep the default setting according to the chosen code (ACI in our case). 

4 Reinforcement

Once the load has been defined, you can proceed to reinforcement design. Before you design one, you can try to run the Topology Optimization tool to see what the most effective reinforcement layout should look like if you want to use the selected effective volume of the structure.

For the effective volume of 20%, the structure looks like this (red lines represent compression and blue lines represent tension). You can switch between effective volumes in the ribbon.

Proceed to Input/edit the reinforcement. You will exploit the items created by the template.

Modify the distance of stirrups of groups ST1 and ST2:

Continue and modify the group GB1, the bars will be lengthened to the left edge. And also change the distance of layers:

At the end edit the length of the bar of the group IB1.

5 Calculation and Check

Start the analysis by clicking Calculate in the ribbon. The analysis model is automatically generated, the calculations are performed, and you can see the summary of checks displayed together with the values of check results.

In the Summary tab, you can find the table with governing checks. Let’s take a peek at it:

In the first row of the table, you can see that the governing ULS check is the strength of the reinforcement in combination C1.

You can also notice the Increment column. The values P100%, V100% say that the calculation finished successfully, and all the permanent and variable load was applied on the structure.

If you click on the row with the SLS summary, the following table appears:

In terms of SLS checks the one for the column in the combination C3 is governing. You can see that the structure satisfied this check as well.

Notice the abbreviations (LT) after the names of the combinations. IDEA StatiCa Detail calculates with different elastic modulus for the short-term (ST) and long-term (LT) effects.

6 Report

At last, we go to the Report Preview/Print. IDEA StatiCa offers a fully customizable report to print out or save in an editable format.

You have designed, optimized, and code-checked a frame joint according to Eurocode.