Example 4: Multibay Two Story Frame

A multi-bay reinforced concrete frame is investigated.

Example 4.1  

This example is of a reinforced concrete multibay two story frame, as shown in Figure 1, subject to gravity loads. The files for this example are:

A model of the frame shown in Figure 1 is created. The number of objects in the model is dependent on the parameter numBay. The (numBay + 1)*3 nodes are created, one column line at a time, with the node at the base of the columns fixed in all directions. Three materials are constructed, one for the concrete core, one for the concrete cover and one for the reinforcement steel. Three fiber discretized sections are then built, one for the exterior columns, one for the interior columns and one for the girders. Each of the members in the frame is modelled using nonlinear beam-column elements with 4 (nP) integration points and a linear geometric transformation object.

For gravity loads, a single load pattern with a linear time series and two vertical nodal loads acting at the first and second floor nodes of each column line is used. The load at the lower level is twice that of the upper level and the load on the interior columns is twice that of the exterior columns.

For the lateral load analysis, a second load pattern with a linear time series is introduced after the gravity load analysis. Associated with this load pattern are two nodal loads acting on nodes 2 and 3, with the load level at node 3 twice that acting at node 2.

The integrator for the analysis will be LoadControl with a load step increment of 0.1. The constraints are enforced with a Plain constraint handler.

Once the components of the analysis have been defined, the analysis object is then created. For this problem a Static analysis is used and 10 steps are performed to load the model with the desired gravity load.

After the gravity load analysis has been performed, the gravity loads are set to constant and the time in the domain is reset to 0.0. A new LoadControl integrator is now added. The new LoadControl integrator has an initial load step of 1.0, but this can vary between 0.02 and 2.0 depending on the number of iterations required to achieve convergence at each load step. 100 steps are then performed.

The output consists of the file Node41.out containing a line for each step of the lateral load analysis. Each line contains the load factor, the lateral displacements at nodes 2 and 3. A plot of the load-displacement curve for the frame is given in Figure 2.