Tutorial 6 of 7 · Advanced · about 30 minutes

Pushover analysis

A pushover analysis pushes a structure sideways, step by step, past the point where its members start to yield, and draws the capacity curve: base shear against roof displacement. This tutorial runs one on a steel moment frame with plastic hinges to ASCE 41-23 and AISC 342-22.

Start from a frame that already carries its gravity loads (see Your first frame and Loads and load combinations).

1. Choose the gravity load #

The pushover starts from the gravity loads that act during an earthquake. Pick the load case or combination to apply first, for example dead load plus a quarter of the live load. It is applied in full before the lateral push begins.

2. Assign the plastic hinges #

Select the beams and open the pushover settings under Response spectrum, time history, pushover, drift check, mesh on the Analyze tab.
Give the beams flexural hinges and the columns hinges that account for their axial load. The hinge properties come from the ASCE 41-23 and AISC 342-22 rules for the member's section and its expected material strength.
Hinges sit at both ends of each member by default; move them in from the face of the joint if your connections call for it.

3. Set the lateral load pattern #

Choose how the push is distributed over the height: proportional to the first mode shape, uniform, or from your own load case. Pick the direction and the control node, usually a node at the roof.

4. Set the target #

Enter the target roof displacement, or let ILX Structures work it out from the seismic design values and the structure's period as ASCE 41-23 describes. Choose the number of steps.

5. Run it #

Pushover is a long run: it goes to the server as a background job, with the progress and a Cancel button. You can keep working on the model meanwhile.

6. Read the capacity curve #

The capacity curve shows base shear against roof displacement. The first bend is where the first hinges form; the flat part is the structure yielding; a drop is a loss of strength.
Step through the curve: the 3D view shows the hinges at each step, colored by their state (elastic, immediate occupancy, life safety, collapse prevention, beyond).
The target displacement is marked on the curve, with the hinge states at that point: the performance of the frame.

7. Report it #

The pushover results, the curve and the hinge table go into the calculation report with the rest of the model (see Reports and hand-off).

Nonlinear buckling (Advanced tab, Stability): buckling with an initial imperfection, followed past the peak.
Material nonlinear (Advanced tab, Specialty): members that yield under any load.
Progressive collapse (Advanced tab, Robustness): remove members one at a time and see which removals the structure survives.

Screens in the app may differ slightly from the words here as the app gains features; the steps stay the same. Questions: services@ilxstudio.com.