Tutorial 3 of 7 · Intermediate · about 30 minutes

Seismic: equivalent lateral force and response spectrum

This tutorial takes a three-dimensional steel frame through the seismic analysis of ASCE 7-22: the site's design values, equivalent lateral forces, a modal analysis, a response spectrum analysis scaled to the code's base shear, and the story drift check.

Start from a 3D frame with floors: the frame of Your first frame repeated along three grid lines, or New from template / example on the Home tab.

1. Give the floors their mass and stiffness #

1. On the Model tab, open Rigid links, diaphragms, offsets, symmetry, soil springs, prestress and add a rigid diaphragm at each floor level. The floor then moves as one piece in its own plane, as a concrete slab on metal deck does.
2. Mass comes from the self-weight and the dead load cases; add the superimposed dead load and any other seismic weight as load cases of the kind Dead, or as nodal masses.

2. Look up the site's seismic design values #

1. On the Seismic tab, click Seismic load (ELF).
2. Enter the site's latitude and longitude, the site class and the risk category, and click Look up. The design spectral values come from the USGS seismic design service.
3. Pick the seismic force-resisting system. Its response modification coefficient, overstrength factor and deflection amplification factor are filled in, and you can change them.

3. Apply the equivalent lateral forces #

1. Enter the period or let ILX Structures estimate it from the building height and system.
2. Choose the load case (E) and the direction, and click Generate. The seismic base shear is distributed over the height as ASCE 7-22 describes and applied at each level, with the accidental torsion if you ask for it.
3. Make the load combinations again (see Loads and load combinations) so the seismic combinations include case E.

4. Run a modal analysis #

1. On the Seismic tab, click Modal analysis and ask for enough modes, for example 12 for a three-story building.
2. When the job finishes, Show modal results lists the period, frequency and mass participation of every mode in each direction. The code asks for enough modes to reach 90% of the mass in each direction; the table shows the running total.
3. Animate mode shows each mode shape moving in the 3D view.

5. Run the response spectrum analysis #

1. Click Response spectrum. The ASCE 7-22 design spectrum is drawn from the values you looked up; you can also enter a site-specific spectrum as a table.
2. Pick the direction, the damping and the way the modes are combined (complete quadratic combination is the usual choice), and run it.
3. ILX Structures compares the response spectrum base shear with the equivalent lateral force base shear and scales the results up when they are lower, as ASCE 7-22 Section 12.9.1 requires. The scale factor is shown with the results.

6. Check story drift #

1. Open Response spectrum, time history, pushover, drift check, mesh on the Analyze tab and pick the drift check.
2. ILX Structures multiplies the elastic drifts by the deflection amplification factor, divides by the importance factor, and compares each story's drift with the allowable drift of your risk category.
3. Stories over the limit are marked in the table and in the 3D view.

Going further #

• Time history: run the model under a recorded ground motion, step by step, from the same menu.
• Pushover: see Pushover analysis.
• Seismic frames: design the members of special moment frames and braced frames to AISC 341-22 in Member design and optimisation.

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.