Seismic Design Basics — Engineering Reference

AISC 341 seismic frame systems: SMF, IMF, SCBF, EBF. R factors, Ry expected strength, strong-column weak-beam check, compact section limits. Free guide.

Overview

Seismic design of steel structures relies on a capacity design philosophy: certain elements (fuses) are designed to yield and dissipate energy during an earthquake, while all other elements in the load path are designed to remain elastic and carry the maximum forces that the fuses can develop. In the U.S., AISC 341 (Seismic Provisions) defines the detailing requirements for steel seismic force-resisting systems (SFRS), and AISC 358 (Prequalified Connections) provides tested connection configurations.

The seismic design base shear is computed per ASCE 7 using the equivalent lateral force procedure or modal response spectrum analysis. The key parameter is the response modification factor R, which reduces the elastic seismic demand in proportion to the system's ductility capacity. Higher R values mean more ductility is expected and less design force is required, but more stringent detailing rules apply.

Steel seismic force-resisting systems

Capacity design principles

The fundamental rule of seismic steel design is that connections, columns, and non-fuse elements must be stronger than the expected strength of the fuse elements:

• Expected yield strength: R_y x F_y, where R_y accounts for the fact that actual steel yield strength exceeds the minimum specified value. For A992: R_y = 1.10, so the expected yield strength is 1.10 x 50 = 55 ksi.
• Strong-column weak-beam (SCWB): For SMF, the sum of column plastic moments at each joint must exceed the sum of beam expected plastic moments: sum(M*_pc) > sum(M*_pb). Per AISC 341 E3.4a: sum(M*_pc) / sum(M*_pb) >= 1.0.
• Connection design force: Moment connections in SMF must develop at least 0.80 x M_p of the connected beam (AISC 358), using the expected yield strength R_y x F_y. This means connections are designed for approximately 1.1 x R_y x F_y x Z_x.

Worked example — SCWB check for SMF joint

Given: W14x176 column (Z_x = 281 in^3, F_y = 50 ksi, A = 51.8 in^2), W24x84 beam framing from both sides (Z_x = 224 in^3, F_y = 50 ksi), P_u = 600 kip axial in column.

1. Column plastic moment (reduced for axial): M*_pc = Z_xc x (F_yc - P_u/A_g) = 281 x (50 - 600/51.8) = 281 x (50 - 11.6) = 281 x 38.4 = 10,790 kip-in per column. Sum for two columns above and below = 2 x 10,790 = 21,580 kip-in.
2. Beam expected plastic moment: M*_pb = R_y x F_y x Z_xb + M_uv (additional moment from shear at plastic hinge). M*_pb = 1.10 x 50 x 224 = 12,320 kip-in per beam. Sum for two beams = 2 x 12,320 = 24,640 kip-in.
3. SCWB ratio: 21,580 / 24,640 = 0.876 < 1.0. FAILS. The column is not strong enough. Options: increase column to W14x211, reduce beam size, or add a reduced beam section (RBS) to lower M*_pb.
4. With RBS (70% flange reduction): M*_pb,RBS ≈ 0.82 x 12,320 = 10,102 kip-in per beam. Sum = 20,204 kip-in. Ratio = 21,580/20,204 = 1.07 >= 1.0. OK.

Compact section requirements for seismic

AISC 341 Table D1.1 requires sections in the SFRS to be highly ductile or moderately ductile depending on the system:

SMF beams and SCBF braces require highly ductile sections. IMF beams require moderately ductile. These limits are tighter than standard AISC 360 compact limits, restricting the range of sections that can be used in seismic applications.

Code comparison — seismic steel design

Common mistakes to avoid

• Using R = 8 without meeting SMF detailing — the R factor is only valid when all AISC 341 requirements are satisfied. Using R = 8 in analysis but failing to provide RBS connections, SCWB checks, or lateral bracing of plastic hinges invalidates the entire design.
• Ignoring the overstrength factor Omega_0 — collectors, diaphragm connections, and column bases must be designed for the amplified seismic force (Omega_0 x E), not the code-level seismic force. For SMF, Omega_0 = 3.0, tripling the connection force at these critical elements.
• Not checking drift with C_d amplification — elastic analysis drift must be multiplied by C_d/I_e to obtain the inelastic drift estimate. For SMF, C_d = 5.5, so an elastic drift of 0.3 in. becomes an inelastic drift of 1.65 in. This amplified drift must be within the 0.020h limit.
• Specifying A36 for seismic members — A36 has R_y = 1.50 (vs. 1.10 for A992), meaning connections must be designed for 50% overstrength rather than 10%. A992 is strongly preferred for all seismic SFRS members.
• Neglecting the protected zone — no drilling, cutting, welding of attachments, or other modifications are permitted in the protected zone (the region where plastic hinging is expected). This includes stud anchors, erection aids, and conduit attachments.

Run this calculation

• Seismic Load Calculator
• Section Properties Database

Related references

• Seismic Design Categories
• How to Verify Calculations
• column compression equations
• beam-column interaction
• effective length factor K
• Braced Frame
• Connection Ductility
• Diaphragm Design
• Foundation Types

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.