------- | ---------- | ---- | ------- | ------------- | ---------- | --------------- | | SMF | 8 | 5.5 | 3.0 | Unlimited | 1.4-1.8 | Poor (flexible) | | IMF | 4.5 | 4.0 | 3.0 | Not permitted | 1.1-1.4 | Moderate | | OMF | 3.5 | 3.0 | 3.0 | 65 ft | 1.0 | Moderate | | SCBF | 6 | 5.0 | 2.0 | Unlimited | 1.0-1.2 | Good | | OCBF | 3.25 | 3.25 | 2.0 | 35 ft | 0.8-1.0 | Good | | EBF | 8 | 4.0 | 2.0 | Unlimited | 1.3-1.6 | Good | | BRBF | 8 | 5.0 | 2.5 | Unlimited | 1.2-1.5 | Good | | SPSW | 7 | 6.0 | 2.0 | Unlimited | 1.5-2.0 | Very good | | Dual SMF+SCBF | 7 | 5.5 | 2.5 | Unlimited | 1.3-1.6 | Good |

Cost index normalized to OCBF = 1.0 for the lateral system only (excludes gravity framing). Cd = deflection amplification factor. Omega_0 = overstrength factor.

Multi-code R-factor / behavior factor comparison

Australian AS 1170 uses a structural performance factor Sp instead of R. European EN 1998 uses behavior factor q. Canadian NBCC uses Rd x Ro.

Height limits by Seismic Design Category

NL = No Limit. NP = Not Permitted. Per ASCE 7-22 Table 12.2-1.

Seismic detailing requirements by system

Seismically compact limits (AISC 341 Table D1.1)

Drift performance comparison

SMF systems are inherently flexible. Wind serviceability (h/400 typical limit) often governs SMF design, not seismic drift.

Cost comparison by building height

Dollar signs indicate relative lateral system cost. NP = not practical or not permitted. Outrigger systems become economical above 20 stories.

Worked example -- system selection for a 12-story office

Building: 12 stories, 48 m tall (158 ft), SDC D, office occupancy, floor plate 40 m x 30 m.

OMF not permitted above 65 ft in SDC D. IMF not permitted in SDC D. Viable options: SMF, SCBF, EBF, BRBF, SPSW.

Drift check (approximate): Cs = 0.08, W = 60,000 kN, base shear V = 4,800 kN. For SMF with R = 8, design story drift at 1/R force level is h/600. Amplified drift = 5.5 x h/600 = h/109. ASCE 7 drift limit is 0.020h (h/50). SMF passes seismic drift, but service wind may exceed h/500 because moment frames are inherently flexible.

For SCBF with R = 6, the design base shear is higher. However, braced frames have 3-5 times the lateral stiffness of moment frames. An SCBF system typically satisfies wind drift limits without supplemental damping.

Cost comparison for this 12-story building:

• SCBF: approximately 15% more than non-seismic braced frame
• SMF: approximately 40-60% more (heavy connections, strong-column-weak-beam)
• BRBF: approximately 25-35% more (proprietary brace cores)

Recommendation: SCBF if architectural program allows diagonal braces on the facade or in the core. BRBF if braces must be hidden. SMF only if floor plan requires completely open perimeter.

Dual systems and combinations

A dual system combines a moment frame with a braced frame (or shear wall). The moment frame acts as a backup, providing redundancy and ductility. ASCE 7 requires the moment frame in a dual system to independently resist at least 25 percent of the design base shear.

In practice, many buildings use different systems in different directions. Braced frames in the short direction and moment frames in the long direction. Each direction is independently checked.

Multi-code design approach

Common mistakes

1. Selecting SMF for drift-sensitive buildings without checking service wind. SMF systems pass seismic drift checks because the Cd-amplified drift is compared to generous limits (0.020h). But service-level wind drift may exceed h/500, causing curtain wall damage and occupant discomfort.

2. Ignoring height limits for SDC D and above. OMF, OCBF, and some bearing wall systems have absolute height limits in high seismic zones. Exceeding these requires switching to a more ductile system.

3. Assuming braced frames are always cheaper than moment frames. For buildings under 4 stories in low seismic zones, the simpler gravity connections of an OMF system can be cheaper than gusset plates, heavy braces, and foundation upgrades for braced frames.

4. Not considering construction speed. Moment frame connections (welded flanges, field quality control, NDE inspection) are slower to erect than bolted brace connections. For schedule-critical projects, braced frames or BRBF often win on total cost.

5. Using SCBF in long buildings without sufficient brace bays. Long buildings may not have enough braced bays in the transverse direction. The lack of redundancy (rho = 1.3 penalty) increases the design force.

6. Forgetting redundancy requirements. ASCE 12.3.4 requires rho = 1.3 when the loss of a single element results in more than a 33% reduction in story strength. Plan enough brace bays or moment frame lines to achieve rho = 1.0.

7. Specifying BRBF without considering procurement. Buckling-restrained braces are proprietary and have long lead times (12-16 weeks). If the schedule does not accommodate this, SCBF or EBF should be used instead.

Frequently asked questions

What is the most common lateral system for steel buildings? Concentrically braced frames (SCBF or OCBF) are the most common for buildings up to 10 stories. They are economical, well-understood, and provide excellent drift control.

When should I use moment frames? When the architectural program requires open perimeters without diagonal braces, or when very high ductility is needed for seismic performance. Common in hospitals, laboratories, and open-plan offices.

What is the R-factor? The response modification factor (R) reduces the elastic seismic design force to account for structural ductility and overstrength. R = 1 means the structure must resist the full elastic force. R = 8 means the design force is 1/8 of elastic.

BRBF vs SCBF -- which is better? BRBF provides higher ductility (R = 8 vs R = 6), more uniform force distribution, and better energy dissipation. However, BRBs are proprietary and cost 25-35% more. SCBF is simpler and adequate for most buildings.

Can I mix systems in the same building? Yes. ASCE 7 permits different systems in orthogonal directions. Each direction is designed independently with its own R, Cd, and Omega_0 values. Different systems at different building levels are also permitted.

What about steel moment frames with concrete shear walls? Concrete shear walls (or concrete-filled steel tube walls) can serve as the primary lateral system in dual systems. The steel frame provides gravity support and the 25% backup requirement for the dual system R-factor.

How does building height affect system selection? Below 4 stories, almost any system works and OCBF is most economical. 4-10 stories, SCBF dominates. 10-20 stories, BRBF or dual systems become competitive. Above 20 stories, outrigger systems, tube systems, or bundled tubes are needed for drift control.

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Related references

• Seismic Detailing
• Frame Analysis Methods
• Moment Frame Connections
• Diagonal Bracing
• Outrigger Systems
• Braced Frame Design
• Steel Grades
• How to Verify Calculations

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.