Steel Structural Systems — Lateral System Selection Guide

Steel structural system selection: moment frames, braced frames, shear walls, and dual systems. R-factors, height limits, drift performance, and relative cost.

System selection overview

Choosing the lateral force resisting system (LFRS) is the most consequential early decision in steel building design. It determines member sizes, connection complexity, fabrication cost, architectural flexibility, and seismic performance. The choice depends on seismic demand (Seismic Design Category), building height, architectural requirements, and economy.

For wind-governed buildings (low seismic zones), braced frames offer the lowest cost because the connections are simple bolted gussets and the bracing stiffness controls drift efficiently. For seismic-governed buildings, the system must provide both strength and ductility, and the code-assigned R-factor (or equivalent) determines how much the elastic seismic force is reduced.

System comparison table

System	R (ASCE 7)	Rd x Ro (CSA)	q (EC8)	Height limit SDC D	Relative cost index	Drift control
SMF	8	5.0 x 1.5	6.5	Unlimited	1.4-1.8	Poor (flexible)
IMF	4.5	3.0 x 1.5	4.0	Not permitted SDC D+	1.1-1.4	Moderate
OMF	3.5	2.0 x 1.3	2.0	65 ft	1.0	Moderate
SCBF	6	4.0 x 1.3	4.0	Unlimited	1.0-1.2	Good
OCBF	3.25	2.0 x 1.3	2.5	35 ft	0.8-1.0	Good
EBF	8	4.0 x 1.5	6.0	Unlimited	1.3-1.6	Good
BRBF	8	4.0 x 1.5	6.0	Unlimited	1.2-1.5	Good
SPSW	7	5.0 x 1.3	5.0	Unlimited	1.5-2.0	Very good

Cost index is normalized to OCBF = 1.0 for the lateral system only (excludes gravity framing).

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

Drift check (approximate): using Cs = 0.08 and building weight W = 60,000 kN, base shear V = 4,800 kN. For SMF with R = 8, the design story drift is amplified by Cd = 5.5. A typical SMF inter-story drift at 1/R force level is h/600. Amplified drift = 5.5 x h/600 = h/109. The ASCE 7 drift limit is 0.020h (h/50). So SMF passes drift easily in this case, but actual drift at service wind may exceed H/500 because moment frames are inherently flexible.

For SCBF with R = 6, the design base shear is higher (V = 4800 x 8/6 = 6,400 kN design base shear adjustment not exactly like this, but the force is higher). However, braced frames have 3-5 times the lateral stiffness of moment frames at the same weight. An SCBF system typically satisfies wind drift limits without supplemental damping or outriggers.

Cost comparison: for this 12-story building, SCBF costs approximately 15 percent more than a non-seismic braced frame (for capacity design of connections, brace slenderness limits). SMF costs approximately 40-60 percent more (heavy beam-column connections, heavier columns for strong-column-weak-beam). BRBF costs approximately 25-35 percent more (proprietary brace cores, special inspections).

Recommendation: SCBF if architectural program allows diagonal braces on the facade or in the core. BRBF if braces must be hidden and the owner accepts the BRB procurement cost. SMF only if the floor plan requires completely open perimeter (no braces, no walls).

Code references for system selection

Aspect	ASCE 7-22	AS 1170.4	EN 1998-1	NBCC 2020
System table	Table 12.2-1	Table 6.5(A)	Table 6.2	Table 4.1.8.9
Height limits	Table 12.2-1 columns	AS 1170.4 Cl. 6.5	EN 1998-1 Cl. 6.3	NBCC 4.1.8.10
Drift limits	Table 12.12-1	Cl. 5.5.4	Cl. 4.4.3.2 (0.075h x q)	4.1.8.13 (0.025hs)
Redundancy factor	rho = 1.0 or 1.3	Not used	Not used	Not explicitly used

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. Dual systems receive higher R-factors than the braced frame alone (R = 7 for dual SMF + SCBF vs R = 6 for SCBF alone).

In practice, many buildings use different systems in different directions. Braced frames in the short direction (fewer bays, braces fit in core) and moment frames in the long direction (open perimeter). This is permitted as long as each direction is independently checked.

Common pitfalls

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. Always check wind serviceability separately.
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 limits requires switching to a more ductile (and expensive) system.
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 (with partitions or cladding providing supplemental stiffness) can be cheaper than the gusset plates, heavy braces, and foundation upgrades required for a braced frame.
Not considering construction speed. Moment frame connections (welded flanges, field quality control, NDE inspection) are slower to erect than bolted brace connections. For projects where schedule is critical, braced frames or BRBF often win on total cost even if material cost is higher.

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Related references

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.