Steel Framing Systems — Lateral System Selection, R-Factors & Height Limits
Selecting the lateral force-resisting system (LFRS) is the most consequential early decision in steel building design. The system choice determines member sizes, connection types, architectural flexibility, construction cost, and the building's maximum height. ASCE 7-22 Table 12.2-1 lists permitted systems by Seismic Design Category (SDC), and AISC 341-22 provides the detailing requirements for each seismic system.
Lateral system comparison
| System | R | Omega_0 | Cd | Max height (SDC D) | Stiffness | Architectural flexibility |
|---|---|---|---|---|---|---|
| SMF (Special Moment Frame) | 8 | 3 | 5.5 | No limit | Low–Medium | High (no braces) |
| IMF (Intermediate Moment Frame) | 4.5 | 3 | 4 | NP (SDC D/E) | Medium | High |
| SCBF (Special Concentrically Braced) | 6 | 2 | 5 | No limit | High | Moderate |
| OCBF (Ordinary Concentrically Braced) | 3.25 | 2 | 3.25 | 35 ft (SDC D/E) | High | Moderate |
| EBF (Eccentrically Braced) | 8 | 2 | 4 | No limit | High | Moderate–High |
| BRBF (Buckling-Restrained Braced) | 8 | 2.5 | 5 | No limit | High | Moderate |
NP = Not Permitted. The R factor reduces seismic design forces: higher R means lower design forces but requires more stringent detailing. SMF and EBF share the highest R = 8, giving the lowest seismic design forces.
System selection by building type
Low-rise (1–3 stories, warehouse, retail): OCBF or SCBF with braces in exterior walls or interior bays. Low cost, rapid erection, minimal connection complexity. OCBF is limited to 35 ft in SDC D, which restricts it to roughly 2 stories in most configurations.
Mid-rise office (4–10 stories): SCBF or BRBF for the lateral system, with composite beams for the floor. SCBF provides the best economy. BRBF eliminates the brace buckling problem, allowing smaller braces and more predictable behavior, but at higher brace cost.
High-rise (10+ stories): SMF (perimeter frames for architectural openness) or dual systems (SCBF + SMF). Dual systems combine the stiffness of braces with the redundancy of moment frames, with R = higher of the two systems (typically R = 7–8). Wind loading often governs over seismic above 15–20 stories.
Industrial, single-story with crane: Portal frames (rigid base columns with tapered or prismatic rafters) designed to AISC 360 without seismic detailing (R = 3 per ASCE 7 Table 12.2-1 for OMF). Moment connections at the knee (rafter-to-column junction).
Worked example — base shear comparison
Given: 6-story steel office building, SDC D, SDS = 1.0, SD1 = 0.50, building period T = 0.8 sec, W = 12,000 kips.
Using the equivalent lateral force method (ASCE 7-22 Section 12.8):
Cs = SDS / (R/Ie) but not less than SD1 / (T × R/Ie)
| System | R | Cs (controls) | Base shear V (kips) |
|---|---|---|---|
| SMF | 8 | 0.50/0.8/8 = 0.078 | 938 |
| SCBF | 6 | 0.50/0.8/6 = 0.104 | 1,250 |
| OCBF | 3.25 | 1.0/3.25 = 0.308 | 3,692 (but NP > 35 ft) |
| EBF | 8 | 0.50/0.8/8 = 0.078 | 938 |
The design base shear for SMF is 25% less than SCBF — but SMF connections (RBS, CJP welds, panel zone reinforcement) cost significantly more per connection. Total project cost optimization depends on the number of bays and connection count.
Gravity system
The gravity system (the beams, girders, and columns that carry floor loads but are not part of the LFRS) is designed to AISC 360 without seismic detailing. Typical gravity connections:
- Simple shear tabs (single plate) for beam-to-girder and beam-to-column connections
- Seated connections for light beams to column webs
- Double angles for older construction
Gravity columns in SDC C, D, E, and F must satisfy a minimum SCWB check and splice requirements per AISC 341-22 Section D2.5, even though they are not part of the LFRS. This ensures gravity columns can maintain load-carrying capacity through the building's lateral drift.
Diaphragm action
The floor slab (concrete on metal deck) acts as a rigid or semi-rigid diaphragm, transferring lateral forces from their point of application to the LFRS elements. ASCE 7-22 Section 12.10 provides diaphragm design forces:
Fpx = max(sum_Fi × wpx / sum_wi , 0.2 × SDS × Ie × wpx)
The collector (drag) elements that transfer diaphragm forces into the LFRS frames must be designed for the amplified seismic force (Omega_0 × Fpx) per AISC 341-22 Section D1.3.
Code comparison
ASCE 7-22 / AISC 341-22 (USA): System selection per ASCE 7 Table 12.2-1. Detailing per AISC 341. SDC determines which systems are permitted and their height limits. Dual systems allowed with R = max(R of either system).
AS 1170.4-2007 / AS 4100-2020 (Australia): Structural ductility factor mu replaces R-factor. mu = 1 (non-ductile) to 4 (fully ductile braced frames, equivalent to SCBF). Performance factor Sp = 0.67–1.0. AS 1170.4 Table 14 gives height limits by ductility category and soil class. Australia uses a displacement-based approach for importance level 4 structures.
EN 1998-1 (Eurocode 8): Behavior factor q replaces R. For moment frames: q = 4 (DCM) to 6.5 (DCH). For concentric braces: q = 2 (DCM) to 4 (DCH). System selection per EN 1998-1 Section 6.3. Height limits are not directly specified — instead, the capacity design requirements and second-order effects practically limit heights for lower-ductility systems. Dual frames (EN 1998 Section 6.10) allowed with q up to 4 × alpha_u/alpha_1.
CSA S16-19 / NBCC 2020 (Canada): Rd (ductility) and Ro (overstrength) factors. Type D (ductile) moment frames: Rd = 5.0, Ro = 1.5. Type MD concentrically braced: Rd = 3.0, Ro = 1.3. Height limits per NBCC Table 4.1.8.9. Canada permits tension-only bracing for Type CC (Rd = 1.5) limited to 20m.
Common mistakes engineers make
Selecting OCBF for buildings in SDC D without checking height limits. OCBF is limited to 35 ft in SDC D and E (ASCE 7 Table 12.2-1). A 3-story building with 15 ft floors = 45 ft, which exceeds the limit. SCBF or BRBF is required instead.
Mixing lateral system types without using dual system rules. If moment frames resist part of the lateral force and braced frames resist the rest, the building must be classified as a dual system with specific R and Cd values — not designed as two independent systems with different R factors applied separately.
Ignoring gravity column requirements in seismic design categories. AISC 341-22 Section D2.5 requires gravity columns in SDC D+ to satisfy splice force requirements and stability under expected drifts. Simply designing gravity columns for axial load alone is not sufficient.
Underestimating the cost of SMF connections. Each SMF connection (RBS cuts, CJP groove welds, UT inspection, demand-critical filler metal, panel zone reinforcement) costs 3-5 times more than a simple shear tab. For buildings with many bays, SCBF in a few bays is often cheaper than perimeter SMF, even though SMF uses a higher R factor.
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Related references
- How to Verify Calculations
- seismic lateral system selection
- SCBF and OCBF design
- moment frame design
- steel beam capacity calculator
- structural engineering unit converter
- Purlin Girt
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.