Chapter 19 — Steel Framing Systems (Gravity, Lateral, and Load Path)

1. Gravity Framing Systems

Gravity framing carries dead and live load down through a hierarchy of elements, each spanning to the next, until the load reaches a column and finally a foundation. The most common arrangement is one-way framing: a metal deck spans a short distance between closely spaced filler beams, the filler beams span onto girders at the column lines, and the girders frame directly into columns. Two-way framing (a slab or deck spanning in both directions onto a denser grid, or waffle/flat-plate concrete systems) is less common in steel because a corrugated deck has an inherently strong and weak direction, but two-way steel framing appears in heavy industrial or transfer-level construction where load must be shared among many supports.

• Metal deck spans perpendicular to the closely spaced filler beams, typically 3 in deep composite deck spanning 8–12 ft unshored, or non-composite deck for roofs spanning less.
• Filler (infill) beams spaced 8–10 ft o.c., spanning 30–45 ft between girders; usually composite with the slab via shear studs.
• Girders at column lines carry the concentrated reactions of the filler beams; often composite, sometimes deeper to control deflection over longer spans.
• Open-web steel joists replace filler beams on long-span (≥ 40 ft), lighter live-load floors/roofs — cheaper per square foot but limited to lighter concentrated loads and larger deflections.

2. Composite Floors and Deck Spans

In a composite floor, shear studs welded through the metal deck to the beam top flange bond the concrete slab to the steel beam so they act as one T-shaped section, dramatically increasing stiffness and strength for the same beam weight. Deck profiles are typically 1.5 in, 2 in, or 3 in deep; a 3 in deep, 20-gauge composite deck can span roughly 10–12 ft unshored with normal-weight topping. Total slab thickness (deck depth + topping) is usually 4.5–6.5 in, chosen to satisfy both strength and the 2-hour fire rating typical of office construction.

3. Lateral Framing Systems

A lateral system resists wind and seismic loads and controls drift. The major steel families are:

• Concentric braced frames (CBF) — diagonal members whose centerlines intersect at a common work point, so the system behaves as an efficient, stiff truss. Configurations include X-bracing (two diagonals per bay, very stiff, blocks door/window openings), chevron (V or inverted-V) bracing (diagonals meet at mid-span of the beam, allowing a door on either side but inducing an unbalanced vertical force on the beam if a brace buckles), and single-diagonal bracing (one diagonal per bay, most architecturally open but must resist tension and compression alternately, requiring symmetric detailing across the building).
• Eccentrically braced frames (EBF) — the diagonal intentionally misses the beam-column joint, leaving a short "link" beam segment that yields in shear and provides ductility (similar to a fuse) while keeping the brace itself close to elastic. Combines the stiffness of a CBF with much of the ductility of a moment frame.
• Moment frames (MF) — rigid beam-column connections resist lateral load through frame bending rather than diagonal bracing; the most architecturally open system (no diagonals blocking space) but also the most flexible, often governed by drift rather than strength.
• Dual systems — a moment frame combined with a braced frame or shear wall; ASCE 7 requires the moment frame alone to resist at least 25% of the design story shear, providing redundancy if the stiffer bracing yields or degrades.
• Shear-wall interaction — in composite steel/concrete buildings, concrete or CMU shear walls (often at stair/elevator cores) can act as the primary lateral system, with the steel gravity frame designed only for gravity loads and "riding along" with the wall's lateral drift; compatibility of steel beam connections to the wall's drift must be checked.

Additional Design Aids & Stratified Equations

4. Load Path: Cladding → Diaphragm → Lateral System → Foundation

Every lateral load must have a complete, continuous load path. Wind pressure first strikes the exterior cladding, which spans (typically vertically) to floor-level girts or the slab edge; that force enters the floor diaphragm (the composite slab acting as a deep horizontal beam/plate); the diaphragm distributes the force to the vertical lateral elements (braced frames, moment frames, or shear walls) in proportion to their relative stiffness; those elements carry the load down as shear and overturning moment to the foundation, which must resist sliding, overturning, and — for braced frames especially — net uplift at the base.

5. Diaphragm Behavior — Rigid vs Flexible

A diaphragm is classified as rigid if its in-plane deflection is small compared to the story drift of the vertical elements it connects (ASCE 7 §12.3.1) — typically a reinforced concrete slab or a well-connected composite steel deck slab. A rigid diaphragm distributes story shear to the vertical lateral elements in proportion to their relative rigidity (stiffness), and also transfers torsional shear if the center of rigidity and center of mass don't coincide. A diaphragm is flexible (bare steel deck without adequate topping, or a wood diaphragm) if its own deflection is large relative to the story drift — a flexible diaphragm instead distributes shear to vertical elements by tributary area, like a simple beam spanning between the lines, ignoring relative stiffness and torsion.

6. Transfer Forces, Collectors, and Drag Struts

Where the vertical lateral elements are not uniformly distributed under the diaphragm (e.g., a braced bay is offset from a re-entrant corner, or a lateral line stops at one level and must "transfer" to a different line below), the diaphragm must collect the distributed shear along a wall or frame line and deliver it as a concentrated axial force to the vertical element. A collector (also called a drag strut) is the beam or added steel member designed to carry this axial "dragged" force in tension and compression, in addition to its normal gravity bending. Collector forces are frequently amplified by the seismic overstrength factor Ω₀ (ASCE 7 §12.10.2.1) because collector failure is a non-ductile, brittle failure mode that must be protected against yielding of the ductile lateral system elsewhere.

7. Bay-Size Economics and Typical Spans

The classic "sweet spot" for typical office/commercial steel is roughly a 30 ft × 40–45 ft bay: wide enough to minimize the number of columns (and their foundations), narrow enough that beam and deck sizes stay light and deflection-controlled rather than strength-controlled.

8. Expansion Joints

Buildings longer than roughly 200–300 ft (IBC/AISC rule-of-thumb, varies by climate and cladding type) are typically split by a full structural expansion joint — two independent structural frames placed close together with a gap — to accommodate thermal expansion/contraction and to limit differential movement demands on cladding, MEP, and finishes. Expansion joints also often mark a natural break for lateral system redundancy: each independent structure needs its own complete, self-sufficient lateral system, since forces cannot reliably transfer across the joint.

9. System Selection Matrix

10. Seismic Force-Resisting System R Factors (ASCE 7 Table 12.2-1)

Higher R means lower design seismic force but demands more ductile detailing (capacity design of connections and adjoining members per AISC 341) — R is a "discount" for guaranteed ductile behavior, not a free reduction in actual earthquake demand.

11. Constructability and Erection Sequence

A framing system's cost and schedule depend heavily on how it goes together in the field, not just its steel weight. Considerations include: connection standardization (repeating a handful of connection types across the whole building speeds shop fabrication and field erection); temporary bracing/stability of columns before the permanent bracing or diaphragm is installed (governed by AISC's Code of Standard Practice and OSHA erection requirements); crane reach and lift weight limiting maximum piece size, especially for long-span trusses; and pour sequencing for composite floors so that shoring/re-shoring, if used, does not overload lower floors.

Design Workflow

1. Establish bay geometry and gravity framing hierarchy (deck → beam → girder → column) based on architectural grid and occupancy loads.
2. Determine seismic design category and wind exposure; select candidate lateral systems from the selection matrix and R-factor table.
3. Locate lateral elements to minimize torsional eccentricity and provide a short, direct load path from every diaphragm area.
4. Classify each diaphragm rigid or flexible; distribute story shear accordingly (relative rigidity vs tributary area).
5. Identify collector/drag-strut lines at re-entrant corners, transfers, and discontinuities; size for Ω₀-amplified force where required.
6. Check drift against ASCE 7 Table 12.12-1 (seismic) or H/400–H/500 (wind); iterate system choice if drift governs.
7. Coordinate erection sequence, temporary bracing, and expansion-joint locations with the construction manager before finalizing member sizes.