Load Path in Steel Buildings — Gravity and Lateral Force Transfer

How gravity and lateral loads travel through steel buildings: deck to beams to columns to foundations. Diaphragm, collector, chord, and brace force flow.

Overview

A structural load path is the continuous chain of members and connections that transfers applied loads from their point of application to the foundation and ultimately into the ground. Every load -- gravity, wind, seismic, snow, or construction -- must follow a complete, uninterrupted path. A broken or missing link in the load path is one of the most common causes of structural failure, particularly during extreme events like earthquakes and hurricanes.

ASCE 7-22 Section 1.3.1 states: "A continuous load path, or paths, with adequate strength and stiffness shall be provided to transfer all forces from the point of application to the final point of resistance." This requirement is code-mandated, not optional.

Gravity load path

Gravity loads (dead, live, snow) travel downward through the structure in a predictable sequence:

1. Applied load on floor or roof surface (slabs, metal deck, concrete fill)
2. Deck/slab distributes load to supporting floor beams (joists or wide-flange beams)
3. Floor beams transfer reactions to girders (via shear connections)
4. Girders transfer reactions to columns (via shear or moment connections)
5. Columns transfer axial load to base plates, then anchor rods, then foundations
6. Foundations distribute load to soil or rock

At each step, the connection between elements must be designed to transfer the full reaction. The most common gravity load path failure is a missing or under-designed beam-to-girder connection -- particularly when beams are added during tenant fit-out without proper engineering.

Lateral load path

Lateral loads (wind, seismic) travel horizontally through a more complex path:

1. Wind pressure or seismic inertia acts on the building envelope or floor masses
2. Floor diaphragm (metal deck + concrete) acts as a deep horizontal beam, collecting lateral forces and transferring them to the lateral-force-resisting system (LFRS)
3. Collectors (drag struts) -- beams that "collect" diaphragm forces and deliver them to the LFRS elements (braced frames or moment frames)
4. Chords -- the diaphragm edges act as flanges of the horizontal beam, carrying tension and compression
5. LFRS elements (braced frames, moment frames, shear walls) -- resist lateral forces through frame action or bracing action, delivering forces to the foundation
6. Foundation (spread footings, piles, mat) -- resists overturning moment, base shear, and uplift

Worked example -- diaphragm force flow in a 3-story building

Given: 3-story building, 120 ft x 80 ft plan, one braced frame on each short (80-ft) end. Seismic story force at roof level F_3 = 100 kips. SDS = 1.0, Ie = 1.0, Omega_0 = 2.0 (SCBF).

Step 1 -- Diaphragm shear: Each braced frame resists half the story force = 100/2 = 50 kips. The diaphragm acts as a beam spanning 120 ft between braced frames. Maximum diaphragm shear at the braced frame = 50 kips.

Step 2 -- Unit shear in diaphragm: v = 50,000 lb / 80 ft = 625 plf along the braced frame line. The metal deck and its connections must resist this unit shear. A 20-ga composite deck with 3.25" LW concrete typically provides 800-1400 plf (adequate).

Step 3 -- Collector force: If each braced frame is 20 ft wide within the 80-ft building width, the collector must drag forces from the remaining 60 ft. Collector force = 625 _ (80 - 20)/2 = 18,750 lb = 18.75 kips per side. Amplified for overstrength: Omega_0 _ 18.75 = 2.0 * 18.75 = 37.5 kips (ASCE 7-22 Section 12.10.2 for SDC C-F). The collector beam and its connections must resist this axial force in addition to gravity reactions.

Step 4 -- Chord force: Diaphragm moment M = w _ L^2 / 8. For uniform load w = 100/120 = 0.833 kip/ft: M = 0.833 _ 120^2 / 8 = 1500 kip-ft. Chord force = M / building width = 1500/80 = 18.75 kips (tension in one perimeter beam, compression in the other).

Step 5 -- Brace force: Total base shear at one braced frame = sum of story shears (say 150 kips total). If the braced frame has a single diagonal at 45 degrees: brace axial force = 150 / cos(45) = 212 kips. The brace, gusset plate, and beam-column connections must all resist this force.

Connection continuity

Every connection along the load path must have adequate strength and stiffness:

Key design considerations

• Redundancy -- buildings with multiple LFRS elements in each direction are more robust than those with a single braced frame. ASCE 7-22 Section 12.3.4 assigns a redundancy factor rho = 1.3 for non-redundant systems, increasing seismic design forces by 30%.
• Load path at openings -- floor openings (stairs, elevators, mechanical shafts) interrupt the diaphragm. Headers and edge beams must transfer forces around the opening. Large openings may require a strut-and-tie analysis.
• Transfer structures -- when lateral systems are offset between floors, a transfer diaphragm or transfer truss must redirect the forces at the transition level. Transfer forces can be 3-5x larger than the typical floor diaphragm force.
• Erection stability -- during construction, before the permanent LFRS is complete, a temporary load path must exist per OSHA 29 CFR 1926.756. This typically involves temporary bracing, guy wires, or construction sequence planning.

Multi-code comparison

ASCE 7-22 (USA): Section 1.3.1 mandates continuous load path. Section 12.1.3 requires a complete lateral-force-resisting system. Section 12.10 specifies diaphragm forces (Fpx equation with 0.2SDS and 0.4SDS bounds). Section 12.10.2 requires overstrength (Omega_0) amplification for collectors in SDC C-F. Redundancy factor rho per Section 12.3.4 penalizes systems with fewer than two LFRS elements per direction. AISC 360-22 Chapter J governs connection design throughout the load path.

AS 1170.0-2002 / AS 4100-2020 (Australia): AS 1170.0 Clause 3.2 requires a continuous load path from point of application to the foundation. AS 1170.4 (seismic) requires lateral load paths including diaphragms and collectors, though the prescriptive detailing is less explicit than ASCE 7. AS 4100 Clause 4.2 requires connections to be designed for the forces they transfer. Redundancy is addressed through the structural ductility factor (mu) in AS 1170.4 Table 6.5 -- lower ductility factors are assigned to less redundant systems.

EN 1990 / EN 1998-1 (Europe): EN 1990 Section 2.1 establishes the fundamental requirement that structures transfer all applied loads to the ground. EN 1998-1 Clause 4.2.1.5 requires floor diaphragms to have sufficient in-plane stiffness and resistance to distribute seismic forces. Clause 4.4.2.5 specifies that diaphragms and horizontal bracings must transmit seismic actions to vertical lateral systems. Capacity design per Clause 6.5.5 requires non-dissipative connections (collectors, chords) to be designed for overstrength forces (gamma_ov = 1.25 times the plastic resistance of the dissipative element).

NBCC 2020 / CSA S16-19 (Canada): NBCC Clause 4.1.2.1 requires a continuous load path. CSA S16 Clause 27.1.5 requires diaphragms to transfer seismic forces between levels. Capacity design applies to collectors: their connections must develop the probable resistance (R_y*Fy) of the connected lateral system elements per CSA S16 Clause 27.5.4.2. The Rd factor in NBCC accounts for system ductility and redundancy -- lower Rd values result in higher design forces for less ductile or less redundant systems.

Common mistakes

1. Missing collector connections at the brace-to-frame interface. The collector beam must have connections designed for the collector axial force (tension and compression) in addition to the gravity reaction. Standard shear tabs transfer only vertical reaction -- bolted flange plates or welded flanges are needed for the amplified collector force per ASCE 7 Section 12.10.2.

2. Ignoring diaphragm capacity at re-entrant corners. L-shaped or U-shaped floor plans create stress concentrations at the re-entrant corner. ASCE 7-22 Section 12.3.3.4 requires reinforcing collectors and additional deck connections at these locations. Without them, the diaphragm can tear at the corner during a seismic event.

3. Assuming the deck always provides adequate diaphragm capacity. Metal deck diaphragm capacity depends on deck gauge, rib orientation, attachment pattern, and span. Light-gauge deck (22-ga) with widely spaced connections may provide only 150-300 plf -- well below the demand in many buildings. Always verify against SDI diaphragm tables.

4. Not designing base plates for uplift. In buildings with high aspect ratios or light dead loads, seismic or wind overturning can produce net column uplift. Anchor rods must resist the full factored tension, and the footing must resist pullout through self-weight or pile tension. This is particularly critical for the 0.9D + E load combination.

5. Discontinuous load path at transfer levels. When the lateral system changes configuration between floors (podium levels, setbacks, bracing offsets), the transfer diaphragm must carry the accumulated lateral force from above to the new system below. Transfer forces are additive to the Fpx forces and frequently govern diaphragm and collector design at transition levels.

Run this calculation

• Load Combinations Calculator
• Section Properties Database

Related references

• Load Combinations ASCE 7
• Load Combinations
• Diaphragm Design
• Braced Frame Design
• Base Plate Design
• Steel Connection Types
• 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.