High-Rise Steel Structures — Tube Systems, Outriggers & Wind Engineering

High-rise steel buildings (generally above 10 stories or 120 ft) face unique design challenges that do not arise in low-rise construction: wind-induced drift and acceleration, dynamic response to vortex shedding, premium for height (the percentage of steel tonnage devoted solely to lateral resistance), and progressive collapse considerations. As buildings grow taller, the lateral system consumes an increasing proportion of the total structural cost — from roughly 10% at 10 stories to 40%+ at 60 stories.

Structural systems by height range

The tube concept

A framed tube uses closely-spaced perimeter columns (typically 10–15 ft o.c.) connected by deep spandrel beams to create a rigid perimeter tube that resists lateral loads like a hollow cantilever. The tube concept was pioneered by Fazlur Khan (SOM) in the 1960s.

Framed tube behavior: Under lateral load, the tube's flange faces (perpendicular to the wind) carry axial tension and compression, while the web faces (parallel to the wind) carry shear. "Shear lag" reduces the effectiveness of columns far from the web-flange corner — axial stress in the middle of the flange face is less than at the corners.

Braced tube (diagrid): Adding diagonal braces to the perimeter (like the John Hancock Center, Chicago) eliminates shear lag and creates a nearly pure cantilever response. Steel tonnage drops dramatically compared to a framed tube for the same height.

Bundled tube: Subdividing the floor plan into multiple tubes (like the Willis Tower, Chicago) reduces the shear lag effect by shortening the flange face length of each individual tube.

Outrigger-belt truss systems

Outrigger trusses extend from the core to the perimeter columns at one or more levels, engaging the perimeter columns as tension-compression couples to resist overturning moment. Belt trusses wrap around the perimeter at the outrigger level, distributing the outrigger force to adjacent columns.

Optimal outrigger location: For a single outrigger, the optimal location is approximately 0.45H from the top (where H = building height). For two outriggers, approximately H/3 and 2H/3. Each outrigger typically reduces the top-of-building drift by 20–30%.

Design forces: The outrigger truss resists a couple: T = C = M_core_at_outrigger_level / L_lever_arm, where the lever arm is the distance between exterior column lines. For a 50-story building with a core moment of 2,000,000 kip-ft at the outrigger level and an 80 ft lever arm: T = C = 2,000,000 / 80 = 25,000 kips — enormous forces that require heavy trusses and transfer elements.

Worked example — wind drift check for a 30-story office tower

Given: 30-story steel office tower, story height = 13 ft, total H = 390 ft. Braced core with perimeter gravity columns. Wind base shear V = 800 kips, triangular load distribution. Target drift: H/400.

Step 1 — Allowable drift: delta_allow = 390 × 12 / 400 = 11.7 in at roof level.

Step 2 — Required lateral stiffness (approximate cantilever model): For a uniformly distributed load, roof drift delta = w × H^4 / (8 × E × I_eff). For triangular: delta = 11 × V × H³ / (120 × E × I_eff). I_eff_required = 11 × 800 × (390 × 12)³ / (120 × 29000 × 11.7) = 11 × 800 × (4680)³ / (120 × 29000 × 11.7) = 11 × 800 × 1.026 × 10^11 / (40,716,000) = 9.03 × 10^14 / 4.07 × 10^7 = 2.22 × 10^7 in^4.

This enormous moment of inertia cannot be achieved with interior bracing alone — perimeter engagement (outriggers or tube action) is essential. A braced core alone with typical W14 columns on a 30 × 30 ft grid provides roughly I = 5 × 10^6 in^4, less than 25% of what is needed.

Step 3 — Add outrigger: With a 2-story outrigger truss at level 14 engaging perimeter columns at 90 ft spacing, the effective I increases by approximately 3× (outrigger contribution), bringing total I_eff to roughly 2 × 10^7 in^4. Supplemental perimeter moment frame stiffness provides the remaining capacity.

Wind acceleration (occupant comfort)

Building occupants perceive motion through acceleration, not displacement. ASCE 7-22 does not provide acceleration limits, but industry practice follows:

Wind tunnel testing is standard practice for buildings above 200 ft to determine accurate wind loads and accelerations, including the effects of surrounding buildings, directional wind climate, and aerodynamic shape modifications (corners cut, tapered form, setbacks).

Tuned mass dampers (TMDs) reduce peak acceleration by 30–50% by providing out-of-phase inertial forces at the building's natural frequency. TMDs are passive (pendulum or sloshing type) or active (servo-controlled mass). Typical TMD mass = 0.5–2% of the building's modal mass.

Code comparison

ASCE 7-22 / AISC 360-22 (USA): Wind design per ASCE 7 Chapter 26-30 (Directional Procedure or Wind Tunnel). Drift limits are project-specific (H/400 to H/500 common). No code-mandated acceleration limit. AISC Design Guide 3 covers serviceability considerations including drift and vibration. P-Delta analysis per AISC 360 Chapter C (Direct Analysis Method) is essential for slender frames.

AS 1170.2-2021 / AS 4100-2020 (Australia): Wind actions per AS 1170.2, which uses regional wind speed maps specific to Australian cyclone and non-cyclone regions. Acceleration limits per AS 1170.2 Appendix G (5-year return period: 8–12 milli-g for office, 5–7 milli-g for residential). AS 4100 drift limits per Section 3.5.4 (H/500 for cladding protection). Australia's wind provisions are among the most prescriptive for acceleration.

EN 1991-1-4 / EN 1993 (Eurocode): Wind actions per EN 1991-1-4. Peak velocity pressure qp depends on terrain roughness and orography factors. EN 1990 Annex A1.4.4 requires serviceability check for wind-induced acceleration. ISO 10137 provides human comfort criteria (often adopted by Eurocode national annexes). Drift limits typically H/500 per national annex.

Common mistakes engineers make

1. Designing for strength without checking acceleration. A building can meet all strength and drift criteria but still have unacceptable occupant comfort. Wind-induced acceleration governs above approximately 30 stories and often requires added damping (TMD) or increased mass (heavier cladding, concrete core).

2. Ignoring the P-Delta effect in slender frames. For buildings with a height-to-width ratio above 4:1, P-Delta effects amplify drift by 15–30%. A first-order analysis grossly underestimates actual drift and column moments. Always run geometric nonlinear (P-Delta) analysis for high-rise frames.

3. Using equivalent static wind loads for dynamic-sensitive buildings. ASCE 7 Chapter 26 equivalent static method is only valid for rigid buildings (natural frequency > 1 Hz). Most steel buildings above 15 stories have fn < 1 Hz and require the analytical procedure (Section 26.11) or wind tunnel testing. Using the simplified method underestimates across-wind and torsional response.

4. Neglecting differential shortening between core and perimeter. In tall buildings, the core columns carry more load than perimeter columns, causing differential axial shortening. Over 30+ stories, this can accumulate to 1–2 inches, causing floor slope and cladding distress. Compensate by adjusting fabrication lengths.

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Related references

• How to Verify Calculations
• Structural System Selection
• Frame Analysis Methods
• Wind Loading
• steel beam capacity calculator
• structural engineering unit converter

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.