Steel Outrigger Systems — Tall Building Lateral Design

Steel outrigger systems for tall buildings: belt trusses, optimal outrigger location, virtual outriggers, construction sequence effects, and core moment reduction.

What is an outrigger system?

An outrigger system connects a building's central core to perimeter columns through stiff horizontal trusses or walls. When wind or seismic forces push the core laterally, the outrigger mobilizes the perimeter columns as a tension-compression couple, dramatically reducing core overturning moment and lateral drift. The concept dates back to sailing ship masts but became a standard tall-building strategy in the 1960s.

A single outrigger at the optimal height can reduce the peak core moment by 25-30 percent. A two-outrigger arrangement, positioned at approximately one-third and two-thirds of the building height, can reduce it by 40-50 percent. The perimeter columns engaged by the outrigger carry significant additional axial load, which must be accounted for in foundation design.

Outrigger configuration types

Optimal outrigger placement

For a single outrigger on a uniform building, the theoretical optimum location is approximately 0.455H from the top (about mid-height). For two outriggers the optimal positions are roughly 0.312H and 0.685H from the top. These assume uniform lateral loading and constant core stiffness.

Drift reduction by number and location of outriggers

In practice, outriggers are placed at mechanical floors where the two-story-deep truss can be accommodated within the plant room height. Shifting from the ideal location by two or three stories typically costs only 3-5 percent of the theoretical drift reduction.

Outrigger truss typical dimensions

Chord forces in outrigger trusses can reach 2,000-5,000 kips for supertall buildings. Built-up box sections are common for these extreme forces.

Column axial load increase from outrigger action

Perimeter columns engaged by outriggers must be designed for combined gravity + outrigger-induced axial forces. Foundation piles or mats must be sized for these increased loads.

Worked example — single outrigger drift reduction

Consider a 40-story steel building, total height H = 160 m, with a braced core (EI_core = 1.2 x 10^12 N-m^2) and a single outrigger at 0.45H = 72 m from the top.

Perimeter column spacing from core centroid: L_out = 18 m. Each column area = 400 cm^2 (A992 steel, E = 200 GPa). Outrigger truss flexural rigidity EI_out = 8 x 10^10 N-m^2. Uniform wind load w = 12 kN/m.

Without outrigger, tip drift = wH^4 / (8 EI_core) = 12 x 160^4 / (8 x 1.2 x 10^12) = 0.082 m = 82 mm.

The outrigger restraining moment M_o reduces this. Using the compatibility method, equate the core rotation at the outrigger level to the rotation induced by the column axial shortening/elongation through the outrigger truss. For this configuration the drift reduces to approximately 58 mm, a 29 percent reduction, bringing the drift ratio from H/1950 to H/2760, well within the common H/500 serviceability limit.

Worked example — outrigger truss member forces

Given: 50-story building, single outrigger at mid-height (H/2 = 100m). Core overturning moment at outrigger level M = 200,000 kN-m. Outrigger spans 15m from core center to perimeter column.

Step 1 — Column couple force: F_col = M / L_out = 200,000 / 15 = 13,333 kN (tension on windward, compression on leeward)

Step 2 — Truss chord force (2-story deep truss, d = 4m): Chord force = F_col x L_out / (2 x d) = 13,333 x 15 / (2 x 4) = 25,000 kN

Step 3 — Diagonal force (45-degree panel): F_diag = F_col / sin(45) = 13,333 / 0.707 = 18,858 kN

Step 4 — Chord section selection: Required area = 25,000 / (0.90 x 345) = 80.5 cm^2. Use built-up box 400x400x25mm (A = 93.8 cm^2).

Belt truss design

A belt truss at the outrigger level engages additional perimeter columns beyond those directly connected to the outrigger. The belt truss distributes the outrigger force across multiple columns.

Belt trusses are standard practice for buildings over 40 stories. The additional steel tonnage is typically 5-10% of the outrigger truss weight but reduces column foundation loads by 30-50%.

Construction sequence effects

Outrigger connections are never made simultaneously at all levels. The construction sequence creates locked-in forces from differential shortening between the core and perimeter columns.

For buildings over 30 stories, staged construction analysis is essential. A linear elastic model with all connections active from the start over-predicts outrigger effectiveness by 15-25%.

Code comparison for lateral drift limits

All codes treat wind drift limits as serviceability guidelines, not mandatory limits. Seismic drift limits are mandatory and checked at the strength level.

Notable outrigger buildings

Most buildings over 200m tall use some form of outrigger system. The trend toward 3-6 outrigger levels reflects the push for taller, slimmer towers.

Common pitfalls

1. Ignoring differential column shortening. Perimeter columns loaded by the outrigger shorten under sustained gravity load. If the outrigger is connected before the building is topped out, the differential shortening between core and columns introduces locked-in forces that can exceed the wind-induced outrigger force. Delayed connection or shimmed connections are standard practice.
2. Under-sizing the outrigger truss. The outrigger must be stiff enough relative to the core and columns to be effective. A flexible outrigger provides negligible drift reduction. The stiffness parameter (EI_out / EI_core) x (H / L_out) should be checked parametrically.
3. Neglecting belt truss design. Without a belt truss, only the columns directly connected to the outrigger engage. Adjacent columns see almost no additional load, wasting the building's perimeter capacity.
4. Forgetting construction-sequence analysis. A linear elastic model with all connections active from the start over-predicts outrigger effectiveness. Staged construction analysis is essential for buildings over 30 stories.
5. Not checking column foundation capacity for outrigger-induced loads. The outrigger adds 30-70% to perimeter column axial loads. Foundation piles or mats may need upsizing.

Frequently asked questions

What is the optimal location for a single outrigger? Approximately 0.455H from the top of the building (about mid-height). In practice, this is adjusted to coincide with a mechanical floor.

How much does an outrigger reduce drift? A single outrigger at the optimal height reduces core overturning moment by 25-30% and tip drift by 20-25%. Two outriggers achieve 40-50% moment reduction.

What is a virtual outrigger? A virtual outrigger uses floor diaphragms and belt trusses to transfer moment without a direct truss-to-core connection. It provides 50-70% of the effectiveness of a conventional outrigger while avoiding congested core connections.

When do I need a belt truss? For buildings over 40 stories where outrigger forces on 2 columns exceed practical column or foundation capacity. The belt truss distributes load to 4-8 perimeter columns.

How do I handle construction sequence? The preferred approach is delayed connection: complete the core and columns to their full height, measure actual differential shortening, then connect the outrigger with calibrated shims. This minimizes locked-in forces.

What is a damped outrigger? Viscous dampers placed at the outrigger-to-column connections instead of rigid links. They provide 2-5% supplemental damping to the building while still reducing drift. Effective for wind-sensitive towers where occupant comfort governs.

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Outrigger System Design Principles

Outrigger systems are lateral load-resisting systems that connect a building's central core to perimeter columns through stiff horizontal trusses or deep beams (outriggers). This engagement converts the otherwise cantilevering core into a composite system with significantly greater lateral stiffness and reduced drift.

The fundamental behavior: when the core bends under lateral load, the outrigger truss forces the perimeter columns to develop axial forces (tension on the windward side, compression on the leeward side). These axial forces create a restoring couple that resists the overturning moment, reducing both core moments and lateral deflection.

Key design principles:

1. Stiffness compatibility — The outrigger must be stiff enough relative to the core to effectively engage the perimeter columns. A flexible outrigger deflects without mobilizing the columns, providing minimal benefit.
2. Force transfer path — The overturning moment is transferred from the core through the outrigger to the perimeter columns as axial forces. The connections at all three points (core-to-outrigger, outrigger-to-column) must be designed for these forces.
3. Thermal effects — Differential shortening between core and perimeter columns due to creep, shrinkage, and thermal effects can induce significant forces in the outrigger. These must be accounted for in design.
4. Construction sequence — Outrigger connections are often detailed to allow delayed engagement after most differential shortening has occurred, then welded or bolted rigidly.

Optimal Outrigger Location

The effectiveness of an outrigger system depends strongly on where the outriggers are placed along the building height. Research and experience have established the following principles for optimal outrigger placement.

The drift reduction values assume a core-outrigger stiffness ratio of approximately 1:3 (core lateral stiffness to perimeter column axial stiffness mobilized through the outrigger).

Belt Truss Interaction

A belt truss (or belt wall) is a horizontal truss or deep beam that runs around the building perimeter at the same level as an outrigger. Belt trusses serve two critical functions.

1. Column force distribution — The belt truss engages all perimeter columns (not just the ones directly connected to the outrigger), distributing axial forces more evenly and increasing the effective lever arm.
2. Perimeter frame action — By rigidly linking the perimeter columns, the belt truss creates a perimeter tube that resists lateral load through frame action, further reducing drift.

Combined outrigger + belt truss systems can achieve drift reductions of 50–70% compared to a free-standing core, making them competitive with tube-in-tube systems for buildings in the 40–80 story range.

Core-Outrigger Stiffness Ratio

The efficiency of an outrigger system depends on the stiffness ratio between the core bending stiffness and the outrigger-column axial stiffness system. This ratio is expressed as:

α = (EI_core / H²) / (EA_col × d² / H)

where:
  EI_core  = flexural rigidity of the core
  EA_col   = axial rigidity of engaged perimeter columns
  d        = distance from core centerline to perimeter columns
  H        = total building height

Optimal range: 1 < α < 10
  α < 1:  Columns too stiff relative to core — outrigger forces are high
  α > 10: Core too stiff relative to columns — outrigger ineffective
  α ≈ 3–5: Balanced system — maximum drift reduction per unit cost

Notable Outrigger System Case Studies

Typical Outrigger Member Sizes

The following table provides typical member sizes for outrigger truss systems in steel-framed buildings. Actual sizes depend on specific project loads, building height, and seismic requirements.

Note: CFST = concrete-filled steel tube. Sizes shown are typical ranges — actual design must be project-specific.

Related references

• Transfer Structures
• Steel Building Envelope
• Steel High-Rise Systems
• Braced Frame Design
• Steel Truss Design
• Connection Types
• Steel Grades
• 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.