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.

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.

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.

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.

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.

Types of outrigger configurations

• Conventional outrigger — steel truss spanning from core to perimeter column, typically one or two stories deep. Diagonal members carry shear; chords carry moment. Connections to the core wall are moment connections; connections to columns are pin or moment depending on design intent.
• Belt truss — a perimeter truss at the outrigger level that engages columns not directly connected to the outrigger. Spreads the load to more columns, reducing individual column forces by 30-50 percent compared to direct outrigger alone.
• Virtual outrigger — uses floor diaphragms and belt trusses to transfer moment without a direct truss connection to the core. Avoids congested core connections but provides roughly 50-70 percent of the effectiveness of a direct outrigger.
• Damped outrigger — places viscous dampers at the outrigger-to-column connection instead of rigid links. Adds supplemental damping (2-5 percent of critical) to the building while still reducing drift. Particularly effective for wind-sensitive towers.

Common pitfalls

• 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.
• 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.
• 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.
• 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.

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Related references

• Transfer Structures
• Steel Building Envelope
• Steel High-Rise Systems
• How to Verify Calculations
• 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.