Transfer Structures — Beams, Trusses & Deep Members

Steel transfer structures: transfer beams vs transfer trusses, depth sizing rules, construction sequence analysis, deflection management, and connection design for massive reactions.

What is a transfer structure?

A transfer structure redirects gravity loads from columns above to columns or walls below that are on a different grid. This occurs when the upper-floor column layout does not align with the lower-floor structure — common in mixed-use buildings (residential tower over retail podium), hotels (lobby column-free spans under room floors), and renovation projects.

Transfer structures carry very large concentrated loads — often 5,000-20,000 kN per column — and must span 8-20 m between supports. They are among the heaviest and most critical members in a building. A failed transfer beam is a progressive collapse initiator.

Transfer beams vs transfer trusses

Transfer beams are deep plate girders or built-up sections, typically 1.0-2.5 m deep (span/6 to span/10). They are conceptually simple — a deep beam carrying point loads — but heavy. A 15 m span transfer beam carrying 10,000 kN might weigh 8-15 tonnes per meter of length (built-up plate girder with 50 mm thick flanges and 25 mm web with stiffeners).

Transfer trusses span the same distance using a 1-2 story deep triangulated framework. The truss depth is the full story height (3.5-4.5 m), giving a span/depth ratio of 3:1 to 5:1. Transfer trusses are much lighter than equivalent beams because the large lever arm reduces chord forces. However, they occupy usable floor space and require careful architectural integration.

Rule of thumb: if the transfer span exceeds 10 m and the total load exceeds 5,000 kN, a transfer truss is usually more economical than a plate girder.

Worked example — transfer beam preliminary sizing

A transfer beam spans 12 m between concrete core walls, supporting two columns at third points. Each column applies a factored load of 6,000 kN (combined dead + live from 20 stories above).

Maximum moment at mid-span (2 point loads at L/3): M = P x L / 3 = 6,000 x 12 / 3 = 24,000 kN-m per load, but the correct formula for two equal point loads at third points gives M_max = P x L / 3 = 6,000 x 4 = 24,000 kN-m.

Actually, for two equal point loads P at a = L/3 from each support: M_max = P x a = 6,000 x 4.0 = 24,000 kN-m (occurs between the loads, which is constant moment).

Using Grade 350 steel (Fy = 350 MPa), phi = 0.9: Required Zx = M / (phi x Fy) = 24,000 x 10^6 / (0.9 x 350) = 76,190 cm^3 = 76,190,000 mm^3.

This is far beyond any rolled section (largest W-shapes have Zx around 25,000 cm^3). A built-up plate girder is needed.

Try depth d = 2,000 mm, flange width bf = 600 mm, flange thickness tf = 60 mm, web tw = 25 mm. Approximate Zx (ignoring web contribution to flanges): Zx is approximately bf x tf x (d - tf) = 600 x 60 x (2000 - 60) = 69,840,000 mm^3 = 69,840 cm^3. Close but insufficient.

Increase flange thickness to tf = 70 mm: Zx approximately = 600 x 70 x 1930 = 81,060,000 mm^3 = 81,060 cm^3 > 76,190 cm^3. OK.

Girder weight: flanges = 2 x 0.6 x 0.07 x 7,850 = 659 kg/m. Web = 2.0 x 0.025 x 7,850 = 393 kg/m. Stiffeners (estimated 10% of web) = 39 kg/m. Total approximately 1,091 kg/m = 1.09 tonnes/m. Over 12 m: 13.1 tonnes for the girder alone.

Shear at support: V = P = 6,000 kN. Web shear capacity = 0.6 x Fy x d x tw x phi = 0.6 x 350 x 2000 x 25 x 0.9 / 1000 = 9,450 kN > 6,000 kN. OK.

Construction sequence effects

Transfer structures are sensitive to construction sequence because the loads they carry accumulate as upper floors are built. Key considerations:

• Staged analysis is mandatory. A single-step analysis with all loads applied simultaneously under-predicts deflection and over-predicts column reactions because it ignores the deflected shape that develops as each floor is placed.
• Shoring and propping. If the transfer beam deflects significantly under early floor loads, columns above develop differential settlements. Temporary shoring below the transfer beam during construction limits deflection until the beam has sufficient gravity load to stabilize.
• Post-tensioning sequence. Some transfer beams use external or internal post-tensioning to offset dead load deflection. Stressing must be timed relative to the construction load sequence.

Code references

Transfer beams in high-importance buildings are often designed to stricter deflection limits (L/600 to L/1000) to limit differential settlement of supported columns.

Common pitfalls

• Ignoring construction-stage deflection. A transfer beam that deflects 30 mm under 10 floors of self-weight before the cladding is installed will cause 30 mm of column shortening that the cladding must accommodate. If the cladding is designed for 10 mm movement, it cracks.
• Under-sizing the web for shear. Transfer beams carry enormous point loads. The web shear at the support equals the full column reaction (6,000 kN in the example). Web stiffeners at the support reaction point are always required.
• Not checking web sidesway buckling. When a heavy column bears on the top flange and the bottom flange is not braced, AISC J10.4 may govern. This limit state is often forgotten for transfer beams where the bottom flange hangs free below the floor slab.
• Designing for strength only, not deflection. Transfer beam deflection directly causes differential settlement of the columns it supports. A 20 mm mid-span deflection under service load means the center columns settle 20 mm relative to the perimeter — this affects partitions, finishes, and M/E systems.

Run this calculation

• Section Properties Database
• Beam Calculator

Related references

• Steel Framed Walls
• Beam Design Guide
• Plate Girder Design
• Steel Outrigger Systems
• How to Verify Calculations
• steel beam capacity calculator

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.