Preventing Underground Garage Roof Collapses: The Post-Tensioned Wide Beam and Slab Solution

Introduction

Underground garage roof slabs, often subjected to heavy soil cover (typically over 1.5 meters) for landscaping, require careful structural design to balance safety and economy. Several collapses during backfilling have highlighted the vulnerabilities of certain structural systems. BICP has developed the Taidi solution—a post-tensioned wide beam and slab system—to address these challenges.

Client Concerns

• Buried garage roof slabs must control story height and excavation depth while ensuring safety under soil surcharge and construction loads.
• Traditional beam-slab systems offer high safety redundancy but increase story height and excavation. Flat plate systems require cautious evaluation for buried roof applications.
• The optimal solution should be selected based on safety, code compliance, excavation, drainage, and buoyancy control costs.

Solution Highlights

• Taidi is a post-tensioned wide beam and slab solution for buried underground garage roofs, targeting commercial and industrial underground spaces.
• The wide beam system balances safety and economy, reducing story height and excavation by approximately 500 mm compared to traditional beam-slab systems.
• It lowers associated costs (excavation, drainage, buoyancy control), enabling comprehensive comparison during the design phase.
• Compared to flat plate systems, Taidi emphasizes punching shear safety redundancy and smoother code approval.

Technical Comparison

Common Failure Patterns of Flat Plate Systems

Several garage roof collapses share common features:

• All used conventional reinforced concrete flat plate (two-way slab) systems.
• No structural drop panels or shear capitals were provided.
• Collapses were sudden (low ductility).
• Occurred during backfilling (significant overloading).
• Failure initiated at column-slab connections (punching shear).

Why Flat Plate Systems Are Unsuitable for Buried Garage Roofs

While flat plate systems offer advantages in reduced story height and cost, they have low safety redundancy and poor ductility. The external load from backfilling is highly uncertain, making flat plates inappropriate for this application.

The Post-Tensioned Wide Beam and Slab Alternative

For a typical 8.1m x 8.1m column grid with 1.5m soil cover, the post-tensioned solution provides:

• 350 mm reduction in story height and excavation depth.
• Approximately 130 RMB/m² savings in structural cost.
• 10% reduction in exterior waterproofing area.
• Design live load reduced from 35 kN/m² to 20 kN/m².
• Lower buoyancy and dewatering costs.
• Reduced cracking and leakage risk.

The post-tensioned wide beam and slab system offers:

• Economy between traditional beam-slab and flat plate systems.
• Safety redundancy comparable to traditional beam-slab systems and significantly higher than flat plates.

Future Research

BICP plans to extend the study to 32 typical garage configurations, varying:

• Column grids: 8.1m x 8.1m, 8.1m x 10.4m, 8.4m x 8.4m, 8.4m x 10.8m.
• Civil defense requirements: with/without.
• Live loads: normal, fire truck.
• Soil cover depths: 1m, 1.5m, 2m, 2.5m, 3m.

Collaboration with owners and designers is welcome.

Applicability and Notes

Taidi is designed for buried underground garage roofs with post-tensioned wide beams. Key checks include soil cover, construction surcharge, code approval, and punching shear safety. This article is a summary for preliminary evaluation; detailed design should consider specific loads, column grids, geotechnical conditions, construction schedule, operational requirements, and local codes.