Load-Bearing Performance of Prestressed Concrete Steel Tube Truss Composite Floor Slab

Literature Overview

The paper by Lü Xiao, Jia Guanghao, Zhang Xin, and Wang Shun (Shandong Jianzhu University and Shandong Provincial Medical Industrial Design Institute, 2023) investigates the structural behavior of prestressed concrete steel tube truss composite floor slabs through finite element analysis and parametric studies. Funded by the National Natural Science Foundation Key Project (Grant 52038006) and Shandong Provincial Natural Science Foundation (Grant ZR2021ME190), the research was published in Journal of Xi'an University of Architecture and Technology (Vol. 55, No. 6, pp. 791-800). This work addresses the design optimization of a composite floor system that combines prestressed concrete with steel tube truss structural elements for long-span floor applications.

Structural System Description

The prestressed concrete steel tube truss composite floor slab is a prefabricated composite structure consisting of:

• A bottom slab with embedded prestressing tendons
• Steel tube truss elements providing vertical structural depth
• A top cast-in-place concrete layer forming the composite action
• Grout or mortar filling the void spaces between structural elements

The steel tube truss serves as the primary load-bearing structural element, with the prestressed bottom slab and cast-in-place top layer acting as flanges of a composite beam system. The steel tubes are typically arranged in a triangular or Warren truss configuration to maximize structural efficiency.

Finite Element Model Validation

The authors developed ABAQUS finite element models validated against existing experimental data. The load-deflection curves from FEA simulations showed good agreement with test results, confirming the model's reliability for parametric analysis. The FEA model incorporated:

Parametric Analysis Results

The parametric study evaluated the influence of seven key design variables on floor slab load-bearing capacity:

The finding that concrete strength has relatively low influence is particularly significant for cost optimization. This means that the structural performance is primarily governed by the steel tube properties and prestressing design rather than requiring high-strength concrete, which reduces material costs and improves constructability.

Steel Tube Selection and Fabrication Requirements

For the steel tube truss elements in composite floor slabs, specific manufacturing considerations apply:

1. Tube geometry: Circular steel tubes (typically 76-219 mm outer diameter) are preferred for their uniform stress distribution under multi-axial loading. Rectangular or square tubes may be used for compact truss configurations but require careful buckling analysis.
2. Wall thickness optimization: The parametric study confirms that wall thickness is a critical parameter. Tubes with insufficient wall thickness will experience local buckling before the truss reaches its full structural capacity. For typical floor slab applications (spans 6-12 m), wall thicknesses of 4-8 mm are recommended.
3. Material grade selection: The steel tubes should meet minimum yield strength requirements of 235-355 MPa (Q235-Q355 per GB/T 3091 or EN 10210). Higher grades provide diminishing returns due to the composite action with concrete.
4. Welding of truss joints: The connection between steel tubes at truss nodes is critical. Options include:

• Gusset plate connections with fillet welds (most common)
• Direct tube-to-tube welding with proper joint preparation
• Bolted connections with sleeve couplings (preferred for field assembly)

1. Surface preparation: The exterior surface of tubes must be clean and free of mill scale to ensure adequate bond with surrounding concrete and grout. Shot blasting or mechanical derusting to Sa 2.5 grade is recommended.

Design Optimization Insights

The parametric analysis reveals several important design principles:

1. Prestress optimization: Increasing prestress force improves load capacity and crack control but introduces higher fabrication complexity and cost. An optimal prestress level should balance structural performance with economic considerations.
2. Tube wall thickness vs. diameter trade-off: For a given span, increasing tube diameter provides more effective structural depth but increases weight and cost. The parametric study suggests that wall thickness optimization provides better returns than diameter increases for most practical span ranges.
3. Grout strength importance: The relatively high influence of grout/mortar strength is somewhat counterintuitive but reflects the critical role of the grout in transferring shear forces between the steel truss and concrete layers. Low-strength grout creates a weak interface that limits composite action.
4. Span limitation: The composite slab system becomes less efficient at very large spans (>15 m) due to the increasing weight of the self-supporting elements and the need for higher prestress levels.

Quality Control and Construction Considerations

For successful implementation of this composite floor system, the following quality control measures are essential:

• Steel tube incoming inspection: dimensional tolerance, wall thickness uniformity, surface condition, and material certification verification
• Truss fabrication quality: weld inspection (MT or UT), geometric accuracy, straightness tolerance
• Prestressing operation: proper jacking sequence, tendon anchorage quality, and stress verification
• Grout placement: proper mixing, pumping, and vibration to ensure complete filling without voids
• Interface bond verification: pull-off testing or ultrasonic inspection of the composite interface

Study Insights and Conclusions

This research provides valuable design guidance for the optimization of prestressed concrete steel tube truss composite floor slabs, identifying the key parameters that govern structural performance and enabling cost-effective design decisions. The finding that concrete strength has relatively minor influence while prestress level, tube wall thickness, and grout strength are critical parameters offers practical design flexibility. For steel pipe manufacturers, this application represents a growing market segment requiring consistent quality in tube dimensions, surface preparation, and material properties, with particular emphasis on weld quality at truss joints and adequate wall thickness uniformity throughout the tube length.