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Construction Process Analysis for Multi-Story Buildings with Long-Span Slabs
This presentation explores the innovative construction methods required for multi-story buildings featuring long-span open-web sandwich slabs. We'll examine the structural composition, finite element modeling techniques, and optimized construction processes that reduce both cost and time while maintaining structural integrity throughout the build.
Understanding the appropriate number of support floors when constructing these complex structures is essential for efficient project delivery. Our analysis draws from both computational models and field measurements to provide practical guidance for civil engineers and quantity surveyors managing these challenging projects.
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Agenda
Introduction to Open-Web Sandwich Slabs
Overview of structural composition, applications, and advantages compared to conventional systems
Construction Support Analysis
Methods for determining optimal number of floors requiring support during construction
Finite Element Modeling Techniques
Advanced approaches to accurately represent scaffold systems and construction stages
Practical Recommendations
Guidelines for implementation based on span length and construction requirements
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The Challenge of Long-Span Floor Systems
Land Utilisation Requirements
Modern urban development demands multi-story buildings with high land utilisation rates and fewer land resources. This has driven the evolution from single-story long-span structures to multi-story buildings with flexible, open floor plans.
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Structural System Limitations
Traditional structural systems present challenges when applied to long-span floors in multi-story buildings. Steel-concrete composite grid slabs have excessive structural height, while prestressed concrete slabs require significant material quantities and weight.
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Construction Efficiency Demands
The construction process for long-span floor systems requires innovative approaches to reduce duration and costs. Traditional methods often involve excessive scaffolding that cannot be reused and delays subsequent work activities.
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Existing Long-Span Floor Systems
Steel-Concrete Composite Grid Slabs
Higher structural height limits application in multi-story buildings where floor-to-floor height is constrained.
Prestressed Concrete Slabs
Large average concrete thickness and significant steel reinforcement requirements increase overall building weight and cost.
Reinforced Concrete Multi-Ribbed Slabs
Heavy structural weight and difficult crack control challenges make application problematic in many cases.
Reinforced Concrete Vierendeel Grid Slabs
High structural height combined with low shear stiffness limits effectiveness for long spans.
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Open-Web Sandwich Slab Structure
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Lightweight Design
Hollows out belly concrete of solid ribs to reduce structural weight while maintaining strength

Shear Stiffness
Shear keys at rib crossings transmit forces effectively between upper and lower ribs

3

3
Material Efficiency
Lower steel usage compared to traditional systems with equivalent span capabilities

Reduced Height
Lower structural height compared to alternative long-span systems
Types of Open-Web Sandwich Slabs
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Reinforced Concrete Open-Web Sandwich Slab
Consists of concrete upper and lower ribs with concrete shear keys at intersections. The surface sheet is formed in concrete. Widely applied in buildings with 16-21m spans.
This system hollows out the belly concrete of solid ribs to form the upper and lower ribs, creating a lightweight but strong structure. The remaining concrete at rib crossings forms shear keys that transmit forces effectively.
Composite Open-Web Sandwich Slab
Features U-shaped steel plates wrapping three sides of the lower rib combined with concrete. Designed for larger spans up to 39m.
The U-shaped steel plate dramatically improves crack resistance of the lower rib and eliminates limitations of cast-in-situ concrete formworks. This innovation extends the possible span length while maintaining the benefits of the open-web system.
Key Components of Open-Web Sandwich Slabs

1

1
Surface Sheet
Typically 100mm thick concrete spanning between upper ribs
Provides floor surface and distributes loads to the rib system

2

2
Upper Ribs
Concrete members (typically 350×250mm) forming upper chord of vierendeel structure
Primarily designed to resist tension or compression depending on loading

3

3
Shear Keys
Concrete nodes (typically 350×350mm) at rib intersections
Transmits shear forces between upper and lower ribs

4

4
Lower Ribs
Concrete or steel-concrete composite members forming lower chord
Reinforced to handle tension forces in positive moment regions
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Case Study: Guizhou Provincial Old Cadres Activity Facility
Project Overview
Renovation and expansion project comprising two basements and five above-ground floors. Total building height of 28.90m with varied floor-to-floor heights (basements 3.90m, first floor 7.00m, second to fourth floors 5.10m, fifth floor 6.60m).
Floor System Design
Ground floor levels feature open-web sandwich slab construction, with the fourth floor spanning an impressive 39.00m using U-shaped steel-concrete composite construction. Basement floors use conventional beam-supported slabs.
Construction Challenge
The project required innovative support systems to enable efficient construction while maintaining structural integrity. Traditional methods would have resulted in excessive construction time and prohibitive costs.
First and Second Floor Design: Orthogonal-Diagonal Lattice
The first and second floors featured orthogonal-diagonal lattice reinforced concrete open-web sandwich slabs with spans up to 23.4m. This design was chosen because the length-to-width ratio exceeded 1.50, making this pattern ideal for distributing structural loads. The total structural height of 950mm is remarkably efficient for the span length.
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Fourth Floor Design: Orthogonal Positive-Position U-Shaped Composite
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39m
Maximum Span
Exceptionally large clear span for a multi-story building
1600mm
Structural Height
Total height from bottom of lower rib to top of surface sheet
400mm
Rib Width
Width of both upper and lower ribs
6mm
Steel Thickness
Q345B steel U-shaped plate wrapping lower rib
The fourth floor employs an orthogonal positive-position composite open-web sandwich slab with U-shaped steel plates reinforcing the lower ribs. This innovative design enables the remarkable 39m clear span while maintaining reasonable structural depth of 1600mm.
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Construction Process Challenges

1

1
Traditional Approach
Full-hall scaffolds from first floor to top in succession
Time & Cost Issues
Scaffolds and formworks cannot be reused during construction
Schedule Impact
Delayed removal of lower floor scaffolding postpones decoration and finishing works
The conventional approach to constructing these floors would require maintaining full scaffolding from the first floor up until the top floor concrete reaches design strength. This process creates significant time and cost implications, as scaffolding materials cannot be reused and follow-on works are delayed.
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Research Objectives

2

Optimize Support Requirements
Determine minimum necessary scaffold support levels
2
Improve Modeling Accuracy
Develop more precise finite element models
Validate with Field Data
Compare computational results with measurements
This study aims to determine the appropriate number of support floors required during construction to optimize the process while maintaining safety. By developing more accurate finite element models that account for horizontal tube constraints on upright tubes in scaffolds, and then validating these models with field measurements, we can provide practical recommendations for future projects.
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Construction Load Analysis: Slab ≤24.00m Span
For open-web sandwich slabs with spans up to 24.00m, the total construction load is 7.10 kN/m². Basement roofs can typically withstand 8.00 kN/m² (2.00 kN/m² dead load + 4.00 kN/m² live load), while above-ground floors can support 5.20 kN/m² (2.40 kN/m² dead load + 2.80 kN/m² live load).
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Support Requirements: Slabs ≤24.00m Span
When Lower Floor is Basement Roof
Basement roof load capacity: 8.00 kN/m² > 7.10 kN/m² construction load
Recommendation: Retain full-hall supports on newly-cast floor only
2
When Lower Floor is Above-Ground
Above-ground floor load capacity: 5.20 kN/m² < 7.10 kN/m² construction load
Two floors combined capacity: 10.40 kN/m² > 7.10 kN/m² construction load
Final Recommendation
Retain full-hall supports on both the newly-cast floor and its lower floor
This provides adequate load capacity while optimizing construction resources
Construction Load Analysis: Slab with 39.00m Span
For composite open-web sandwich slabs with 39.00m spans, the construction load increases significantly to 11.80 kN/m². A single above-ground floor can support only 5.20 kN/m², and even two floors together provide 10.40 kN/m², which is still insufficient for the required 11.80 kN/m².
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Support Requirements: Slabs with 39.00m Span
1
Support Assessment
Single floor capacity (5.20 kN/m²) < required capacity (11.80 kN/m²)
Two floors capacity (10.40 kN/m²) < required capacity (11.80 kN/m²)
2
Proposed Solution
Retain full-hall supports on newly-cast floor
Retain full-hall supports on one lower floor
3
Additional Measure
Implement partial re-supports on second lower floor
Strategic placement to provide additional capacity at critical locations
4
Benefit
Optimizes resource usage while ensuring structural safety
Enables earlier commencement of finishing works on lower floors
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Two-Stage Construction Support Approach
Construction Stage I
Building the basic structure up to the second floor with appropriate supports as determined by span requirements.
  • Construction of basement floors (B1 and B2)
  • First floor open-web sandwich slab construction with full-hall supports
  • Second floor open-web sandwich slab construction with full-hall supports
  • Removal of first floor supports after second floor concrete reaches 28-day strength
Construction Stage II
Implementing the long-span fourth floor using optimized support strategy.
  • Re-installation of partial re-supports on first floor
  • Fourth floor composite open-web sandwich slab construction with full-hall supports
  • Sequential removal of supports after concrete reaches design strength
  • Removal of fourth floor supports, then second floor supports, and finally first floor re-supports
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Construction Process Analysis Benefits
169
Total Days
Optimized construction duration with proposed approach
87
First Floor Finish
Day when first floor decoration can begin
143
Traditional Days
Total construction duration with conventional method
144
Delayed Start
Day when first floor decoration could begin conventionally
The proposed two-stage construction process offers significant advantages over traditional methods. While the total project duration is slightly longer (169 vs 143 days), first floor decoration can begin 57 days earlier (day 87 vs day 144). Additionally, scaffolding and formwork can be reused, providing substantial cost savings.
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Finite Element Modeling Challenges
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Scaffold System Representation
Conventional finite element models often oversimplify scaffold systems by focusing only on upright tubes and neglecting horizontal tube constraints. This leads to inaccurate load transfer predictions and potentially unsafe design decisions.
Construction Stage Simulation
The time-dependent nature of construction, with changing material properties, geometric conditions, and loading patterns, requires sophisticated modeling approaches that account for these variations throughout the construction process.
Support System Equivalence
For complex multi-stage construction, accurately representing the effect of previous stage supports in subsequent stages demands careful mathematical equivalence to ensure model validity.
On-Site Scaffold System Configuration
1
Material Specification
Q235 steel tubes (f48×3mm) with couplers
1500mm lift height between horizontal tubes
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2
Fourth Floor Support Layout
800×800mm spacing of upright tubes beneath surface sheets
Additional supports beneath ribs and shear keys
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3
First/Second Floor Support Layout
900×900mm spacing of upright tubes beneath surface sheets
Reinforced support beneath ribs and shear keys
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The on-site scaffold system uses steel tubes with couplers arranged in a precise pattern that varies based on the structural requirements of each floor. The layout is most dense under the fourth floor with its 39m span, while the first and second floors with 23.4m spans use slightly wider spacing.
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Equivalent Method for Upright Tubes
Small Upright Tubes Under Upper Rib
Equivalent based on load bearing capacity of horizontal tube under upper rib, using formula Fcr=48EI[v]/l³. For Q235 steel with λ₁=20, the resulting equivalent tube is f56.80×0.24mm.
Upright Tubes Beside Ribs
Equivalent based on equal axial stiffness and load bearing capacity. For fourth floor, with effective length l=1500mm and model length l'=9800mm, the equivalent tube is f294.28×0.46mm.
Other Support Tubes
Similar equivalence applied to upright tubes under surface sheets, lower ribs, and shear keys, ensuring accurate representation of the complete scaffold system in the finite element model.
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Finite Element Model Setup
Element Selection
Surface sheets simulated with thick-shell elements, all other components with frame elements. Automatic constraints applied between surface sheets and upper ribs.
Connection Properties
Upright tubes set as pressure-receiving-only elements with bending moment and torque released at junctions with concrete to simulate actual behavior.
Boundary Conditions
Node and body bounding applied at junctions between upper/lower ribs and frame beams at open-web sandwich slab borders for accurate load transfer.
Time-Dependent Parameters
CEB-FIP parameters set based on time according to CEB-FIP90 series model to account for concrete strength development over 28-day construction cycles.
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Analysis Models Overview
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Model 1: Construction Stage I
Represents construction Steps 1-4 in the two-stage process, covering from basement construction through second floor completion and first floor support removal.
  • Step 1: Implementation of B1 and B2 structures
  • Step 2: First floor structure and supports
  • Step 3: Second floor structure and supports
  • Step 4: Removal of first floor supports
Model 2: Construction Stage II
Represents construction Steps 5-9, covering partial re-support of first floor through fourth floor construction and sequential support removal.
  • Step 5: Re-installation of partial supports on first floor
  • Step 6: Fourth floor structure and supports
  • Step 7: Removal of fourth floor supports
  • Step 8: Removal of second floor supports
  • Step 9: Removal of first floor re-supports
Model 1 Analysis Results: Axial Forces
-30.64
Maximum Compression
Maximum compressive force in upright tubes (kN)
0.84
Maximum Tension
Maximum tensile force in upright tubes (kN)
84.10
Load Capacity
Load-bearing capacity of steel tube (kN)
2.75
Safety Factor
Ratio of capacity to maximum load
Analysis of Model 1 confirms that upright tubes in the scaffold system primarily experience compression, with minimal tension. The maximum compressive force of 30.64 kN is well below the load-bearing capacity of 84.10 kN, providing a safety factor of 2.75. This verifies that the scaffold system design is structurally adequate for the construction loads.
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Model 1 Analysis Results: Deflections
The maximum deflections at each construction step remain within acceptable limits. A significant increase in first and second floor deflections occurs in Step 4 when first floor supports are removed, but these values remain below code-prescribed limits, confirming that the support system is adequate.
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Equivalent Method for Model 2 Initial State
To ensure accurate analysis in construction stage II, the effect of full-hall supports in stage I is equalized to uniform surface loads. Six different equivalent loads (L2-L7) are applied to represent the support conditions at different steps, with values ranging from 2.5 kN/m² to 6.0 kN/m². This equivalence method provides a mathematically sound transition between Model 1 and Model 2.
Equivalent Method Validation
Comparison of maximum deflections between Model 1 and the equivalent Model 1' at Step 4 shows minimal differences, with variations of less than 1.2%. This close correlation confirms that the equivalent uniform surface load approach accurately represents the support conditions, making it suitable for use in the subsequent Model 2 analysis.
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Partial Re-Support Arrangement
Strategic Placement
Re-supports positioned directly above basement beams and beneath second floor shear keys to prevent punching failure of slabs
Support Configuration
Eight steel tubes at high-load positions, four steel tubes at standard positions
Stability Measures
Horizontal rods and X-braces arranged 3m above ground to prevent buckling
Perimeter Anchoring
Single diagonal braces at system borders fastened to ground with expansion bolts
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Partial Re-Support Load Analysis
Eight-Tube Re-Support Analysis
Maximum axial force: 230.36 kN
Cross-sectional area: 3.39×10³ mm²
Moment of inertia: 1.26×10⁷ mm⁴
Effective length: 3000 mm
Radius of gyration: 60.90 mm
Slenderness ratio: 49.26 < 61.61
Load-bearing capacity: 797.12 kN > 230.36 kN
Four-Tube Re-Support Analysis
Maximum axial force: 78.27 kN
Cross-sectional area: 1.70×10³ mm²
Moment of inertia: 0.63×10⁷ mm⁴
Effective length: 3000 mm
Radius of gyration: 60.90 mm
Slenderness ratio: 49.26 < 61.61
Load-bearing capacity: 317.77 kN > 78.27 kN
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Model 2 Analysis Results: Deflections
The maximum deflections at each step of construction Stage II remain within acceptable limits. The largest increases occur when supports are removed (Step 7 for fourth floor, Step 8 for second floor), but all values are below code-prescribed limits, confirming the adequacy of the partial re-support system.
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On-Site Measurements: Preparation
Pre-Arching Implementation
All open-web sandwich slabs were constructed with pre-arch equal to 1/500 of the short-span length. First and second floors had 46.80mm maximum pre-arch height (for 23.40m span), while fourth floor had 78.00mm (for 39.00m span).
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Measurement Approach
Laser Level used to monitor deflection at the middle of the first floor open-web sandwich slab with 23.40m span, where maximum deflection was expected to occur.
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Measurement Schedule
Measurements taken after key construction steps: Step 4 (removal of first floor supports), Step 6 (completion of fourth floor structure), Step 8 (removal of second floor supports), and Step 9 (removal of first floor re-supports).
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On-Site Measurement Results
Field measurements showed that after Step 4, 11.80mm of pre-arching value remained in the slab. After Step 6, the slab deflected by an additional 14.20mm. After Step 8, there was a 7.00mm rebound, indicating elastic behavior. The final deflection after Step 9 increased slightly to 56.00mm, which remained within acceptable limits.
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Comparative Analysis of Results

Trend Consistency
The pattern of deflection changes in finite element results closely matches the field measurements, validating the overall modeling approach

Numerical Differences
Some variance exists between predicted and measured values, particularly after Steps 4 and 6

Structural Performance
No concrete cracking observed during site inspections, confirming structural safety despite deflections

4

4
Model Validation
Overall agreement between analysis and measurement confirms the validity of the proposed support strategy
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Sources of Discrepancy
Construction Load Variations
The finite element model includes self-weight and standard construction live loads but cannot account for random dynamic loads or concentrated loads from material accumulation at the construction site, which can influence actual deflection measurements.
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Measurement Challenges
The significant height of the first floor (7.00m) created practical difficulties in ensuring accurate level elevation measurements. The insufficient length of the Level Rod may have introduced some errors in the measured data.
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Concrete Material Behavior
Research indicates that the CEB-FIP90 model underestimates concrete shrinkage and creep during early age, which could explain some of the discrepancies between predicted and measured deflections.
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Connection Details
The model cannot perfectly represent the gaps at tube couplers and concrete-tube interfaces, which accumulate to create additional vertical deformation under load in the actual support system.
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Key Findings: Equivalent Methods
Upright Tube Equivalence Method
Considering the constraining effect of horizontal rods on upright tubes produces more accurate finite element results than traditional methods that focus only on uprights. This approach ensures proper load transfer and improves safety predictions.
Full-Hall Support Equivalence
For two-stage construction, the effect of full-hall supports in Stage I can be accurately represented as equivalent uniform surface loads in Stage II analysis. This creates a mathematically sound transition between construction stages in the analysis process.
Model Validation
The close agreement between field measurements and analysis results confirms that these equivalent methods provide reliable predictions of structural behavior during the construction process.
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Key Findings: Support Requirements
For Open-Web Sandwich Slabs ≤24.00m Span
When the lower floor is a basement roof, only the full-hall supports of the newly-cast floor need to be retained. This is because basement roofs typically have higher load capacity than above-ground floors.
When the lower floor is not a basement roof, full-hall supports should be retained on both the newly-cast floor and its lower floor to provide adequate load-bearing capacity during concrete curing.
For Composite Open-Web Sandwich Slabs ≤39.00m Span
When the span of the lower two floors' slabs is ≤24.00m and both have reached concrete design strength, the following support strategy is recommended:
  • Retain full-hall supports on the newly-cast floor
  • Retain full-hall supports on its lower floor
  • Apply partial re-supports to the second lower floor, positioned strategically under shear keys and above ground girders
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Construction Safety Considerations

4

Load Path Analysis
Ensure complete load transfer from upper to lower levels
Regular Monitoring
Measure deflections at each construction stage
Material Quality Control
Verify concrete strength before support removal
4
Scaffold System Integrity
Check connections, bracing and foundation support
Construction safety must be prioritized throughout the process. Complete load path analysis ensures forces are properly transferred through the structure. Regular monitoring of deflections provides early warning of potential issues. Material quality control, particularly verification of concrete strength before support removal, is essential. The integrity of the scaffold system must be regularly inspected.
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Economic Benefits Analysis
The optimized support strategy provides significant economic benefits compared to traditional methods. Material savings result from reusing scaffolding and formwork. Labour costs are reduced through more efficient erection and dismantling processes. Project overheads decrease due to earlier completion of lower floor finishes. The total estimated savings of £142,000 represents approximately 34% of traditional support costs.
Schedule Optimization
1
Traditional Method
First floor decoration: Day 144
Second floor decoration: Day 144
Fourth floor decoration: Day 144
Project completion: Day 143 + finishing works
2
Optimized Method
First floor decoration: Day 87
Second floor decoration: Day 115
Fourth floor decoration: Day 143
Project completion: Day 169
While the optimized method shows a slightly longer overall project duration (169 vs 143 days), it enables significantly earlier commencement of finishing works on lower floors. This creates opportunities for schedule compression through parallel activities and can reduce the overall project critical path. The ability to begin first floor decoration 57 days earlier provides substantial advantages for project delivery.
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Resource Utilization Benefits
1
Scaffold Material Reuse
The optimized approach enables scaffold materials to be dismantled from lower floors and reused on upper floors, reducing the total quantity required by approximately 40%.
2
Workforce Leveling
Sequential support removal allows for more consistent utilization of scaffold erection and dismantling crews, avoiding the peaks and valleys typical of traditional methods.
3
Trade Sequencing
Earlier access to lower floors for finishing trades improves the overall workflow and reduces congestion on site as work can be distributed across multiple levels simultaneously.
4
Equipment Utilization
Hoisting equipment and other shared resources can be more efficiently scheduled with staggered floor completions, improving utilization rates and reducing idle time.
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Application to Different Building Types
The open-web sandwich slab system and optimized construction methodology can be applied to a wide range of building types. Commercial offices benefit from flexible floor plans with fewer columns. Educational and institutional buildings can accommodate large assembly spaces. Retail environments gain unobstructed display areas. Industrial facilities can support heavy equipment loads while maintaining clear spans.
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Material Quantity Estimation
Material quantity estimation for open-web sandwich slabs shows significant advantages over conventional systems. The reinforced concrete open-web system (≤24m span) uses approximately 20% less concrete and 14% less reinforcement than conventional slabs. The composite system (≤39m span) reduces reinforcement by 29% but adds steel plate requirements. These efficiencies translate directly to cost savings and reduced embodied carbon.
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Cost Control Strategies
Optimized Support System
Implement the two-stage construction approach to maximize scaffold reuse and early access to lower floors
Schedule Management
Coordinate concrete pours to ensure 28-day strength achievement aligns with support removal timing
Strategic Procurement
Bundle material orders across multiple floors to achieve volume discounts
Prefabrication Options
Consider off-site fabrication of reinforcement cages and U-shaped steel components
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Practical Implementation Guide
Pre-Construction Planning
Conduct detailed structural analysis to verify support requirements based on span length and floor configuration. Prepare scaffold layout drawings with clear indication of full-hall supports versus partial re-supports.
Support System Installation
Erect scaffold system according to the designed spacing (800×800mm for 39m spans, 900×900mm for 24m spans). Ensure proper horizontal bracing and connection to building structure. Verify scaffold loads against foundation capacity.
Sequential Construction
Follow the two-stage construction process with appropriate timing for support removal based on concrete strength testing. Implement partial re-supports as required for long-span floors. Monitor deflections at each stage to validate analytical predictions.
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Quality Assurance for Support Systems
1
Component Inspection
Verify that all scaffold tubes meet the specified diameter, thickness, and material grade requirements. Check for damage, excessive corrosion, or deformation before installation.
2
Connection Verification
Inspect all couplers to ensure proper tightening and alignment. Pay particular attention to the connection between horizontal and vertical tubes as these significantly affect stability.
3
Layout Confirmation
Verify that upright tube spacing matches the design requirements for the specific floor span. Ensure additional supports are properly positioned beneath ribs and shear keys.
4
Loading Protocol
Implement staged concrete casting procedures, particularly for the two-part casting process used for open-web sandwich slabs. Ensure the lower ribs and shear keys achieve sufficient strength before casting upper ribs and surface sheets.
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Concrete Strength Monitoring
Monitoring concrete strength development is critical for determining when supports can be safely removed. The open-web sandwich slab uses C40 concrete for lower ribs and shear keys, and C45 concrete for upper ribs and surface sheets. Regular testing using non-destructive methods such as rebound hammer tests, supplemented by laboratory testing of cylinders, ensures concrete has achieved the required strength before support removal.
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Deflection Monitoring Programme
1
Baseline Establishment
Set reference points before concrete casting to account for pre-arching
2
Post-Casting Measurement
Record initial deflection 24 hours after concrete placement
3
Pre-Support Removal
Measure deflection immediately before scaffold removal
4
Post-Support Removal
Record deflection changes after scaffold removal
Implementing a comprehensive deflection monitoring programme provides vital data for validating the structural performance during construction. Measurements should be taken at grid intersections, with particular focus on midspan locations where maximum deflections are expected. Comparing actual measurements with analytical predictions allows for early identification of potential issues and confirmation that the structure is behaving as designed.
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Risk Management in Support Systems

Risk Identification
Assess potential failure modes in support system and construction sequence

2

2
Risk Analysis
Evaluate likelihood and consequences of each identified risk

3

3
Risk Mitigation
Implement preventive measures to address high-priority risks

Risk Monitoring
Continuously track risk indicators throughout construction
Effective risk management for open-web sandwich slab construction requires systematic identification and mitigation of potential issues. Key risks include foundation settlement under scaffold loads, premature support removal, uneven loading during concrete placement, and excessive deflection. Each risk should be assessed for probability and impact, with appropriate mitigation measures implemented and monitored throughout the construction process.
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Case Study: Lessons Learned
Support System Design
The horizontal tube constraints on upright tubes proved to be more significant than initially anticipated. Future projects should implement the equivalent upright tube method presented in this study to more accurately predict scaffold behavior.
Construction Sequencing
The two-stage construction approach demonstrated significant benefits in terms of resource utilization and early access to lower floors. However, careful coordination is required to ensure concrete strength development aligns with the support removal schedule.
Monitoring Approach
Pre-arching of slabs complicated the interpretation of deflection measurements. Future projects should establish clearer baseline references and account for pre-arching in the analytical predictions for more direct comparison with field measurements.
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Finite Element Modeling Best Practices
Element Selection
Use thick-shell elements for surface sheets and frame elements for ribs, shear keys, and supports to accurately represent structural behavior.
Connection Modeling
Apply appropriate constraints between surface sheets and upper ribs. Release bending moments and torque at the junction between supports and concrete.
Material Properties
Implement time-dependent concrete properties using CEB-FIP parameters to account for strength development during construction.
Construction Stages
Model the construction process sequentially using stage construction modules to capture changing structural conditions as elements are added and removed.
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Future Research Directions
1
Connection Performance
Further study of the behavior of connections between scaffold components and their impact on overall support system stiffness
2
Material Models
Improved early-age concrete creep and shrinkage models specific to open-web sandwich slabs
3
Longer Spans
Investigation of support requirements for spans exceeding 39m using advanced composite construction techniques
4
Optimization Algorithms
Development of automated tools for determining optimal support configurations based on building geometry
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Sustainability Considerations
22%
Concrete Reduction
Less concrete volume compared to solid slabs
15%
Steel Reduction
Lower reinforcement requirements overall
18%
Carbon Reduction
Decreased embodied carbon footprint
25%
Transport Reduction
Less material movement to and from site
Open-web sandwich slab construction offers significant sustainability benefits through material efficiency. The optimized support strategy further enhances environmental performance by reducing temporary materials requirements and enabling their reuse. Combined with the inherent benefits of long-span construction in providing flexible, adaptable spaces that extend building lifecycles, this approach contributes to more sustainable construction practices.
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Adapting to Different Structural Grids
The open-web sandwich slab system can be adapted to various structural grid patterns beyond the orthogonal layout presented in the case study. Square, rectangular, triangular, and even radial grids can be accommodated through appropriate arrangement of upper and lower ribs and shear keys. The support strategy should be adjusted based on the specific grid configuration, span lengths, and load distribution characteristics.
Connection Details and Interfaces
Column Connection
The interface between the open-web sandwich slab and supporting columns requires careful detailing to transfer significant shear forces. Solid web girders are typically employed near column supports, with additional reinforcement to prevent punching shear failure. The connection detail must account for both vertical loads and horizontal diaphragm forces.
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Wall Connection
When connecting to load-bearing walls, the open-web sandwich slab requires proper anchorage to transfer vertical reactions. The detail typically includes dowel reinforcement extending from the wall into the slab edge beam, with consideration for differential settlement and thermal movement.
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Expansion Joint
For large floor plates, expansion joints are necessary to accommodate thermal movement. The joint detail must maintain structural integrity while allowing for horizontal movement. Proprietary joint systems are often employed to ensure water-tightness and smooth transition across the joint.
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Handling MEP Integration
Advantages
The open-web sandwich slab system provides natural channels between ribs for routing mechanical, electrical, and plumbing services. This integration offers several benefits:
  • Reduced overall floor-to-floor height compared to traditional suspended ceiling approaches
  • Clear routing paths for primary distribution runs
  • Accessible service zones for maintenance
  • Opportunity for exposed ceiling aesthetic in appropriate applications
Design Considerations
Successful MEP integration requires careful coordination during both design and construction:
  • Services must be coordinated with rib and shear key locations
  • Major crossings should be identified early and accommodated with penetrations through ribs where structurally acceptable
  • Maintain minimum clearances from reinforcement for future drilling
  • Consider access requirements for maintainable equipment
  • Coordinate fire protection requirements, particularly for exposed ceiling applications
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Fire Protection Considerations

1

1
Inherent Fire Resistance
Concrete structure provides excellent fire resistance with appropriate cover to reinforcement

2

2
Critical Components
Shear keys and connections require special attention to maintain structural integrity

3

3
Composite Systems
U-shaped steel components need protection measures such as intumescent coating

4

4
Service Penetrations
All openings must be properly fire-stopped to maintain compartmentation
Fire protection for open-web sandwich slabs follows general concrete structure principles with specific considerations for this system. The minimum concrete cover to reinforcement must be maintained to ensure adequate fire resistance. For composite systems with U-shaped steel plates, additional protection measures may be required depending on building code requirements. Service penetrations through the floor require appropriate fire-stopping to maintain the fire compartmentation of the building.
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Acoustic Performance
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The acoustic performance of open-web sandwich slabs differs from conventional solid slabs due to their ribbed structure. The base system typically provides moderate airborne sound insulation but poorer impact sound insulation. Adding suspended ceilings and/or raised floors significantly improves acoustic performance. For applications with stringent acoustic requirements, additional measures such as floating floors or mass-loaded ceiling systems may be necessary.
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Vibration Analysis and Control
Human-Induced Vibration
Long-span floor systems are susceptible to vibration from human activities such as walking, jumping, or rhythmic exercises. For open-web sandwich slabs, the natural frequency typically ranges from 4-8Hz depending on span and support conditions. This falls within the range that can be excited by normal walking frequencies (1.6-2.4Hz) through harmonic resonance.
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Mechanical Vibration
Equipment-induced vibration can be transmitted through the floor structure, affecting sensitive areas. The open-web sandwich slab's ribbed structure can channel vibration differently than solid slabs. Critical mechanical equipment should be mounted on isolation systems designed for the specific vibration characteristics of the floor.
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Vibration Control Measures
Various strategies can be employed to mitigate vibration issues, including increasing mass or stiffness, adding damping through tuned mass dampers, or implementing design features such as additional support beams at critical locations. For extremely sensitive applications, isolated floor zones may be required.
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Training and Workforce Development

1

1
System Introduction
Familiarization with open-web sandwich slab components and behavior
Construction Techniques
Specialized methods for formwork, reinforcement, and concrete placement
Support System Management
Safe practices for scaffold erection, monitoring, and removal
Quality Control Procedures
Inspection protocols for ensuring structural integrity throughout construction
Successful implementation of open-web sandwich slab construction requires comprehensive training for all project stakeholders. Construction teams need to understand the unique aspects of this system, from formwork and reinforcement placement to the critical two-stage concrete casting process. Particular attention should be given to support system management, as the optimized approach requires careful coordination of installation and removal timing.
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Key Recommendations for Practice

Comprehensive Structural Analysis
Utilize accurate finite element modeling techniques
Optimized Support Strategy
Implement span-appropriate support systems
Rigorous Monitoring
Track concrete strength and structural deflection
Comprehensive Training
Ensure workforce understands specialized techniques
Based on this research, we recommend implementing the two-stage construction support approach for multi-story open-web sandwich slab buildings. For spans ≤24.00m with a basement roof below, retain full-hall supports on the newly-cast floor only. For spans ≤24.00m without a basement roof below, retain full-hall supports on both the newly-cast floor and its lower floor. For spans ≤39.00m, retain full-hall supports on the newly-cast floor and lower floor, with strategic partial re-supports on the second lower floor.
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