Structural Capacity Assessment: Converting from Structural Steel to Light Gauge Steel Platforms

Executive Summary

This technical paper provides structural engineers with a methodology to assess and validate the conversion of traditional structural steel HVAC platforms to light gauge steel (LGS) systems. With an 83% reduction in platform dead load, LGS systems enable previously unfeasible retrofit projects and provide significant capacity reserves for new construction.

1. Introduction

Increasing rooftop mechanical equipment densities, particularly in data centres and commercial buildings, often exceed existing structural capacity when using traditional structural steel platforms. Light gauge steel platforms offer a paradigm shift by dramatically reducing imposed loads while maintaining required performance standards to AS/NZS 1170 series.

2. Fundamental Load Comparison

2.1 Platform Weight Comparison

Platform TypeWeight (kg/m²)10m × 10m PlatformStructural Steel85-1208,500-12,000 kgEasyMech MR (LGS)12-141,200-1,400 kgSpan+ (LGS)15-181,500-1,800 kg

2.2 Total Load Impact

For 20 tonnes of HVAC equipment on a 100m² platform:

Structural Steel:

• Total design load (ULS): 73,500 kg

LGS Platform:

• Total design load (ULS): 63,180 kg
• Load reduction: 14% total, 86% platform weight

3. Existing Structure Assessment Methodology

3.1 Assessment Process

Step 1: Document Review

• Original structural drawings
• Current equipment schedules
• Previous modifications
• Design capacity reports

Step 2: Capacity Calculations

For existing rafter capacity:

Available capacity = φMb - Mexisting
Utilisation = (Mexisting + Mnew) / φMb
Acceptable if < 0.95

Step 3: Load Distribution Analysis

Load AspectStructural SteelLGS PlatformSupport Points4-8 concentrated20-50 distributedPeak IntensityP/4 to P/8P/20 to P/50Load PathPoint loads → ColumnDistributed → Purlin/Rafter

4. Reinforcement Avoidance Analysis

4.1 Reinforcement Triggers

Structural steel platforms typically require reinforcement when:

• Rafter capacity utilisation >85%
• Deflection exceeds L/250
• Connection capacity reached

Typical reinforcement costs:

• Rafter strengthening: $500-800/m
• New portal frame: $25,000-40,000
• Foundation augmentation: $50,000-100,000

4.2 LGS Advantage Calculation

Example: 2,000m² warehouse retrofit

• Required: 15 × 3-tonne RTUs
• Existing capacity: 1.5 kPa additional

Structural Steel: 28.5 kg/m² new load = Reinforcement requiredLGS Platform: 23.4 kg/m² new load = Within capacity

5. Detailed Calculation Example

Office Building Retrofit

Parameters:

• Existing: 2.8 kPa used of 4.0 kPa capacity
• New: 8 × 4-tonne units on 300m² platform

Structural Steel Option:

Platform weight = 28,500 kg
Total new load = 6.17 kPa
Available = 1.2 kPa
RESULT: Major reinforcement required ($350,000)

LGS Option:

Platform weight = 3,900 kg
Effective new load = 4.41 kPa (with distribution factor)
Total = 7.21 kPa < 8.0 kPa capacity
RESULT: Acceptable with minor upgrades ($45,000)

6. Additional Benefits

6.1 Wind Load Reduction

• Open mesh design: 30-40% less wind force
• Lower drag coefficient (0.8-1.2 vs 1.2-2.0)
• Reduced overturning moments

6.2 Seismic Performance

Seismic force reduction = 8,600 kg × Cs per platform
For Cs = 0.15: Force reduction = 12.9 kN

6.3 Installation Benefits

AspectStructural SteelLGS PlatformInstallation time5-10 days1-2 daysCrane size50-tonne20-tonneHot worksRequiredNoneRevenue loss (DC)$250K-2M$50-400K

7. Decision Matrix

FactorWeightStructural SteelLGS PlatformStructural Impact30%2/109/10Initial Cost25%3/108/10Installation Time20%3/109/10Future Flexibility15%4/108/10Maintenance10%5/109/10Weighted Score3.2/108.6/10

8. Case Study: Sydney Data Centre

Challenge: Add 20 chillers to facility at 95% structural capacity

Traditional approach: New building ($15M) or major upgrade ($3.5M)

LGS Solution:

1. Progressive platform replacement saved 43,000 kg
2. New platforms for expansion: 2,800 kg
3. Net capacity for 40 tonnes of new equipment
4. Total cost: $850,000
5. Saving: $2.65M minimum

9. Specification Guidelines

9.1 When to Specify LGS

• Structural capacity >80% utilized
• Reinforcement cost >50% of platform cost
• Installation time critical
• Seismic/wind governed design
• Future flexibility required

9.2 Essential Information Checklist

• Existing structural drawings
• Current load assessment
• Equipment layout and weights
• Wind/seismic parameters
• Future expansion plans

9.3 Standard Specification

"HVAC platforms shall be lightweight steel construction utilizing G550 cold-formed sections. Maximum dead load 15 kg/m². Design for minimum 2.5 kPa live load to AS/NZS 1170. No hot works required for installation."

10. Conclusion

Converting to light gauge steel platforms provides quantified benefits:

Load Reduction: 83% platform weight, 15-30% total roof loadCost Savings: 40-60% total project costTime Savings: 70-90% installation timeFuture-proofing: Modular reconfiguration without structural modification

The assessment methodology ensures conversions are technically sound and economically optimal. For capacity-constrained projects, LGS platforms transform unfeasible projects into straightforward implementations.

Critical takeaway: The 83% weight reduction often eliminates the need for structural reinforcement, saving hundreds of thousands in upgrade costs while achieving faster installation and improved long-term flexibility.

Technical Support: Con-form Group Engineering provides complimentary structural assessments. Contact: engineering@con-formgroup.com.au | 1300 882 490

‍