Comparative Design Utilization Sensitivity of Industrial Steel Buildings under Increased Roof and Wind Actions

Technical Summary: Comparative Design Utilization Sensitivity of Industrial Steel Buildings under Increased Roof and Wind Actions

Problem Statement
Industrial steel buildings, particularly those utilizing rigid portal frames, are frequently subjected to revised service demands, such as the addition of photovoltaic systems, suspended utilities, or maintenance loads, as well as updated wind action data during their service life. A critical challenge in structural assessment is that modest changes in load can shift the governing member or load combination, potentially moving a different structural element to a critical failure state. While system-based studies suggest that gravity-to-wind proportions and redistribution influence reliability, there is a need to understand how specific, controlled load perturbations affect the governing code-check utilization ratios in professional practice models. This study addresses the question of whether a common percentage increase in load produces a uniform utilization response across different building configurations.

Methodology
The study employs a comparative screening approach using three verified, three-dimensional STAAD.Pro models derived from completed professional engineering assignments in India. The models represent distinct industrial facilities:

• Case A: Bansal Steel & Power Limited, Shed B (Transverse: 52.92 m, Length: 90.00 m, Height: 9.98 m).
• Case B: Tata Motors Body Solutions Limited, Dharwad (Transverse: 105.20 m, Length: 150.00 m, Height: 14.90 m).
• Case C: LIBGEM Solutions/Gravita industrial shed (Transverse: 65.00 m, Length: 67.50 m, Height: 15.87 m).

The analysis retained the original geometry, supports, material properties, and Indian steel design parameters (IS 800:1984) for member checks. The study did not perform nonlinear collapse or reliability analysis but focused on elastic member verification.

Six controlled sensitivity scenarios were applied to the baseline models:

1. Baseline (original loads).
2. Roof collateral load increased by 25% (Factor 1.25).
3. Roof collateral load increased by 50% (Factor 1.50).
4. Wind load increased by 10% (Factor 1.10).
5. Wind load increased by 20% (Factor 1.20).
6. Combined roof (1.25) and wind (1.10) increase.

The primary metric was the governing utilization ratio (demand-to-capacity) across all checked steel members. A ratio exceeding 1.000 indicated a failed member check. The study also tracked the "migration" of the governing member identity across scenarios.

Key Results

• Baseline Performance: All three models passed their original design checks with governing utilization ratios of 0.897 (Case A), 0.917 (Case B), and 0.977 (Case C).
• Roof Load Sensitivity: A 25% increase in roof load kept all cases below unity. However, a 50% increase caused Cases B and C to exceed unity (1.073 and 1.011, respectively), while Case A remained safe at 0.954. Case A was comparatively insensitive to added roof load over the tested range, whereas Case B showed comparable sensitivity to both roof and wind actions.
• Wind Load Sensitivity: Wind amplification proved to be the strongest driver for failure in Cases A and C. Case C was the most wind-sensitive model, exceeding unity at a wind factor of 1.10 (1.101), whereas Cases A and B remained below 1.0. At a wind factor of 1.20, all cases failed, with Case C reaching a utilization ratio of 1.235.
• Governing Member Migration: The identity of the critical member changed significantly with load type and magnitude. For instance, in Case B, the baseline was governed by a tapered column, but amplified roof scenarios shifted the governing member to a tapered rafter. In Case A, the governing member shifted from a short ISMB300 segment to an angle bracing member under high roof loads.
• Non-Linear Response: The results demonstrated that a common percentage load increase does not produce a common utilization response. Case C, which started with the highest baseline utilization, was not necessarily the most sensitive to all load types; its response was highly dependent on whether the load was wind or roof-driven.

Key Contributions

• Empirical Evidence of Non-Proportionality: The study provides concrete evidence that scaling baseline utilization ratios is insufficient for assessing structural safety under new load conditions. The response is model-specific and depends on geometry, framing density, and load distribution patterns.
• Identification of Critical Load Drivers: The research distinguishes between buildings where wind action is the primary constraint (Cases A and C) versus those where roof collateral loads are equally or more critical (Case B showed comparable sensitivity to both).
• Documentation of Member Migration: The paper highlights that screening existing buildings requires tracking the migration of the governing member. A defensible assessment process must involve a complete re-analysis and code-check set rather than applying a global multiplier to existing baseline ratios.

Significance and Claims
The paper positions itself as a comparative screening study rather than a comprehensive reliability or collapse analysis. Its significance lies in providing a compact workflow for engineers to prioritize detailed reassessments when industrial steel buildings face additional roof services or altered wind demands.

The author claims that the study identifies the specific load intervals where unity is exceeded, guiding where refined analysis should focus. They emphasize that while the results highlight the need for re-analysis, the study does not authorize additional loading or revised wind demands without project-specific verification of the physical structure. The findings support the system-oriented view that demand distribution and governing locations change with the wind-to-gravity proportion, reinforcing the necessity of full model re-evaluation over simplified scaling methods. The study acknowledges limitations, including the small sample size (three models), the use of IS 800:1984, and the exclusion of factors such as drift, fatigue, and connection flexibility.