Correlated Geometric–Ponding Nonlinearity in Steel Space-Frame Roofs: A Review of Causes, Failures and Design Implications

Correlated GeometricPonding Nonlinearity in Steel Space-Frame Roofs: A Review of Causes, Failures and Design Implications

Hani Upadhyay

Civil Engineer

SJH Engineering, P.C., Princeton, USA

Abstract

Flat and low-slope long-span roofs are susceptible to significant strength deterioration when rainwater accumulation combines with geometric nonlinearity, especially under insufficient or obstructed drainage conditions. This study examines ponding mechanisms, real-world case studies, existing code provisions, and analytical methodologies. Comparative tables encapsulate significant failures and methodologies, whilst schematic representations depict ponding mechanisms and operations. The objective is to combine knowledge and emphasize design impli cations to assist engineers in averting rain-induced structural failures. The synthesis underscores the necessity for nonlinear, climate-adaptive design approaches and predictive maintenance strategies to ensure structural safety under evolving

precipitation patterns.

Keywords: Ponding, precipitation loads, roof drainage, spatial framework, geometric nonlinearity, structural failure assessment.

INTRODUCTION

Rain-induced ponding instability has resulted in numerous failures of large-span space-frame roofs globally [1], [2]. Investigations regularly identify insufficient drainage, low roof slopes, and obstructed siphonic systems as persistent factors in such failures. Although design regulations recognize rain and ponding loads, they seldom account for nonlinear structural interactions that precipitate abrupt failure [3]. The objectives of this review are:

(1) to summarize documented case studies and the lessons derived from them, (2) to evaluate the stipulations and constraints of existing codes and standards, (3) to assess analytical techniques for ponding instability, and (4) to identify pragmatic workflows and ramifications for engineering practice.

MECHANISMS OF PONDING IN ROOF STRUCTURES

Ponding arises from a positive feedback loop: as rainwater collects, roof deflection exacerbates local depressions, which subsequently hold increased volumes of water. The nonlinear loaddeflection interaction can swiftly lead to instability and collapse [4], [5]. Investigations of siphonic drainage systems have shown that partial obstruction can significantly diminish flow capacity even in advanced systems [11][14], [21]. These studies collectively emphasize that structural design alone is insufficient without redundant drainage planning and regular inspection. Refer to Figure 1.

Figure 1. Schematic of ponding mechanism in a roof system.

LITERATURE REVIEW AND CASE STUDIES

Understanding the phenomenon of rain-induced ponding in steel space-frame roofs has evolved through decades of empirical observation, simulation, and code-based refinement. Numerous researchers have analyzed real-world failures, laboratory tests, and analytical developments to identify the key parameters influencing structural instability. The following section summarizes major contributions to the field and their implications for design and maintenance.

SIPHONIC DRAINAGE PERFORMANCE STUDIES

Several foundational investigations by Arthur and associates focused on the dynamics of siphonic drainage systems. Their work, spanning from 2001 to 2007, was later integrated into the ASTM F2021-17 (2022) and ASPE/ANSI 45-2025 standards [11][14], [21]. These studies highlighted the efficiency of siphonic priming under ideal conditions while emphasizing that partial obstruction can significantly reduce discharge capacity. Their findings established that even advanced siphonic systems require redundancy, inspection, and fail-safe mechanisms to prevent catastrophic water accumulation on flat roofs.

EXPERIMENTAL AND NUMERICAL ANALYSES OF ROOF FAILURES

Türker and colleagues performed detailed numerical and forensic analyses following the collapse of a large terminal roof structure subjected to intense rainfall [16]. Their finite element (FE) simulations accurately reproduced the failure sequence, revealing how parapet confinement exacerbated ponding pressure and localized deformation. The study concluded that traditional elastic design methods underestimated instability risk by neglecting geometric nonlinearity and fluidstructure coupling (Figure 2).

Figure 2. Drainage blockage and parapet schematic.

ANALYTICAL METHOD DEVELOPMENT FOR SLOPED ROOFS

Burnham and Denavit expanded upon the first-order approximation framework to incorporate the effects of roof slope and varying rainfall intensity [15]. Their analytical model delineated threshold conditions under which the FOA remains valid and identified cases demanding full nonlinear simulation. This contribution bridged the gap between simplified design tools and advanced analysis, offering engineers practical methods for assessing long-span roof stability under variable slopes.

VALIDATION OF NONLINEAR PONDING FORMULATIONS

Denavit and Scott validated the nonlinear analysis procedures for ponding loads by developing benchmark problems for model verification [4]. Their research provided experimental and numerical evidence that nonlinear formulations can precisely capture the interdependence between water depth and roof deflection. This verification enhanced confidence in adopting nonlinear models for design and evaluation, especially in flexible roof systems.

SIMPLIFIED ANALYTICAL APPROACH: FIRST-ORDER APPROXIMATION

Denavit introduced the enhanced first-order approximation (FOA) as a computationally efficient tool for evaluating ponding effects in roof design [5]. The FOA method simplifies the complex nonlinear coupling between geometry and water load, making it practical for preliminary design. However, the study also cautioned that its accuracy diminishes for very low-slope or geometrically irregular roofs, necessitating validation against nonlinear results in critical cases.

FAILURE OF INDUSTRIAL STEEL TRUSS ROOFS

Tüfekci and associates investigated the collapse of a steel truss factory roof that failed under heavy rainfall and insufficient drainage [10]. Their FE simulations demonstrated that blocked or undersized drains significantly increased ponding head, leading to progressive deflection and eventual instability. The findings highlighted the inadequacy of elastic methods in predicting water-deflection feedback and reinforced the importance of drainage design and maintenance.

BUCKLING AND STRUCTURAL INSTABILITY STUDIES

Vatansever examined the buckling of truss members within a space truss roof under ponding loads [9]. The study revealed that slender compression bars in grid systems are particularly vulnerable to secondary geometric effects introduced by accumulated water. This work established a direct correlation between member slenderness ratios, ponding-induced deflection, and global stability loss, emphasizing the need for geometric optimization in design.

FORENSIC INVESTIGATION OF ROOF COLLAPSE IN TURKEY

Piroglu and associates conducted a detailed forensic investigation of a collapsed steel space truss roof in Turkey [8]. The investigation identified obstructed drainage outlets and low roof slope as primary causes of rapid water accumulation. Their analysis demonstrated that localized depressions on the roof surface acted as water traps, accelerating ponding formation. The findings concluded that ponding instability can develop within hours of heavy rainfall if maintenance and inspection procedures are neglected (Figure 3).

Figure 3. Example collapsed space-frame roof (adapted frm [8])

Table I. Documented Space-Frame Roof Failures due to Ponding

DISCUSSION

Collectively, these studies establish a comprehensive understanding of the interplay between water accumulation, roof flexibility, and drainage performance. The consistent pattern across global failures indicates that roof slope, drainage redundancy, and geometric stiffness are dominant predictors of ponding instability. Analytical models such as FOA provide valuable first-stage assessments, but the transition to nonlinear analysis is crucial for accurate and reliable results in modern long-span roofs. Integrating lessons from both experimental research and forensic case studies offers engineers a path toward safer, climate-resilient roof designs.

CODE PROVISIONS AND GAPS

• ASCE 7-22: Defines rain load as a function of ponding head but assumes static equilibrium; geometric stiffness degradation is neglected. It requires designers to check for stability under accumulated water, yet lacks a prescribed analytical method.

• AISC/SJI DG40: Introduces approximate formulas for amplified deflection but still assumes uniform geometry and constant rainfall rate. It offers design charts for safe roof bay dimensions but does not cover dynamic rainfall events.

• ASTM F2021-17 (2022): Focuses on hydraulic efficiency of siphonic drains. It provides equations for pipe sizing, but not structural consequences of partial blockage.

• ASPE/ANSI 45-2025: Incorporates siphonic design and maintenance guidelines but omits structural interaction under nonlinear ponding.

• Eurocode (implicit reference): While Eurocode EN 1991-1-3 covers snow and rain loads, it lacks direct ponding provisions.

Critical limitation: None of these codes integrates time-dependent rainfall intensity or coupled structural hydraulic effects. There is an urgent need to introduce nonlinear feedback coefficients and maintenance inspection clauses.

Table II. Comparison of Codes and Standards for Ponding

PONDING ANALYSIS METHODS

Analysis methodologies comprise: (i) First-order Elastic (basic, imprecise), (ii) Amplified First-order Analysis (effective, approximate), and (iii) Full Nonlinear Analysis (precise, computationally demanding). Refer to Table III.

Table III. Ponding Analysis Methods: Advantages and Limitations

PRACTICAL WORKFLOW FOR ENGINEERS

A proposed process includes: (1) defining geometry, (2) computing the hydrostatic water field, (3) conducting nonlinear analysis, (4) iterating until equilibrium or instability is achieved, and (5) evaluating stability and drainage sufficiency. Refer to Figure 4.

Figure 4. Flowchart of ponding analysis workflow.

• Geometry & Material Definition:

  Create an accurate 3D model capturing slope variations, gutter locations, and structural connectivity. Assign nonlinear material properties for steel (E = 200 GPa, yield 250 MPa).

• Hydrostatic Field Computation:

  Calculate water height (h) distribution using continuity equations or simplified parabolic approximations. Apply this as a surface load incrementally.

• FOA / Elastic Screening:

  Perform FOA analysis to determine whether the structure is sensitive to ponding (e.g., if amplified deflection

  > L/200, proceed to nonlinear stage).

• Nonlinear Coupled Analysis:

  Use incrementaliterative solvers (NewtonRaphson) to simulate loaddeflection until equilibrium or divergence. Observe the slope of the loaddeflection curve; negative slope indicates instability.

• Validation & Reporting:

Compare results with code-specified rain load thresholds and drainage design. Recommend reinforcement, additional drains, or slope adjustment if the computed ponding head exceeds allowable limits.

RESEARCH GAPS AND FUTURE DIRECTIONS

Despite major advancements, several gaps persist in the understanding and prevention of ponding-induced failures. A key limitation is the lack of large-scale experimental validation for fluidstructure interaction (FSI) models. Current simulations rely heavily on theoretical assumptions and small-scale prototypes, limiting real- world applicability [4], [5]. Moreover, many design codes still use rainfall intensitydurationfrequency (IDF) data based on outdated meteorological records [15], [16]. Simplified geometry and isotropic stiffness assumptions fail to capture the behavior of modern irregular roof systems [8], [9]. Long-term effects of corrosion, fatigue, and drain clogging are rarely modeled, even though they directly influence load paths.

Future studies should focus on:

• Developing AI-based predictive tools using sensor and weather data to forecast ponding risk in real time;

• Building open-access databases that compile roof geometries, rainfall events, and failure case historis for model validation;

• Introducing climate-resilience factors in design codes analogous to seismic importance factors; and

• Promoting international collaboration for full-scale roof testing and forensic analysis [15], [17], [22].

A transition toward resilience-based design frameworks, integrating sustainability, monitoring, and predictive analytics, is essential for preventing ponding-induced collapses in future structures.

CONCLUSIONS

Rain-induced ponding remains one of the most underestimated failure mechanisms in long-span roof structures. This review consolidates global evidence of how drainage inefficiencies, geometric nonlinearity, and insufficient design provisions can combine to trigger sudden and catastrophic collapse. Analytical advancements such as nonlinear finite-element modeling and improved first-order approximations have enhanced prediction accuracy, yet their adoption in routine engineering practice remains limited.

To advance structural resilience, engineers must integrate geometric nonlinearity, drainage performance, and climate variability into unified design frameworks. Routine inspection and predictive maintenance should be treated as integral components of roof safety rather than afterthoughts. Moving forward, the inclusion of real-time monitoring technologies and adaptive design strategies will enable early detection of instability and reduce the likelihood of failure.

Ultimately, the path toward safer and more sustainable roof systems lies in bridging the gap between theoretical modeling, code implementation, and on-site maintenance. A multidisciplinary approach combining structural analysis, environmental engineering, and data-driven monitoring will ensure that future roof designs remain robust under evolving climatic conditions.

Figure 5. Example loaddeflection curve showing ponding instability.

REFERENCES

1. ASCE, Guide to the Rain Load Provisions of ASCE 7-16, 2018. doi:10.1061/9780784415535

2. ASCE, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-22).

3. AISC/SJI, Design Guide 40: Rain Loads and Ponding, 2024.

4. M.D. Denavit, M.H. Scott, Strength and Reliability of Structural Steel Roofs Subjected to Ponding Loads, J. Struct. Eng., vol. 147, no. 2, 04020318, 2021. doi:10.1061/(ASCE)ST.1943-541X.0002882.

5. M.D. Denavit, Approximate ponding analysis by amplified first-order analysis, Eng. Struct., vol. 197, 109428, 2019. doi:10.1016/j.engstruct.2019.109428.

1. F. Piroglu, et al., Site investigation of damages occurred in a steel space truss roof structure due to ponding, Eng. Fail. Anal., vol. 36,

   pp. 301313, 2014. doi:10.1016/j.engfailanal.2013.10.018.

2. C. Vatansever, Investigation of buckled truss bars of a space truss roof system, Eng. Fail. Anal., vol. 106, 104156, 2019. doi:10.1016/j.engfailanal.2019.104156.

3. M. Tüfekci, et al., Numerical Investigation of the Collapse of a Steel Truss Roof, Appl. Sci., vol. 10, 7769, 2020. doi:10.3390/app10217769.

4. S. Arthur, J.A. Swaffield, Siphonic roof drainage system analysis utilising unsteady flow theory, Build. Environ., vol. 36, no. 8, pp. 939948, 2001. doi:10.1016/S0360-1323(00)00049-4.

5. S. Arthur, et al., Operational performance of siphonic roof drainage systems, Build. Environ., vol. 40, no. 6, pp. 788796, 2005. doi:10.1016/j.buildenv.2004.08.015.

6. S. Arthur, G. Wright, Siphonic roof drainage systemspriming focused design, Build. Environ., vol. 42, no. 6, pp. 24212431, 2007. doi:10.1016/j.buildenv.2006.08.021.

7. ASTM, F2021-17(2022), Standard Guide for Design and Installation of Plastic Siphonic Roof Drain Systems.

8. A. J. Burnham and M. D. Denavit, Approximate Ponding Analysis for Sloped Roofs, ASCE J. Struct. Eng., Part C, vol. 30, no. 4, 2025. doi:10.1061/JSDCCC.SCENG-1809.

9. H. T. Türker, M. Tüfekci, and E. Tüfekci, Collapse of a Steel Space Truss Roof due to Rainwater and Ponding Effects, ASCE J. Performance of Constructed Facilities, 2025. doi:10.1061/JPCFEV.CFENG-5027.

10. AISC/SJI, Design Guide 40: Rain Loads and Ponding, American Institute of Steel Construction & Steel Joist Institute, 2024.

11. Steel Joist Institute, Roof Bay Analysis Tool with Ponding Analysis, v5.x, 20232025.

12. ASCE, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 722), 2022.

13. S. K. Ghosh Associates Inc., Significant Changes in ASCE 722, presentation, 2024.

14. ASPE/ANSI 452025, Siphonic Roof Drainage Systems Design Standard, 2025.

15. Pondering Ponding, Modern Steel Construction, July 2024.