Forces on buildings with openings and orientation in a steady post-tsunami free-surface flow

Steady free-surface flows around buildings occurring during flood or tsunami events can produce major damages and a quantification of the post-peak forces is essential for safety and resilience of coastal structures. The loading process is highly affected by the flow Froude number and the drag coefficient, commonly defined for highly subcritical flows, while field observations of tsunami flows reported Froude numbers close to one, visually appearing as a choked regime. The present experimental study addresses the hydrodynamic forces on emerging buildings in a subcritical choked regime, focusing on the effect of openings and orientation. Laboratory experiments indicated a substantial difference in flow depths between the up- and downstream side of the building for increasing Froude numbers. The presence of openings induced a flow through the building lowering the difference in flow depth, limiting the effect of the hydrostatic component of the loading process. The formulation of an empirical resistance coefficient CR allowed a combination of both form drag and hydrostatic forces. Whilst for impervious buildings CR was consistent with reference studies, for buildings with openings CR was directly dependent upon porosity. Contrarily to the unsteady flow conditions, results showed that the sidewalls also played a role in the loading process. Buildings with a rotated orientation resulted in slightly larger surfaces exposed to the flow and larger horizontal forces. Nevertheless, these were applied at lower cantilever arms, thus reducing the tilting moment. Altogether, this study provides experimentally-derived parameters that will support hydraulic engineers in the design of coastal structures.

Extreme Hydrodynamic impact onto buildings

Davide Wüthrich

Dam failures, impulse waves, tsunamis and storm surges are disasters that challenge humanity, often leading to massive casualties and important economic losses. These events generate highly unsteady flow conditions in the form of surges or turbulent bores, associated with extreme loading on infrastructure. In the past, the impact on these flows on buildings was considered too rare and wave induced forces neglected in the design process. However, recent events showed that some measures can guarantee human safety and reduce reconstruction costs. As such, vertical shelters able to withstand extreme loading are fundamental and an estimation of induced forces necessary. Field studies showed that buildings with openings perform better during wave impact and the objective of this research is to evaluate and analyse damage potential of resilient buildings hit by both surges and bores. In particular, the influence of openings as a mitigation measure on the loading process is addressed and quantified. The research is based on an experimental approach. Wave formation is achieved through the vertical release of a water volume from an upper reservoir, generating dry bed surges and wet bed bores in the downstream horizontal channel. A variation of the released discharge results into waves with different hydrodynamic properties. A detailed methodology to hydraulically characterize the generated waves in terms of their water depths and flow velocities is presented and good agreement with the classical dam-break case for both dry surges and wet bed bores is demonstrated. Particular attention is given to wave front celerity and velocity profiles measured behind the wave front, showing some features typical of open channel flows. Buildings were reproduced using aluminium structures installed on a force plate, providing a detailed time history of impact forces and moments. The impact was characterised by high splashes, followed by a quasi-steady flow around the building. For the impervious structures, dry bed surges resulted into horizontal forces proportional to the momentum flux. For wet bed bores, an attenuation of the peak force was constantly observed and the introduction of a reduction coefficient was necessary to achieve a realistic force estimation. In this study the effect of openings and overflow were tested. Four building configurations with seven opening values ranging from 0 to 84% were investigated using 12 standard waves. The flow through the structure reduced the upstream water depths, providing safer vertical shelters. In terms of loading, the openings produced a linear reduction of the maximum horizontal force, if compared to the impervious case. The configuration with an impervious back wall showed similar results to the impervious building. Openings on the side walls had no influence on the resulting maximum load. Analysis in terms of peak time, wave height at maximum force and impulse pointed out some key differences between forces exerted by surges and bores. The time occurrence of the maximum tilting moment corresponded to the maximum horizontal force and the resulting cantilever arm was constant for all opening configurations. In case of overflow, milder loading conditions were constantly measured. Finally, new formulae to estimate the hydrodynamic load are introduced, taking into account the effect of openings within an adapted resistance coefficient, providing relevant information for the design of safer infrastructures.

EPFL2018

Experimental study on forces exerted on buildings with openings due to extreme hydrodynamic events

Ioan Nistor, Michael Pfister, Anton Schleiss, Davide Wüthrich

Previous studies and field surveys showed that specific structural designs can decrease the load on free-standing buildings along the coast, providing safer vertical shelters. This experimental study investigated the effect of openings in buildings (windows, doors and foyers) on horizontal forces and tilting moments induced by both dry bed surges and wet bed bores. Four configurations with seven porosity values ranging from 0% (impervious) to 84% (highly permeable) were systematically tested. Due to the presence of openings, the flow through the building reduced the upstream water depths. The porosity resulting from the presence of openings was shown to produce a linear reduction of the maximum horizontal force, when compared to the corresponding impervious building. The configuration with an impervious back showed results similar to those measured for the fully impervious buildings. The occurrence of the maximum tilting moment was shown to coincide with the maximum horizontal force and an estimation of the cantilever arm was therefore possible. The latter was constant for all configurations, independent of the geometry of the openings. Finally, two equations to predict the maximum horizontal force and the tilting moment were proposed, taking into account the effect of building openings within the resistance coefficient. These showed good agreement with experimental data and previous studies. These findings provide engineers with practical information for the design of safer vertical shelters in tsunamiprone areas.

2018

Experimental study on the hydrodynamic impact of tsunami-like waves against impervious free-standing buildings

Ioan Nistor, Michael Pfister, Anton Schleiss, Davide Wüthrich

Tsunamis, landslide-generated waves, and dam failures are rare, but highly destructive phenomena, associated with extreme loading on infrastructure. Recent events showed that specific measures must be taken to guarantee safety of both people and the built environment. This experimental study investigates the forces and moments exerted on free-standing buildings that are induced by both surges and bores. The hydrodynamic impact was characterized by high splash, subsequently followed by a quasi-steady flow around the structure. For dry bed surges, the time history of the horizontal force was proportional to the momentum flux per unit width. For wet bed bores, an attenuation of the peak force due to the presence of an aerated front was observed and the introduction of a reduction coefficient was necessary to achieve a realistic force estimation. Additional force analysis in terms of peak time, wave height at maximum force and impulse also pointed out some key differences between forces exerted by dry bed surges and wet bed bores. The occurrence of the maximum tilting moment on the building coincided with the maximum horizontal force and an evaluation of the cantilever arm was possible. These findings provide engineers with practical information for the design of safer coastal structures.

2018

Extreme Hydrodynamic impact onto buildings

Davide Wüthrich

EPFL2018