Rain Loads Research Articles

A brief compendium of water entry results derived from laboratory tests of various types of wall assemblies

There is an increase in the use of hygrothermal models to complete the performance evaluation of walls assemblies, either in respect to design of new assembles or the retrofit of existing wall assemblies. To this end there are guides available in which is provided information on moisture loads to wall assemblies. This includes, for example, Criteria for Moisture-Control Design Analysis in Buildings given in ASHRAE 160, Assessment of moisture transfer by numerical simulation provided in EN 15026, and NRC’s “Guidelines for Design for Durability of the Building Envelope”. The designer of a new assembly or evaluator of an existing wall is tasked with having to determine what moisture loads to apply to the wall and where to apply this load within the assembly. Typically there is little or no information that is readily available regarding moisture loads to walls and thus the suggested hourly moisture load, as given in ASHRAE 160, is 1% by weight of the total driving rain load to the wall (i.e. kg/m2-hr). In this paper, a brief compendium of water entry test results derived from laboratory tests of various types of wall assemblies is provided from which estimates of moisture loads to different types of wall can be developed. Water entry test results are given of wood frame walls typically used in housing, but also metal-glass curtain walls and other commercial wall assemblies, where possible, in terms of driving loads to the wall.

In-Situ Moisture Assessment in External Walls of Historic Building using Non-Destructive Methods

Abstract In-situ measurements of a case study building located in Riga old town near the river Daugava has been carried out in this work. Performed measurements are moisture level of historic masonry, and interstitial monitoring of temperature and relative humidity between the layers of internal insulation and external wall. Obtained results are compared with outdoor weather data. Results show that during the cold months of the year no rising damp problem. No interstitial condensation under the internal insulation, and no risk of mold growth occur. However, the façade of the building tends to be highly influenced by the outdoor weather, and the moisture of the masonry increases during the rain load events.

Relation between degradation of polymeric components in crystalline silicon PV module and climatic conditions: A literature review

During their outdoor service, photovoltaic (PV) modules are exposed to different set of external stresses that can affect their efficiency and lifetime such as UV irradiation, temperature and humidity cycles, rain, snow and wind loads, hail, sand and/or salt. Moreover, internal stresses such as choice of materials and design (additives, morphology and compatibility of materials) can drive degradation of PV modules as well. The failure data collection started already in 1980s and is still ongoing. Most of these studies are focused mainly on power data or categorizing PV failure modes concerning solely climate, EVA or cell-technology. However, relationship between climate, degradation of polymeric materials and degradation of PV modules has not been reported so far. Hence, within this study a lot of effort was put into finding the appropriate data and publications with as many as possible data on PV, climate and materials degradation. As a result, a great overview of climate and polymeric materials induced PV failure modes is given. Based on the reported data, the topic of new, climate-based design of PV modules is discussed. This paper provides numerous number of selected references that can be beneficial in further studies on degradation of PV modules, design of PV modules and choice of materials.

Failure analysis of a lattice transmission tower collapse due to the super typhoon Rammasun in July 2014 in Hainan Province, China

The failure mechanisms of lattice transmission towers under the extreme wind conditions of typhoons are not well known and such knowledge is important for the improvement of overhead transmission line (OHL) design and to ensure the safe operation of transmission lines in typhoon-prone areas. This paper presents a case study on failure analysis of 13 collapsed lattice transmission towers in a110 kV transmission line during the super typhoon Rammasun that occurred in July 2014 in Hainan Province of China. A systematic computational procedure was developed to conduct the failure analysis with particular focus on modeling the fluctuating turbulent wind loads combined with the heavy rain loads. The results show that the failure mode and location obtained from the dynamic analysis are in good agreement with the post-event field observations. The collapse of the suspension lattice transmission towers was triggered by the buckling of the main leg members in the third panel from the ground level due to the extreme high axial stresses. Key design suggestions like improving the design wind speed and adopting an equal-strength beam approach are proposed to reduce the risk of lattice transmission tower failure and ensure continued serviceability and safe operations under the typhoon-wind conditions.

Effect of Raindrop Size Distribution on Rain Load and Its Mechanism in Analysis of Transmission Towers

Rain load acting on a transmission tower is large enough to receive attention according to previous studies and, therefore, a parametric analysis is necessary to understand its mechanism. In this paper, the effects of different raindrop size distributions on both rain pressure and tower response are studied. First, a theoretical method is proposed to estimate the shape parameters of the gamma raindrop size distribution based on the conservation of rain intensity. The influence of raindrop spectrum on rain pressure in free wind field is then investigated. The results reveal that raindrop spectrum has great effect on the rain pressure distribution and its time interval has a large impact on the total rain pressure. In addition, the time interval has greater influence than the spectrum when the rain intensity is constant. At last, six different raindrop spectra are employed to simulate the tower response induced by wind and rain loads, of which the results indicate that the raindrop spectrum has a significant effect on the tower response. The maximum increasing percentage of rain load relative to wind load can reach up to 13.7%, indicating that the influence of rain load is quite remarkable and should be considered in analyzing the response of transmission towers.

Performance of hydrophobized historic solid masonry – Experimental approach

The hygrothermal conditions in historic solid masonry are expected to change for the worse, with the application of internal insulation. Nevertheless, internal insulation plays a role in a holistic energy retrofit of historic buildings. With careful considerations and correct application, hydrophobic treatment may help remedy moisture ingress from external rain loads. This study includes experimental investigations of the effect on hygrothermal performance of various hydrophobization treatments on both brick and air lime mortar. An investigation of water migration through masonry applied with imitated climatic loads is also reported. The study showed a larger efficiency of hydrophobization on specimens of brick compared to the efficiency of hydrophobization of specimens of air lime mortar, which may be problematic in cases where mortar joints are the primary means for water ingress. Silane-based treatments generally proved to be most efficient in brick, whereas a variety of other active components were most successful in air lime mortar treatment. The investigation of water migration showed a distinct effect of silane, cream hydrophobization, though most evident in the external part of the brick.

Wind Load Characteristics and Action Mechanism on Internal and External Surfaces of Super-Large Cooling Towers under Wind-Rain Combined Effects

By focusing on wind-rain two-way coupling algorithm, simulation iterations of wind field and raindrops in the world highest cooling tower (210m) in northwest China were carried out using continuous phase and discrete phase models based on CFD numerical simulation. Firstly, influence laws of 9 wind velocity-rainfall intensity combinations on wind-induced rainfall, raindrop additional force, and equivalent pressure coefficient on internal and external surface of the tower body were discussed. On this basis, speed flow line, turbulence energy strength, raindrop running speed, and track on the tower body in the wind-rain coupling field were disclosed. Finally, qualitative and quantitative contrastive analyses on wind pressure, rain pressure, and equivalent pressure coefficient on internal and external surfaces of the tower body were conducted under different working conditions. Thus, the most unfavorable wind-rain combination was identified. Calculation formulas of equivalent internal and external pressure coefficients of super-large cooling towers were fitted from nonlinear least square method. Research results demonstrate that the 3D effect of equivalent internal and external pressure coefficients with considerations to wind-rain two-way coupling is more prominent. Particularly, there is strong transition on the windward region of the external surface and leeside region at bottom of internal surface. The quantity of caught raindrops on the structural surface is negatively related to wind velocity but is positively related to rainfall intensity. Rain load and rainfall coefficients on the external surface are significantly higher than those on the internal surface. Equivalent internal pressure coefficient has a sharp reduction on the leeside region under different working conditions. Besides, equivalent internal pressure coefficient of different meridians decreases with the increase of height. The maximum and minimum are -0.574 and -0.282, respectively. The proposed equivalent internal and external pressure coefficients of super-large cooling tower can predict wind load under extreme climate conditions accurately.

Velocity Ratio of Wind-Driven Rain and Its Application on a Transmission Tower Subjected to Wind and Rain Loads

AbstractWind-driven rain (WDR) refers to raindrops that have a horizontal velocity in the wind field. The WDR intensity on building envelopes as well as other issues such as wind–rain-induced vibra...

Theoretical and testing investigation of wind–rain coupling loads on some typical bluff bodies

The article presents a mathematical theoretical framework and fitting parameters with aspects of joint probability distribution of wind and rain, separate wind and rain action, and coupled wind and rain effects on steady and unsteady forces acting on some typical bluff bodies. Gumbel and copula functions were first selected to describe the joint probability distribution of wind speed and rain intensity. Then, two models, a raindrop impact model and an equivalent air density model, were adopted to quantify the loading action considering only separate wind and rain action, and simplified coupled effects with superimposition of wind and rain show that it would be accurate enough to neglect separated rain influence in steady wind and rain loading conditions. Furthermore, wind tunnel testing has been carried out under coupled wind and rain conditions with the help of a high-precision raining simulation system in TJ-1 wind tunnel on various reduced-scale models with some typical cross sections, such as circular and rectangular, thin plate, and streamlined box, and their aerodynamic loading and wind–rain-induced performance have been systematically compared. It has thus been found that the coupling effects of wind and rain should not be neglected in steady and unsteady force models.

Modelling of turbulent dispersion for numerical simulation of wind-driven rain on bridges

Wind-driven rain (WDR) is responsible for many potential negative effects on bridges, such as structural cracking, aggregate erosion, steel corrosion and storm water management problems and so on. Hence, accurate evaluations of the WDR effects on bridges are essential to provide solutions for preventing material degradation and improving durability capability of bridges. However, in most previous WDR numerical studies, the turbulent dispersion of raindrops was neglected. In this paper, the turbulent dispersion is integrated into Eulerian multiphase model to investigate the WDR effects on a bridge with rectangular cross-section. Especially, the influences of the turbulent dispersion are discussed in detail by comparing the WDR simulation results for the cases with and without consideration of the turbulent dispersion in terms of WDR flow fields, volume fraction, specific catch ratio, catch ratio, rain loads and aerostatic force coefficients. The results indicate that the turbulent dispersion for a certain range of raindrop size is needed to be taken into account for obtaining accurate WDR simulation results for bridges.

Large Swing Behavior of Overhead Transmission Lines under Rain-Load Conditions

In recent years, flashover accidents caused by large swings of overhead conductors that frequently occurred under rain-wind condition, greatly jeopardized the normal operation of power transmission systems. However, the large swing mechanism of overhead conductor under the simultaneous occurrence of rain and wind is not clear yet. Thus, a unified model is proposed with derived stability criterion to analyze the large swing of the overhead conductor. The analytical model is solved by finite element method with the aerodynamic coefficients obtained from simulated rain-wind tests, taking into account the effect of wind velocities, upper rivulet motion, rainfall rates, and rain loads on the large swings of overhead transmission lines. The results show that the proposed model can capture main features of the large swing of overhead conductor, this swing being probably due to the upper rivulet’s motion, by which negative aerodynamic damping occurs at a certain range of wind velocity (10 m/s). Furthermore, the peak swing amplitude of the overhead conductors under rain-wind condition is larger than that under wind only, and the rain loads cannot be neglected.

Nonlinear vibrations of CFRP cables excited by periodic motions of supports in cable-stayed bridges

Owing to its excellent non-corrosiveness and fatigue resistance, a carbon fiber-reinforced polymer (CFRP) stay cable is an ideal alternative to overcome corrosion and fatigue problems associated with the traditional steel cable. However, stay cables are prone to various oscillations under wind, rain, and traffic loading. The vibrations of CFRP stay cables excited by periodic motions of the girder and/or pylons were studied and compared with those of steel cables. A nonlinear dynamic model for in-plane and out-of-plane vibrations of stay cables was proposed. Particularly, the geometrical nonlinearity of the cables was considered in this model. On the basis of this model, numerical solutions were obtained for CFRP cables and steel cables with the same conditions. Furthermore, the effects of important parameters on vibrations were discussed. These parameters included cable tensions, excitation amplitudes, and damping ratios. Results demonstrate that small excitation amplitudes may lead to forced vibrations or parametric vibrations with substantial amplitudes when natural frequencies of the cables are approximately half or one time of excitation frequencies. The maximal vibration responses of CFRP cables are weaker than those of steel cables when their lengths are substantial. As static tensions of the cables decrease, the “beating” frequencies and the maximal amplitudes of the vibrations increase.

Experimental assessment of rainwater management of a ventilated façade

Façades are not watertight systems in which the exterior surface of the cladding plays the role of the water shedding surface; the air space is the drainage plane; the exterior surface of the thermal insulation layer acts as water-resistive barrier, and the interior layer of the inner leaf is supposed to be the air barrier. The degree of watertightness of ventilated façades relies on the characteristics of the materials used, the geometry of the external cladding element, and edge profile of the joints. Hence, in this article, we extrapolate the laboratory testing to develop a holistic approach on the water management of ventilated façades. The laboratory test showed that 50% of the water deposited to the surface of the wall splashes back, 22% creates a runoff film along the exterior surface of the cladding, 27% infiltrates into the cavity behind the cladding, and <1% reaches the inner layer of the wall. Given these results, the moisture load has been determined for a range of wind-driven rain loads and vertical joint configurations. Such information can be used as input to hygrothermal simulations.

積雪後の降雨に配慮した屋根雪荷重の評価に関する研究 －その６ 小型模型実験による軒どい排水性能と積雪後の雨荷重との関係について－

積雪後の降雨に配慮した屋根雪荷重の評価に関する研究 －その６ 小型模型実験による軒どい排水性能と積雪後の雨荷重との関係について－

Wind-driven rain on a building façade in urban environment

The amount of rain water which impinged on the building envelope has a significant effect on the building envelope’s thermal performance, the indoor air humidity and the users’ health and comfort. As an important exterior boundary condition, wind-driven rain (WDR) loads estimation methodology could also be used in building energy simulation (BES) and heat and moisture transport (HAM) tools. As available WDR calculation models either ignore or simplify the influence of the complicate urban environment, their results have great discrepancy with the test results on a residential building facade which is even only moderate sheltered. In this paper, systematically models are built to study the shelter effect by surrounding buildings on WDR,. The building density of the street canyon and the buildings’ arrangement style of the street canyon are two parameters that had been taken into consideration. Simulation cases with three packing densities and two typical buildings’ arrangements are displayed. Predominant influencing factors are pointed out.

Analysis of time-resolved wind-driven rain on an array of low-rise cubic buildings using large eddy simulation and an Eulerian multiphase model

Time-resolved wind-driven rain (WDR) load is investigated on an array of low-rise cubic buildings using an Eulerian multiphase (EM) model together with large eddy simulation (LES). The influence of wind-flow unsteadiness on the unsteady behavior of raindrops and the WDR intensity is discussed in detail. The wind-flow field predicted with LES has been validated with wind-tunnel measurements. The mean WDR intensity values obtained using the EM model are found to be in agreement with in-situ WDR measurements. The time-resolved simulations show that the instantaneous specific catch ratio values of smaller droplets fluctuate a lot around their mean values due to higher influence of turbulence. Instantaneous specific catch ratios for the smallest raindrops are mainly dictated by local turbulent structures present in the shear layer or below the rooftop level. On the other hand, the motion of larger raindrops is mainly influenced by the larger-scale motions above the cubes. It is also shown that, running means of the specific catch ratio over a time window of 100–300 s of physical time stabilize to a constant value.

Robust moisture reference year methodology for hygrothermal simulations

Hygrothermal models allow designers to evaluate the hygrothermal performance of building envelopes. A key question in hygrothermal modeling however remains in the proper selection of representative exterior moisture conditions, i.e. a moisture reference year. A Climatic Index combining wind-driven rain load and potential evaporation is developed allowing considering the effect of climatic variability on the performance of the building envelope. The proposed Climatic Index combines thus both the wetting load and evaporation potential. This index is suitable for the evaluation of the level of moisture damage risk of wall assemblies where typical moisture problems are mainly caused by rainwater uptake or ingress.The Climatic Index is determined for four cities in Switzerland. The hygrothermal performance of the assemblies is simulated and evaluated using a hygrothermal indicator, called the RHT Index. A clear correlation between Climatic Index and RHT Index is found. However, the moisture reference year selected based solely on a 10% level criterion of the Climatic Index may not correspond to the year with the most significant moisture damage risk. Therefore, a new procedure is proposed which combines a first selection of three years around the 10% level criterion, followed by a careful comparison of these years based on hygrothermal simulations and selection of the year with the largest RHT Index as moisture reference year. The combination of Climatic Index and RHT Index from hygrothermal simulations allows for the selection of moisture reference years that are around the 10% level, but excluding the ambiguity.

Dynamic analysis of transmission tower-line system subjected to wind and rain loads

In order to study the dynamic response of transmission tower-line system induced by wind and rain loads, the calculating method of wind and rain loads acting on the transmission conductor is presented. The wind tunnel test, conducted by Kikuchi et al. (2003), is introduced to determine the drag coefficient under both wind and rain excitations, which is a key parameter to estimate the wind and rain loads for the transmission conductor. Then the three-span conductor and tower-line system are modeled to perform the dynamic analysis subjected to wind and rain loads, of which the results demonstrate that the maximum percentage of average displacement induced by the rain loads relative to the wind loads can reach to 22.00%, indicating strongly that the effect of rain load on the response of tower-line system is not negligible. Meanwhile the influence of wind attack angle on the response of tower-line system induced by wind and rain loads is investigated, and the results show that the most unfavorable wind attack angle is 90° and the rain load has no effect on the most unfavorable wind attack angle. The conductors made up of trapezoidal wires are used to examine the effects of wind and rain loads, and therefore some of the models and findings of the present contribution may not always be applicable. All in all, it can be stated that the rain load has an important effect on the response of tower-line system and should be given more attention.

Hygrothermal performance of cross-laminated timber wall assemblies: A stochastic approach

Cross-laminated timber (CLT) panels have the potential market in North America for building mid-rise or even taller structures due to their good structural and fire safety performance, carbon storage capacity, light weight, and prefabricated nature. However, prolonged exposure to moisture during construction and in service is a durability concern for most wood products including CLT. This paper presents the evaluation of the hygrothermal performance of CLT wall assemblies through simulations using a stochastic approach to account for the uncertainty of material properties, boundary conditions, and environmental loads. The moisture content of the CLT panel is used as the performance indicator. The influential factors are categorized into continuous and discrete random variables. The influence of the continuous random variables including material properties, boundary conditions and cladding ventilation rate is investigated under conditions represented by different combinations of the discrete random variables including wall configurations, orientation and rain leakage rate. The critical factors that lead to high moisture content are identified under each condition and the interaction between the discrete and continuous random variables is analyzed. The stochastic results show that the CLT wall assembly with low vapor permeance material placed outboard the CLT panel has a higher risk of moisture problem than CLT wall assembly with high vapor permeance material placed outboard the CLT panel. The sensitivity analysis indicates that the significance of the stochastic parameters on the hygrothermal performance of CLT panel depends on environmental loads, especially the rain load, and wall configurations.

The Spell Definition in ISO-15927 and its Impact on the Rain Deposition on the Building Facade

Wind-driven rain (WDR) is one of the most important causes for water damage in buildings. Therefore, the first crucial step to assess the hygrothermal performance of the building envelope, is the appropriate estimation of the amount of rainwater striking the building's façade. ISO 15927 offers the annual average index –mainly to assess the moisture content of absorbent surfaces, and the spell index – more related to the likelihood of water penetration through joints. To calculate these indices, assumptions are made concerning the length of the period of ‘no rain’, called ‘spell definition’. Obviously, the choice of this spell definition will characterise the WDR-amount. In this paper, WDR measurements of a 3-storey building in Vancouver, Canada are used to investigate how this spell definition affects the rain load. Different filter criteria are used to exclude errors due to measurement equipment. By means of the catch ratio as a dimensionless parameter, the results of the analysis for different spell definitions are compared to hourly and 5-min data. It is concluded that longer spell definitions result in lower catch ratios and an underestimation of the WDR load. Hourly data turns out to be a more conservative approach for WDR-assessment of this case study, but is able to represent the spread on the catch ratio most closely to the original 5min-dataset.