Seismic Damage Observations of Precast Hollow-Core Floors From Two Full-Scale Super-Assembly Tests.

Serious concerns about the life safety risk of hollow-core floors during earthquakes were raised following the collapse of hollow-core units during the 1994 Northridge earthquake and in subsequent laboratory tests. To enhance the understanding of the seismic performance of existing hollow-core floors, a substantial experimental programme of two large-scale super-assembly tests with hollow-core floors was carried out. Each test specimen consisted of a two-bay by one-bay concrete frame with full-scale hollow-core floors, which were constructed using typical 1980s floor detailing. The specimens were loaded with a simulated earthquake record applied quasi-statically. This paper discusses the progression of hollow-core floor damage observed in both super-assembly experiments. The main findings include the early onset of cracks in the unreinforced webs of the hollow-core units at 0.5% interstorey drift. The tests also demonstrated the detrimental effect of web cracking on the gravity load-carrying capacity of hollowcore floors. Additionally, hollow-core units that are seated at intermediate columns (so-called ‘beta units’) were found to get damaged more heavily than those supported away from the columns. Moreover, several transverse cracks were observed in the floor soffit away from the support and beyond the provided seating retrofits. Lastly, the extent of floor damage was found to be sensitive to the ground motion, with pulse-type motions (pushing the structure in one direction) tending to cause more severe floor damage than far-field motions with multiple cycles. The paper also outlines key challenges and recommendations for web crack inspections

Load-Path and Stiffness Degradation of Floor Diaphragms in Reinforced Concrete Buildings Subjected to Lateral Loading - Part I, Experimental Observations.

An experimental investigation into the degradation of load-paths in damaged diaphragms was conducted to provide answers to the New Zealand structural engineering community following concerns that strut-and-tie load-paths could not cross wide cracks that develop around the floor perimeter during earthquake loading demands. A full-scale super-assembly concrete moment frame specimen with a hollow-core flooring system installed was subjected to realistic drift deformations to induce damage in the floor, followed by in-plane shear deformation demands to assess the ability of the diaphragm to transfer load between frames at different floor damage levels. It was found that compression struts could form across much wider cracks in floors than previously anticipated. This was due to contact compressive stresses forming via loose aggregate that lodged within rugged sinusoidal wide floor cracks. Additionally, it was found that diaphragm compression struts can only transfer to the primary lateral load resisting frame through beam plastic hinges acting in minor axis shear following gaps opening between the floor and columns at moderate drift demands. Smooth floor to column interfaces did not provide the same residual rubble aggregate binding compressive load path observed in cracks within the floor. The primary driver of diaphragm shear stiffness degradation was found to be torsional softening of the perimeter beams of the floor. This was caused by simultaneous bi-directional demands applied to longitudinal beam bars and a phenomenon known as the bowstring effect applying large torsional demands through the beam-floor continuity reinforcement. The diaphragm strength and rate of shear stiffness degradation was found to be highly reliant on earthquake directionality. A set of generalised equations was developed to describe the rate of diaphragm shear stiffness degradation with respect to magnitude and directionality of drift demands. Part I of II in this journal series details the full-scale super-assembly experiment conducted on a floor diaphragm at different damage states and the observed behaviour during testing

Predicting ozone fluxes, impacts and critical levels on European forests (PRO3FILE)

The impact of tropospheric ozone pollution on European forests is a key topic of concern and discussion between the public, stakeholders, and scientists. Despite increasing knowledge on the effects of ozone on plant physiological functions, its impacts at a higher organization level, i.e., on individual tree diameter increment and forest growth are highly uncertain and vary among studies. The contrasting dose-response relationships reported may arise from the different data used as input in terms of sample size and characteristics, and/or from differing methodological choices. The proposed study aims to make use of over 200 UNECE/ICP Forests long-term monitoring plots across Europe where ozone concentrations have been measured since 2000, in parallel to forest and vegetation variables. Ozone related effects and critical levels on selected endpoints such as tree growth will be derived by quantifying ozone fluxes and applying multiple and various statistical techniques that consider for confounding abiotic and biotic environmental factors (see Cailleret et al. 2018). Data sources from various networks (ICP Forests, EMEP, ECMWF, Swiss Long-term Forest Ecosystem Research LWF, Swiss NFI) will be combined for calibration and validation purposes. The output will be an important contribution to the objectives of the UNECE Working Group on Effects acting under the umbrella of The 1999 Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone (UNECE 2015) of The Convention on Long-range Transboundary Air Pollution (CLRTAP). The project has been funded by the Swiss Federal Office for the Environment FOE

Evaluation of simulated ozone effects in forest ecosystems against biomass damage estimates from fumigation experiments

Regional estimates of the effects of ozone pollution on forest growth depend on the availability of reliable injury functions that estimate a representative ecosystem response to ozone exposure. A number of such injury functions for forest tree species and forest functional types have recently been published and subsequently applied in terrestrial biosphere models to estimate regional or global effects of ozone on forest tree productivity and carbon storage in the living plant biomass. The resulting impacts estimated by these biosphere models show large uncertainty in the magnitude of ozone effects predicted. To understand the role that these injury functions play in determining the variability in estimated ozone impacts, we use the O-CN biosphere model to provide a standardised modelling framework. We test four published injury functions describing the leaf-level, photosynthetic response to ozone exposure (targeting the maximum carboxylation capacity of Rubisco (Vcmax) or net photosynthesis) in terms of their simulated whole-tree biomass responses against data from 23 ozone filtration/fumigation experiments conducted with young trees from European tree species at sites across Europe with a range of climatic conditions. Our results show that none of these previously published injury functions lead to simulated whole-tree biomass reductions in agreement with the observed dose-response relationships derived from these field experiments and instead lead to significant over- or underestimations of the ozone effect. By re-parameterising these photosynthetically based injury functions, we develop linear, plant-functional-typespecific dose-response relationships, which provide accurate simulations of the observed whole-tree biomass response across these 23 experiments

DO₃SE modelling of soil moisture to determine ozone flux to forest trees

The DO₃SE (Deposition of O₃ for Stomatal Exchange) model is an established tool for estimating ozone(O₃) deposition, stomatal flux and impacts to a variety of vegetation types across Europe. It has been embedded within the EMEP (European Monitoring and Evaluation Programme) photochemical model to provide a policy tool capable of relating the flux-based risk of vegetation damage to O₃ precursor emission scenarios for use in policy formulation. A key limitation of regional flux-based risk assessments has been the assumption that soil water deficits are not limiting O₃ flux due to the unavailability of evaluated methods for modelling soil water deficits and their influence on stomatal conductance (gsto), and subsequent O₃ flux. This paper describes the development and evaluation of a method to estimate soil moisture status and its influence on gsto for a variety of forest tree species. This DO₃SE soil moisture module uses the Penman-Monteith energy balance method to drive water cycling through the soil-plantatmosphere system and empirical data describing gsto relationships with pre-dawn leaf water status to estimate the biological control of transpiration. We trial four different methods to estimate this biological control of the transpiration stream, which vary from simple methods that relate soil water content or potential directly to gsto, to more complex methods that incorporate hydraulic resistance and plant capacitance that control water flow through the plant system. These methods are evaluated against field data describing a variety of soil water variables, gsto and transpiration data for Norway spruce (Picea abies), Scots pine (Pinus sylvestris), birch (Betula pendula), aspen (Populus tremuloides), beech (Fagus sylvatica) and holm oak (Quercus ilex) collected from ten sites across Europe and North America. Modelled estimates of these variables show consistency with observed data when applying the simple empirical methods, with the timing and magnitude of soil drying events being captured well across all sites and reductions in transpiration with the onset of drought being predicted with reasonable accuracy. The more complex methods, which incorporate hydraulic resistance and plant capacitance, perform less well, with predicted drying cycles consistently underestimating the rate and magnitude of water loss from the soil. A sensitivity analysis showed that model performance was strongly dependent upon the local parameterisation of key model drivers such as the maximum gsto, soil texture, root depth and leaf area index. The results suggest that the simple modelling methods that relate gsto directly to soil water content and potential provide adequate estimates of soil moisture and influence on gsto such that they are suitable to be used to assess the potential risk posed by O₃ to forest trees across Europe

DO₃SE modelling of soil moisture to determine ozone flux to forest trees

The DO₃SE (Deposition of O₃ for Stomatal Exchange) model is an established tool for estimating ozone(O₃) deposition, stomatal flux and impacts to a variety of vegetation types across Europe. It has been embedded within the EMEP (European Monitoring and Evaluation Programme) photochemical model to provide a policy tool capable of relating the flux-based risk of vegetation damage to O₃ precursor emission scenarios for use in policy formulation. A key limitation of regional flux-based risk assessments has been the assumption that soil water deficits are not limiting O₃ flux due to the unavailability of evaluated methods for modelling soil water deficits and their influence on stomatal conductance (gsto), and subsequent O₃ flux. This paper describes the development and evaluation of a method to estimate soil moisture status and its influence on gsto for a variety of forest tree species. This DO₃SE soil moisture module uses the Penman-Monteith energy balance method to drive water cycling through the soil-plantatmosphere system and empirical data describing gsto relationships with pre-dawn leaf water status to estimate the biological control of transpiration. We trial four different methods to estimate this biological control of the transpiration stream, which vary from simple methods that relate soil water content or potential directly to gsto, to more complex methods that incorporate hydraulic resistance and plant capacitance that control water flow through the plant system. These methods are evaluated against field data describing a variety of soil water variables, gsto and transpiration data for Norway spruce (Picea abies), Scots pine (Pinus sylvestris), birch (Betula pendula), aspen (Populus tremuloides), beech (Fagus sylvatica) and holm oak (Quercus ilex) collected from ten sites across Europe and North America. Modelled estimates of these variables show consistency with observed data when applying the simple empirical methods, with the timing and magnitude of soil drying events being captured well across all sites and reductions in transpiration with the onset of drought being predicted with reasonable accuracy. The more complex methods, which incorporate hydraulic resistance and plant capacitance, perform less well, with predicted drying cycles consistently underestimating the rate and magnitude of water loss from the soil. A sensitivity analysis showed that model performance was strongly dependent upon the local parameterisation of key model drivers such as the maximum gsto, soil texture, root depth and leaf area index. The results suggest that the simple modelling methods that relate gsto directly to soil water content and potential provide adequate estimates of soil moisture and influence on gsto such that they are suitable to be used to assess the potential risk posed by O₃ to forest trees across Europe