Christian Frier

Measurements of Hybrid Ventilation Performance in an Office Building

Abstract This paper focuses on hybrid ventilation performance in an office building. It presents measurement results from the new headquarters of the company Bang & Olufsen, which served as one of the case study buildings in IEA ECBCS Annex 35. Ventilation and control strategy, as well as operational experience of the hybrid ventilation system is presented. Measurement results include long-term values of temperatures, CO2 and energy use for assisting fans and heating of ventilation air as well as electricity use for appliances. They include short term values of thermal comfort, temperature distribution in selected parts of the building, CO2 levels, and various tracer gas measurements for examining ventilation effectiveness, air and contaminant distribution, multizone internal air flows and air flows between the building and the outside environment. Results show an office environment with an acceptable thermal comfort level and a high level of IAQ, but with relatively high energy use for heating. The ventilation principle works well with high ventilation effectiveness when the system is operated.

Finite element reliability analysis of chloride ingress into reinforced concrete structures

For many reinforced concrete structures corrosion of the reinforcement is an important problem since it can result in maintenance and repair actions. Further, a reduction of the load-bearing capacity can occur. In the present paper the Finite Element Reliability Method (FERM) is employed for obtaining the probability of exceeding a critical chloride concentration level at the reinforcement bars, both using Monte Carlo Simulation (MCS) and the First Order Reliability Method (FORM). The chloride ingress is modelled by the Finite Element Method (FEM) and the diffusion coefficient, surface chloride concentration and reinforcement cover depth are modelled by stochastic fields, which are discretized using the Expansion Optimum Linear Estimation (EOLE) approach. The response gradients needed for FORM analysis are derived analytically using the Direct Differentiation Method (DDM). As an example, a bridge pier in a marine environment is considered and the results are given in terms of distributions of time for initiation of corrosion.

Sensitivity Study of Stochastic Walking Load Models

On flexible structures such as footbridges and long-span floors, walking loads may generate excessive structural vibrations and serviceability problems. The problem is increasing because of the growing tendency to employ long spans in structural design. In many design codes, the vibration serviceability limit state is assessed using a walking load model in which the walking parameters are modelled deterministically. However, the walking parameters are stochastic (for instance the weight of the pedestrian is not likely to be the same for every footbridge crossing), and a natural way forward is to employ a stochastic load model accounting for mean values and standard deviations for the walking load parameters, and to use this as a basis for estimation of structural response. This, however, requires decisions to be made in terms of statistical distributions and their parameters, and the paper investigates whether statistical distributions of bridge response are sensitive to some of the decisions made by the engineer doing the analyses. For the paper a selected part of potential influences are examined and footbridge responses are extracted using Monte-Carlo simulations and focus is on estimating vertical structural response to single person loading.

Flooring-Systems and Their Interaction with Usage of the Floor

Some flooring-system designs might be sensitive to their vibrational performance, as there might be the risk that serviceability-limit-state problems may be encountered. For evaluating the vibrational performance of the flooring-system at the design stage, decisions need to be made by the engineer in charge of computations. On a flooring-system often passive humans and/or furniture are present. Often these masses and their way of interacting with the floor mass are ignored in predictions of vibrational behavior of the flooring-system. The paper explores and quantifies how these masses can influence central parameters describing the dynamic behavior of the flooring-system.

Sensitivity of Footbridge Response to Load Modeling

The paper considers a stochastic approach to modeling the actions of walking and has focus on the vibration serviceability limit state of footbridges. The use of a stochastic approach is novel, but useful, as it is more advanced than the quite simplistic deterministic load models seen in many design codes. Using a stochastic approach, however, requires a number of decisions to be made (statistical distributions and associated parameters) for walking parameters. These decisions might have an impact on the outcome of serviceability evaluations (bridge acceleration levels), but it is often not a simple matter to foresee their impact. The paper contributes by examining how some of these decisions influence the outcome of serviceability evaluations. The sensitivity study is made focusing on vertical footbridge response to single person loading.

Non-structural Masses and Their Influence on Floor Natural Frequencies

Excessive floor vibrations are problematic and may potentially render a floor unfit for its intended use. A design-stage check of vibrational performance of a floor design would encompass design-stage estimates of floor dynamic characteristics such as floor natural frequencies. Non-structural masses such as furniture might be present on the in-service floor. For a prediction of floor dynamic characteristics it is not common to account for the fact that non-structural masses elevated above the floor plane may contribute with inertial energy as a result of their horizontal motion occurring during vertical floor vibration. The paper addresses this subject by setting up a finite element model for the floor, which also accounts for an elevation of the non-structural masses. It is shown how different configurations of non-structural masses influence floor natural frequencies. For the investigations, the elevations and weights of the masses are modelled as random variables and Monte Carlo simulations are used for setting up the random configurations of non-structural masses across the floor area.

Footbridge Vibrations Predicted by Stochastic Load Model

Actions of humans on footbridges may result in structural vibrations that may be annoying to bridge users potentially rendering footbridges unfit for their intended use. Hence, it is useful to make predictions of footbridge vibrational performance already at the design stage involving estimation of levels of vibrations in the footbridge. Nowadays both deterministic and stochastic approaches are available for such evaluations. The have primary focus on probability-based approaches for predicting levels of floor vibrations. The predictions involve employing Monte Carlo simulations and the initial setting up of a stochastic framework describing the action of a walking person. The paper investigates the influence of selected decisions made by the engineer when setting up the basis for the prediction of levels of vibration in the footbridge.

Probabilistic Analysis of Modal Properties for Floor Systems with Uncertain Support Conditions

Traffic and construction work as well as internal sources may cause vibration of floors in buildings. Potentially, this leads to annoyance for people living or working in the buildings—especially when resonance occurs as a result of excitation frequencies coinciding with eigenfrequencies of the floors. Hence, proper design of floors requires insight into the dynamic properties of the system in order to avoid resonance. In this context, the boundary conditions for the floor—or the connections to the main structure—play an important role. A floor clamped along the entire edge reacts differently than a floor which is simply supported. However, whereas the floor system may well be described in terms of material and geometry, an assessment of the supports can be difficult. Often, calculated eigenmodes and eigenfrequencies do not match those identified for a real floor system and this is, to a great extent, due to uncertain and poorly described supports. Hence, the paper suggests a probabilistic approach focussing on the dynamic properties of the floor given uncertain support conditions. Especially, a rectangular concrete floor, representative of a floor in an office or residential building, is assessed regarding its eigenfrequencies. A stochastic model is introduced for the rotational stiffness of the supports, and a numerical analysis is performed in order to quantify how uncertainty related to the supports for the floor system transfers into uncertainty of its eigenfrequencies.

Predicting Footbridge Vibrations Using a Probability-Based Approach

Vibrations in footbridges may be problematic as excessive vibrations may occur as a result of actions of pedestrians. Design-stage predictions of levels of footbridge vibration to the action of a pedestrian are useful and have been employed for many years based on a deterministic approach to modeling the action of a pedestrian. The paper employs a probability-based approach to modeling the action of a pedestrian by considering randomness in the behavior of the pedestrian crossing the footbridge. The paper describes the approach and studies implications (sensitivity) of selected decisions made when setting up the probabilistic framework for the predictions of footbridge response.

Stochastic Load Models and Footbridge Response

Pedestrians may cause vibrations in footbridges and these vibrations may potentially be annoying. This calls for predictions of footbridge vibration levels and the paper considers a stochastic approach to modeling the action of pedestrians assuming walking parameters such as step frequency, pedestrian mass, dynamic load factor, etc. to be random variables. By this approach a probability distribution function of bridge response is calculated. The paper explores how sensitive estimates of probability distribution functions of bridge response are to some of the decisions to be made when modelling the footbridge and when describing the action of the pedestrians (such as for instance the number of load harmonics). Focus is on estimating vertical structural response to single person loading.