UK National Annex Research Articles

Viewpoint. Nominal q-values for roofs: is it time to talk them down, up or both?

Alastair Hughes argues for a number of changes in the way roof loading is dealt with in the UK National Annex to EN 1991, including a reduction in nominal roof loading q.

Shear enhancement in RC beams without shear reinforcement simultaneously loaded within 2d and at 3d from supports

Shear resistance is increased by arching action when reinforced concrete (RC) beams are loaded within around twice the beam effective depth (d) of supports. The vast majority of laboratory tests have investigated shear enhancement in beams under three- or four-point loading with all loads applied within 2d of supports. This type of loading is representative of deep beams but not slender beams and slabs which are invariably loaded within the span when subject to point loads near supports. The latter problem arises in many guises in practice but is scarcely researched. The motivation for looking into the problem was the difference in design approaches adopted in EC2 and the superseded UK code BS8110. The former accounts for shear enhancement by reducing the design shear force while the latter increases the design shear resistance within 2d of supports. The UK National Annex to Eurocode 2 Part 2 Concrete Bridges adopts the approach of BS8110 for members without shear reinforcement. The paper presents the results of tests on four simply supported RC beams without shear reinforcement in which the loading arrangements were chosen to investigate the influence on shear enhancement of applying loads both within and outside 2d of supports. Digital image correlation (DIC) system was used to investigate the crack kinematics and shear resisting mechanisms of each beam. The BS8110 approach to shear enhancement is shown to be potentially unsafe for beams without shear reinforcement. Good predictions of strength were obtained with EC2 as well as strut and tie modelling in which the concrete strength was determined using the Modified Compression Field Theory.

Glass Suspension Footbridge: Human-Induced Vibration, Serviceability Evaluation, and Vibration Mitigation

Glass suspension footbridges or glass-bottomed suspension footbridges, such as the Zhangjiajie Grand Canyon Glass Bridge in China, are increasingly constructed in scenic regions as tourist attractions due to the glass-bottomed and transparent characteristics. Being installed in the second phase as a constant load, the glass deck is not designed to participate in the load bearing of the whole structure and thus the structure requires larger load-bearing capacity from other parts and a slightly different load-bearing system. More importantly, the glass deck weakens structural integrity and results in slightly smaller structural stiffness, which may result in globally lower natural frequencies. These structures may inevitably be more sensitive to the pedestrian-induced excitations. This paper investigates the human-induced vibrations of a typical glass suspension footbridge in China. The investigations are first performed based on intensive parametric studies. Except for the boundary conditions, the interlayers of glass, the vertical rise–span ratio and the axial stiffness of main cables, other governing parameters of the modal parameters for this type of structures are also identified, including the pavement load of the bridge deck system, the central buckle, and the prestress of wind-resistant cables. It may guide designers to take effective measures to improve the dynamic behavior of this type of structure. Next, the vibration serviceability assessments were performed based on different standards and guidelines such as the UK National Annex (NA) to Eurocode 1 (2008), the Bro2004 specification of Sweden, the HiVoSS guideline, and the ISO standard. Comparisons show that the UK NA to Eurocode 1, the HiVoSS guideline, and the ISO standard are more suitable for the vibration serviceability evaluations. These three codes consider the pedestrian-induced vibrations in both lateral and vertical directions and also consider both a small number of pedestrians and crowd-induced loads with different densities. Furthermore, comfort limits are not defined by only one limiting value but sorted into different levels. This may provide reference for the vibration serviceability evaluations of the other glass suspension footbridges. The assessments also show that changing the structural parameters can only partially reduce vibration levels and may result in different modal parameters to dominate the vibration responses. Thus, they recommend taking vibration mitigation measures to improve the vibration serviceability and suggest performing careful response calculations to ensure the effectiveness of vibration serviceability design if via changing the structural parameters. Finally, tuned mass dampers (TMDs) are installed to the structure. The results show that the popular TMD may effectively reduce the vibration levels.

Vibrations of metal web joist timber floors with strongbacks

The aim of this research is to assess the effects of joist spacing, strongbacks and ceiling on the dynamic response of timber floors with metal web joists so as to evaluate the vibrational design criteria, for example, modal frequencies, modal shapes, damping and unit point load deflection, required by Eurocode 5-1-1 and the UK National Annex for timber floors. In general, joist spacing, strongbacks and ceiling do not greatly influence the fundamental frequency and damping ratio, but affect higher modal frequencies. The measured damping ratio for the fundamental mode is 0·86% on average. The use of strongbacks considerably reduces the number of first-order modes below 40 Hz, and provides easier fulfilment of the velocity design criterion. The test results indicate that the decrease in joist spacing, the increase in number, size and stiffness of strongbacks, and the use of a ceiling all greatly reduce the maximum displacement of the floors. On average, the calculated displacements based on the equations in the UK National Annex are close to those measured.

Effects of axial and rotational restraints on concrete-filled tubular columns under fire

This paper presents a study of the effects of axial and rotational restraints on the fire response of concrete filled tubular (CFT) columns. The fiber model presented by the authors in a previous paper Ibañez et al. (2013) is employed to simulate the fire behavior of CFT columns within non-sway frames. Parametric studies are performed to analyze the influence of the different factors affecting the problem. Consequently, the opposite effects that the restraining frame has on the fire response of CFT columns are corroborated. On one hand, the restrained thermal elongation induces restraining forces which negatively affects the column. However, a beneficial effect is produced by the rotational restraint which positively modifies the boundary conditions of the heated column. Besides, another favorable effect comes from the gradual redistribution of the internal forces as the heated column loses its mechanical capacity, resulting in higher FRR. In a second step, current provisions given by Eurocode 4 Part 1.2 (EC4) are analyzed together with those given in the UK National Annex to Eurocode 4. Finally, these values are also used in the assessment of the simple calculation model presented by the authors in a previous paper Ibañez et al. (2015) and subsequently a proposal is made obtaining reasonably accurate but slightly safe results since the guidelines of CEN-Horizontal Group Fire were followed. As a result, the fire effective length value of 0.5L given by EC4 is found to lead to unsafe results, being more appropriate to adopt the value of 0.7L suggested in the UK National Annex.

Cube, cylinder, core and pull-off strength relationships

This limited experimental investigation examined the relationships between the compressive strengths of cubes, cylinders, cores and the estimated compressive strengths derived from pull-off tests for a relatively low-strength structural-grade concrete (<35 N/mm2). Test specimens were cast and tested at 7, 14, 28, 56 and 84 days. The relationships of the trends of the test results to the trends of results of standard cube specimens and standard cylinder specimens were compared. It was found that the mean strength of each type of specimen tended to increase as a function of the natural logarithm of the specimen age. The mean strength of cylinders of length/diameter ratio 2·0 was found to be slightly greater (by about 7·5%) than the generally accepted value of 80% of the mean cube strength. Core results were corrected using correction factors defined in BS 6089 and the UK national annex to BS EN 12504-1. The mean corrected cube strength of cores taken from cubes was approximately 12% greater than the mean companion cube strength. The mean corrected cylinder strength of cores taken from cubes was approximately 5% greater than the mean companion cylinder strength. The potential cube and cylinder strengths of cores taken from slabs cured under different environmental conditions correlated well with companion cube and cylinder strengths, respectively, at 28 days. The pull-off test results gave a variable but, on average, slightly conservative estimate of the cube compressive strength of the relatively low-strength structural-grade concrete, using a simple general linear estimated compressive cube strength to tensile strength correlation factor of 10.

Vibrational performance of timber floors constructed with metal web joists

Metal web engineered timber joists are increasingly being used nowadays to replace traditional solid timber joists for constructing intermediate-span timber floors in low-rise houses and long-span floors in commercial buildings. Vibrational performance criteria become crucial for serviceability design of timber floors and often control the design. This paper presents an experimental study carried out on the dynamic performance of a series of full-scale floors constructed with metal-web joists with focus on modal frequencies, modal shapes and damping. The effects of a range of floor component configurations including spacing of joists, type, size, number and location of strongback, ceiling, etc., were examined. In general, joist spacing, strongback bracings and ceiling did not significantly influence the fundamental frequency and the corresponding damping ratio, required for the design of timber floors, but they did influence higher modal frequencies. The measured damping ratio for the fundamental mode was 0.9% on average, which is below the recommended value of 1% in Eurocode 5 Part 1–1 and much smaller than the recommended value of 2% in the UK National Annex to the code. The use of strongbacks, however, could considerably reduce the number of first-order modes below 40Hz, used to determine the unit impulse velocity response, and in turn it could result in easier fulfilment of velocity design criterion.

Discussion: The design of temporary excavation support to Eurocode 7

Most would accept CIRIA C580 (Gaba et al., 2003) as a guide to previous good practice. The author’s conclusion that designs to Eurocode 7 give lower margins of safety than are recommended in CIRIA C580 is indeed surprising, and as he states is contrary to Eurocode 7 requirements. This may follow from the way that the so-called ‘single source principle’ is applied in the paper. Eurocode 7 (Clause 2.4.2) states that this principle may apply ‘in some situations’. Neither Eurocode 7 nor the UK National Annex (BSI, 2004b) describes these situations, but it is doubtful that designers may invoke it wherever they choose. Engineers should aim to apply the partial factors given in the National Annex in the way envisaged when they were set down. Unfortunately the National Annex does not tell us what this is.

Effect of fire-induced damage on the uniaxial strength characteristics of solid timber: A numerical study

The advent of the structural Eurocodes has allowed civil engineers to be more creative in the design of structures exposed to fire. Rather than rely upon regulatory guidance and prescriptive methods engineers are now able to use such codes to design buildings on the basis of credible design fires rather than accepted unrealistic standard-fire time-temperature curves. Through this process safer and more efficient structural designs are achievable. The key development in enabling performance-based fire design is the emergence of validated numerical models capable of predicting the mechanical response of a whole building or sub-assemblies at elevated temperature. In such a way, efficiency savings have been achieved in the design of steel, concrete and composite structures. However, at present, due to a combination of limited fundamental research and restrictions in the UK National Annex to the timber Eurocode, the design of fire-exposed timber structures using numerical modelling techniques is not generally undertaken.The 'fire design' of timber structures is covered in Eurocode 5 part 1.2 (EN 1995-1-2). In this code there is an advanced calculation annex (Annex B) intended to facilitate the implementation of numerical models in the design of fire-exposed timber structures. The properties contained in the code can, at present, only be applied to standard-fire exposure conditions. This is due to existing limitations related to the available thermal properties which are only valid for standard fire exposure. In an attempt to overcome this barrier the authors have proposed a 'modified conductivity model' (MCM) for determining the temperature of timber structural elements during the heating phase of non-standard fires. This is briefly outlined in this paper. In addition, in a further study, the MCM has been implemented in a coupled thermo-mechanical analysis of uniaxially loaded timber elements exposed to non-standard fires. The finite element package DIANA was adopted with plane-strain elements assuming two-dimensional heat flow. The resulting predictions of failure time for given levels of load are discussed and compared with the simplified 'effective cross section' method presented in EN 1995-1-2.

Lateral buckling of plate girders with flexible restraints

Since publication of BS 5400: part 3 in 2000, the design check of paired plate girders during erection has become more onerous in the UK. This change has been brought about by the addition of two main features in BS 5400: part 3—a change to the mode of buckling considered in deriving girder slenderness and the addition of more conservative buckling curves when the effective length for buckling differs from the half-wavelength of buckling. BS EN 1993-1-1 requires no such reduction in resistance. The work presented in this paper was prompted by a proposal to modify the rules of BS EN 1993-2 for bridges in the UK's national annex to be more like those in BS 5400: part 3. The authors believed this to be unnecessary. This paper investigates buckling cases where the effective length for buckling is shorter than the half-wavelength of buckling and demonstrates that the series of correction curves used in BS 5400: part 3 are unnecessary and that the BS EN 1993-1-1 method is satisfactory and slightly conservative. The paper also outlines the design process to BS EN 1993 using both elastic critical buckling analysis and non-linear analysis. The case studies considered are a simple pin-ended strut with intermediate restraints, a pair of braced girders prior to hardening of the deck slab and a half-through deck with discrete U-frame restraints. For the latter two cases, the results predicted by BS 5400: part 3 and BS EN 1993-1-1 are compared with the results of non-linear finite-element analyses.

Strategies for the management of bridges for vehicular impacts

Impacts on bridges by road vehicles are a relatively frequent occurrence and can lead to delays for road users, injuries or fatalities for vehicle occupants and in some cases risks of major incidents from derailments on railway lines carried by highway bridges. This paper gives an overview of the scale of the problem in the UK taken from data gathered by Network Rail and the Highways Agency (HA). The primary reasons for bridge-deck strikes are given and a review is undertaken of various industry good practice guides and strategies to prevent or deal with these incidents. The background to the development of the current design and assessment requirements is given, including some research and development projects undertaken by the HA. The design requirements from the Eurocode for accidental actions and an outline of risk methodology being developed in the proposed UK national annex to the Eurocode are also presented.

A Parametric Evaluation of Multi-Webbed Composite Joists Based on EC5

The introduction of engineered beams offers long span, low cost and light weight joists in comparison to traditional solid timber sections. However, the vibration criteria in the design codes may significantly reduce the permissible span of the joists for domestic construction. This paper investigates the influence of geometric and material variability on permissible spans of a series of multi-webbed composite joists, which are designed in accordance with the requirements of Eurocode 5 and its UK National Annex. Geometric variability comprises four different profiles, namely; I-beams, Double I-beams, Box beams and Box I-beams/joists. The variations between the profiles are web-flange connection details and the number of webs, which vary from one to three depending on the profile. Material variability involves two different web materials, OSB and plywood, which are considered together with two different flange materials, C16 and C24. This study shows that serviceability would in general govern design requirements in UK timber flooring systems. Out of four different sub-criteria defined under the serviceability conditions, the deflection under unit point load is the governing condition in most cases. It was found that the correct choice of materials and beam profile can enhance the permissible span by up to 34%. When lateral restraint is not provided, lateral stability becomes the controlling criterion as beam depth increases. An inverse relationship has been found between the permissible span and the beam depth for an I-beam profile, whereas for the Double I-beams a constant permissible span was observed for varied beam depths. For the Box beams and Boxed I-beams, the permissible span will increase in proportion to beam depth. This study also shows that the effect of lateral stability on solid timber joists is less pronounced than for engineered beams.