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Issue 23/24

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The Structural Engineer

Bursting forces in columns are caused by cranked vertical reinforcement. Although these bursting forces are well documented, few Codes of Practice deal with the effects adequately. British, South African and American Codes are critically evaluated in this regard. An equation is developed to determine the amount of bursting steel required which is in the form of column ties. A finite element study and columns tests are carried out to evaluate the effects of the bursting forces and as a contribution to the design process. Recommendations are also given describing practical measures for design. M. Gohnert, C. Morris and K. Webber

The Structural Engineer

Mr A. N. Beal (M) (Thomason Partnership) This paper adds valuable experimental results to the database of concrete column tests, particularly for very high strength concrete. However, there is an unfortunate, misleading statement in the synopsis: 'These tests ... have indicated that the Codes are safe design documents for concrete strengths as high as 96N/mm²'. In fact, the tests reported in the paper covered only short-term loading and the 'conclusions' state that 'BS 8110 might not be conservative enough for long-term loading...'. Outside of the laboratory, very few concrete columns are subjected to only short-term loading: indeed, most real concrete columns are subjected to high proportions of long-term load, so this is a point of fundamental importance.

The Structural Engineer

Dr S. B. Desai (F) (Department of the Environment, Transport & the Regions) You state, under eqn (8) of the paper, that the core will remain uncracked if the principal shear stress voldV does not exceed fct/3, where fct is the concrete tensile strength given by fct= 0.3(f'c)2/3. This implies that the shear strength is related to (fc)2/3. Although this relationship may agree with that given in EC 2, it tends to give unsafe results for higher values off,. BS 8110 relates concrete shear strength to the cube root of compressive strength, which is more realistic.

The Structural Engineer

Heather Stanley recently became Managing Director of Yolles Partnership, the Canadian company, an unusual lead position for a woman in her mid-thirties in the construction industry. As Director of its first office in the UK since 1997, she is responsible for 22 staff. The post offered work on larger buildings and more management experience than her jobs to date. Kathy Stansfield

The Structural Engineer

Six large, pretensioned concrete girders of I-shape cross-section were tested to failure. Two parameters were introduced, i.e. shearspan- to-effective-depth ratio and partial prestressing ratio. The test results are timely and useful, as, to the authors’ knowledge, there is no current Code provision for the design of prestressed deep girders. A strut-and-tie approach is also proposed, and the failure criteria are based on Mohr-Coulomb’s equation for biaxial compression-tension stress state and Kupfer's equations for biaxial compression-compression stress state. The accuracy of the method is verifled by experimental results from six girders. Comparison study shows that the model predictions are in good agreement with test results. K.H. Tan and K. Tong

The Structural Engineer

This article was prompted by the contributions in Verulam (2 March 1999) by Paul Rose and John Tanner that appeared under the heading ‘How big is a dominant opening' Professor Nicholas Cook

The Structural Engineer

This paper provides an overview of the design of external, reinforced concrete beam-column joints for monotonic 1oading.There is little consensus over the variables that influence joint behaviour. Existing test data are analysed to determine the influence of factors, including concrete strength, column loading, joint aspect ratio, reinforcement detailing, and joint stirrups, on joint shear strength. The most significant conclusions are that joint shear strength reduces with increasing joint aspect ratio and is increased by joint stirrups, but by less than is commonly assumed. The conclusion that joint shear strength is dependent on joint aspect ratio is at variance with many design recommendations, including EC 8 and ACI/ASCE Committee 352. A new design method is proposed for external beam-column joints based on a comprehensive analysis of all existing test data. The proposed design method is compared with other design methods and is shown to be more realistic. R.L. Vollum and J.B. Newman

The Structural Engineer

This viewpoint is written on a wholly personal basis and may not reflect the views of the trade and professional associations with which I am associated, or those of my employer. I write after a decade of concern, discussion, meetings and seminars in connection with EC 3, the proposed European Steel Structures Design Code, and with a current feeling of complete frustration and despair. Joe Locke

The Structural Engineer

The Bann River Bridge is located on the M1 Motorway in Northern Ireland. The bridge was assessed as part of the National Assessment Programme. Localised durability problems were identified in the areas of the expansion joints, which had failed, allowing deicing salt contamination to parts of the bridge soffit and parts of the substructure. The bridge fniled the loading requirements of the National Assessment Programme, achieving 45% of the required HA loading. The preferred repair and strengthening option involved the repair of the contaminated areas. However concrete replacement repairs could not be applied economically to some of the contaminated areas owing to access problems and the extent of the contamination. Electrochemical repair techniques were therefore regarded us the only viable repair option. The Department of the Environment Roads Service (NI) decided to carry out an electrochemical chloride extraction demonstration on purt of the bridge, to ascertain the effectiveness of the technique. The University of Ulster was commissioned to provide independent assessment of the success of the treatment. During the autumn of 1997 the demonstration was carried out by Martech Services Ltd, and the range of tests performed by the team from the Universily of Ulster proved its success. J.A. Cromie, A.I. Abu-Tair and J.F. Lyness

The Structural Engineer

Settlement cracks and cavity wall insulation Andrew Shaw, from Wakefield, W Yorks., writes: My practice investigates inter alia building movement, frequently subsidence-related. I wonder whether the pages of Verulam can be used to air a matter that somewhat perplexes me?