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Improved crack width calculation method to BS 8007 for combined flexure and direct tension

(The Structural Engineer, 17 May 2005)

Synopsis

The British Code of Practice for the design of water-retaining structures, BS 8007, includes recommendations for the calculation of design crack widths for sections under flexure and for sections under direct tension. However, it does not provide recommendations for sections under combined flexure and direct tension

In an earlier article, Erhard Kruger has shown the separate equations for flexure and direct tension to be based on similar premises. A method was proposed to proportion the tensile stiffening force to the two layers of reinforcement by considering horizontal and moment equilibrium of the stiffening forces. This method results in the “neutral axis” of the stiffening strain diagram not coinciding with the neutral axis of section under the applied forces. Some literature suggests, however, that the stiffening strains should emanate from the neutral axis position.

In this article Erhard Kruger and Robin Atkinson proposes an improved method to proportion the stiffening strain to the reinforcement layers in the two faces of the section under combined flexure and direct tension.

It is shown how the design crack widths for these cases can be determined. Equations are developed with variables allowing for different reinforcement ratios and concrete cover at each face.

Erhard Kruger of HGK Consulting CC and Robin Atkinson of Howes Atkinson Crowder LLP discusses this in an article which appeared in The Structural Engineer of 17 May 2005.

(Please note: This article must be read in conjunction with the article, Crack width calculation to BS 8007 for combined flexure and direct tension (The Structural Engineer, 17 Sep. 2002) by Erhard Kruger.)

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Crack width calculation to BS 8007 for combined flexure and direct tension

(The Structural Engineer, 17 Sep. 2002)

Synopsis

The British Code of Practice for the design of water-retaining structures, BS 8007, includes recommendations for the calculation of design crack widths for sections under flexure and for sections under direct tension.  However, it does not provide recommendations for sections under combined flexure and direct tension. Furthermore, little guidance in this regard is given in the literature. Neither Anchor nor Batty provides a rational approach to allow for the effect of tension stiffening for the case of combined flexure and direct tension in their handbooks.

Since combined flexure and direct tension often occurs in structural elements of certain water-retaining structures,  for example in the horizontal direction of walls of rectangular or square tanks, a need for a rational approach to calculate crack widths for this case exists.

In this article Erhard Kruger examines the procedures given in the code, and shows the separate equations for flexure and tension in the code to be based on similar premises.  The situation where tension exists across the whole of the section under combined flexure and direct tension is then examined, and the limiting values of the depth of the neutral axis for this to occur, are calculated. Equations are developed for surface strains and the stiffening effect of the concrete.

Similar equations are developed for the second case where some compression is present on one face of the section under combined flexure and direct tension.

It is shown how the design crack widths for these cases can be determined.  Equations are developed with variables allowing for different reinforcement ratios and concrete cover at each face.

Erhard Kruger of HGK Consulting CC discusses this in an article which appeared in The Structural Engineer of 17 September 2002.

(Please note: This article must be read in conjunction with the following article by Erhard Kruger and Robin Atkinson, Improved crack width calculation method to BS 8007 for combined flexure and direct tension (The Structural Engineer, 17 May 2005).)

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Saving a doomed reservoir- Rehabilitation of Waterval Reservoir

(The Structural Engineer, 2 Jun. 2010)

Synopsis

The Waterval reservoir, with a capacity of 45,5Mℓ is situated in Florida, Roodepoort, a suburb of Johannesburg, South Africa.  Structural drawings are dated 9 February 1953, suggesting that the reservoir was constructed during 1953 and 1954.  The reservoir is owned and operated by Rand Water, who supplies potable water to Pretoria, Johannesburg and surrounds.

The reservoir is cylindrical and has a gravity type mass concrete wall and reinforced concrete floor, roof and columns. The wall has a complicated section, and only the top 1,1 m is reinforced. The roof slab is supported on the wall and 120 circular columns.

The reservoir is surrounded by an earth embankment, the level of which is approximately 1 m below the top of the reservoir wall. Due to the slope of the terrain, the height of the fill varies along the perimeter, with the embankment being at its highest on the southern side of the terrain, where the reservoir is at its closest to the residential area.

During the nineties large cracks developed in the reservoir wall. With time, the width of these cracks increased, resulting in some of the cracks being up to 20 mm wide. Various unsuccessful attempts were made to seal these cracks with sealants and bandages.  Eventually the reservoir had to be de-commissioned in 2001 due to excessive leakage and concerns regarding the structural integrity of the reservoir especially since the largest cracks in the wall were occurred closest to the residential area.

In May 2006 Rand Water called for tenders and proposals to rehabilitate the reservoir, and Nyeleti Consulting, with HGK Consulting CC as specialist sub-consultant, was awarded the tender.

Erhard Kruger of HGK Consulting CC and Marelize Mostert of Nyeleti Consulting discuss how this reservoir was successfully rehabilitated in an article which appeared in The Structural Engineer of 2 June 2010.

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Rehabilitation of Clifton Avenue Reservoir

(Urban Management, Jan./Feb. 2000)

Synopsis

The 14 Mℓ Clifton Avenue reservoir is situated in a dolomitic area in Centurion near Pretoria, South Africa. The cylindrical prestressed concrete reservoir with a domed roof was constructed in 1971. The reservoir wall is free-sliding and is supported on a bearing located on the wall footing.

In  May 1997 the reservoir started to show signs of leakage. Remedial work to the waterproofing bandages on joints in the reservoir done by the Greater Pretoria Metropolitan Council (GPMC) proved unsuccessful. At first filling of the reservoir after rehabilitation of the bandages leakage was detected on the outside of the reservoir at the wall-to-floor joint, indicating settlement of the floor slab and wall footing. GPMC then appointed a consulting engineering company, BKS, to resolve the problem. Erhard Kruger and Emuël Kok, who at the time were employed at BKS, were responsible for the design of the remedial measures. Erhard Kruger was responsible for the structural remedial work and Emuël Kok for the geotechnical rehabilitation.

Successful rehabilitation was dependent on an accurate diagnosis of the cause of the problem and assessment of the geotechnical conditions. Monitor points were installed on the wall footing and movement at these points were monitored over a period of 18 months. The floor slab of the reservoir was also surveyed. Gradual settlement of up to 196 mm occurred over about 25% of the floor area. Cracks also appeared in the ground surface around the reservoir. The settlement caused the bandage between the wall and floor to tear in this area. Eventually a gap of 250 mm formed beneath the settled floor slab, which rendered the reservoir unserviceable.

Dynamic cone penetrometer tests were conducted and boreholes were drilled inside and outside the reservoir to assess the geotechnical conditions.

Erhard Kruger and Emuël Kok discuss how this reservoir was successfully rehabilitated in an article which appeared in Urban Management Volume 31 of January/February 2000. Click the link to download the complete article.

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(Article reproduced with the permission of the publisher, TE Trade Events; www.tetradeevents.com)