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Galvanizing in engineeringand the environment

Braithwaite type pressed steel

water storage tank

Johannesburg, South Africa

Braithwaitetype pressed

steel water storage tank Johannesburg, South Africa

The Application

The concept of liquid storage intanks constructed by means of aseries of modular steel panels boltedtogether on site was originallydeveloped and patented in 1901

by Braithwaite in the UnitedKingdom, an organisation which hascontinued to be active to this day.

Large storage tanks constructedto this design from pressed steelsections such as the one in this casehistory are generally mounted onsteel towers and used extensively asin this case by municipal authorities.

Environmental conditions

South Africa is known for its manyatmospheric conditions from

relatively benign to extremelycorrosive. Although Johannesburghas no doubt suffered frompollution from numerous sources,one such source being the oldOrlando and Kelvin Power Stations(see article in magazine No. 26),where huge amounts of coal wereburnt monthly, the atmosphere iscurrently relatively benign. Thearea of Johannesburg currently fallsinto corrosion category (C1 or C2in accordance with ISO 9223)andtherefore has a zinc corrosion rate

of less than 1mm per year.(See Corrosion Category table inmagazine No. 23, page 62).

Our findings

The Braithwaite water tank andstand is situated on the easternrampart of the Johannesburg Fort.The water tank steel panels havethe same pressed steel form that

is used to manufacture presentday Braithwaite tanks. Braithwaitecommenced business in 1901.

Although the coating on thefasteners has failed and are nowdiscolouring the remaining hotdip galvanized coating on thepressed steel panels, they are stillstructurally sound but would shortlyrequire over coating or replacement.

The coating on the pressed steelpanels ranges from 2.7 mils (68m)to 3.6 mils (91m), which in terms

of the life of the water tank, isimpressive to say the least.

Should the water tank have beenordered prior to 1937 the Britishspecication BS 729 would not havebeen in existence as it was rstpublished in that year.

However, BS 729 requires thatfor steel thickness less than 5mmbut greater and equal to 2mm acoating mass of 460 grams/m2(coating thickness 64m) andfor steel thickness greater than

5mm a coating mass of 610grams/m2 (coating thickness 85m), bedeposited on the steel.

continued over the page

Close-upofpanels

andbolts.

View

ontherooflookingnorth.

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continued over

Braithwaitetype pressedsteel water storage tank

Acknowledgement

Hot Dip Galvanizers Association,Southern Africawww.hdgasa.org.za

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http://www.hdgasa.org.za/http://www.hdgasa.org.za/

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As it was difcult to assess thesteel thickness of these panels, wemust assume that they were mostprobably 5 to 6mm thick, as per the

local equivalents.

Conclusion

In order to establish when the watertank and structure was installed,we discussed the history of the fortwith Mr Herbert Prins a HeritageArchitect,who was kind enoughto share some facts in trying toestablish the age of the water tank.The following sequence of eventsis known:

The Prison within the Rampart wasbuilt in 1893.

The rampart (the embankment ontwo sides of the Fort) was builtin 1896.

Boer war commenced in 1899 andended in 1902.

Paul Kruger built several forts inthe Johannesburg / Pretoria areaat much the same time as the onein Johannesburg.

Johannesburg surrendered in May1900 and the Boer garrison movedout at the same time the Britishgarrison moved in.

After the Boer war the Britishgarrison moved out (some timebetween 1902 and 1907 when theTransvaal received responsiblegovernment).

In 1909 the womens goal wasbuilt.

In 1928 the Awaiting Trial prisonerblock was built.

It would seem that there wasa strong possibility that theBraithwaite water tanks wereordered from the United Kingdom toaccommodate water for the entiregoal including the Awaiting Trialprisoner block and that suggeststhat the tanks may have beenin existence since about 1930.Should this be correct, then thehot dip galvanizing has performedconsiderably well over the 76 oddyears. Several coating thickness

readings were taken at between2.7 to 3.6 mils (about 68 to 91m)see photos. On one bolt thecoating thickness read as 0.99 mils(25m), but generally the fastenerswere starting to corrode and arediscolouring the hot dip galvanizedsurface of the plates. This makessense, as they would have had anoriginal coating thickness of about45 to 70m and with a zinc corrosionrate of less than a micron a year,would be 70 to 80 years old. Nosurface inspection was done on the

inside of the tank. The structureholding up the water tanks is fairlycorroded but could, if required, berestored. The components couldbe abrasively blasted and hot dipgalvanized and if the tanks werealso re-galvanized, the tank andtank stand would provide a further70 odd years of service free life atthe site.

Braithwaitetype pressed steel waterstorage tank Johannesburg, South Africa

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Generalview

ofwatertanks.

Maximumc

oatingthickness,

3.6mils(91m)onpanel.

Minimumc

oatingthickness,

2.7mils(68m)onpanel.

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