huangyhg

errors in rcc spreadsheets' crackwidth calculation
this is probably only of interest to uk members.
i recently had call the check the crackwidth calculations for the design of a basement wall that an engineer had provided me using the rcc spreadsheets available from the concrete centre. on doing so i discovered that the spreadsheets rcc61 basement wall and rcc62 retaining wall both have a number of errors in the crackwidth calculations to bs8110 1997.
the most significant of these results in the sheet typically underestimating the crackwidth by 15-20% for widths greater than 0.1mm. thus a number of designs will be reported as passing the serviceability design checks when in fact they should fail. the error arises because of a formula error due to a misplaced decimal point – the stiffening factor for crackwidths less than 0.1mm is applied for all widths less than 1.0 mm (ie all designs). the error appears to be present in all versions of the spreadsheets from the just issued v3 ones going back to when first issued in 1999 so has consequences for any structure designed using the sheets over the past 9 years.
i have reported the errors to the concrete centre but as i have been rather underwhelmed by their response so far i thought it appropriate to publicise to the wider community so that at least anyone else using these spreadsheets can take note of the errors. annotated copies of a spreadsheet highlighting the errors are attached. i'll post the second in a reply. i have not checked others of the rcc spreadsheets to see if they duplicate the error.
i leave it to those more expert in the structural engineering issues to judge how serious the error is – nothing's likely to fall down but presumably some structures might not last quite as long as intended.
regards,
charles
acc brennan
the second file: rcc62
charles
i haven't designed a basement wall for a couple of years until tis week. i was landed with one this week and i designed it using this very spreadsheet. i obviously hand checked some of the critical stability calcs but not the crack widths. now low and behold, just as the rc drawings are whisked out the door i spot your thread charles. a busy weekend now lies ahead for me checking crack widths for the issued reinforcement!!!
i was interested to hear about the creator's apathy towards your discovery. i had a similar problem when i highlighted flaws in a major suppliers software. it has since been proven that the errors i highlighted were genuine.
thanks for the info from across the pond... i've picked up copies of the rcc sheets and they, for the most part, are well done.
dik
for information - for those wishing to perform checks the calculations in the rcc14 crackwidth spreadsheet are correct (in my inexpert view) so you can most easily check figures by plugging the design specification into this spreadsheet.
charles
ps - further to my previous post if you do use rcc14 for checks you will get minor variation in the crackwidth figure returned and those from a corrected rcc61/62 because the spreadsheets use two different numerical estimates for the modulus of elasticity of concrete (ec) based on the fcu value entered. the variation should only be in the order of approx 1% at most for reasonable fcu values.
try looking at punching shear on foundation pads using the eurocode version. seems to be out by a factor of 10.
i passed on the comment by herewegothen in reference to foundation pads which drew the response below from the spreadsheet authors. if anyone has a further view on whether an error exists then perhaps they can pass it on to concretecentre.com
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thank you for relating the comment "try looking at punching shear on foundation pads using the eurocode version. seems to be out by a factor of 10." via owen.
our spreadsheet author rod webster has looked at tcc81 foundation pads and also compared it with rcc81. punching shear resistance is very similar to both codes. his guess is that whoever made the "10 times" statement may have omitted to deduct the bearing pressure within the critical punching perimeter (cl 6.4.4(2)). this can make a very significant difference.
we will not take the issue further. if there appears to still be a problem we can only really resolve it if we are given the cell references in question. sending the spreadsheet to us with the questions would help enormously.

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