Column reinforcement changes when cover changes

[Μηχανικος]

Hello everyone, i am using robot provided reinforcment to design a column.

Initially the provided reinforcement  calculated by Robot is 8Φ20 = 2513mm2     (with cover = 40mm)

Then the provided reinforcement  calculated by Robot is 12Φ20 = 3769mm2     (with cover = 30mm)

so why when i decrease my cover, the area of steel reinforcement increases?

[Artur.Kosakowski]

Could you send the file with both these columns please? Thank you.

[Μηχανικος]

Below are the links for both columns.

http://www.mediafire.com/view/t44c48dlyqie822/1._Design_Response_Spectrum_-_Column_Design.rtd

http://www.mediafire.com/view/af2f5ctbieg16i0/1._Design_Response_Spectrum_-_Column_Design_2.rtd

These are exactly the same file except that in one i changed the cover distance within the Rc design module provided reinforcement

Analysis -> calculation options -> General -> Cover -> c >= 30 or 20mm

when i use c >= 30 the actual cover in the calculation is 40mm

when i use c >= 20 the actual cover in the calculation is 30mm

[Artur.Kosakowski]

For 20 mm cover EC in 5.4.3.2.11 b imposes limits on main rebars tied by stirrups spacing as 20 cm (for 3 bars at at the side that would be 20.3 cm). For 30 mm cover the spacing is smaller that this code limit. 

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[Μηχανικος]

Hi there is no such section (5.4.3.2.11 b) in EN 1992-1-1:2004 

[Μηχανικος]

Hi there is no such section (5.4.3.2.11 b) in EN 1992-1-1:2004

Even if this is the case i dont think this is what is happening in my case. also i dont understand for which spacing limit you are referring to?

ok so when my cover is 40mm (30mm cover + 10mm deviation) i get 8 bars of 20mm diameter

and clear spacing between main bars is [500 - (2 x 40mm cover) + (2 x 10mm links) + (3 x 20mm main bars)]/2 = 170 mm

clear spacing between main bars is = 170mm

when i decrease the cover to 30mm i get 12 bars of 20mm.

and clear spacing between main bars is [500 - (2 x 30mm cover) + (2 x 10mm links) + (4 x 20mm main bars)]/3 = 113.33mm

clear spacing between main bars is = 113.33mm

what i am saying here is that if you decrease the cover and keep the same amount of main bars then the spacing between bars increases so why is Robot adding more bars?

[Artur.Kosakowski · Accepted solution]

 You can find the paragraph 5.4.3.2.11 b in  EN 1998-1:2004 (EC8, not EC2) which is used as you included seismic in your design:

The distance is understood as axial spacing rather than clear space between bars.

When cover is 40mm (30mm cover + 10mm deviation) and you get 8 bars of 20mm diameter, distance between axes of those bars = [500 - (2 x 40mm cover) -(2 x 6mm links) - (2 x 20mm main bars)/2]/2= 194 mm

This spacing is correct (194mm <200mm).

When you decrease the cover to 30mm and get 12 bars of 20mm, spacing between axes of main bars is [500 - (2 x 30mm cover) - (2 x 6mm links) - (2 x 20mm main bars)/2]/3 = 136mm

For 8 bars fi 20mm you would have [500 - (2 x 30mm cover) - (2 x 6mm links) - (2 x 20mm main bars)/2]/2 = 204mm which would be above the limit (>200mm)

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Μηχανικος]

ohhhh i see, you are great, thank you for all your help, really appreciate this.