Precast boundary wall 200mm thick(Reinforcement mesh double layer or single?)

[precast123]

Hello everyone

I have a boundary wall panel 200mm thick(6.2m L x 4.5m H) with outside elevation having 50mm recess covering 70% of panels elevation. Its subjected to wind pressure of 1.0 kN/m2 laterally(which can be from both direction) front and back. Can I use single wire mesh in the middle or have to provide two layer wire mesh for flexure due to wind. Is there any restriction on this in BS8110 or ACI 318, that upto certain wall thickness single layer to be used or double layer to be used? Thanks.

*Note that if I use single layer d = 100mm from back elevation and d = 50mm in front elevation.

 

[mike20793]

If it is low seismic or not a shear wall, you can use one layer of reinforcing for that wall thickness.

 

[phuduhudu]

I don't think there is a restriction in the BS. If most of one side has a recess of 50mm then you can only consider it to be 150mm thick actually unless there are continuous ribs of 200mm thickness. You could try a design with the reinf in the centre of the 150mm which would give you an effective depth of 75mm and check if it works for bending under the wind load. Looking at the span/depth ratio here I don't think that works if it has to span 4.5m. Even at a solid 200mm thick with reinf at 25mm cover front and back I think you would struggle.

 

[JAE]

ACI has a restriction where you have to use double reinforcement in walls thicker than 10" (255mm).



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[precast123]

Panel will not crack if the concrete cover to reinforcement is large(75mm or 100mm) ?

 

[precast123]

I estimated for a single layer with d = 75 mm, Ast = 10mm dia mesh @ 200mm C/C = 393 mm2 and if double layer is used with d= 120mm, Ast = 8mm dia mesh @ 200mm C/C = 251 mm2 x 2 = 502 mm2. If ignore the 28% of extra steel I am using in case of double layer, which will be a better option?

 

[precast123]

2.4. British standards BS 8110-1997

According to BS 8110-1997 [23], the width of flexural cracks at a particular point on the surface of a member depends primarily on three factors:

(a)The proximity to the point considered of reinforcing bars perpendicular to the cracks; concrete cover.
(b)The proximity of the neutral axis to the point considered; h–x.
(c)The average surface strain at the point considered.
BS 8110-1997 recommends that the strain in the tension reinforcement is limited to 0.8 fy/Es (i.e. 0.8 * steel yield strain) and the design surface crack width should not exceed the appropriate value. Cracking should not lead to spoil appearance. So for members that are visible, the calculated maximum crack width should not exceed 0.30 mm. Also, cracking should not lead to steel corrosion, so for members in aggressive environment the calculated maximum crack width should not lead to a loss of the performance of the structure.

BS 8110-97 provisions are based on Beeby [12] empirical equations,

equation(10)
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where acr = distance from the point considered to the surface of the nearest longitudinal bar; εm = average strain at the level where the cracking is being considered; cmin = minimum cover to the tension steel; h = overall depth of the member; x = depth of neutral axis.
For cracked section, the value of εm is expressed as:

equation(11)
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where ε1 = strain at the level considered, calculated ignoring the stiffening effect of the concrete in the tension zone, b = width of the section at the centroid of the tension steel, a′ = distance from the compression face to the point at which the crack width is being calculated.
According to BS 8110-1997 [23], in assessing the strains, the modulus of elasticity of the concrete should be taken as half the instantaneous values.

 

[precast123]

http://www.sciencedirect.com/science/article/pii/S1110016812000385

 

[phuduhudu]

If you have done the full section capacity design check and serviceability checks to BS8110 for deflections and crack widths and it works then you are fine. Emprirical rules about maximum covers etc are only there to avoid having to do those detailed checks.