Design parameters for longitudinal bars (EC2)

Tekla Structural Designer
2021
Tekla Structural Designer

Design parameters for longitudinal bars (EC2)

For each of these parameters, any user defined limits (as specified on the appropriate Reinforcement Settings page within Design Options) are considered in addition to the EC2 or NA recommendations.

Maximum diameter of reinforcement

At Section 8.8 of BS EN 1992-1-8:2004, additional rules are specified when "large diameter bars" are used in the design. A large diameter bar is defined as being a bar with a diameter larger than φlarge where φlarge is an NDP value.

For design in accordance with EC2 Recommendations;

φlarge
  =  
32 mm

For design in accordance with UK NA, Irish NA, Malaysian NA and Singapore NA;

φlarge
  =  
40 mm

In the current release the provisions of Section 8.8 are not implemented. If the design results in a bar size with φ > φlarge then a warning is displayed.

Note:

Clause 7.3.3 (2) indicates that cracking can be controlled either by restricting the bar diameter or the max spacing. Tekla Structural Designer adopts the latter approach using Table 7.3N - therefore the maximum bar diameters specified in Table 7.2N are not checked.

Minimum distance between bars

The minimum clear horizontal distance between individual parallel bars, s cl,min, is given by;1

scl,min

MAX [k1*φ, dg+k2, scl,u,min, 20 mm]

where

k1
  =  
the appropriate NDP
k2
  =  
the appropriate NDP
dg
  =  
the maximum size of aggregate
φ
  =  
the maximum diameter of adjacent bars, φi and φj
scl,u,min
  =  
user specified minimum clear distance between bars
Note:

To allow you to make decisions regarding access for concrete compaction or size of aggregate, a value for the minimum clear distance between bars can be specified on the appropriate Reinforcement Settings page within Design Options - separate values being set for bars in the top of a beam and for those in the bottom of a beam.

The minimum clear vertical distance between horizontal layers of parallel bars, scl,min, is given by;

scl,min
MAX[k1*φ, dg+k2, 20 mm]

For design in accordance with UK NA,EC2 Recommendations, Irish NA, Malaysian NA and Singapore NA;

k1
  =   
1.0
k2
  =   
5.0 mm

Maximum spacing of tension bars

The maximum centre to centre bar spacing for crack control, s cr,max, is dependent on the maximum allowable crack width, wmax, specified in the beam properties from a menu of values which are: 0.20mm, 0.30mm or 0.40mm with a default value of 0.30mm.

The service stress in the reinforcement, σs, is given by;

σs
  =  
(As,reqd/As,prov)*(fyks)*R PL

where

As,reqd
  =  
area of reinforcement required for the maximum design Ultimate Limit State moment, MEd
As,prov
  =  
area of reinforcement provided
RPL
  =  
permanent load ratio

In the beam properties you are required to supply a value for the permanent load ratio, RPL. A default of 0.65 has been assumed, but you are advised to consider if this is appropriate and adjust as necessary.

The maximum allowable centre to centre bar spacing, scr,max, is then obtained from table 7.3N (shown below) by looking up the calculated value of the service stress in the reinforcement, σs, using interpolation between values of σs

Steel Service Stress,

σs (N/mm2)

Max Allowable bar Spacing, scr,max
wmax = 0.40 mm wmax = 0.30 mm wmax = 0.20 mm
≤ 160 300 300 200
200 300 250 150
240 250 200 100
280 200 150 50
320 150 100 Warning
360 100 50 Warning
>360 Warning Warning Warning

Maximum spacing of tension bars (slabs not exceeding 200mm)

In accordance with clause 7.3.3(1) of EC2 for slabs not exceeding 200mm in overall depth and not subjected to significant axial tension the maximum limit on centre to centre bar spacing is governed by clause 9.3 only and there is no need to perform specific checks on the bar spacings to control cracking. These limits are applied to all slabs and then the additional limit in the next section are applied to slabs greater than 200mm thick.

From clause 9.3 the maximum limit on bar spacings can be somewhat subjective so these limits will be user definable with conservative defaults as follows :-

Principal bars (NDP) (cl. 9.3.1.1(3))

smax = 2h but ≤ 250mm

Secondary bars (NDP) (cl. 9.3.1.1(3))

smax = 3h but ≤ 400mm

Bars are classed as secondary if both the following are true:

  1. The design moment for bars in this direction is lower than the design moment for bars in the other direction.
  2. The calculated reinforcement requirement based on the design moment is less than the minimum reinforcement requirement.

Minimum area of reinforcement

The minimum area of longitudinal tension reinforcement, A s,min, is given by;2

As,min
MAX[kmin1*bw*d*(fctm/fyk), kmin2*bw*d]

where

kmin1
  =  
the appropriate NDP value
kmin2
  =  
the appropriate NDP value
fctm
  =  
mean value of the axial tensile strength of the concrete
fyk
  =  
characteristic yield strength of the reinforcement

For design in accordance with UK NA,EC2 Recommendations, Irish NA, Malaysian NA and Singapore NA;

kmin1
  =  
0.26
kmin2
  =  
0.0013
Note:

Note that there is no requirement to have a minimum area of compression reinforcement.

The minimum area of longitudinal tension reinforcement for crack control, As,min,cr is given by;3

As,min,cr
0.4*k*fctm*Acts

where

k
  =  
1.0 when h ≤ 300
0.65 when h ≥ 800
fctm
  =  
mean value of axial tensile strength of concrete
0.30*fck(2/3) for concrete grades ≤ C50/60
2.12*ln(1+((fck+8)/10)) for concrete grades > C50/60
σs
  =  
the interpolated reinforcement service stress from appropriate for the bar spacing of the reinforcement provided
Act
  =  
area of concrete in tension just before formation of first crack
  =  
b*y where y = the distance of the Elastic NA from bottom of beam

The minimum area of longitudinal tension reinforcement required, As,min,reqd, is then given by;

As,min,reqd
MAX (As,min, As,min,cr)

Maximum area of reinforcement

The maximum area of longitudinal tension reinforcement, Ast,max, is given by;4

Ast,max
kmax*Ac

The maximum area of longitudinal compression reinforcement, Asc,max, is given by;

Asc,max
kmax*Ac

where

kmax
  =  
the appropriate NDP value
Ac
  =  
the cross sectional area of the beam
  =  
h*bw

For design in accordance with UK NA,EC2 Recommendations, Irish NA, Malaysian NA and Singapore NA;

kmax
  =  
0.04
     
1 BS EN 1992-1-1:2004 Section 8.2(2)
2 BS EN 1992-1-1:2004 Section 9.2.1.1(1)
3 BS EN 1992-1-1:2004 Section 7.3.2(2)
4 BS EN 1992-1-1:2004 Section 9.2.1.1(3)
Was this helpful?
Previous
Next