C HAPTER 9 -D ETAILING RULES RELATIVE TO MEMBERS
The détailing of members concerns the minimum reinforcement and the provisions to take for the reinforcement [EC2-1-1 Sect.9].
I.M
INIMUMBENDINGREINFORCEMENTThe minimum value of reinforcement to place is defined in [EC2-1-1 9.2.1.1] for reinforced concrete beams and in [EC2-1-1 9.3.1.1] for reinforced concrete slabs. For prestressed structures, the minimum reinforcement is given by the national annex that requires arrangement of the minimum reinforcement required by [EC2-1-1 7.3.2].
An example is given in [Appendix VII-2.5].
II.S
URFACE REINFORCEMENTSurface reinforcement is dealt with in two places in Eurocode 2: [EC2-1-1 7.3.3(3)] and [EC2-2/AN 9.1(103)].
Clause [EC2-1-1 7.3.3(3)] allows calculation of surface reinforcement for the sides of very high beams (h > 1.0m), in tensioned zones only, when the major reinforcement is concentrated on a part of the height only.
For a rectangular beam in simple bending, of width b and tensioned height ht , the value of the reinforcement is given by:
As,peau = kc×k×fct,eff×Act/fyk = 0.4×0.5×fctm×b×ht / fyk
Clause [EC2-2/AN 9.1(103)] deals in a more general way with surface reinforcement for beam sides, at the same time in tensioned and compressed zones, parallel and perpendicular to the transverse section.
This surface reinforcement is not cumulative with the other steels calculated.
Application to a rectangular beam with height greater than 1.0m, a web thickness of 30cm, a concrete C30/37 and reinforcing steels fyk = 500 MPa:
According to clause [EC2-1-1 7.3.3(3)], the amount of surface reinforcement to place, on the beam sides in a tensioned zone is equal to:
As,peau = 0.4×0.5×2.9×0.30×1.0 / 500 = 3.50 cm² Or 1,75 cm²/m of surface.
Expressed as a percentage of the tensioned area, this value is equal to:
ρ = kc×k×fct,eff / fyk = 0.4×0.5×2.9 / 500 = 0.12%
According to [EC2-2/AN 9.1(103)], the surface reinforcement to place on all the beam’s perimeter is equal to 3cm²/m for the standard exposure classifications (XC4), or 5cm²/m in an aggressive environment (XD, XS).
Detailing Chapter 9 - Detailing rules relative to members
On the application it is stated that the calculation according to [EC2-1-1 7.3.3(3)] gives a value lower than the minimum of 3cm²/m given in [EC2-2/AN 9.1(103)]. In standard cases it is possible to be exempt from the calculation of [EC2-1-1 7.3.3(3)] and to use directly the fixed values in section 9.
III.S
HEARREINFORCEMENTIII.1.Minimum section of shear reinforcement
The corresponding clauses are in [EC2-1-1 9.2.2]III.1.1.Beams
A minimum transverse reinforcement will be required even in the case of elements not needing shear stress reinforcement.
The rate of shear stress reinforcement is given by the expressions:
ρw = Asw / (s × bw × sinα ) [EC2-1-1 Expr.(9.4)]
with a minimum recommended value confirmed by the national annex:
yk ck w,min =(0,08 f )/f
ρ [EC2-1-1 Expr.(9.5N)]
o ρw is the rate of shear reinforcement
o Asw is the area of the shear reinforcement section along the length s
o s is the spacing of the shear reinforcement measured along the longitudinal axis of the element o bw is the web width of the element
o α is the angle between the shear reinforcement and the longitudinal axis [EC2-1-1 9.2.2(1)].
Compared to previous practices the expression (9.5N) leads to less steel for concretes with low characteristic strength and to more steel for higher characteristic strengths..
0.0cm2/m 2.0cm2/m 4.0cm2/m 6.0cm2/m 8.0cm2/m 10.0cm2/m 12.0cm2/m
25 30 35 40 45 50
EC2 Asw/(s.bw)=0.08*racine(fck)/fyk*10000=
BPEL At/(st.bn)=0.4*gs/fyk*10000=
Fig./Tab.III.(1): Variation of minimum ratio of shear reinforcement with characteristic strength of concrete
III.1.2.Slabs
The minimum rate of transverse reinforcement and its minimum value seen above for beams applies equally for slabs. However the slabs that benefit from a transverse load redistribution are exempt from it [EC2-1-1 6.2.1(4) et 9.3.2].
If shear reinforcement are necessary the slab should be at least 200mm thick.
Bridge slabs come under the category of slabs allowing a transverse load redistribution. As such, they may not include shear reinforcement unless the stresses applied to them require it. [Chapter 6-II.2.1]
Eurocode 2 [EC2-1-1 9.3.1.4], however, requires that longitudinal and transverse construction reinforcement be planned for along the free edges as shown in the figure below
For standard-thickness slabs (between 20 and 35cm) and concretes of fck ≤ 50 MPa, a section of 2cm²/m will cover the minimum reinforcement condition near the edges.
Fig./Tab.III.(2): Principle of reinforcement with a free slab edge
III.2.Arrangements for shear reinforcement
The major detailing arrangements of shear stress reinforcement are represented in the figure below [EC2-1-1 9.2.2].
Eurocode 2 part 2 recommends not using open frameworks and open stirrups.
A combination of frameworks and bent-up bars is acceptable.
The inclination of the shear reinforcement should respect the following condition: 45° ≤ α ≤ 90°.
However, at least 50% of the shear reinforcement should be in the form of frameworks and stirrups (this is a recommended value taken up by the national annex).
Detailing Chapter 9 - Detailing rules relative to members
Fig/Tab.III.(3): Example of shear reinforcement Cadres, épingles et étriers intérieurs Inner link alternatives
Cadre extérieur Enclosing link
For beams of effective depth d, the maximum spacing of shear reinforcement courses is limited to:
Longitudinal direction sl,max= 0.75 d (1+cotα) [EC2-1-1 Expr.(9.6N)]
Transverse direction st,max= 0.75 d ≤ 600mm [EC2-1-1 Expr.(9.8N)]
For slabs, the maximum spacing in the longitudinal direction uses the same expression and the maximum transverse spacing is modified and limited to:
Transverse direction st,max ≤ 1.5 d (α = π/2)
With α = angle of inclination of reinforcement on the centroidal axis.
In the case of bent-up bars which are used less in practice, reference should be made to the text of Eurocode 2.
IV.T
ORSIONALREINFORCEMENTThe detailing arrangements of torsional reinforcement are given in [EC2-1-1 9.2.3] and are represented in the figure below.
The frameworks should be anchored by overlaps and/or hangers and be perpendicular to the beam axis.
The maximum spacing of the transverse torsional reinforcement courses is limited to:
Longitudinal direction sl,max = min (0.75 d; U/8; smallest dimension of section)
The maximum spacings of the longitudinal torsional reinforcement are limited to 350mm with at least one bar at each corner.
Previous regulations did not include particular specifications except for longitudinal reinforcement that had to be grouped together in the corners.
a) recommended configurations b) non-recommended configuration Fig/Tab.IV.(1): Examples of torsional reinforcement