Bent Up Bars In Beams

Section

Total ultimate load on beam = 200 x 8.0 = 1600kN

Support reaction = 1600/2 800 kN

Shear, Vtl at face of support = 800 - 200 x 0.3/2 = 770 kN

Shear. Vu distance ct from face of support = 770 200 x 0.65 = 640 kN

1. Check the crushing strength VRd maxof the concrete diagonal strut at the face of the beams support.

From equation 5.6 with 0 = 22

0.124 x 350 x 650(1 - 30/250)30 = 745 kN (<Vnf = 770kN)

From equation 5.7 with 0 45

= 0.18 x 350 x 650( 1 30/250)30 = 1081 kN (>VEf=770kN)

2. Determine angle 0 From equation 5.8(a)

or alternatively from equation 5.8(b)

3. Determine shear resistance of the links

The cross-sectional area /lsw of a 12mm bar = 113 mm2. Thus for the two legs of the link and a spacing of 175 mm

(or alternatively the value could have been obtained from table A4 in the Appendix)

From equation 5.11 the shear resistance, VRJ s of the links is given by

Therefore shear resistance of links 781 kN.

Design shear, VH<i distance d from the face of the support = 640 kN (< 781 kN). Therefore, the beam can support, in shear, the ultimate load of 200 kN/m.

4. Additional longitudinal (ensile force in the tension steel

It is necessary to check that the bottom tension steel has a sufficient length of curtailment and anchorage to resist the additional horizontal tension Ah\t caused by the design shear. These additional tension forces are calculated from equation 5.12. Therefore

This force is added to the MEd/«" diagram, as described in section 7.9. to ensure there is sufficient curtailment of the tension reinforcement and iis anchorage bond ^_length at the supports, as described in section 5.2._j

5.1.3 Bent-up bars

To resist shearing forces, longitudinal tension bars may be bent up near to the suppon-as shown in figure 5.5. The bent-up bars and the concrete in compression are considers to act as an analogous lattice girder and the shear resistance of the bars is determined b> taking a section X-X through the girder.

(a) Single System x | Anchorage length

Bentup Bars Beam

(a) Single System

Bent Bar Shear

(b) Multiple System

Figure 5.5

Bent up bars

(b) Multiple System

From the geometry of part (a) of figure 5.5. the spacing of the bent-up bars is:

and at the section X-X the shear resistance of a single bent-up bar (Vwd) must equal the shear force (VV.a)-

= Vfci = /j.wd<4Sw sin a = A,w sin a = 0.87/ykAsw sin a where is the cross-sectional area of the bent-up bar.

For a multiple system of bent-up bars, as in part (b) of figure 5.5. the shear resistance is increased proportionately to the spacing, .v. Hence:

0,, , . 0.9i/(cota + cotfl) VEd - 0.87/ykAsw sin a x-1

Vfct

This equation is analogous to equation (5.9) for the shear resistance of shear links. In a similar way it can be shown that, based on crushing of the concrete in the compressive struts, the analogous equation to (5.4) is given by:

and the additional tensile force to be provided by the provision of additional tension steel is given by a modified version of equation 5.12:

EC2 also requires that the maximum longitudinal spacing of bent-up bars is limited to 0.6i/(l +cola) and specifies that at least 50 per cent of the required shear reinforcement should be in the form of shear links.

5.1.4 Shear between the web and flange of a flanged section

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Responses

  • jouni salpa
    How to calculate the length of a bent up bar?
    9 years ago
  • nicol
    How to calculate length of bent up bars?
    7 years ago
  • oliwier
    WhAt is lattice factor for bent up bars?
    3 years ago
  • futsum
    Why bent up bars are provided in beams?
    2 years ago
  • Teegan Gibson
    What is meant by shear strength of bent up bars?
    12 months ago
  • Asmeret
    What is bent up bar in beam?
    6 months ago

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